Accurate flow measurements at low flow velocities

Abstract

An air flow measurement system for determining an air flow value indicative of air flow in an associated air duct system with an air duct and a controllable damper to control air flow in the air duct by means of a damper blade which has a controllable position, such as around an axis of rotation. An air flow sensor, preferably an EFV based sensor, is arranged for being positioned inside the associated air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper. The air flow sensor is connected to a flow measurement system, and a processor processes an output from the flow measurement circuit to determine a measured air speed accordingly. The processor can then calculate the air flow value based on the measured air speed, preferably further based on a damper blade position, and a correction value. The measurement system can measure very low air speeds e.g. in a ventilation duct with high accuracy, such as down to 0.2 m / s. To protect the air flow sensor from dirt particles, an obstacle element can be placed upstream of the sensor, i.e. in the form of a pin or rod.

Description

[0001] ACCURATE FLOW MEASUREMENTS AT LOW FLOW VELOCITIES

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the air ventilation systems. More specifically, the invention relates to a method and a device for accurately measuring air flow at low flow velocities in a ventilation duct system having a rotary damper for controlling air flow.

[0004] BACKGROUND OF THE INVENTION

[0005] Existing methods for measurement of air flow in a ventilation system duct is typically based on a measurement device in the form of a flow sensor having a cross positioned at the cross section of a ventilation duct for sensing pressure at various positions of the cross section of the duct. In this way, air flow can be estimated. However, the method fails to measure air flow accurately when the air flow speed is below such as 1 m / s, and especially below 0.5 m / s, which is a common air speed in ventilation systems. Furthermore, such measurement device causes a significant pressure drop and thereby results in a reduced energy efficiency of the ventilation system. Still further, for optimal measurement accuracy, it is required that the flow sensor is positioned in the duct at a significant distance from a duct bend or other obstacle.

[0006] Thus, based on existing measurement technique with imprecise air flow measurements at low air flow speeds, it follows that it is difficult to correctly control the ventilation system as intended. Especially, it is problematic to control the rotary damper in a ventilation system when it is near its closed position and the resulting mean downstream air flow speed is low. This causes malfunction of the ventilation system and can result in poor indoor quality as well as poor energy efficiency.

[0007] Currently, ventilation system are typically time-regulated, whereas Demand Controlled Ventilation (DCV) can help to improve energy efficiency as well as indoor quality, but DCV requires a precise measurement of air flow speed for control of each rotary damper of a ventilation system. SUMMARY OF THE INVENTION

[0008] Thus, according to the above description, it is an object of the present invention to provide an improved method and device for measurement of a precise air flow speed in a duct also at low air flow speeds of below 1 m / s, preferably also below 0.5 m / s.

[0009] In a first aspect, the invention provides an air flow measurement system for determining an air flow value indicative of air flow in an associated air duct system with an air duct and a controllable damper to control air flow in the air duct by means of a damper blade which has a controllable position, such as a position around an axis of rotation, the system comprising:

[0010] - an air flow sensor arranged for being positioned inside the associated air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper,

[0011] - a processor system comprising:

[0012] - a sensor input arranged for connection to the electric interface of the air flow sensor, wherein the sensor input is connected to a measurement circuit, and

[0013] - a processor arranged to process an output from the flow measurement circuit to determine a measured air speed accordingly, wherein the processor is further arranged to calculate the air flow value in response to all of: the measured air speed, a damper blade position, and a correction value.

[0014] The flow measurement system is advantageous, since it has been verified by tests, that it is possible to measure e.g. in a 0160 mm ventilation duct accurate air flow rates below 1 m / s, even down to 0.2 m / s still with a high accuracy. This is made possible based on the insight by inventors that it is possible to place the air flow sensor to measure air flow velocity at a short distance from the damper, especially close to the position where the damper blade opens, and at a position near or at the duct wall. Based on the measured air flow at such position, a considerable air flow velocity can be measured even when the damper blade is in a position where the damper is almost closed, and thus where the air flow velocity in the average cross section of the duct is very low. In this way, when the damper blade position is known, it is possible to translate the measured air flow velocity to an average air flow velocity in the duct. Thus, with a mapping between measured air flow velocities at different damper positions, a correction factor can be determined, and the processor system can calculate and output an air flow value indicative of the air flow velocity in the duct. In this way, it has been found to be possible to determine very low air flow velocities with high precision which exceeds known ventilation duct air flow measurement methods. At air flow velocities of 0.2 m / s existing measurement crosses and ultrasonic techniques are known to have an accuracy of + / - 40%, while the system according to the first aspect of the invention can achieve an accuracy of + / - 5%. This is a significant approvement over known systems which can help to improve performance of ventilation controls, both with respect to more accurate control of flow and temperatures, especially upon operation at low air flow rates, but it can further help to save energy.

[0015] Further, a small air flow sensor can be used, preferably an Elastic Filament Velocimetry (EFV) nanotechnology type of sensor can be used, and the small size combined with the position close to the air duct wall, a very low pressure drop in the air duct due to the presence of the air flow sensor can be obtained, and a low pressure drop helps to save energy in the ventilation system. This can be further improved by using the measurement system in connection with Demand Controlled Ventilation (DCV).

[0016] Still further, the measurement system according to the first aspect is advantageous, since it has been found to be robust with respect to upstream bends of the duct or other obstacles the duct system. Thus, for example the air flow sensor can be positioned a short distance downstream of a duct bend, and still accurate air flow measurements can be performed. This is a known problem with existing air flow measurement systems, for example measurement crosses, which requires a significant distance upstream of the air flow sensor position without any elements causing turbulence.

[0017] Even further, the measurement system according to the first aspect is advantageous in that it is easy to retro-fit into an existing ventilation system. Due to a position at or a few centimetres from the wall of the ventilation duct, it is possible to drill a hole from outside and mount a unit on the outside of the duct wall including the air flow sensor which protrudes through the hole into the desired position inside the duct. Thus, it is easy to replace an existing air flow measurement system in a ventilation system. In preferred embodiments, the processor system is configured to autonomously perform air flow measurements to provide the necessary mapping between the air flow speed measured by the air flow sensor and the average air flow speed in the duct. Thereby, it is easy to retro-fit the measurement system into existing ventilation systems, since it can self-calibrate to output the required average air flow speed in the duct, thus facilitating direct connection to an existing ventilation control system.

[0018] Various embodiments and preferred features will be described in the following.

[0019] The air flow sensor is preferably arranged for being positioned at or near a cross sectional position of the air duct where the damper blade opens in direction downstream. Here, especially in case of a circular cross section of the duct and damper blade, the highest air speed can be expected, and in this way, the air flow sensor is exposed to a rather high air speed in spite the average air speed in the duct is very low, such as below 0.5 m / s. For the same reason, i.e. in case the damper blade has a controlled position around an axis of rotation, the air flow sensor is preferably arranged for being positioned at an angular position which is perpendicular or substantially perpendicular to said axis of rotation, seen in a direction of the air flow direction of the air duct.

[0020] It is to be understood that the damper blade may have a circular or an oval shape. Especially, the damper blade may be circular or oval in combination with the air duct having a circular cross section.

[0021] The damper blade can be controllable by means of an actuator, e.g. an electric actuator, e.g. a stepper motor, or the like that can rotate the damper blade around an axis of rotation. However, the damper blade may additionally or alternatively be manually controllable, e.g. by a turning knob or the like which can be operated by a user from outside the air duct. In some embodiments, the air flow sensor is arranged to measure air flow, such as an air speed or an air velocity directly. In other embodiments, the air flow sensor is arranged to sense air pressure, e.g. an absolute air pressure or a differential air pressure, and wherein the measurement circuit and / or processor is then capable of determining the measured air speed, based on the sensed air pressure. Thus, in case of an electric air flow sensor, such as an Elastic Filament Velocimetry EFV based sensor, the air flow sensor has an electric interface to the flow measurement circuit, while in case of the air flow sensor being an air pressure sensor, the interface to the flow measurement circuit may be based on an air tube connection and / or an electric connection.

[0022] In preferred embodiments, the air flow sensor comprises an Elastic Filament Velocimetry (EFV) based sensor, such as a nano ribbon sensor, e.g. a 5-20 nano ribbon sensor. Such sensor can be very small, within such as 1 mm2, and still it is capable of measuring precisely at low air speeds. The small size allows a low impact on the flow in the duct and thus the presence of the air flow sensor only contributes to a low pressure drop. To improve measurement accuracy it may be preferred to provide a temperature compensation of the EFV sensor.

[0023] It is to be understood that other sensor than EFV based sensors can be used, if preferred, as briefly mentioned above, and still utilizing the finding that a measurement position close to the damper and at or near the duct wall results in a rather high air speed which is significantly higher than the average air speed in the duct, especially at positions of the damper blade near its closed position.

[0024] In some embodiments, the air flow sensor is arranged in proximity to a structure which directs air flow in a direction which is non-parallel with the wall of the air duct, such as perpendicular to the wall of the air duct, so as to cause the air flow sensor to measure an air flow having a direction non-parallel, such as perpendicular to, the wall of the air duct. This has proven in tests to provide a stable flow which further tends to build up less dirt on the sensor which can influence its function over time. For example, with a small EFV based sensor, such position of the sensor can be achieved by placing the small EFV based sensor in a hole in a plane and narrow PCB part which then directs air flow and at the same time provides electric contact between the sensor and at a distal end connects to a measurement circuit implemented at another part of the PCB. The plane and narrow PCB part with the sensor can then be arranged at an angle of such as 60°- 120° with a wall of the duct, so that the plane and narrow PCB part directs air flow towards the sensor in an angle which is significantly different from the flow direction along the duct.

[0025] In some embodiments, the EFV based sensor and the measurement circuit are implemented on different parts of one single PCB. In som embodiments, the EFV based sensor is placed on a first PCB, while the measurement circuit together with components of the processor system, or at least a part of the measurement circuit together with components of the processor system, are placed on a second PCB separate from the first PCB, e.g. electrically connected by wires or by a flexible PCB or the like, e.g. using an electric connector which allows disconnecting the first and second PCBs. Preferably, the entire measurement circuit and the entire processor system are placed on the second PCB.

[0026] In preferred embodiments, the correction value is determined based on a cross sectional dimension of the associated air duct, and on a distance between the air flow sensor and the controllable damper. This can be implemented by the processor system comprising an input configured to receive data indicative of cross sectional dimension of the duct (e.g. a value indicative of diameter in mm or cm), and / or a value indicative of a distance (in mm or cm) between the air flow sensor and a rotation axis of the controllable damper. In this way, the measurement system can adapt to various installations. Alternatively, or additionally, the data can be preset in software in the processor system.

[0027] In some embodiments, the processor system comprises an output arranged for controlling position of the damper blade of the controllable damper, and wherein the processor system is configured to performing a mapping between measured air flow velocities at respective plurality of different damper blade positions and to determine said correction value or a set of correction values in response to said mapping. In this way, the system can autonomously adapt to an unknown installation, and the correction value or a set of correction values can then be stored in a memory of the processor system for use in calculating the air flow value in response to a measured air speed. As an alternative, the correction value or set of correction values can be based on calibration measurements performed by high precision air flow measurement equipment, and wherein the correction value or set of correction values are then stored in a memory of the processor system.

[0028] In some embodiments, the processor system is configured to receive one or more input values and to determine said correction value accordingly. Especially, the processor system may be configured to receive an input indicative of a cross sectional dimension of the air duct, such as a value indicative of a diameter of the air duct. Especially, the processor system may be configured to receive an input indicative of a distance between the controllable damper and a position where the air flow sensor is mounted in the air duct, such as a distance from an axis of rotation of the damper blade.

[0029] The processor system may be configured for connection to a controllable damper via an interface, such as a so-called MODBUS interface. In this way, the measurement system can easily be adapted to an existing ventilation system since such MODBUS interface is widely used.

[0030] In some embodiments, the processor system is configured to receive an input indicative of an intended air flow value, and wherein the processor system is configured to generate an output to the controllable damper for control of the position of the damper blade in response to the input indicative of the intended air flow value. Such embodiment is adapted for connection to a ventilation control system which determines an intended air flow in the duct and communicates the processor system which is then connected to the controllable damper and further configured to calculate a damper blade position estimated to result in the intended air flow in the duct, and finally the control system communicates the controllable damper to change its damper blade position to the desired damper blade position estimated to provide the intended air flow in the duct.

[0031] In some embodiments, the processor system is configured to control the damper blade position by turning an actuator on or off, thereby causing the damper blade to open or close. Especially, the processor system is configured to perform a calibration procedure comprising controlling the damper blade position and to determine a time lapsed from the damper blade being in closed position until the actuator has turned the damper blade to a fully open position. Especially, the processor system is configured to estimate a damper blade position by tracking a time where the actuator has been turned on to cause the damper blade to open and a time where the actuator has been turned on to cause the damper blade to close. Such measurement embodiments are adapted for connection to on / off controllable types of dampers. As a further embodiment, the processor system may be configured to perform a calibration procedure comprising controlling the damper blade position, wherein the actuator comprises a stepper motor, and wherein the processor system is configured to determine a number of steps of the stepper motor from the damper blade being in closed position until the stepper motor has turned the damper blade to a fully open position.

[0032] In some embodiments, there is a rubber gasket around damper blade in order to enable airtight fitting between the air duct wall and the damper blade in closed position. The flexibility of the rubber makes the damper airtight before the damper blade reaches its zero position, i.e. its mechanically fully closed position. The difference between the zero position and the actual closed position has to be measured to enable correct calculation between actual and measured air flow. However, this difference can easily be detected by the air flow measurement system, since the air flow sensor immediately senses when the air flow starts and stops.

[0033] In some embodiments, the processor system comprises a damper blade position input arranged to receive a value indicative of the damper blade position.

[0034] In some embodiments, the processor system comprises an output interface arranged to generate an output in response to the calculated air flow value. Especially, the output interface may be arranged to generate an output indicative of the calculated air flow value. In this way, the measurement system can be used to replace the air flow sensor of an existing ventilation system and thereby allow direct connection to an existing ventilation control system. In some embodiments, the processor system is arranged to generate an output to the controllable damper for controlling damper blade position. This may be according to a standard interface or a dedicated interface to a damper.

[0035] In some embodiments, the processor system is arranged to receive an input indicative of a specified air flow value. Especially, the processor system may be arranged to generate an output to the controllable damper for controlling damper blade position in response to the received input indicative of the specified air flow value. Especially, the processor system may be arranged to determine the output to the controllable damper for controlling position of the controllable damper in response to a difference between the specified air flow value and the calculated air flow value.

[0036] In some embodiments, the processor system is arranged to receive an input indicative of at least one of: a cross sectional dimension of the air duct, and a distance between the air flow sensor and the controllable damper. With such information, the processor system can determine the correction value or a set of correction values, based on measured air velocities, and thus calculate the air flow value accordingly.

[0037] In preferred embodiments, the air duct has a circular cross section with a diameter D. This is a usual type of duct for ventilation systems, especially combined with a rotational damper which has a damper blade with an axis of rotation and an actuator that can determine an angular position of the damper blade between a closed and a fully open position along with a plurality of positions in between the closed and fully open position.

[0038] The inventors have performed air flow simulations as well as performed practical tests on circular cross sectional ducts with different diameters, and with a 0.5 mm x 1.0 mm EFV based air flow sensor, where five parameters have been tested.

[0039] Based on these tests, accuracy of the air flow value have been evaluated, and the inventors have determined preferred guidelines or criteria accordingly for optimal position of the air flow sensor in a duct with diameter D. Preferably, the air flow sensor is arranged for being positioned at a longitudinal distance from an axis of rotation of the controllable damper being within a range of 0.2-2.5 times D, more preferably 0.25-2.0 times D. Preferably, the air flow sensor is arranged for being positioned at a distance from the wall of the duct which is less than 0.15 times D, more preferably less than 0.12 times D. Preferably, the air flow sensor is arranged for being positioned at a longitudinal distance of more than 0.2 times D away from an axis of rotation of the damper blade. Preferably, the air flow sensor is arranged for position at a longitudinal distance between 0.25 times D and 2.0 times D from the damper blade in its closed position.

[0040] In preferred embodiments, system comprises a housing arranged for position outside the air duct. Preferably, the processor system is arranged inside this housing. In preferred embodiments, the air flow sensor is mounted on a first part of a Printed Circuit Board (PCB), and wherein components of the processor system are arranged on a second part of the PCB. Especially, the first part of the PCB is arranged for insertion through an opening in the wall of the air duct, wherein the second part of the PCB is arranged for being positioned in the housing mounted on an exterior part of the air duct. Such embodiment is suited for mounting of the measurement system either for a new design of a ventilation system or for use as retro-fit on an existing ventilation system, where a hole is drilled in the duct wall to allow insertion of the air flow sensor into the duct.

[0041] In some embodiments, the system comprises an obstacle element, such as a pin or a rod, arranged upstream of the air flow sensor to avoid dirt particles on the air flow sensor. This is advantageous if the air flow sensor is positioned in an air flow with dirt particles. For example, in case of an EFV air flow sensor with air from a kitchen hood, it has been found that dirt particles can collect and disturb the sensor function. With an obstacle element in the form of a pin or rod arranged immediately in front of the air flow sensor, the inventors have found that dirt particles can be guided away from the air flow sensor and thereby solve the problem of collection of dirt particles on the air flow sensor.

[0042] Especially, the obstacle element can be a pin or a rod with its length extending in a direction perpendicular to the flow direction. The pin or rod may have a noncircular cross sectional shape and have varying shape in its length. However, in some embodiments, the pin or rod has a circular cross sectional shape, such as having a diameter of 1-20 mm. More specifically, the pin or rod may be arranged at a distance of less than 5.0 times a diameter of the pin or rod away from the air flow sensor or if the shape is not round, the distance should be considered from its widest part, wherein said distance is measured in direction of the air flow from a centre of the pin or rod to the air flow sensor. Such distance has been found to have a good dirt preventing effect. In preferred embodiments, the air flow sensor is arranged at a distance from the wall of the air duct, and wherein the pin or rod has a height which exceeds said distance for at least 0.5 times the diameter or 0.5 times the width. Especially, the air flow sensor is an Elastic Filament based sensor, such as a nano ribbon sensor, arranged on a PCB which is mounted parallel with the pin or rod. It may be preferred that the air flow sensor and the obstacle element are mounted together to form one unit which is arranged to be mounted on or in the duct wall. In this way mounting is facilitated, since the mutual arrangement of the obstacle element and the air flow sensor is prearranged prior to mounting on the air duct.

[0043] In preferred embodiments, the damper blade has a controllable position around an axis of rotation. However, it is to be understood that the measurement system can be used with other types of dampers as well.

[0044] The measurement system can be used for retro-fitting on an existing ventilation system. In this way, the measurement system can be used to replace air flow sensors in existing ventilation system which can thereby benefit of improved performance with respect to measurement accuracy at low air flow speeds as well as a low pressure drop.

[0045] In a second aspect, the invention provides a ventilation system comprising:

[0046] - an air duct system comprising an air duct with an air inlet and an air outlet,

[0047] - a controllable damper arranged to control air flow in the air duct between the air inlet and air outlet by means of a damper blade which has a controllable position, and

[0048] - an air flow measurement system according to the first aspect, wherein the air flow sensor is positioned inside the air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper. In some embodiments, the air flow measurement system comprises a damper position input which is connected to the controllable damper to allow receipt of data indicative of a damper blade position of the controllable damper. Especially, this may be via a standard bus system which is widely used for dampers in ventilation systems.

[0049] In some embodiments, the processor system is arranged to generate an output for control of a position of the damper blade of the controllable damper. Especially, wherein the processor system is arranged to generate the output for control of a position of the damper blade of the controllable damper in response to the calculated air flow value.

[0050] In some embodiments, the processor system is arranged to communicate the determined air flow value. Especially, the ventilation system may comprise a ventilation control unit arranged for connection to the processor system to receive the air flow value, wherein the ventilation control unit is arranged to control the damper blade position in response to the received air flow value.

[0051] The air duct preferably has a circular cross section with a diameter D. Preferably, the air flow sensor is positioned at a longitudinal distance from an axis of rotation of the controllable damper being within a range of 0.2-2.5 times D, more preferably 0.25-2.0 times D. Preferably, the air flow sensor is positioned at a distance from the wall of the duct which is less than 0.15 times D, more preferably less than 0.12 times D. Preferably, the air flow sensor is positioned at a longitudinal distance of more than 0.2 times D away from the axis of rotation of the damper blade.

[0052] In some embodiments, the air duct has a cross sectional area of at least 50 cm2, such at least 100 cm2.

[0053] In some embodiments, the air duct has a cross sectional area of less than 1 m2.

[0054] In preferred embodiments, the air flow sensor comprises an Elastic Filament Velocimetry based sensor. In preferred embodiments, the ventilation system comprises a fan and a ventilation control unit connected, by wire or wirelessly, to the air flow measurement system.

[0055] In a third aspect, the invention provides a method for measuring an air flow in an associated air duct system with an air duct and a controllable damper to control air flow in the air duct by means of a damper blade which has a controllable position, the method comprising:

[0056] - mounting an air flow sensor inside the associated air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper,

[0057] - connecting the air flow sensor to a flow measurement circuit,

[0058] - processing an output from the flow measurement circuit by a processor system to determine a measured air speed accordingly, and

[0059] - calculating by the processor system an air flow value indicative of the air flow in the air duct system in response to all of: the measured air speed, a damper blade position, and a correction value.

[0060] The method may comprise retro-fitting the air flow sensor and the processor system on an existing air duct system.

[0061] The method may comprise drilling a hole in the air duct at a position at or near a cross sectional position of the air duct where the damper blade opens in direction downstream, and mounting the air flow sensor inside the air duct through in the hole.

[0062] The method may comprise determining said correction factor at least in response to a cross sectional dimension of the air duct and a distance between the position of the air flow sensor and the controllable damper.

[0063] In preferred embodiments, the air duct has a circular cross section with a diameter D, and wherein the method comprises mounting the air flow sensor at a longitudinal distance from an axis of rotation of the damper blade being within a range of 0.2-2.5 times D, more preferably as 0.25-2.0 times D. Preferably, the method comprises mounting the air flow sensor at a distance from the wall of the duct which is less than 0.15 times D, more preferably less than 0.12 times the diameter D. Preferably, the method comprises mounting the air flow sensor at a longitudinal distance of more than 0.2 times D away from an axis of rotation of the damper blade.

[0064] In some embodiments, the method comprises performing a mapping of corresponding measured air flow velocities and damper blade positions to determine the correction value.

[0065] In a fourth aspect, the invention provides a kit for mounting on an existing ventilation system, the kit comprising:

[0066] - the air flow measurement system according to the first aspect, and

[0067] - a set of instructions for guiding a user to mount the air flow sensor at a position inside the duct which is specified at least with respect to a distance from a damper of the ventilation system.

[0068] In a preferred embodiment of the kit, the air flow sensor is mounted on a unit comprising a part arranged for mounting on an outside part of the wall of the duct while the air flow sensor is positioned inside the duct inserted through a hole in the duct wall. This allows easy retro-fitting of the air flow system onto an existing ventilation system, basically requiring only drilling a hole in the duct wall and mounting the unit with the air flow sensor inserted for position inside the duct, near the duct wall, and wherein the outside unit, preferably comprising the processor system is arranged inside a housing serving to the protect the outside part of the unit.

[0069] In preferred embodiments, the air duct has a circular cross section with a diameter D, and wherein the set of instructions comprises information to the user for mounting the air flow sensor at a longitudinal distance from an axis of rotation of the damper blade being within a range of 0.2-2.5 times D, more preferably as 0.25-2.0 times D. Preferably, the method comprises mounting the air flow sensor at a distance from the wall of the duct which is less than 0.15 times D, more preferably less than 0.12 times the diameter D. Preferably, the method comprises mounting the air flow sensor at a longitudinal distance of more than 0.2 times D away from an axis of rotation of the damper blade. In preferred embodiments, the air flow sensor comprises an Elastic Filament Velocimetry based sensor.

[0070] The unit is preferably configured for being fastened to an outside part of the duct wall.

[0071] In preferred embodiments, the unit further comprises a dirt preventing obstacle, such as a pin or rod, arranged upstream of the air flow sensor. Such obstacle has been found to be able to prevent dirt particles collecting on the air flow sensor, thereby ensuring reliable function of the air flow sensor even in case of dirt particles in the air flow.

[0072] It is appreciated that the described embodiments and features for the mentioned aspects can be intermixed in any way.

[0073] BRIEF DESCRIPTION OF THE FIGURES

[0074] The invention will now be described in more detail with regard to the accompanying figures of which:

[0075] FIG. 1 illustrates a ventilation duct with a damper and with an air flow measurement system embodiment with an air flow sensor positioned near the duct wall and near the damper,

[0076] FIG. 2 illustrates preferred positions guidelines for position of the air flow sensor in a ventilation duct with diameter D,

[0077] FIG. 3a and 3b illustrate simulated air flow in a longitudinal section of a ventilation duct downstream of a damper at average air flow velocities of 0.2 m / s and 0.8 m / s respectively with colours (grey scales) illustrating local variations in air velocity indicating high air velocities at preferred air flow sensor positions near the damper and near the duct wall,

[0078] FIG. 4a illustrates a specific example of an 0160 mm duct with an air flow sensor positioned at a longitudinal distance L distance from the rotational damper being 110 mm, while FIG. 4b illustrates graphs showing the corresponding translation between measured air flow speed and average air flow in the duct for three different opening angles of the damper, FIG. 5a and 5b illustrate two photos of a specific nano-ribbon EFV based air flow sensor and the associated measurement circuit implemented in a PCB,

[0079] FIG. 6a and 6b illustrate sketches of a longitudinal and cross sectional sections of a duct with the air flow sensor mounted on a flow directing structure thus causing the sensor to measure a flow in a direction different from the longitudinal flow direction along the duct,

[0080] FIG. 7a, 7b, and 7c show sketches of a specific embodiment with a unit having an exterior housing for housing the processor system and measurement system outside the duct, while the air flow sensor is inserted into a hole in the duct wall, FIG. 8 illustrates steps of a method embodiment, and

[0081] FIG. 9a and 9b show respective side view and a top view of a dirt preventing obstacle element serving to protect the air flow sensor from dirt particles.

[0082] The figures illustrate specific ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0083] DETAILED DESCRIPTION OF THE INVENTION

[0084] FIG. 1 illustrates a sketch of an embodiment of the air flow measurement system according to the invention. The system is shown mounted on a ventilation air duct system having an air duct DCT and a controllable damper to control air flow AF in the air duct DCT by means of a damper blade DB which has a controllable position around an axis of rotation AR and with the controlled position being determined by a controllable actuator AT. In this way, air flow AF can be controlled to have a plurality of different positions between a closed position of the damper blade DB (zero air flow AF) and a fully open position of the damper blade DB.

[0085] An air flow sensor AFS is arranged at a position inside the duct DCT, near a wall of the DCT, and downstream of the controllable damper and in proximity to the controllable damper, since this has proven in simulations as well as practical tests to provide a good position for measuring air flow velocity, even at very average low air flow velocities AF in the duct DCT of such as 0.5 m / s or even 0.2 m / s. The air flow sensor AFS, preferably an EFV based sensor, has an electric interface which is connected to a sensor input of which is connected to a measurement circuit which can be positioned either inside the duct DCT, partially inside the duct DCT, of completely outside the duct DCT. A processor system PS comprising a processor and associated memory and control software, is in this embodiment arranged in a housing outside the duct DCT. The processor of the processor system PS is arranged to process an output from the flow measurement circuit to determine a measured air speed accordingly. The processor system has an input connected via a bus BS to the controllable damper to receive data indicative of the damper blade position. The processor is then arranged to calculate the air flow value in response to the measured air speed, the damper blade position, and a correction value. The correction value can be an input regarding dimensions and exact position of the air flow sensor AFS which allows the processor to translate a measured air flow by the air flow sensor AFS to determine an air flow value indicative of the AF in the duct.

[0086] FIG. 2 illustrates an area AFS_P of a preferred position of the air flow sensor for optimal measurement of air flow that can be translated into the air AF of the duct which has a circular cross section with diameter D. The damper has a damper blade which can change position by rotation around a rotation axis AR which is perpendicular to a longitudinal axis of the air flow AF in the duct. Preferably, the air flow sensor is arranged for being positioned at or near a cross sectional position of the air duct where the damper blade DB opens in direction downstream.

[0087] The indicated area AFS_P relates to duct diameter D and distance and angle to the damper, especially the axis of rotation of the damper blade DB. This preferred area AFS_P for position of the air flow sensor is based on computer simulations of air flow as well as practical tests performed by the inventors on circular cross sectional ducts with different diameters, and with a 0.5 mm x 1.0 mm EFV based air flow sensor. The tests have been performed with five parameters:

[0088] 1. Diameter of duct of: 0100 mm, 0160 mm and 0315 mm.

[0089] 2. Different air speeds of: 0.2 m / s, 0.8 m / s, 2.0 m / s, and 3.2 m / s. 3. Different positions of the air flow sensor with respect to longitudinal distance between damper and air flow sensor in relation to duct diameter: 31%, 44%, 69%, 94%, 116%, 144%, and 169%.

[0090] 4. Different damper blade position angles: 20°, 40°, and 80° (where 0° corresponds to a fully closed damper, while 90 corresponds to a fully open damper).

[0091] 5. Different positions of the air flow sensor with respect to distance between the air flow sensor and wall of the duct: 4 mm, 8 mm, and 16 mm.

[0092] The results of the performed tests described above with respect to the most accurate measurements have resulted in the preferred air flow sensor position indicated by the area AFS_P on FIG. 2.

[0093] FIG. 3a and 3b show computer simulations of a flow profile in a duct with a damper blade at a position near its closed position. The air flow direction is indicated by the white arrow, and the duct extends in a longitudinal direction X. The air speed at each position is indicated by a shading corresponding to the simulated air speed. To the left, the corresponding shaded scale is shown, ranging from about 0.2 m / s to 3.0 m / s.

[0094] In FIG. 3a, the average air flow speed in the duct is 0.2 m / s, i.e. a very low value. Still, in spite the very low average air flow value in the duct, it is possible to measure an air speed of about 1 m / s at a short distance after the damper, and close to the duct wall, i.e. the approximate area indicated by the dashed curve. Thus, the simulation indicates, corresponding to the preferred area AFS_P (FIG. 2) for position of the air flow sensor, that the air flow sensor can measure a significantly higher air flow value close to the damper and close to the duct wall, and thereby more accurate measurements can be performed when translated to the corresponding average air flow value in the duct.

[0095] In FIG. 3b, the same damper position as in FIG. 3a is shown, but here the average air flow in the duct is 0.8 m / s, and in the approximate area indicated by the dashed curve, an air speed of more than 3 m / s can be measured. FIG. 3a and 3b serve to indicate that the preferred air flow sensor positions have been found to help to increase the accuracy of even very low air flow speeds in a duct, even down to 0.2 m / s where a + / - 5% accuracy can be achieved and this is verified with an EFV based sensor. This is obtained by the selection of the air flow sensor position where a significantly high air speed can be measured, thereby increasing the measurement accuracy with a given air flow sensor.

[0096] FIG. 4a shows a specific example of an 0160 mm duct with an air flow sensor AFS positioned at longitudinal distance L from the axis of rotation of the damper blade DB, where L=110 mm. Further, the air flow sensor AFS is positioned close to the duct wall, e.g. less than 10 mm from the duct wall. The goal is to determine the average air speed V_D in the duct, while the air flow sensor AFS measures another air speed, namely the air speed V_M at the sensor position.

[0097] FIG. 4b illustrates graphs showing the corresponding translation between measured air flow speed V_M and average air flow in the duct V_D for three different damper blade positions DB_P, namely opening angles 20° (near a closed position), 40° and 80° (near a fully open position). With the translation indicated by the graphs, or converted to a set of correction values, it is possible to calculate the air flow value V_D for the duct in response to a measured air speed V_M based on the input from the air flow sensor AFS as well as knowledge about the damper blade position DB_P.

[0098] FIG. 5a and 5b illustrate photos of a specific nano-ribbon EFV based air flow sensor SNS, NR_EFV and the associated measurement circuit implemented on a PCB, e.g. also comprising components implementing the processor system, or at least a part of the processor system.

[0099] As seen in FIG. 5a, the sensor SNS is positions at or near a distal end of an elongated part of the PCB, and the sensor itself NR_EFV, is positioned in a hole in the PCB, and electrically connected to conductors of the PCB. With this elongated PCB with the sensor positioned near its distal end, the sensor SNS can be positioned inside a duct, while the PCB part with the measurement circuit and e.g. also the processor components can be positioned outside the duct without the need for any physical connector between the two. Further, the elongated PCB part can serve as a flow directing structure which will be further explained in connection with FIG. 6a and 6b.

[0100] FIG. 5b shows a specific sensor in the form of a 11-nano ribbon EFV based sensor NR_EFV. The dimensions of the sensor NR_EFV is about 0.5 mm x 1.0 mm, thus the sensor is very small compared with the dimensions of the duct, and therefore it is suited for a position close to a wall of the duct, and it only creates a minimal pressure drop due to its small size. Still, it has been found that it is capable of measuring air flow speed in a reliable an accurate way.

[0101] FIG. 6a and 6b illustrate sketches of a longitudinal section and a cross sectional sections of a duct with an EFV based air flow sensor SNS mounted on a flow directing structure FDS thus causing the sensor to measure a flow in a direction (indicated by the curved arrow) different from the longitudinal flow direction along the duct wall DW (indicated by the left arrow). The sensor is preferably positioned 6-10 mm from the duct wall DW, as indicated. The sensor SNS actually senses an air speed which is directed perpendicular to the duct air flow, in the illustrated example. Such position of the sensor SNS has been found to tend to collect less dirt in a practical ventilation duct, and further in general a more stable flow has been found with the use of a flow directing structure FDS, and thus more reliable measurement results can be expected compared to a position of the sensor SNS where it senses a flow speed in a direction parallel with the duct wall DW. However, it is to be understood that this can be varied, if preferred, by choosing another angle between the flow directing structure FDS and the duct wall DW.

[0102] In FIG. 6b the position of the sensor SNS in a hole in the flow directing structure FDS is seen. E.g. the flow directing structure FDS can be the elongated PCB part as shown in FIG. 5a, and the sensor SNS can be the sensor NR_EFV shown in FIG. 5b. It is indicated that the width of the flow directing structure 4-8 mm, and thus its size is small compared to a duct diameter of e.g. 160 mm, and thus it only creates a small pressure drop.

[0103] FIG. 7a, 7b, and 7c show sketches of a specific embodiment with a unit having an exterior housing HS for housing the processor system and measurement system outside a ventilation duct VD, while the air flow sensor SNS is inserted into a hole in the duct wall. The unit has a bottom plate BP which faces an exterior part of the duct wall. A sensor hole H_SNS in the bottom plate BP allows the sensor SNS being placed on a distal part of a PCB (e.g. as FIG. 5a) to penetrate this sensor hole H_SNS and via a hole in the duct wall into a position inside the duct. The part of the PCB inside the housing is seen in FIG. 7c, while FIG. 7b shows holding structures PCB_S serving to connect and fix position of the PCB relative to the bottom plate BP. The housing HS serves to connect to a part of the bottom plate BP.

[0104] FIG. 8 illustrates steps of an embodiment of a method for measuring an air flow in an associated air duct system with an air duct and a controllable damper to control air flow in the air duct by means of a damper blade which has a controllable position. The method comprises mounting M_AFS an air flow sensor with an electric interface inside the associated air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper. Next, connecting C_AFS_MS the electric interface of the air flow sensor to a flow measurement circuit. Then, processing P_M_AF an output from the flow measurement circuit by a processor system to determine a measured air speed accordingly. Finally, calculating C_AFV by the processor system an air flow value indicative of the air flow in the air duct system in response to all of: the measured air speed, a damper blade position, and a correction value.

[0105] FIG. 9a and 9b illustrate respective side view (FIG. 9a) and top view (FIG. 9b) of a dirt obstacle element PN placed upstream of the air flow sensor SNS in the air flow AF. Such element PN has been found to be capable of preventing dirt particle in the air flow from collecting on the air flow sensor SNS, thereby avoiding any disturbance of the air flow sensing function of the air flow sensor SNS due to dirt on the sensor SNS. It is to be understood that the dirt obstacle element PN can be combined with any of the above mentioned embodiments including e.g. FIGs. 5, 6 and 7, and it is also suited in combination with an air flow sensor SNS being a Elastic Filament based sensor.

[0106] In FIG. 9a and 9b, the obstacle element in the form of a cylindrical pin PN, i.e. an elongated object with a height H and having a circular cross section with a cross sectional diameter DM. This shape has been found to give a good dirt particle preventing effect to prevent dirt particles from collecting on the air flow sensor SNS.

[0107] With the pin PN arranged immediately upstream of the air flow sensor SNS, dirt particles will be guided away from the sensor SNS, thus preventing dirt particles collecting on the sensor SNS.

[0108] The pin PN is arranged at a distance A from the air flow sensor SNS. The air flow sensor SNS is arranged on a flow directing structure FDS as seen e.g. in FIGs. 5- 7, e.g. a structure formed by a PCB, e.g. such that the air flow sensor SNS is located at a distance of (H-B) from a wall DW of the air duct, if distance B is a distance measured from the top of the pin PN to the location of the sensor SNS. The flow directing structure FDS has a width of P. The width of P is important for creating a pressure difference over the flow directing structure FDS. This pressure difference creates flow through the sensor SNS.

[0109] As seen in the side view of FIG. 9a, the pin PN and the flow directing structure FDS are parallel and perpendicular to the duct wall DW. Preferably, both the pin PN and the flow directing structure FDS with the air flow sensor SNS are arranged on one common structure to form one single unit to be mounted in an opening of the duct wall DW.

[0110] With the distances A, DM, B, H, and P indicated in FIG. 9a and 9b, the most preferred relations are: 1) A < 5.0*DM, 2) B > 0.2*A and B > 0.5*DM, and 3) P > 0.5*DM.

[0111] Thus, for example, if the pin PN has a diameter of 3 mm, the preferred distance A from the sensor SNS is less than 15 mm. Further, with a pin PN diameter of 3 mm, the flow directing structure FDS preferably has a width P of more than 1.5 mm.

[0112] To sum up, the invention provides an air flow measurement system for determining an air flow value indicative of air flow in an associated air duct system with an air duct and a controllable damper to control air flow in the air duct by means of a damper blade which has a controllable position, such as around an axis of rotation. An air flow sensor, preferably an EFV based sensor, is arranged for being positioned inside the associated air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper. The air flow sensor is connected to a flow measurement system, and a processor processes an output from the flow measurement circuit to determine a measured air speed accordingly. The processor can then calculate the air flow value based on the measured air speed, preferably further based on a damper blade position, and a correction value. The measurement system can measure very low air speeds e.g. in a ventilation duct with high accuracy, such as down to 0.2 m / s. To protect the air flow sensor from dirt particles, an obstacle element can be placed upstream of the sensor, i.e. in the form of a pin or rod.

[0113] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms "including" or "includes" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

CLAIMS1. An air flow measurement system for determining an air flow value (V_D) indicative of air flow (AF) in an associated air duct system with an air duct (DCT) and a controllable damper to control air flow in the air duct by means of a damper blade (DB) which has a controllable position, such as around an axis of rotation (AR), the system comprising:- an air flow sensor (AFS) arranged for being positioned inside the associated air duct (DCT) at or near a wall of the air duct (DCT) and downstream of the controllable damper in proximity to the controllable damper,- a processor system (PS) comprising:- a sensor input arranged for connection to the air flow sensor (AFS), wherein the sensor input is connected to a flow measurement circuit, and- a processor arranged to process an output from the flow measurement circuit to determine a measured air speed accordingly, wherein the processor is further arranged to calculate the air flow value (V_D) in response to all of: the measured air speed (V_M), a damper blade position (DB_P), and a correction value.

2. The system according to claim 1, wherein the air flow sensor is arranged for being positioned at or near a cross sectional position of the air duct where the damper blade opens in direction downstream.

3. The system according to claim 1 or 2, wherein the air flow sensor is arranged for being positioned at a position which is perpendicular or substantially perpendicular to an axis of rotation of the damper blade, seen in a direction of the air flow direction of the air duct, such as the air flow sensor being arranged for a position which is within a central part between ends of an axle of rotation of the damper blade seen in a direction of the air flow direction of the air duct.

4. The system according to any of the preceding claims, wherein the air flow sensor is arranged to sense an air velocity or an air speed.

5. The system according to any of claims 1-3, wherein the air flow sensor is arranged to sense an air pressure, such as an absolute air pressure or a differential air pressure.

6. The system according to any of the preceding claims, wherein the air flow sensor comprises an Elastic Filament based sensor, such as a nano ribbon sensor.

7. The system according to any of the preceding claims, wherein the air flow sensor is arranged in proximity to a structure which directs air flow in a direction which is non-parallel with the wall of the air duct, such as perpendicular to the wall of the air duct, so as to cause the air flow sensor to measure an air flow having a direction non-parallel, such as perpendicular to, the wall of the air duct.

8. The system according to any of the preceding claims, wherein said correction value is determined based on a cross sectional dimension of the associated air duct, and on a distance between the air flow sensor and the controllable damper.

9. The system according to claim 8, wherein said correction value is determined further based on a distance of the air flow sensor to the wall of the air duct.

10. The system according to any of the preceding claims, wherein the processor system comprises an output arranged for controlling position of the damper blade of the controllable damper, and wherein the processor system is configured to performing a mapping between measured air flow velocities at respective plurality of different damper blade positions and to determine said correction value in response to said mapping.

11. The system according to any of the preceding claims, wherein the processor system is further configured to receive one or more input values and to determine said correction value accordingly.

12. The system according to claim 11, wherein the processor system is configured to receive an input indicative of a cross sectional dimension of the air duct, such as a value indicative of a diameter of the air duct.

13. The system according to claim 11 or 12, wherein the processor system is configured to receive an input indicative of a distance between the controllable damper and a position where the air flow sensor is mounted in the air duct, such as a distance from an axis of rotation of the damper blade.

14. The system according to any of the preceding claims, wherein the processor system is configured for connection to a controllable damper via an interface, such as a MODBUS interface.

15. The system according to any of the preceding claims, wherein the processor system is configured to receive an input indicative of an intended air flow value, and wherein the processor system is configured to generate an output to the controllable damper for control of the position of the damper blade in response to the input indicative of the intended air flow value.

16. The system according to any of the preceding claims, wherein the processor system is configured to control the damper blade position by turning an actuator on or off, thereby causing the damper blade to open or close.

17. The system according to claim 16, wherein the processor system is configured to perform a calibration procedure comprising controlling the damper blade position and to determine a time lapsed from the damper blade being in closed position until the actuator has turned the damper blade to a fully open position.

18. The system according to claim 17, wherein the processor system is configured to estimate a damper blade position by tracking a time where the actuator has been turned on to cause the damper blade to open and a time where the actuator has been turned on to cause the damper blade to close.

19. The system according to claim 16, wherein the processor system is configured to perform a calibration procedure comprising controlling the damper bladeposition, wherein the actuator comprises a stepper motor, and wherein the processor system is configured to determine a number of steps of the stepper motor from the damper blade being in closed position until the stepper motor has turned the damper blade to a fully open position.

20. The system according to any of the preceding claims, comprising a damper blade position input arranged to receive a value indicative of the damper blade position.

21. The system according to any of the preceding claims, wherein the processor system comprises an output interface arranged to generate an output in response to the calculated air flow value.

22. The system according to claim 21, wherein the output interface is arranged to generate an output indicative of the calculated air flow value.

23. The system according to any of the preceding claims, wherein the processor system is arranged to generate an output to the controllable damper for controlling damper blade position.

24. The system according to any of the preceding claims, wherein the processor system is arranged to receive an input indicative of a specified air flow value.

25. The system according to claim 24, wherein the processor system is arranged to generate an output to the controllable damper for controlling damper blade position in response to the received input indicative of the specified air flow value.

26. The system according to claim 25, wherein the processor system is arranged to determine the output to the controllable damper for controlling position of the controllable damper in response to a difference between the specified air flow value and the calculated air flow value.

27. The system according to any of the preceding claims, wherein the processor system is arranged to receive an input indicative of at least one of: a crosssectional dimension of the air duct, and a distance between the air flow sensor and the controllable damper.

28. The system according to any of the preceding claims, wherein the air duct has a circular cross section with a diameter (D).

29. The system according to claim 28, wherein the air flow sensor is arranged for being positioned at a longitudinal distance (L) from the axis of rotation of the controllable damper being within a range of 0.2-2.5 times the diameter (D) of the air duct, preferably 0.25-2.0 times the diameter (D) of the air duct.

30. The system according to claim 28 or 29, wherein the air flow sensor is arranged for being positioned at a distance from the wall of the duct which is less than 0.15 times the diameter (D) of the air duct, preferably less than 0.12 times the diameter (D) of the air duct.

31. The system according to any of claims 28-30, wherein the air flow sensor is arranged for being positioned at a longitudinal distance of more than 0.2 times the diameter (D) of the air duct away from the axis of rotation of the damper blade.

32. The system according to any of the preceding claims, wherein the processor system comprises a housing arranged for position outside the air duct.

33. The system according to claim 32, wherein the air flow sensor is mounted on a first part of a PCB, and wherein components of the processor system are arranged on a second part of the PCB.

34. The system according to claim 33, wherein the first part of the PCB is arranged for insertion through an opening in the wall of the air duct, wherein the second part of the PCB is arranged for being positioned in the housing mounted on an exterior part of the air duct.

35. The system according to any of the preceding claims, wherein the air flow sensor is mounted on a first PCB, and wherein components of the processorsystem are arranged on a second PCB which is electrically connected to the first PCB, such as the second PCB further having components of the measurement circuit arranged thereon.

36. The system according to any of the preceding claims, wherein the damper blade has a controllable position around an axis of rotation.

37. The system according to any of the preceding claims, being configured for retrofitting on an existing ventilation system.

38. The system according to any of the preceding claims, comprising an obstacle element (PN), such as a pin or a rod, arranged upstream of the air flow sensor (SNS) to avoid dirt particles on the air flow sensor (SNS).

39. The system according to claim 38, wherein the obstacle element (PN) is a pin or a rod with its length extending in a direction perpendicular to the flow direction.

40. The system according to claim 39, wherein the pin or rod (PN) has a noncircular cross sectional shape.

41. The system according to claim 39, wherein the pin or rod (PN) has circular cross sectional shape, such as having a diameter (DM) of 1-20 mm.

42. The system according to claim 41, wherein the pin or rod (PN) is arranged at a distance (A) of less than 5.0 times a diameter (DM) of the pin or rod (PN) away from the air flow sensor, wherein said distance (A) is measured in direction of the air flow from a centre of the pin or rod (PN) to the air flow sensor (SNS).

43. The system according to any of claims 41 or 42, wherein the air flow sensor (SNS) is arranged at a distance (DW) from the wall of the air duct (DCT), and wherein the pin or rod (PN) has a height (H) which exceeds said distance (DW).

44. The system according to any of claims 41-43, wherein the air flow sensor (SNS) is an Elastic Filament based sensor, such as a nano ribbon sensor, arranged on a PCB which is mounted parallel with the pin or rod (PN).

45. The system according to any of claims 38-44, wherein the air flow sensor and the obstacle element (PN) are mounted together to form one unit which is arranged to be mounted on or in the duct wall (DW).

46. A ventilation system comprising- an air duct system comprising an air duct with an air inlet and an air outlet,- a controllable damper arranged to control air flow in the air duct between the air inlet and air outlet by means of a damper blade which has a controllable position, and- an air flow measurement system according to any of claims 1-45, wherein the air flow sensor is positioned inside the air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper.

47. The ventilation system according to claim 46, wherein the air flow measurement system comprises a damper position input which is connected to the controllable damper to allow receipt of data indicative of a damper blade position of the controllable damper.

48. The ventilation system according to claim 46 or 47, wherein the processor system is arranged to generate an output for control of a position of the damper blade of the controllable damper.

49. The ventilation system according to claim 48, wherein the processor system is arranged to generate the output for control of a position of the damper blade of the controllable damper in response to the calculated air flow value.

50. The ventilation system according to any of claims 46-49, wherein the processor system is arranged to communicate the determined air flow value.

51. The ventilation system according to claim 50, comprising a ventilation control unit arranged for connection to the processor system to receive the air flow value, wherein the ventilation control unit is arranged to control the damper blade position in response to the received air flow value.

52. The ventilation system according to any of claims 46-51, wherein the air duct has a circular cross section with a diameter (D).

53. The ventilation system according to claim 52, wherein the air flow sensor is positioned at a longitudinal distance from the axis of rotation of the controllable damper being within a range of 0.2-2, 5 times the diameter (D) of the air duct, preferably 0.25-2.0 times the diameter (D) of the air duct.

54. The ventilation system according to claim 52 or 53, wherein the air flow sensor is positioned at a distance from the wall of the duct which is less than 0.15 times the diameter (D) of the air duct, preferably less than 0.12 times the diameter (D) of the air duct.

55. The ventilation system according to any of claims 52-54, wherein the air flow sensor is positioned at a longitudinal distance of more than 0.2 times the diameter (D) of the air duct away from the axis of rotation of the damper blade.

56. The ventilation system according to any of claims 46-55, wherein the air duct has a cross sectional area of at least 50 cm2.

57. The ventilation system according to any of claims 46-56, wherein the air duct has a cross sectional area of less than 1 m2.

58. The ventilation system according to any of claims 46-57, wherein the air flow sensor comprises an Elastic Filament Velocimetry based sensor.

59. A method for measuring an air flow in an associated air duct system with an air duct and a controllable damper to control air flow in the air duct by means of a damper blade which has a controllable position, the method comprising:- mounting (M_AFS) an air flow sensor inside the associated air duct at or near a wall of the air duct and downstream of the controllable damper in proximity to the controllable damper,- connecting (C_AFS_MS) the air flow sensor to a flow measurement circuit,- processing (P_M_AF) an output from the flow measurement circuit by a processor system to determine a measured air speed accordingly, and- calculating (C_AFV) by the processor system an air flow value indicative of the air flow in the air duct system in response to all of: the measured air speed, a damper blade position, and a correction value.

60. The method according to claim 59, comprising retro-fitting the air flow sensor and the processor system on an existing air duct system.

61. The method according to claim 59 or 60, comprising drilling a hole in the air duct at a position at or near a cross sectional position of the air duct where the damper blade opens in direction downstream, and mounting the air flow sensor inside the air duct through in the hole.

62. The method according to any of claims 59-61, comprising determining said correction factor at least in response to a cross sectional dimension of the air duct and a distance between the position of the air flow sensor and the controllable damper.

63. The method according to any of claims 59-62, wherein the air duct has a circular cross section with a diameter (D), and wherein the method comprises mounting the air flow sensor at a longitudinal distance from an axis of rotation of the damper blade being within a range of 0.2-2, 5 times the diameter (D) of the air duct, preferably as 0.25-2.0 times the diameter (D) of the air duct.

64. The method according to any of claims 59-53, wherein the air duct has a circular cross section with a diameter (D), and wherein the method comprisesmounting the air flow sensor at a distance from the wall of the duct which is less than 0.15 times the diameter (D) of the air duct, preferably less than 0.12 times the diameter (D) of the air duct.

65. The method according to any of claims 59-64, wherein the air duct has a circular cross section with a diameter (D), and wherein the method comprises mounting the air flow sensor at a longitudinal distance of more than 0.2 times the diameter (D) of the air duct away from an axis of rotation of the damper blade.

66. The method according to any of claims 59-65, comprising performing a mapping of corresponding measured air flow velocities and damper blade positions to determine the correction value.

67. A kit for mounting on an existing ventilation system, the kit comprising:- the air flow measurement system according to any of claims 1-45, and- a set of instructions for guiding a user to mount the air flow sensor at a position inside the duct which is specified at least with respect to a distance from a damper of the ventilation system.

68. The kit according to claim 67, wherein the air flow sensor is mounted on a unit comprising a part arranged for mounting on an outside part of the wall of the duct while the air flow sensor is positioned inside the duct inserted through a hole in the duct wall.

69. The kit according to claim 68, wherein said unit further comprises a dirt preventing obstacle, such as a pin or rod, arranged upstream of the air flow sensor.

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