Improvements in and relating to data centres

Abstract

A data centre (101, fig. 1) comprising a cold zone segregated from a hot zone by an air barrier. The air barrier comprises at least one pressure relief flap assembly 108 movable between a first closed

Description

The present invention concerns data centres and a method of cooling servers in a data centre. More particularly, but not exclusively, this invention concerns data centre buildings and methods of operating data centre buildings. Background of the Invention A data centre is understood to be a large group of networked computer servers, typically provided in a dedicated space within a building. For example, a modern data centre may include high density, high capacity IT systems in a specialist, dedicated space having a carefully controlled environment. A state of the art data centre may include multiple data halls each containing hundreds or even thousands of computer servers, typically contained in racks arranged in rows. In general, each server is provided with electrical power and a network connection. During operation, components of servers typically generate heat, which should be dissipated to avoid overheating and damage to equipment. Organisations, and also individuals, treat data centres as mission-critical facilities, meaning that even short periods of downtime due to equipment malfunction are not tolerated. With the increase in demand for 'cloud' computing services, there is a need not only for additional data centre facilities, but also higher capacity facilities. Alongside increases in computing power of individual servers, the number of servers in each rack, and the number of racks in each data hall increases with each new generation of data centres. Various measures have been used to compare and contrast modern data centres, including the number of servers, building area, and power consumption. According to Data Centre Magazine, a hyperscale data centre should exceed 5,000 servers and 10,000 square feet, and the three largest data centre facilities in the world have total areas exceeding 7 million square feet. Hyperscale data centres often include multiple data halls, each being a single continuous space for accommodating servers. An individual data hall may include IT servers having a total power consumption of 3 MW or more. Racks, or cabinets, housing servers are typically arranged in rows separated by air pathways, such as personnel aisles. In order to make efficient use of space, racks have become larger, and aisles longer. For example, a common modern rack design is capable of holding 42 servers stacked one on top the other (a '42U' rack). The dimensions of server racks have become highly standardised, particularly in terms of width. Racks are typically 600mm wide, and a 42U rack may have a height of about 2300 mm, a depth of 1050mm, and once fully loaded with servers may weigh as much as 2.4 tonnes. A single row of racks in the data hall of a Hyperscale data centre may be made up of 22 or more racks, with the data hall accommodating 14 or more rows. Such a high dead load weight concentrated in a relatively small area can present challenges in building design. For example, a single row of racks occupying a floor space of under 16 m2 (i.e. a row of racks taking up 15 m by 1.05 m) may weigh as much as 62.5 tonnes. Air flow management is considered especially important in modern data centres, where there is a continuing drive for lower PUE (power usage effectiveness) values. PUE is the ratio of total data centre power consumption to server power consumption, and so minimising energy use for equipment cooling is often a high priority. WO2010139921 (Bripco BVBA) describes an especially energy efficient data centre layout, in which the data centre is subdivided into segregated hot and cold zones, including alternating hot and cold aisles separated by rows of server cabinets. Each cold aisle is supplied with cooling air from a cold air corridor, that also functions as a personnel access corridor, that leads from an air handling unit. Modern data centre facilities often incorporate fail-safe mechanisms to protect sensitive equipment in the event of malfunction or incorrect system operation. Servers can become damaged rapidly if allowed to overheat, and given the relatively high density of servers in 'hyperscale' data centres in particular, overheating in just one part of a data hall can lead to damage of a large number of servers and severe disruption to services reliant on computing equipment in the data centre. Typically, modern data sensors use temperature and pressure sensors to monitor conditions in the data hall (e.g. in cold and / or hot aisles). It will be appreciated that failure of a sensor could lead to undetected overheating within the data centre. One approach to mitigate against such problems is for the building management system to monitor for irregularities in measurements, and tip control systems into a 'fail-safe' mode if sensor readings fall outside of expected ranges. For example, if a temperature sensor shows a sudden drop / increase in temperature outside what might be expected to be possible, and / or if a sensor provides readings inconsistent with neighbouring sensors. Similarly, the building management system may incorporate a manual override, allowing the operator to switch cooling systems into a fail-safe mode if they are concerned that equipment could be overheating. In fail-safe mode, air supply systems may increase flow of cooling air, for example to flood the data hall with additional cooling air in the vicinity of the sensor suspected to be faulty and / or the area of the data hall that the operator has identified as a potential hot-spot. It will be appreciated that providing servers with an unnecessary increase in supply of cooling air will not damage equipment, whereas starving equipment of cooling air can rapidly cause overheating and damage. To provide a high level of fail-safe protection, it may be desired to rapidly ramp up cooling air supply to the maximum, or near maximum, capacity of air supply equipment. However, a sudden and large increase in supply of cooling air can damage wall and / or ceiling panels in the data hall, particularly in data halls with panels providing segregation between hot and cold zones. For example, a sudden increase in the supply of cooling air into the cold zone of a data hall may create a significant pressure imbalance with the hot zone, and in some cases could dislodge or 'blow out' panels. Aside from the damage caused to the panels themselves, such a situation would likely exacerbate heat management problems since missing panels may provide a route for cooling air to bypass servers, precluding return to normal operation of the data centre. There remains a need for a data centre design that combines efficient use of space with cost-effective operational costs, convenient commissioning and maintenance, and reliable operation. The present invention seeks to mitigate various problems of the prior art. Alternatively or additionally, the present invention seeks to provide an improved data centre building, and method of cooling IT equipment in a data centre building. Summary of the Invention According to a first aspect, the invention provides a data centre comprising a cold zone segregated from a hot zone and at least one air handling unit (e.g. a plurality of air handling units) configured to provide cool supply air (also known as cooling air) into the cold zone and optionally to receive warm return air from the hot zone, wherein the hot zone is segregated from the cold zone by an air barrier. Optionally, the air barrier comprises at least one pressure relief flap assembly movable between a first position and a second position, wherein flow of supply air from the cold zone to the hot zone through a pressure relief opening is inhibited or prevented when the at least one flap assembly is in the first position (i.e. a closed position), and flow of supply air from the cold zone to the hot zone through the pressure relief opening is permitted when the at least one flap assembly is in the second position (i.e. an open position). It will be appreciated that the pressure relief opening, when open, provides fluid communication between the cold zone and the hot zone. Optionally, the at least one pressure relief flap assembly is configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl, and to reclose automatically when differential air pressure between the cold zone and the hot zone falls below a second threshold P2. Preferably, Pl is greater than P2. It may be that each of the pressure relief flap assemblies (when there are more than one) is arranged such that they each open and close automatically and such that the threshold differential air pressure required to open from the closed position is greater than the threshold differential air pressure below which the same pressure relief flap assembly automatically closes. Additionally or alternatively, the data centre of the first aspect of the invention comprises a cold zone segregated from a hot zone by an air barrier, the air barrier comprising a pressure relief flap movable between an open position in which air is permitted to flow via an opening from the cold zone to the hot zone and a closed position, in which the flap closes the opening, the pressure relief flap being urged by a return force towards its closed position and being configured to move automatically from the closed position to the open position in response to a differential air pressure between the cold zone and the hot zone, and to reclose automatically at a lower differential air pressure, such that the force required to move the flap from the closed position is greater than the force required to prevent the flap from moving back to the closed position, once so open. It will be understood that air flow through the pressure relief opening can be deemed "inhibited" when there is a significantly greater resistance to airflow (for example as compared to when air flow is permitted through the pressure relief opening), for example when air flow, if any, through the pressure relief opening is less than about 0.1 m3 / s at a pressure differential of 100 Pa from one side of the pressure relief opening to the other. Air flow through the pressure relief opening can be considered as being "prevented" when no air flow is detected (i.e. air flow is less than about 0.01 m3 / s, or less than about 0.001 m3 / s) at a pressure differential of 100 Pa from one side of the pressure relief opening to the other. It will be understood that differential air pressure is the difference in air pressure between two zones. More particularly, as used herein the term 'differential air pressure' means cold zone air pressure minus hot zone air pressure. In other words, differential air pressure is positive when cold zone air pressure is higher than hot zone air pressure. For example, if air pressure in the cold zone is 101.45 kPa, and air pressure in the hot zone is 101.35 kPa, the differential air pressure is 100 Pa (0.1 kPa). As used herein, 'over-pressure' of the cold zone refers to conditions where the differential air pressure is above a first threshold (e.g. Pl, or Pl(l)). It will also be appreciated that air pressure may vary across the cold zone. For example, the pressure in some areas may be higher than the pressure in others. When reference is made herein to 'cold zone pressure', 'hot zone pressure' or 'differential air pressure', what is meant is air pressure / differential air pressure in the cold / hot zone at or adjacent said flap assembly. It has been found that providing such a flap assembly over an opening between hot and cold zones allows excessive air pressure in the cold zone to be dissipated rapidly without loss of cooling air. For example, such positioning can allow air, which is released from the cold zone to reduce over-pressure, to be returned to the at least one air handling unit for re-use. Such released air is likely to have already been conditioned to an appropriate temperature and / or humidity, and so returning that air to an air handler may be more efficient than, e.g. exhausting the air out of the data centre building (in which case exhausted air may be replaced by ambient air from outside the building). It has been found that having a first threshold Pl higher than the second threshold P2 can allow for better control over airflow in the data centre. In particular, having a resistance to opening that is higher than the reclose force may allow the flap assembly to rapidly move to a more fully open position when Pl is exceeded, allowing a significant and rapid pressure drop in the cold zone. In contrast, it has been found that when opening and reclosing forces are balanced (i.e. Pl is equal to P2), a pressure relief damper may act more as a bleed valve, effectively maintaining the pressure differential at Pl. Embodiments of the present invention allow Pl and P2 to be set to avoid the damper opening unnecessarily, remaining open unnecessarily and repeatedly opening and closing unnecessarily. It will be appreciated that cooling air passing through the flap assembly effectively bypasses IT equipment without being used for cooling, and so the cooling potential of such air is wasted, and the cooling system operates inefficiently, when the flap assembly is open. It will be understood that the data centre may, for example, be configured to maintain a pressure differential between the cold zone and the hot zone, such that air pressure in the cold zone is higher than air pressure in the hot zone (e.g. at least about 3 Pa higher, such as at least about 5 Pa higher). It has been found that maintaining a moderate positive pressure in the cold zone can help to ensure an adequate supply of cooling air to servers in the data centre. It will be appreciated that one flap assembly may, for example, comprise or consist of a single flap or a plurality offlaps. Each of a plurality offlaps of the same flap assembly may be configured to move independently or be connected so that they move together. It has been found that an assembly with a single flap can provide for particularly reliable and simple operation. It will be appreciated that 'automatic' opening and reclosing means that the at least one flap assembly will open and reclose without requiring any intervention from the data centre operator. It has been found that such an arrangement can ensure that the flap assembly responds to any over-pressure in the cold zone rapidly and reliably. Optionally, the flap assembly operates independently of a central building control system. It will be understood that in such an arrangement, the flap assembly does not require any command from a building management or control system to operate. It has been found that such an arrangement can ensure that the flap assembly operates in the event of control system malfunction or failure, which may be especially important if cold zone over-pressure is a result of control system malfunction or failure. Optionally, the at least one flap assembly opens and recloses automatically under mechanical control, for example wherein the flap assembly is free from electronic actuation or control systems (e.g. if the flap assembly is mounted in a pressure relief damper unit, the unit is free from electronic actuation or control systems). It has been found that mechanical automation of the at least one flap assembly improves reliability, particularly in the event of localised power loss, control malfunction or error, and reduces maintenance requirements. Optionally, the or each flap is a hinged or pivotable flap. Optionally, the flap assembly is mounted in a pressure relief damper unit. Optionally, the pressure relief damper unit comprises an indicator for showing that the at least one flap assembly has opened and reclosed, for example since a previous inspection of the pressure relief damper unit. Optionally, the indicator is a replaceable and / or resettable visual indicator, for example which can be read by a building operator when performing an inspection of the data centre. Additionally or alternatively, the indicator comprises an electronic sensor, for example a sensor arranged to communicate with a building management system to report on automatic operation of the pressure relief damper unit. Optionally, the pressure relief damper unit comprises a frame to which the or each flap is secured by a hinge or pivot. For example, such a frame may be attached around or to the perimeter of the opening, optionally with an air-tight seal between the frame and other parts of the barrier. Optionally, the pressure relief damper unit comprises at least one closure device for holding the at least one flap assembly in the first closed position when the differential air pressure is below the first threshold Pl. Optionally, the closure device comprises a latch assembly. Optionally, the closure device comprises at least one magnet for retaining the at least one flap assembly in the first closed position. For example, when the pressure relief damper unit comprises a frame to which at least one flap is connected by a pivot and / or hinge, one or more magnets may be secured to the frame and / or the flap. In such an arrangement, the one or more magnets may be arranged for example to engage with one or more corresponding magnets or magnetic components on the flap and / or the frame (i.e. a magnet on the frame may engage with a magnet or magnetic component on the flap, and vice versa). It may be that the or each magnetic component is in the form of a ferromagnetic plate or other type / shape of material that is magnetically attracted to a permanent magnet. It may be that the frame and / or flap is made at least in part from a material that is inherently attracted to a permanent magnet, obviating the need for a separately provided magnetic component. Preferably, the one or more magnets are permanent magnets (as opposed to, for example, electromagnets). It has been found that a magnet closure system may provide a reliable and consistent force resisting opening, thereby allowing the at least one flap assembly to open at a pre-determined differential air pressure. Optionally, the closure device comprises a plurality of magnets, for example a plurality of magnets mounted on each flap and / or a plurality of magnets mounted on the frame. It will be appreciated that having a plurality of magnets may help to avoid undue distortion of the flap, such as when subjected over-pressure in the cold zone. Optionally, the at least one closure device is configured to hold the flap assembly in the first closed position when differential air pressure is below the first threshold Pl. For example, the cold zone-facing surface area and weight of the flap assembly, and the retaining force of the closure device (e.g. a latch assembly and / or magnets) are configured to control the first threshold Pl. It will be appreciated that a flap assembly having a large surface area facing the cold zone will require a higher retaining force to maintain the flap assembly in the first closed position when differential air pressure is below the first threshold Pl. Similarly, it will be appreciated that gravitational forces acting on the flap assembly should be taken into consideration when specifying the retaining force of the closure device. Optionally, the at least one flap assembly is configured to reclose with the assistance of gravity. For example, the flap assembly may be arranged so that the centre of gravity of each flap when in the first closed position is located at a point lower than the centre of gravity of the flap when in the second open position. Optionally, the flap assembly is arranged (e.g. positioned) to open upwards. Optionally, each flap may be a planar flap having a lower surface facing the cold zone and an opposed upper surface facing the hot zone. In such arrangements, each flap preferably moves generally into or towards the hot zone when moving from the first closed position to the second open position. Optionally, the flap assembly is movable from the second open position to the first closed position entirely under the control of gravity (e.g. the flap assembly is mounted in a pressure relief damper unit free from any device or mechanism for urging or causing the flap assembly to reclose). It has been found that a flap assembly that recloses under gravity is particularly reliable and consistent in operation. Optionally, the flap assembly is arranged / configured so that it recloses under gravity when differential air pressure reaches or drops below the second threshold P2. For example, the cold zone-facing surface area and weight of each flap are configured to control the second threshold P2. It will be appreciated that a flap having a larger surface area facing the cold zone will be more resistant to returning to the first closed position as cold zone air passing through the pressure relief opening presses against it. It will be appreciated that when the flap assembly is to be moved back to the first closed position by gravity, gravitational forces acting on the flap assembly to urge it towards the first closed position should overcome forces exerted by air pressure pushing the flap assembly open when differential air pressure is at or below the second threshold P2. Optionally, the pressure relief damper unit comprises a reclosure device for urging the flap assembly from the second open position to the first closed position. Optionally the reclosure device comprises a spring, for example a spring arranged to press against the flap assembly in order to urge the flap assembly towards the first closed position. Optionally, when the pressure relief damper unit comprises a reclosure device, the configuration of the closure device takes account of any additional closure force provided by the reclosure device holding the flap assembly in the first closed position. For example, the cold zone-facing surface area and weight of each flap, and the retaining forces of the closure device (e.g. magnets) and the reclosure device (e.g. spring) are configured to control the first threshold Pl. Optionally, the flap assembly is movable from the second open position to the first closed position under the control of the reclosure device and optionally gravity. Optionally, the flap assembly is movable from the second open position to the first closed position entirely under the control of the reclosure device (e.g. the unit is arranged so that gravity acts against movement of the flap assembly from the second open position to the first closed position). Optionally, the unit is configured and / or arranged so that the flap assembly recloses under control of the reclosure device when differential air pressure reaches or drops below the second threshold P2. For example, the cold zone-facing surface area, orientation and weight of each flap and closure force of the reclosure device are configured to control the second threshold P2. It will be appreciated that when the flap assembly is to be moved back to the first closed position under control of the reclosure device, the closure force exerted by the reclosure device (optionally with the assistance of gravitational forces urging the flap assembly closed) should overcome force exerted by differential air pressure pushing the flap assembly open (and optionally any gravitational forces urging the flap assembly open) when the differential air pressure is at or below the second threshold P2. Optionally, the first threshold Pl is at least about 70 Pa, such as at least about 90 Pa. Optionally, the first threshold Pl is in the range of from about 70 Pa to about 200 Pa, such as about 90 Pa to about 160 Pa. It has been found that such first threshold Pl values can avoid damage to data centre air entrainment components, such as blanking panels and / or ceiling panels, while also allowing significant levels of airflow when cooling demand is high, especially when it is high and rising. It will be understood that a particular first threshold Pl should be chosen depending on the airflow design requirements of the data centre. Optionally, the second threshold is at least about 3 Pa, such as at least about 5 Pa. It has been found that such second threshold P2 values can avoid backflow of air from the hot zone to the cold zone. Optionally, the second threshold P2 is no more than about 25 Pa, such as no more than about 15 Pa. It has been found that such second threshold P2 values can allow the flap assembly to remain at least partially open if the oversupply of cooling air continues at the same level that caused Pl to be exceeded or if cooling air oversupply increases further. It may be that the flap assembly remaining at least partially open while differential air pressure drops can help maintain cold zone pressure at a stable level and / or avoid rapid low pressure / high pressure cycles in the cold zone, for example with air pressure cycling between P2 and Pl as the flap assembly recloses and reopens. Optionally, the second threshold P2 is in the range of from about 3 Pa to about 25 Pa, such as about 5 Pa to about 15 Pa. Optionally, P2 (when measured in Pa) is about 1-15%, such as about 3-10% of Pl (when measured in Pa). Optionally, Pl is greater than P2 by at least about 10 Pa and preferably by at least about 50 Pa. Optionally, each pressure relief flap assembly is sized and configured to permit a flow rate of air through the pressure relief opening of at least about 2 m3 / s, such as at least about 4 m3 / s, at a differential air pressure of 100 Pa. Optionally, each pressure relief flap assembly is sized and configured to permit a flow rate of air through the pressure relief opening of about 2 m3 / s to about 8 m3 / s, such as about 4 m3 / s to about 6 m3 / s, at a differential air pressure of 100 Pa. Optionally, the at least one pressure relief flap assembly provides an open cross-sectional area when fully open (also referred to as a free vent area) of at least 0.3 m2, such as at least 0.6 m2. Optionally, the at least one pressure relief flap assembly provides an open cross-sectional area when fully open of 0.3 m2 to 1.5 m2, such as 0.6 m2 to 1 m2. It will be appreciated that 'fully open' is the operating position in which the flap assembly is open to its greatest extent. Optionally, the air barrier comprises a plurality of pressure relief flap assemblies, e.g. comprising at least one primary flap assembly and at least one secondary flap assembly. Optionally, at least one primary flap assembly and at least one secondary flap assembly are positioned at least partially over a common pressure relief opening (e.g. the primary and secondary flap assemblies together closing a single pressure relief opening when the flap assemblies are each in their first closed positions). Additionally or alternatively, a primary flap assembly is positioned over a primary pressure relief opening in the air barrier, and a secondary flap assembly is positioned over a secondary pressure relief opening (e.g. a primary flap assembly closes a primary pressure relief opening, and a secondary flap assembly closes a secondary pressure relief opening, e.g. separate from the primary pressure relief opening, when the flap assemblies are each in their first closed positions). Optionally, primary and secondary flap assemblies are mounted in a single pressure relief damper unit, for example a plurality offlap assemblies is mounted in a single frame structure. Additionally or alternatively, a primary flap assembly is mounted in a primary pressure relief damper unit, and a secondary flap assembly is mounted in a secondary (e.g. separate) pressure relief damper unit. Optionally, the first and second threshold pressures (Pl, P2) of the primary and secondary flap assemblies are set independently. For example, a primary flap assembly may have a first threshold pressure Pl that is the same as or different to the first threshold pressure Pl of a secondary flap assembly; and / or a primary flap assembly may have a second threshold pressure P2 that is the same as or different to the second threshold pressure P2 of a secondary flap assembly. Optionally, said primary flap assemblies are configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl(l), and to reclose automatically when differential air pressure between the cold zone and the hot zone falls below a second threshold P2(l), preferably wherein Pl(l) is greater than P2(l); and said secondary flap assemblies are configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl(2), and to reclose automatically when differential air pressure between the cold zone and the hot zone falls below a second threshold P2(2), preferably wherein Pl(2) is greater than P2(2). Optionally, Pl(2) is greater than Pl(l). In other words, said primary flap assemblies open at a lower cold zone / hot zone pressure differential than the secondary flap assemblies. Optionally, Pl(2) is at least about 30 Pa greater than Pl(l), for example at least about 40 Pa greater that Pl(l). Optionally, Pl(2) is about 30 to 70 Pa greater than Pl(l), such as about 40 to 60 Pa greater than Pl(l). Optionally, Pl(2) (when measured in Pa) is about 25-100%, such as about 40-60%, greater than of Pl(l) (when measured in Pa). It has been found that having primary / secondary flap assemblies with different opening threshold pressure differentials can provide further protection against overpressure while also avoiding excessive bypassing of air and / or pressure drop through the opening of more flap assemblies than necessary to reduce the overpressure to acceptable levels. For example, if overpressure in the cold zone results from a relatively modest or temporary increase in supply of cooling air, one or more primary flap assemblies opening may allow rapid reduction in the overpressure while avoiding unnecessary loss of cooling air from the cold zone. On the other hand, if overpressure results from a large or prolonged increase in supply of cooling air, one or more secondary flap assemblies may provide for larger amounts of cooling air to exit the cold zone. Optionally, P2(l) and P2(2) are substantially the same, for example wherein P2(l) is within about 5 Pa of P2(2), such as when P2(l) is within 2 about Pa of P2(2), e.g. P2(l) is about equal to P2(2). It will be understood that absolute and relative values of Pl(l), P2(l), Pl(2) and P2(2) may, for example, fall within the ranges described above for Pl and P2. Optionally, the data centre comprises at least one data level divided into said cold zone and hot zone. Optionally the data centre comprises a plurality of data levels, such as at least three data levels. Optionally, each data level accommodates at least fifteen, optionally at least twenty, rack storage areas positioned on a floor. Each rack storage area may have a width and a length and is arranged to accommodate a row of at least ten, such as at least twenty, server racks. Each server rack may be capable of accommodating at least 20 servers, such as at least 30 servers. Preferably, the rack storage areas separate alternating hot aisles and cold aisles, the cold aisles together forming at least a portion of the cold zone and the hot aisles together forming at least a portion of the hot zone. Each cold aisle and each hot aisle may have a width and a length. Optionally, the data centre (such as each level of the data centre) comprises a plurality of air handling units for supplying cooling air to the servers via the cold aisles, for example for supplying cooling air into the cold zone. Preferably, each air handling unit comprises an adiabatic cooling unit. Optionally, the data centre (for example each level of the data centre) comprises a cold air supply corridor forming a portion of the cold zone, the cold air supply corridor being arranged to transport cold air from the air handling units to the cold aisles, and preferably being suitable for providing personnel access to the cold aisles. The cold air supply corridor is in fluid communication with the cold aisles, for example through openings arranged along a first side of the corridor. Optionally, said openings are provided with doors having adjustable vents for controlling air flow into the cold aisle from the cold corridor. Optionally, the adjustable vent is controlled in dependence on cold aisle air pressure and / or differential air pressure between the cold aisle and a neighbouring hot aisle. Such a door with an adjustable vent may help to direct airflow towards cold aisles with high demand for cooling air. It will be appreciated that overpressure may develop in the cold air supply corridor particularly quickly if such adjustable vents are partially or fully closed. Examples of suitable vented doors are disclosed in WO2010139921 (Bripco BVBA), the contents of which are incorporated herein by reference. In use, the cold air supply corridor is in fluid communication with at least one air handling unit (optionally a plurality of air handling units), for example through one or more supply openings arranged along a second side of the corridor (e.g. opposite the first side). Optionally, the air barrier comprises a cold corridor portion separating the cold corridor from the hot zone. Optionally said at least one pressure relief flap assembly is positioned in the cold corridor portion of the air barrier, thereby providing fluid communication between the cold corridor and the hot zone when the at least one pressure relief flap assembly is in the second open position. It has been found that positioning said pressure relief flap assembly in the cold corridor can allow cold zone overpressure to be relieved prior to a build-up of air pressure in cold aisles. For example, if overpressure results from a sudden increase in supply of cooling air into the cold corridor by an air handling unit, pressure relief flap assemblies opening in the cold corridor may provide a route for excess cooling air to exit the cold corridor prior to entry into the cold aisles. Optionally, the cold corridor portion of the air barrier comprises x pressure relief openings and the data centre comprises y air handling units, wherein x is equal to or greater than y. Optionally, the air barrier comprises a plurality of primary pressure relief flap assemblies distributed along the cold air supply corridor, a first secondary pressure relief flap assembly located towards a first end of the cold air supply corridor, and a second secondary pressure relief flap assembly located towards a second opposed end of the cold air supply corridor. Optionally, the plurality of primary flap assemblies include at least one intermediate primary flap assembly positioned between the secondary flap assemblies and at least one primary flap assembly located adjacent each secondary flap assembly. It will be understood that flap assemblies are adjacent when spaced apart by less than 1 m, such as less than 0.5 m. Optionally, the at least one intermediate primary flap assembly is spaced apart from each of the first and second secondary flap assemblies by at least 2 m, such as at least 3 m. It will be understood that primary and secondary flap assemblies may incorporate any features of primary and secondary flap assemblies described herein above. It has been found that such positioning and arrangement of primary and secondary flap assemblies can provide a particularly effective arrangement. For example, airflow models suggest that overpressure often develops towards the centre of the cold corridor and spreads out towards the ends, and in that scenario such positioning of the flap assemblies may allow the primary flap assemblies to open first, leaving the secondary flap assemblies to open only if overpressure continues to develop to the point that the pressure differential threshold of the secondary flap assemblies is reached at one or both ends of the corridor. Optionally, the cold air supply corridor comprises: a plurality of supply openings for receiving cooling air from at least one air handling unit, the plurality of supply openings including at least one outer supply opening positioned towards each opposed end of the cold air supply corridor and at least one intermediate supply opening positioned along the cold air supply corridor between the first and second outer supply openings; and a plurality of pressure relief flap assemblies, including at least one primary pressure relief flap assembly located proximate each intermediate supply opening (e.g. proximate each supply opening), and at least one secondary pressure relief flap assembly located proximate each outer supply opening. It will be understood that a pressure relief flap assembly is proximate a supply opening when the centre position of the flap assembly along the length of the air supply corridor is within 4 m, such as within 2 m, of the centre position of the supply opening along the length of the supply corridor. It will be appreciated that the flap assembly and the supply opening may be positioned independently at any point around the periphery of the corridor, such as in the walls, ceiling or floor, preferably walls or ceiling, of the corridor, and that a position along the length of the corridor is the distance from one end of the corridor. Optionally, the data centre comprises a first outer air handing unit fluidly communicating with the cold air supply corridor through at least one said outer supply opening positioned towards a first end of the cold air supply corridor, a second outer air handling unit fluidly communicating with the cold air supply corridor through at least one said outer opening positioned towards a second end of the cold air supply corridor, and at least one intermediate air handling unit fluidly communicating with the cold air supply corridor through at least one said intermediate supply opening, for example wherein each air handling unit is positioned alongside the cold air supply corridor such that the intermediate air handling unit is positioned between the first and second outer air handling units. Optionally, the cold air supply corridor may be positioned between, such as sandwiched between, the air handling units and the cold aisles. Optionally, the air handling units may supply cooling air into a first side of the corridor, and the cold aisles may receive cooling air from a second opposed side of the corridor. Additionally or alternatively, the air handling units and cold aisles may be distributed along the length of the cold corridor on opposed sides, for example wherein an end of each cold aisle communicates with the cold corridor. Optionally, the cold air supply corridor extends perpendicular to the cold aisles. For example, the cold air supply corridor has a width and a length, the length extending perpendicular to the cold aisles. Optionally, the air barrier comprises at least one said pressure relief flap assembly proximate each air handling unit, for example in line with at least a portion of each air handling unit along the length of the cold corridor. Optionally, at least one of said pressure relief openings is positioned along the cold corridor opposite, above or adjacent (preferably opposite or above) each air handling unit along the corridor. It has been found that providing such a pressure relief opening proximate to each air handling unit helps to provide rapid relief from overpressure, for example when one air handling unit supplies excess cooling air. Optionally, the data centre (e.g. each level of the data centre) comprises a warm air return path for transporting warm air from the hot aisles to the air handling units, the warm air return path forming a portion of the hot zone. Optionally, cold aisles are positioned along a first side of the cold air supply corridor, and the plurality of air handing units is positioned along a second side of the cold air supply corridor opposite the first side (and thus opposite the cold aisles). Optionally, the rack storage areas, cold aisles, hot aisles, cold air supply corridor, warm air return path, and the air handling units are positioned on one floor of the data centre. It has been found that such an arrangement confers a number of advantages. For example, a cold air supply corridor allows for high volumes of cooling air to be distributed to cold aisles at low velocity, the corridor allowing air from the air handling units to mingle and provide all cold aisles with cooling air even in the event of limited performance or failure of one or more air handling units. Furthermore, positioning multiple air handling units along the cold air supply corridor (e.g. along a side of the corridor) has been found to allow an especially high density of racks on each level, for example by allowing longer aisles with larger server racks without compromising availability of cooling air. The present inventors have further recognised that positioning air handling units along a common air supply corridor helps to make efficient use of space inside the building, for example because equipment redundancy can be provided across the level, rather than for each cold aisle, while also keeping air handling equipment in close proximity to the cold aisles. Yet further, such an arrangement allows each data level to be more self-contained, allowing for a staged commissioning of the building. Optionally, the cold corridor transports cooling air to at least five cold aisles (e.g. at least seven cold aisles), and receives cooling air from at least three air handling units (e.g. at least four air handling units). It will be understood that a rack storage area is a space sized and configured to accommodate a row of server racks, for example a row of at least 20 server racks. Optionally, the server racks that may be accommodated are 42U or larger server racks, meaning racking capable of holding 42 or more servers. In an air-cooled data centre, IT equipment (which becomes hot during use) is cooled by contact with cooling air. Such cooling air may be supplied by one or more air handling units. As used herein: a 'cold aisle' of an operational data centre is a space adjacent a row of racks (for example between opposed rows of racks) from which rack-mounted IT equipment is able to draw cooling air; a 'hot aisle' is a space adjacent a row of racks (for example between opposed rows of racks) into which rack-mounted IT equipment is able to expel warm air. Typically, operational efficiency is improved by segregating hot aisles and cold aisles, thereby avoiding cooling air in a cold aisle mixing with warm air from a hot aisle before being used to cool IT equipment. Optionally, each cold aisle has a width of from 1 m to 2 m, such as 1.4 m to 1.6 m. Optionally, each hot aisle has a width of from 1 m to 2 m, such as 1 m to 1.4 m. Optionally, each rack storage area has a width of from 1 m to 1.5 m, such as about 1.2 m. Optionally, the cold and / or hot aisles are sized to permit personnel access. Optionally, each data level comprises at least three, such as at least four air handing units in fluid communication with the, or each, cold air supply corridor. Optionally, each data level comprises no more than eight, such as no more than six, air handling units in fluid communication with the, or each, cold air supply corridor. It will be understood that each data level may, for example, comprise one or more additional cold air supply corridors and associated cold and hot aisles, air handling units and warm return space. Optionally, each such cold air supply corridor, associated cold and hot aisles, air handling units and warm return space forms a data hall. Optionally, each data level comprises a plurality of data halls, such as at least three data halls, for example at least four data halls. Optionally, the data centre is a multi-level data centre comprising a plurality of data levels, for example at least four, optionally at least five, data levels. Optionally, the data centre comprises a plurality of server racks located in the rack storage areas. Optionally, each server rack accommodates at least 20, such as at least 30, for example at least 40, servers. Optionally, the racks together with at least a portion of the air barrier segregate the hot and cold aisles, for example wherein the air barrier comprises a plurality of air entrainment panels arranged to cooperate with racks to segregate the hot and cold aisles. Optionally, the data centre building is a hyperscale data centre building, for example accommodating at least 5,000 servers (rack-mountable items of IT equipment), and / or having an IT floor-space of at least 10,000 square feet. It will be understood that IT floor-space is the sum total of space occupied by rack storage areas, cold aisles and hot aisles, in the data centre building. Optionally, each data level comprises at least 15 rack storage areas each able to accommodate at least 20 racks, for example wherein each data level is configured to accommodate at least 300 racks, for example 300 racks capable of housing at least 40 severs. In such an arrangement, each data level is optionally capable of accommodating at least 12,000 servers. Optionally, each data level has a single IT area comprising rack storage areas, cold aisles and hot aisles. It will be appreciated that each data level may for example accommodate two or more separate IT areas. Optionally, each IT area is configured to accommodate at least 12,000 servers. Optionally, each IT area occupies a floor space of at least 1,000 square metres. A hyperscale data centre building may be one that is configured, when fully operational, to be capable of operating at total power levels of at least 40MW. Optionally, the cold air supply corridor has a length extending perpendicular to and being in fluid communication with the cold aisles of the data level, the cold aisles being arranged to receive from the cold air supply corridor cooling air supplied to the cold air supply corridor by a plurality of air handling units. A suitable cold corridor / cold aisle arrangement is described in WO2010139921 (Bripco BVBA), the contents of which are incorporated by reference. Specifically, that document describes a data centre layout which utilises personnel space for transport of large volumes of cooling air at low velocity, improving operating efficiency and making better use of space. Preferably, the air supply corridor is a personnel corridor, for example having a height of at least 3 m, such as at least 3.5 m, and a width of at least 2 m, such as at least 2.5 m, and optionally a length of at least 30 m, such as at least 40 m. Optionally, the air handling units are positioned alongside and distributed along the length of the cold air supply corridor. Optionally, during operation of the data centre, cooling air from the air handing units enters the corridor through openings in the side of the corridor along its length. In such an arrangement, pressure relief openings may be provided, for example, opposite and / or above said openings along the length of the cold air supply corridor. Optionally, the warm air return space is a warm air plenum. Optionally, the warm air plenum extends above at least a portion of: the cold aisles, the cold air supply corridor, and / or and the air handling units. It has been found that such an arrangement provides a particularly space efficient layout, for example by avoiding the need for extra floor space to accommodate the warm air return space. Optionally, the air barrier segregates the warm air plenum from one or more of: the cold aisles, and the cold air supply corridor. Optionally, the air barrier comprises a plurality of air entrainment panels. Optionally, one or more of said air barrier pressure relief openings provide direct fluid communication between the cold corridor and the warm air plenum, for example when the pressure relief flap assembly(s) of said openings are in the second open position. Optionally, the air barrier (for example when in the form of one or more air entrainment panels) is at least partially suspended from a data level ceiling structure, and preferably entire suspended from the ceiling structure. It will be understood that the ceiling structure may be part of the loadbearing structure of the building, for example forming the floor of a level above. Optionally, the plurality of data levels comprises an uppermost data level, the ceiling structure of the uppermost data level forming at least a portion of a roof of the multi-level data centre. Additionally or alternatively the plurality of data levels comprises a lowermost data level, the floor of the lowermost data level being the ground floor of the multi-level data centre. Optionally, each data level includes one or more cold aisle service cassettes suspended from the ceiling structure. Service cassette systems are also disclosed in WO2017129448 (Bripco BVBA), the contents of which are incorporated by reference. Specifically, that document discloses cassettes suspendable from a building structure and equipped with air entrainment panels and data centre services. Preferably, each cold aisle service cassette comprises components of a plurality of data centre services, such as at least three services. For example, components may include a conduit for fire suppressant fluid, a cable tray, electrical power components (such as cabling and / or a bus-bar), networking cabling, lighting equipment and / or sensor equipment (such as motion, temperature and / or humidity sensor equipment). Optionally, each cold aisle service cassette comprises a portion of the air barrier formed by the one or more aisle air entrainment panels configured to cooperate with racks of IT equipment to segregate the hot and cold aisles. Optionally, each cold aisle service cassette comprises a portion of the air barrier formed by one or more ceiling air entrainment panels configured to segregate cold aisles from a warm air plenum disposed between the ceiling air entrainment panels and said ceiling structure, said warm air plenum being in fluid communication with one or more hot aisles adjacent said cold aisle. For example, each data centre level may comprise a warm air plenum for receiving hot air from hot aisles and transporting hot air to air handling units and / or exhaust vents on an outer wall of the data centre. Optionally, said warm air plenum is positioned above the cold aisles, for example being segregated from said cold aisles by a portion of the air barrier, e.g. formed by one or more ceiling air entrainment panels. Optionally, the data centre is arranged so that warm air has a direction of flow through the hot zone, for example wherein the hot zone comprises a warm air flow path extending from an upstream location (e.g. adjacent to at least one rack storage area) to a downstream location (e.g. adjacent to at least one hot air exhaust outlet and / or air handling unit recirculation inlets). It will be appreciated that, in use, warm air generally flows along the warm air flow path away from the upstream location towards the downstream location. Optionally, the hot zone has a cross-sectional area that varies along the length of the warm air flow path, for example the hot zone having a transition portion with a cross-sectional area that increases along its length from a first end to a second end. Preferably, the first end is upstream of the second end along the warm airflow path. In other words, warm air flows through the transition portion from the first end to the second end during use of the data centre. Optionally, the at least one pressure relief flap assembly is positioned to permit direct fluid communication between the cold zone and the transition portion of the hot zone, and / or between the cold zone and a portion of the hot zone immediately adjacent and downstream of the transition portion, when in the second open position. It has been found that in such an arrangement, airflow in the hot zone may provide less resistance against the flap assembly opening and better facilitate escape of cooling air from the cold zone through the pressure relief opening. Optionally, the transition portion of the hot zone is located in a warm air plenum, for example above cold aisles and / or the cold corridor, preferably above the cold corridor. Optionally, the warm air plenum has an internal height that increases along the length of a transition portion (e.g. from a first end of the transition portion to a second end), thereby providing an increase in cross-sectional area of the warm air plenum along the length of the transition portion. Optionally, at least a portion of the air barrier forms a bottom of the warm air plenum and at least a portion of a ceiling of the cold corridor, for example wherein said portion of the ceiling includes a sloped ceiling section above which the internal height of the warm air plenum increases from one end of the sloped ceiling section to the other. It will be appreciated that such a sloped ceiling may, for example, provide a corresponding reduction in internal height of the cold corridor from said one end of the sloped ceiling section to the other. It will be understood that the internal height of the plenum may, for example, be the distance between the air barrier forming the bottom of the warm air plenum and a surface (such as part of the building structure) forming the top of the warm air plenum, while the internal height of the corridor may, for example, be the distance between the air barrier forming the ceiling of the corridor and a surface (such as part of the building structure) forming the floor of the corridor. Optionally, said at least one pressure relief opening is located in said portion of the ceiling of the cold corridor, for example in the sloped ceiling section. Optionally, the at least one pressure relief flap assembly is located in the portion of the air barrier forming said sloped section of the ceiling of the cold corridor. Optionally, the sloped ceiling section is angled at 45° to 90° from horizontal, such as 45° to 70° from horizontal. It will be understood that the floor of the data centre building is horizontal, and 90° from horizontal is vertical. Preferably, the at least one flap of the pressure relief flap assembly is substantially coplanar with the sloped portion when in the first closed position. Additionally or alternatively, the at least one flap is planar and angled at 45° to 90° from horizontal, such as 45° to 70° from horizontal when in the first closed position. Preferably, the pressure relief flap assembly is arranged to open upwards (e.g. into the warm air plenum). It has been found that such arrangements are particularly advantageous. For example, locating pressure relief openings in the cold corridor allows overpressure to be controlled before excess pressure develops in the cold aisles, positioning pressure relief openings in the ceiling avoids unwanted contact between the flap assembly and personnel, and an upwards opening flap assembly may be able to reclose under gravity. Optionally, each air handling unit has a first end for receiving air to be treated and a second end for discharging treated air. Preferably, the first end is opposed to the second end and the adiabatic cooling unit is positioned between the first end and the second end. Optionally, the second end is positioned at the second side of the cold air supply corridor. Optionally, the air handling unit has an airflow path that extends in a straight line from the first end to the second end, for example wherein the airflow path extends in a direction parallel to the length of the cold aisles and perpendicular to the length of the cold air supply corridor. It has been found that with such an arrangement, cooling air provided by the air handling units tends to flow efficiently across the cooling air supply corridor into ends of the cold aisles. Optionally, the data centre comprises a plurality of controllable intake vents for admitting ambient air into the data centre building, optionally wherein each intake vent is in fluid communication with an air handling unit. Optionally, each air handling unit is in fluid communication with at least one intake vent. Optionally, the data centre comprises a plurality of controllable outlet vents for exhausting air out of the data centre building, optionally wherein each outlet vent is in fluid communication with the warm air return path. Optionally, each data level comprises a plurality of controllable recirculation vents for controlling admission of warm air from the warm air return path to the air handling units. Optionally, each controllable inlet vent is positioned at or adjacent the second end of an air handling unit. In such an arrangement, ambient air may for example directly enter the air handing unit. Optionally, each controllable outlet vent is positioned above an air handling unit. For example, it may be that each controllable outlet vent is positioned on a portion of the building envelope defining a side of the warm air plenum immediately above an air handling unit. Optionally, the data centre (e.g. each data level of the data centre) comprises at least three air handling units for providing cooling air to cold aisles. Optionally, the air handling units are direct air handling units. It will be understood that a direct air handling unit provides cooling air that comprises or consists of a portion of ambient air from outside the data centre building, optionally where said ambient air is treated to adjust its temperature and / or humidity. Direct air handling units, and suitable control methodologies are disclosed in WO2011 / 148175A1 (Bripco BVBA), the contents of which are incorporated herein by reference. Alternatively, the air handling units are indirect air handling units. Suitable indirect air handling units are disclosed in WO2016 / 207323A1 (Bripco BVBA), the contents of which are incorporated herein by reference. It will be understood that an indirect air handling unit provides air that consists substantially of (for example consists entirely of) air from inside the data centre building, optionally having been treated to adjust its temperature and / or humidity. Optionally, an indirect air handling unit comprises a heat exchanger for transferring heat from air inside the building to air from outside the building, for example wherein internal air travels through the air handling unit along an internal airflow path segregated from an external airflow path along which air from outside the building travels. Preferably, the air handling units comprise adiabatic cooling units, and optionally are free from mechanical cooling units (such as direct expansion cooling units). It will be understood that a direct expansion mechanical cooling unit provides cooling by contacting air with coils containing a refrigerant that had been compressed then allowed to expand. Suitable adiabatic cooling units include so-called wetted matrix coolers (in which air to be cooled is passed through an array of a wetted material) and spray coolers. According to a second aspect there is provided a data centre building comprising at least four hot aisles (e.g. at least 10 hot aisles) interleaved with at least three cold aisles (e.g. at least 9 cold aisles) positioned on a first side of a cold air supply corridor, at least three air handling units positioned on a second side of the corridor opposite the first side, and a warm air plenum positioned above the corridor, wherein at least a portion of an air barrier forms an intermediate ceiling separating the corridor from the warm air plenum. It will be understood that the intermediate ceiling so formed may be the ceiling of the corridor, and form the bottom of the warm air plenum. Optionally, the data centre building is arranged so that, in use, cooling air may be supplied to the cold aisles from the air handling units via the cold air supply corridor, and warm air may be returned to the air handling units from the hot aisles via the warm air plenum. Optionally, the barrier comprises a sloped ceiling portion, which may for example provide an increase in internal height of the warm plenum between a first portion of the plenum located above the first side of the corridor and a second portion of the plenum located above the second side of the corridor. Optionally, the air barrier comprises at least one pressure relief flap assembly movable between a first closed position and a second open position, wherein flow of cooling air from the cold corridor to the warm air plenum through a pressure relief opening is inhibited or prevented when the at least one flap assembly is in the closed position, and flow of cooling air from the cold zone to the hot zone through the pressure relief opening is permitted when the at least one flap assembly is in the second open position. Optionally, the at least one pressure relief flap assembly is located in the sloped ceiling portion. Additionally or alternatively, a data centre building of the second aspect of the invention comprises a cold air supply corridor, an overhead warm air plenum, multiple hot aisles interleaved with multiple cold aisles, the cold aisles being configured to receive cooling air from the cold air supply corridor and the warm aisles being configured to supply warm air to the warm air plenum; wherein the warm air plenum is formed in part by a ceiling to the aisles and / or corridor, the ceiling having a first portion having a first height and a second portion having a second height, which is higher than the first height, and wherein a pressure relief flap is located in the ceiling between the first and second portions of the ceiling, at least a portion of the pressure relief flap being at a third height, which is between the first and second heights. Optionally, the first portion of the barrier has an internal height hl and the second portion has a height h2, wherein hl is less than h2, for example wherein hl is 0.3 m, such as 0.5 m less than h2. Optionally, the sloped portion is angled at 45° to 90° from horizontal, such as 45° to 70° from horizontal. Preferably, the at least one flap of the flap assembly is substantially coplanar with the sloped portion when in the first closed position. Additionally or alternatively, the flap assembly comprises at least one planar flap angled at 45° to 90° from horizontal, such as 45° to 70° from horizontal when in the first closed position. Optionally, the data centre building of the second aspect of the invention incorporates any feature described in relation to the data centre of the first aspect of the invention, and vice versa. In particular, it may be that at least one pressure relief flap assembly of the second aspect of the invention is as described according to the first aspect of the invention (e.g. having the structure, configuration and arrangement of said flap assembly, and / or being a plurality of flap assemblies comprising primary and secondary flap assemblies, as described in relation to the first aspect of the invention). Optionally, the data centre building comprises at least three pressure relief flap assemblies positioned in the ceiling separating the corridor from the warm air plenum, for example in the sloped ceiling portion. Optionally, at least one flap assembly (e.g. at least one primary flap assembly) is positioned along the length of the corridor proximate each air handling unit, and optionally at least one secondary flap assembly is positioned towards each end of the cold corridor, primary and secondary flap assemblies being as described according to the first aspect of the invention. According to a third aspect, there is provided a method of operating a data centre, the data centre comprising a cold zone segregated from a hot zone and at least one air handling unit (e.g. a plurality of air handling units), wherein the hot zone is segregated from the cold zone by an air barrier. Optionally, the air barrier comprises at least one pressure relief flap assembly movable between a first closed position and a second open position, wherein flow of supply air from the cold zone to the hot zone through a pressure relief opening is inhibited or prevented when the at least one flap assembly is in the closed position, and flow of supply air from the cold zone to the hot zone through the pressure relief opening is permitted when the at least one flap assembly is in the second open position. It will be appreciated that the pressure relief opening provides fluid communication between the cold zone and the hot zone. Optionally, the at least one pressure relief flap assembly is configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl, and to reclose automatically when differential air pressure between the cold zone and the hot zone falls below a second threshold P2. Preferably, Pl is greater than P2. Optionally, the data centre operated according to the third aspect of the invention comprises any feature described in relation to the first or second aspects of the invention, and vice versa. Optionally, the method comprises operating the at least one air handling unit to supply cooling air into the cold zone, and optionally to cool warm received from the hot zone. Optionally the method comprises operating the data centre for a first period under a first condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone below Pl, optionally to provide a differential pressure of at least about 3 Pa, such as at least about 5 Pa. Optionally, the method comprises subsequently operating the data centre for a second period under a second condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above Pl. Optionally, the method comprises subsequently operating the data centre for a third period under a third condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above P2 and below Pl. Optionally, the method comprises subsequently operating the data centre for a fourth period under a fourth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone below P2. Optionally, the method comprises subsequently operating the data centre for a fifth period under the first condition. Optionally, under the first condition the at least one pressure relief flap assembly is in the first closed position, under the second condition the at least one pressure relief flap assembly moves from the first closed position to the second open position, under the third condition the pressure relief flap assembly remains in the second open position, and under the fourth condition the pressure relief flap assembly remains moves from the second open position to the first closed position. It will be appreciated that in the fifth period the at least one pressure relief flap assembly remains in the first closed position. Optionally, transition from the first period to the second period is triggered by a command sent to at least one air handling unit to increase supply of cooling air. It will be appreciated that such a command may be initiated automatically by a data centre management system (e.g. software) or by a manual operator. Optionally, transition from the second period to the third period is triggered by automatic operation of the flap assembly, e.g. without requiring any command from a source external to the flap assembly. Optionally, transitions from the third period to the fourth period and from the fourth period to the fifth period are triggered by a command sent to at least one air handling unit to reduce supply of cooling air. It will be understood that the method optionally comprises sending such commands to at least one air handling unit to increase or decrease supply of cooling air. Optionally the method comprises operating the data centre for a first period under a first condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone below Pl( 1), optionally to provide a differential pressure of at least about 3 Pa, such as at least about 5 Pa. Optionally, the method comprises subsequently operating the data centre for a second period under a second condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above Pl(l) but below Pl(2). Optionally, the method comprises subsequently operating the data centre for a third period under a third condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above P2(l) but below Pl(2). Optionally, the method comprises subsequently operating the data centre for a fourth period under a fourth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above Pl(2). Optionally, the method comprises subsequently operating the data centre for a fifth period under a fifth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above P2(l), above P2(2) and below Pl(2) and / or below Pl(l). Optionally, the method comprises subsequently operating the data centre for a sixth period under a sixth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone below P2(l) and below P2(2). Optionally, the method comprises subsequently operating the data centre for a seventh period under the first condition. Optionally, under the first condition primary and secondary pressure relief flap assemblies are each in the first closed position, under the second condition at least one primary pressure relief flap assembly moves from the first closed position to the second open position, under the third condition the at least one primary pressure relief flap assembly remains in the second open position, under the fourth condition primary pressure relief flap assemblies remain open and at least one secondary pressure relief flap assembly moves from the first closed position to the second open position, under the fifth condition primary pressure relief flap assemblies and the at least one secondary pressure relief flap assembly remain open, under the sixth condition primary pressure relief flap assemblies and the at least one secondary pressure relief flap assembly move from the second open position to the first closed position. It will be appreciated that in the seventh period primary and secondary pressure relief flap assemblies remain in the first closed position. Optionally, transition from the first period to the second period is triggered by a command sent to at least one air handling unit to increase supply of cooling air. It will be appreciated that such a command may be initiated automatically by a data centre management system (e.g. software) or by a manual operator. Optionally, transition from the second period to the third period is triggered by automatic operation of at least one primary flap assembly, e.g. without requiring any command from a source external to the at least one primary flap assembly. Optionally, transition from the third period to the fourth period is triggered by a command sent to at least one air handling unit to further increase supply of cooling air. Optionally, transition from the fourth period to the fifth period, is triggered by automatic operation of at least one secondary flap assembly, e.g. without requiring any command from a source external to the at least one secondary flap assembly. Optionally, transitions from the fifth period to the sixth period and from the sixth period to the seventh period are triggered by a command sent to at least one air handling unit to reduce supply of cooling air. It will be understood that the method optionally comprises sending such commands to at least one air handling unit to increase or decrease supply of cooling air. It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1 shows a side cross-sectional view of data centre according to the invention; Figure 2 shows an enlarged portion of the drawing of Figure 1; Figure 3 shows a further enlarged portion of the drawing of Figure 1; Figure 4 shows an end cross-sectional view through a portion of the data centre of Figure 1; Figure 5 shows an enlarged view of a portion of Figure 4; Figure 6a shows a plan view of data level of the data centre of Figure 1; Figure 6b shows the plan view of Figure 8a, annotated to indicate the positions of the views of Figures 1 and 4; Figure 7 illustrates steps of a method of cooling servers in the data centre building of Figures l-6b; Figure 8 illustrates steps of a further method of cooling servers in the data centre building of Figures l-6b; Figures 9a and 9b show schematic representations of a model of a data centre according to the invention; Figure 10 shows a graphical representation of differential air pressure vs. air handling unit fan speed in a test performed with the model of Figures 9a and 9b. Detailed Description Figure 1 shows a side cross-sectional view of a multi-level data centre building 101 according to the invention. The building 101 comprises a mechanical and electrical services level 102a on the ground floor, and three data levels 102b, 102c, 102d on the floors above. The mechanical and electrical services level 102a houses ancillary equipment, including 'uninterrupted power supply' (UPS) equipment, and power and data distribution equipment, not shown in Figure 1. Each data level 102b-d comprises twenty rack storage areas arranged between alternating hot and cold aisles, with each rack storage area accommodating a row of 31 server racks 105. The server racks each hold up to 42 servers. The view shown in Figure 1 cuts along a cold aisle, and thus illustrates the fronts of server racks 105. Each data level 102b-d comprises a plurality of air handling units for providing cooling air to the cold aisles. Figure 1 shows air handling units 120b-d of data levels 102b-d, respectively. Illustrated in Figure 1 are the side covers and access doors of the air handler units 120b-d. In the embodiment show in Figure 1, the air handling units 120b-d are positioned inside the building. It will be appreciated that air handling units may, alternatively, be positioned outside the building, for example supported on gantry structures positioned adjacent a side of the building. When the data centre 101 is operational, cooling air is supplied by air handler units 120b / c / d into cold corridors 150b / c / d, respectively. The cold corridors 150b / c / d run perpendicular to cold aisles, receiving cooling air from the multiple air handlers on each data level and transporting said cooling air to the multiple cold aisles. From the cold aisles, cooling air passes into server racks 105, thereby cooling the servers and becoming warm air and exiting into the hot aisles. Warm air passes upwards from the hot aisles to overhead warm air return plenums 153b / c / d on each level, and then into a warm air return space 154b / c / d above the air handler units 120b / c / d. From that space, warm air may be ejected from the building through warm air exhaust vents (not shown in Figure 1) and / or passed into mixing chambers 155b / c / d upstream of the air handler units 120b / c / d for mixing with ambient air from outside the building 101. Ambient air enters the mixing chambers 155b / c / d through air intake vents (not shown in Figure 1). Pressure relief dampers 108b / c / d are positioned in an air barrier forming the ceiling of cold corridors 150b / c / d, the pressure relief dampers 108b / c / d each comprising a flap assembly in the form of a single hinged flap mounted in a frame and kept in the first closed position by a magnet-based closure device. The flaps are 0.9 m x 0.9 m, providing an opening with a cross-sectional area of 0.81 m2 when fully open. The pressure relief damper units allowing cooling air to pass directly from the cold corridors 150b / c / d into warm air return plenums 153b / c / d immediately above the cold corridors 150b / c / d if differential air pressure between the cold corridors 150b / c / d and the warm air return plenums 153b / c / d exceeds 100 Pa (Pl(l)) - the pressure relief damper units 108b / c / d shown in Figure 1 comprise primary flap assemblies. Note that support pillars 104 do not prevent lateral air flow between the air handler units 120b / c / d and the cold corridors 150b / c / d, nor through the warm air return plenums 153b / c / d. Figure 2 shows an enlarged portion of the drawing of Figure 1, annotated with the same reference numerals as used in Figure 1. As shown in Figure 2 with reference to air handler unit 120d, the air handler unit 120d is made up of multiple sections. The sections include a filter bank 121d, a fan bank 122d, an adiabatic cooler (such as a wetted matrix cooling system) 123d, and an air blender 124d. An access door 125d provides access to the mixing chamber 155d, and further access doors 126d, 127d and 128d provide access to the section of the air handler 120d. The air handler sections are mounted in a support frame 129d. Figure 2 is annotated with dashed arrows showing air flow within data level 102d, cooling air flowing from air handler unit 102d into the cold corridor 150d, from the cold corridor 150d into the cold aisle. Following use for cooling servers in IT racks 105, warm air flows upwards from the adjacent hot aisle into the warm air plenum 153d, over the top of the air handler 150d and back down to the inlet of the air handler 150d. In Figure 2, pressure relief damper 108d is shown with the flap assembly in the second open position, thus allowing air from the cold corridor 150d to pass directly up into the warm air plenum 153. As shown in Figure 2, the pressure relief damper unit 108d is positioned in a sloped section of the air barrier forming the ceiling of the cold corridor 150d, the sloped section and the flap (when in the first closed position) being angled at about 60° from horizontal. As shown in Figure 2, air flow through the warm air plenum 153d does not significantly act to resist opening of the flap because of its position in a section of the ceiling that slopes downwards away from the direction of airflow in the warm air plenum 153d. Dashed arrows showing air flow within data level 102c are also shown in Figure 2, with the same overall movement of air as in data level 102d, except that the flap assembly of pressure relief damper 108c is in the first closed position, and so cooling air does not pass directly from the cold corridor 150c to the warm air plenum 153c. Figure 3 shows a further enlarged portion of the drawing of Figure 1, annotated with the same reference numerals as used in Figure 1. In Figure 3, a portion of a row of IT racks 105 is shown with a corresponding row of service cassettes 106 suspended above them. The service cassettes 106 are shown comprising air entrainment panels 107, which form part of the air barrier segregating hot and cold zones of the data centre. Further suspended air entrainment panels 110 form the ceiling separating the cold corridor 150c from the warm air plenum 153c, thereby making up another part of the air barrier. The air entrainment panels 107 of the service cassettes 106 segregate the hot and cold aisles. Openings 119 in panels 107 are provided to accommodate services passing between hot and cold aisles. The openings include adjustable covers to maintain air segregation between aisles. Droprods 112 fastened to brackets 111 support the service cassettes 106. Further droprods 113 secured to slot channel beams (not shown in Figure 3) fastened to brackets 111 support the air entrainment panels 110. The pressure relief damper unit 108d is shown with its flap assembly in the first closed position. Figure 4 shows an end cross-sectional view through a portion of the data centre 101 of Figure 1, labelled with the same reference numerals as used in Figures 1-3. As in Figure 1, Figure 3 shows services level 102a and data levels 102b-d. The view of Figure 3 cuts across cold aisles 151 and hot aisles 152. Figure 5 shows an enlarged view of a portion of Figure 4. As shown in Figure 5, service cassettes 106 are suspended from brackets 111 mounted on the structure of the building. Figure 6a shows a plan view of data level 102c of the data centre 101 of Figure 1, labelled with the same reference numerals as used in Figures 1-5 (except that, for clarity, the 'c' suffix is omitted). In the floorplan shown in Figure 6a, the IT area of the data level includes cold aisles 151, hot aisles 152, rack storage areas accommodating racks 105, and connecting corridor 114 (which links cold aisles 151 together). In use, cooling air is provided by air handler units 120, which deliver cooling air into one side cold corridor 150, which transports and distributes cold air to cold aisles 151 through vented access doors 116, which provide controlled fluid communication and personnel access between the cold corridor 150 and cold aisles 151. The cold corridor 150 and cold aisles form the cold zone, while the hot aisles 152 and the warm air return plenum (not shown in Figure 8a) form the hot zone. Personnel doors 117 provide access to hot aisles 152 from the cold corridor 150, while also maintaining air segregation between the cold corridor 150 and the hot aisles 152. It will be appreciated that personnel doors could additionally or alternatively be provided at the other end of hot aisles 152. Warm air is ejected from the building through controllable exhaust vents 203, and fresh ambient air is drawn in through controllable intake vents 205. Two pressure relief damper units are provided for each air handling unit in the cold corridor 150, including eight primary pressure relief damper units 108 and two secondary pressure relief damper units 109, the secondary pressure relief damper units 109 having flap assemblies configured to open at a pressure differential of 150 Pa (Pl(2)). The secondary pressure relief damper units 150 are provided towards the outer ends of the cold aisle 150, with the eight primary damper relief units 108 arranged therebetween. The flap assemblies of both the primary and secondary pressure relief damper units 108,109 are configured to reclose at a differential pressure of about 5 Pa. Figure 6b shows the plan view of Figure 6a, annotated to indicate the positions of the views of Figures 1 and 4. Line A-A indicates the position of the view of Figure 1. Line B-B indicates the position of the view of Figure 4. Figure 7 illustrates steps of a method according to an embodiment of the invention, the method being a method of cooling servers in the data centre building 101 of Figures l-6b. In step X01, air handling units 120 are operated to supply cooling air into the cold corridor of the cold zone, and in step X02, air handling units 120 are operated to cool warm air received from the warm air plenum 153 of the hot zone. By those steps, cooling air is transported along an airflow path leading from the air handling units 120 to servers in racks 105 via the cold air supply corridor 150 and the cold aisles 151, and from the servers to the air handling units 120 via the warm area comprising hot aisles 152 and warm air plenum 153. The airflow path is located within the building 101. Figure 8 illustrates steps a further method according to an embodiment of the invention, the method against being for cooling servers in the data centre building 101 of Figures l-6b. In each step of the method, air handling units 120 are operated to supply cooling air into the cold corridor 150 of the cold zone and to cool warm air received from the warm air plenum 153 of the hot zone, each step representing a period of operation during which one or more air handling units 120 are operated to provide the stated cold zone / hot zone differential air pressure. In step Xll, the differential air pressure is below 100 Pa (Pl(l)), and the flap assemblies of the primary and secondary pressure relief damper units 108,109 are closed; in step X12, differential air pressure is above 100 Pa (Pl(l)) but below 150 Pa (Pl(2)), and one or more flap assemblies of primary pressure relief damper units 108 open automatically; in step X13, the differential air pressure is between 5 Pa (P2( 1)) and 100 Pa (Pl( 1)), and the one or more flap assemblies of primary pressure relief damper units 108 remain open; in step X14, the differential air pressure is above 150 Pa (Pl(2)), the flap assemblies of primary pressure relief damper units 108 are open and at least one flap assembly of secondary pressure relief damper units 109 open automatically; in step X15, the differential air pressure is between 5 Pa (P2(l)) and 150 Pa (Pl(2)), and the flap assemblies of primary pressure relief damper units 108 and the at least one flap assembly of secondary pressure relief damper units 109 remain open; in step X16, the differential air pressure is below 5 Pa (P2(l) and P2(2)), and the flap assemblies of the primary and secondary pressure relief damper units 108,109, reclose; in step X17, the differential air pressure is below 100 Pa (Pl(l)), and the flap assemblies of the primary and secondary pressure relief damper units 108,109 remain closed. The transitions from step Xll to X12 and from step X13 to X14 are caused by a command 301, 302 sent to at least one air handling unit to increase supply of cooling air, while the transition from step X15 to X16 is caused by a command 303 sent at least one air handling unit to decrease supply of cooling air. In the transition from step X16 to X17, supply of cooling air may remain constant or increase. According to certain embodiments, the method may comprise transitioning directly from step X13 to X16, said transition being caused by a command 304 sent to at least one air handling unit to decrease supply of cooling air. The transition from step X12 to X13 is caused by automatic opening 401 of at least one flap assembly of at least one primary pressure relief damper 108. The transition from step X14 to X15 is caused by automatic opening 402 of at least one flap assembly of at least one secondary pressure relief damper 109. Figures 9a and 9b show top plan and side cross-sectional schematic views of the layout of a model data centre 501 constructed to test operation of the data centre of the invention. To simulate low IT load, electrical heaters are positioned in 10% of rack spaces, the other unoccupied spaces being infilled with blanking panels. Air flow through the model data centre 501 is represented by dashed arrows, which show air travelling through the air handling unit 520, across cold corridor 550 and into cold aisles 551, through heaters into the hot aisle 552, then upwards into the warm air plenum 553. The model data centre 501 is shown operating in 'fresh air mode', meaning that warm air is exhausted from the warm air plenum 553 through exhaust vents 590, and replaced by ambient air drawn into the air handing unit 520 from outside the data centre. The model data centre 501 is fitted with primary pressure relief damper PRD-1 (having a flap assembly configured to open at a cold corridor 550 / warm air plenum 553 differential air pressure of 100 Pa), and secondary pressure relief damper PRD-2 (having a flap assembly configured to open at a cold corridor 550 / warm air plenum 553 differential air pressure of 150 Pa). PRD-1 is provided with an airflow sensor to measure airflow through the damper when the flap assembly is open, and pressure sensors are provided in the cold corridor 550 and warm air plenum 553 adjacent PRD-1, thereby providing an indication of the differential air pressure. Figure 10 provides a graphical representation of results obtained with the model data centre 501 of Figures 9a and 9b. In the test, air handling unit 520 fan speed was gradually increased from 42% to 92% of maximum, while differential pressure was recorded using the cold corridor 550 and warm air plenum 553 pressure sensors and open / close status of PRD-1 and PRD-2 was observed. At fan speeds from 42% to 60%, differential pressure gradually rises to 100 Pa, while PRD-1 and PRD-2 remain closed. At a fan speed of 62%, PRD-1 was observed to have opened, and differential pressure dropped to 12 Pa. At fan speeds of 62% to 78%, PRD-1 remained open while differential pressure gradually rose again to 100 Pa. At a fan speed of 80%, PRD-2 was observed to open, and differential pressure dropped to 65 Pa. At fan speeds of 80% to 92%, PRD-1 and PRD-2 both remained open, and differential pressure rose to a maximum of 120 Pa. At the end of the test, no buckling or damage to blanking panels between the aisles was observed. Although the pressure sensors indicated a differential pressure of 100 Pa when PRD-2 opened, it is believed that the proximity of the sensor to PRD-1 affected the reading, and that a pressure differential of 150 Pa was reached across PRD-2 before it opened. The test demonstrated that the pressure relief damper units were capable of relieving over-pressure in the cold zone (avoiding damage to blanking panels), while also avoiding excessive pressure swings. In particular, the staged opening of PRD-1 and PRD-2 is believed to have limited maximum cold zone overpressure while also maintaining effective flow of cooling air into the cold aisles. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be 5 construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not 10 be desirable, and may therefore be absent, in other embodiments.

Claims

1. A data centre, wherein the data centre comprises:a cold zone segregated from a hot zone by an air barrier, andat least one air handling unit configured to provide cool supply air into the cold zone,wherein the air barrier comprises at least one pressure relief flap assembly movable between a first closed position and a second open position, wherein flow of supply air from the cold zone to the hot zone through a pressure relief opening is inhibited or prevented when the at least one flap assembly is in the closed position, and flow of supply air from the cold zone to the hot zone through the pressure relief opening is permitted when the at least one flap assembly is in the second open position;wherein the at least one pressure relief flap assembly is configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl, and to reclose automatically when differential air pressure between the cold zone and the hot zone falls below a second threshold P2;and wherein Pl is greater than P2.

2. A data centre according to claim 1, wherein the flap assembly comprises at least one hinged or pivotable flap mounted in a frame of a pressure relief damper unit.

3. A data centre according to claim 2, wherein the pressure relief damper unit comprises at least one closure device having a magnet for retaining the at least one flap assembly in the first closed position.

4. A data centre according to claim 2 or claim 3, wherein the pressure relief damper unit comprises a reclosure device for urging the flap assembly from the second open position to the first closed position.

5. A data centre according to any one of claims 1 to 3, wherein the at least one flap assembly is configured to move from the second open position to the first closed position entirely under the control of gravity.

6. A data centre according to any preceding claim, wherein the first threshold Pl is in the range of from about 70 Pa to about 200 Pa, such as about 90 Pa to about 160 Pa, and P2 (when measured in Pa) is about 5-15% of Pl (when measured in Pa).

7. A data centre according to any preceding claim, wherein the at least one pressure relief flap assembly is sized and configured to permit a flow rate of air through the pressure relief opening of at least about 2 m3 / s, such as at least about 4 m3 / s, at a differential air pressure of 100 Pa, when the flap assembly is in the second open position.

8. A data centre according to any preceding claim, wherein the hot zone comprises a warm air flow path extending from an upstream location to a downstream location, the hot zone having a transition portion with a cross-sectional area that increases from a first upstream end to a second downstream end, and wherein the at least one pressure relief flap assembly is positioned to permit direct fluid communication between the cold zone and the transition portion of the hot zone, and / or between the cold zone and a portion of the hot zone immediately adjacent and downstream of the transition portion, when the at least one pressure relief flap assembly is in the second open position.

9. A data centre according to any preceding claim, wherein the air barrier comprises a plurality of said pressure relief flap assemblies, the plurality of pressure relief flap assemblies including at least one primary flap assembly and at least one secondary flap assembly,wherein, each primary flap assembly is configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl(l), and to reclose automatically whendifferential air pressure between the cold zone and the hot zone falls below a second threshold P2(l), and wherein Pl(l) is greater than P2(l);wherein each secondary flap assembly is configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl(2), and to reclose automatically when differential air pressure between the cold zone and the hot zone falls below a second threshold P2(2), and wherein Pl(2) is greater than P2(2);and wherein Pl(2) is greater than Pl(l).

10. A data centre according to claim 9, wherein Pl(2) is about 30 to 70 Pa greater than Pl(l), such as about 40 to 60 Pa greater than Pl(l).

11. A data centre according to claim 9 or claim 10, wherein the first thresholds Pl(l) and Pl(2) are each in the range of from about 70 Pa to about 200 Pa, such as about 90 Pa to about 160 Pa, the second threshold P2(l) (when measured in Pa) is about 5-15% of Pl(l) (when measured in Pa), and the second threshold P2(2) (when measured in Pa) is about 1-15% of Pl(2) (when measured in Pa).

12. A data centre according to any preceding claim, the data centre comprising a plurality of said air handling units and at least one data level divided into said cold zone and hot zone, the at least one data level comprising:at least fifteen rack storage areas positioned on a floor, each rack storage area being arranged to accommodate a row of at least ten server racks capable of accommodating at least 20 servers, wherein the rack storage areas separate alternating hot aisles and cold aisles, anda cold air supply corridor arranged to transport cold air from the air handling units to the cold aisles, and preferably being suitable for providing personnel access to the cold aisles,wherein the cold aisles and the cold air supply corridor together form at least a portion of the cold zone, and the hot aisles together form at least a portion of the hot zone;wherein the air barrier comprises a cold corridor portion separating the cold corridor from the hot zone, and wherein the at least one pressure relief flap assembly is positioned in the cold corridor portion of the air barrier, thereby providing fluid communication between the cold corridor and the hot zone when the at least one pressure relief flap assembly is in the second open position.

13. A data centre according to claim 12 when dependent on any one of claims 9 to 11, wherein the air barrier comprises:a plurality of primary pressure relief flap assemblies distributed along the cold air supply corridor,a first secondary pressure relief flap assembly located towards a first end of the cold air supply corridor, anda second secondary pressure relief flap assembly located towards a second opposed end of the cold air supply corridor;wherein the plurality of primary flap assemblies include at least one intermediate primary flap assembly positioned between the secondary flap assemblies and at least one primary flap assembly located adjacent each secondary flap assembly.

14. A data centre according to claim 13, wherein the cold air supply corridor comprises a plurality of supply openings for receiving cooling air from at least one air handling unit, the plurality of supply openings including at least one outer supply opening positioned towards each opposed end of the cold air supply corridor and at least one intermediate supply opening positioned along the cold air supply corridor between the first and second outer supply openings,wherein at least one primary pressure relief flap assembly is located proximate each supply opening, and at least one secondary pressure relief flap assembly is located proximate each outer supply opening.

15. A data centre according to any one of claims 12 to 14, wherein the at least one data level comprises a warm air return plenum for receiving hot air from hot aisles and transporting hot air to air handling units and / or exhaust vents on an outer wall of the data centre, wherein the warm air return plenum forms a portion of the hot zone and extends above at least a portion of: the cold aisles, the cold air supply corridor, and / or and the air handling units;wherein at least a portion of the air barrier forms a bottom of the warm air return plenum and at least a portion of a ceiling of the cold corridor, wherein said portion of the ceiling includes sloped ceiling section above which internal height of the warm air plenum increases from an upstream end of the sloped ceiling section to a downstream end, thereby providing an increase in cross-sectional area of the warm air return plenum at the downstream end as compared to the cross-sectional area at the upstream end;and wherein the at least one flap assembly is positioned in said sloped ceiling section.

16. A data centre according to claim 15, wherein the sloped ceiling section is angled at 45° to 90° from horizontal, optionally wherein the at least one flap is planar and angled at 45° to 90° from horizontal when in the first closed position.

17. A data centre building, wherein the data centre building comprises at least four hot aisles interleaved with at least three cold aisles positioned on a first side of a cold air supply corridor, at least three air handling units positioned on a second side of the corridor opposite the first side, and a warm air plenum positioned above the corridor, wherein at least a portion of an air barrier forms a ceiling separating the corridor from the warm air plenum, thedata centre building being arranged so that, in use, cooling air is supplied to the cold aisles from the air handling units via the cold air supply corridor, and warm air is returned to the air handling units from the hot aisles via the warm air plenum,wherein the barrier comprises a sloped ceiling portion providing an increase in internal height of the warm plenum between a first portion of the plenum located above the first side of the corridor and a second portion of the plenum located above the second side of the corridor,wherein the air barrier comprises at least one pressure relief flap assembly movable between a first closed position and a second open position, wherein flow of cooling air from the cold corridor to the warm air plenum through a pressure relief opening is inhibited or prevented when the at least one flap assembly is in the closed position, and flow of cooling air from the cold zone to the hot zone through the pressure relief opening is permitted when the at least one flap assembly is in the second open position, and wherein the at least one pressure relief flap assembly is located in the sloped ceiling portion.

18. A data centre building according to claim 17, wherein the first portion of the barrier has an internal height hl and the second portion has a height h2, wherein hl is less than h2, for example wherein hl is about 0.3 m less than h2.

19. A data centre building according to claim 17 or claim 18, wherein the sloped portion is angled at 45° to 90° from horizontal, optionally wherein the at least one flap assembly comprises at least one planar flap angled at about 45° to about 90° from horizontal when in the first closed position.

20. A data centre building according to any one of claims 17 to 19, wherein the at least one flap assembly is as described in any one of claims 1 to 7.

21. A data centre building according to claim 20, wherein the at least one flap assembly comprises a plurality of flap assemblies as described in any one of claims 9 to 11.

22. A data centre building according to claim 21, wherein the data centre building comprises at least three pressure relief flap assemblies positioned in the sloped ceiling portion, wherein at least one primary flap assembly is positioned along the length of the corridor proximate each air handling unit, and at least one secondary flap assembly is positioned towards each end of the cold corridor.

23. A method of operating a data centre, wherein the data centre comprises: a cold zone segregated from a hot zone by an air barrier, and at least one air handling unitwherein the hot zone is segregated from the cold zone by an air barrier, the air barrier comprising at least one pressure relief flap assembly movable between a first closed position and a second open position, wherein flow of supply air from the cold zone to the hot zone through a pressure relief opening is inhibited or prevented when the at least one flap assembly is in the closed position, and flow of supply air from the cold zone to the hot zone through the pressure relief opening is permitted when the at least one flap assembly is in the second open position;wherein the at least one pressure relief flap assembly is configured to open automatically when differential air pressure between the cold zone and the hot zone exceeds a first threshold Pl, and to reclose automatically when differential air pressure between the cold zone and the hot zone falls below a second threshold P2, and wherein Pl is greater than P2;wherein the method comprises operating the at least one air handling unit to supply cooling air into the cold zone.

24. The method of claim 23, comprising:operating the data centre for a first period under a first condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone below Pl;subsequently operating the data centre for a second period under a second condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above Pl;subsequently operating the data centre for a third period under a third condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above P2 and below Pl;subsequently operating the data centre for a fourth period under a fourth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone below P2; and,subsequently operating the data centre for a fifth period under the first condition;wherein:under the first condition the at least one pressure relief flap assembly is in the first closed position;under the second condition the at least one pressure relief flap assembly moves from the first closed position to the second open position;under the third condition the pressure relief flap assembly remains in the second open position; and,under the fourth condition the pressure relief flap assembly remains moves from the second open position to the first closed position;and wherein:transition from the first period to the second period is triggered by a command sent to at least one air handling unit to increase supply of cooling air;transition from the second period to the third period is triggered by automatic operation of the flap assembly;transitions from the third period to the fourth period and from the fourth period to the fifth period are triggered by a command sent to at least one air handling unit to reduce supply of cooling air.

25. The method of claim 23, comprising:operating the data centre for a first period under a first condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone below Pl(l);subsequently operating the data centre for a second period under a second condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above Pl(l) but below Pl(2);subsequently operating the data centre for a third period under a third condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above P2(l) but below Pl(2);subsequently operating the data centre for a fourth period under a fourth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above Pl(2);subsequently operating the data centre for a fifth period under a fifth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressure between the cold zone and the hot zone above P2(l), above P2(2) and below Pl(2) and / or below Pl(l);subsequently operating the data centre for a sixth period under a sixth condition in which the at least one air handling unit supplies cooling air into the cold zone to provide a positive differential air pressurebetween the cold zone and the hot zone below P2(l) and below P2(2);and,subsequently operating the data centre for a seventh period under the first condition;wherein:under the first condition primary and secondary pressure relief flap assemblies are each in the first closed position;under the second condition at least one primary pressure relief flap assembly moves from the first closed position to the second open position;under the third condition the at least one primary pressure relief flap assembly remains in the second open position;under the fourth condition primary pressure relief flap assemblies remain open and at least one secondary pressure relief flap assembly moves from the first closed position to the second open position;under the fifth condition primary pressure relief flap assemblies and the at least one secondary pressure relief flap assembly remain open; and,under the sixth condition primary pressure relief flap assemblies and the at least one secondary pressure relief flap assembly move from the second open position to the first closed position;and wherein:transition from the first period to the second period is triggered by a command sent to at least one air handling unit to increase supply of cooling air;transition from the second period to the third period is triggered by automatic operation of at least one primary flap assembly;transition from the third period to the fourth period is triggered by a command sent to at least one air handling unit to further increase supply of cooling air;transition from the fourth period to the fifth period, is triggered by automatic operation of at least one secondary flap assembly; and,transitions from the fifth period to the sixth period and from the sixth period to the seventh period are triggered by a command sent to at least one air handling unit to reduce supply of cooling air.

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