Ult freezer

Abstract

An ultra low temperature (ULT) freezer (1, fig 1) having a cooling chamber (2) accessible via a door (3) and is cooled by a refrigeration circuit 5. The refrigeration circuit comprises at least a firs

Description

[0001] This invention relates to ultra-low temperature freezers and methods of operating ultra-low temperature freezers.

[0002] Ultra-low temperature freezers are intended for the safe storage of, for example, blood plasma, vaccines, human cells, tissues and laboratory samples at ultra-low temperatures. As used herein, the terms “ultra-low temperature freezer” and “ULT freezer” are used interchangeably to indicate a freezer configured to maintain a cooling chamber at a set point temperature which is >-95°C, preferably >-95°C and <-15°C, more preferably >-95°C and <-45°C. The ULT freezers of the invention may have i) a single set point temperature, for example a set point temperature of -86°C or a set point temperature of -50°C; or ii) multiple set point temperatures, for example a first set point temperature of -86°C and a second set point temperature of -50°C, notably with the desired set point temperatures being selected by a user of the ULT freezer.

[0003] One of the challenges of ULT freezers is to configure the cooling system to operate appropriately in each of a plurality of different cooling modes, notably in each of a cool-down mode, a fast recovery mode and steady state mode. In the cool-down mode the temperature of the cooling chamber is reduced from ambient temperature (typically 20 °C to 30°C but sometimes as high as 43°C) to its desired set point temperature (for example -82°C), notably to prepare the cooling chamber to receive goods to be stored. It is, of course, desirable to minimise the time required for the cool-down. Once the cooling chamber has been cooled down and the ULT freezer is in operation at its set point temperature, opening the door (for example to remove goods from the cooling chamber) or loading goods having a temperature above the set point temperature into the cooling chamber causes an increase in the cooling chamber temperature. It is then desirable to rapidly reduce the temperature of the cooling chamber back to its set point temperature using the fast recovery mode. Then, once the temperature of the cooling chamber has been reestablished and stabilised around the set point temperature the cooling circuit is operated in the steady-state cooling mode; this is used to maintain the cooling chamber around the set point temperature i.e. within a pre-defined temperate range around the set-point temperature e.g. -82°C ±1.5°C.

[0004] US6,324,856 explains that ULT refrigeration system parameters are generally designed for efficient operation in the steady state mode and that this does not adequately meet the greater cooling demands encountered during the cool down mode or during the fast recovery mode. US6,324,856 addresses this by controlling refrigerant flow in the higher temperature first stage of a vapour compression, cascade ultra-low temperature refrigeration system in response to higher-than-desired second stage evaporator outlet temperature. A temperature sensor at the evaporator output sends a signal to a controller. The controller operates a valve to increase refrigerant flow in the first stage, increasing system capacity. The valve allows some refrigerant to bypass the normal or primary flow control device, flowing through a second flow control device. The increased flow in the first stage results in increased pressure in the first stage side of the heat exchanger. The heat exchanger transfers heat from the second stage side to the first stage side in a cascade refrigeration system. The increased first stage pressure results in increased refrigerant flow in the second stage. The increased flow provides more efficient system operation when large cooling demands are present. When evaporator outlet temperature returns to the desired range, the controller operates the valve to restore the normal refrigerant flow path, reducing refrigerant flow in the first stage to the normal condition. This allows the refrigeration system to reach desired operating conditions more quickly on initial system start-up, during periods of frequent access or when an abnormally large heat load is placed on the system.

[0005] Another way of addressing these requirements is to provide the ULT freezer with a vapour compression cooling circuit comprising a variable speed compressor. In the cooldown mode, the compressor is generally run at high speed so as to reach the set-point temperature in the cooling chamber as quickly as possible. During the fast recovery mode the compressor is also run at high speed to provide rapid removal of heat from the cooling chamber. During the steady-state cooling mode, the compressor is operated at low speed. For example, WO2011 / 041374 A2 describes a ULT freezer in which each of the first and second compressors of a two-stage cascade refrigeration system is a variable speed compressor and a specific control system is used to control operation of the refrigeration circuit.

[0006] The aim of the present invention is to provide further improvements to ULT freezers, notably to further optimise their operation and efficiency.

[0007] The present invention provides, in accordance with one aspect, an ULT freezer as set out in claim 1; other aspects are set out in other independent claims. The dependent claims define preferred or alternative embodiments.

[0008] In one of its aspects, the present invention is based upon the realisations that: i) an advantageous way of providing a combination of desired temperature management and low power operation for a ULT freezer is to provide a vapour compression refrigeration circuit with i) a variable speed compressor and ii) a boost function which allows for particularly low power consumption in a steady-state cooling mode; ii) that such a boost function can be provided by an expansion valve which provides a greater throttling effect during a steady-state cooling mode that during a fast recovery cooling mode; and iii) that such a boost function allows for operation at lower compressor speeds during the steady-state cooling mode. Thus, the present invention provides a steady-state cooling mode which is particularly energy efficient, referred to herein as an “energy efficient cooling mode”.

[0009] Thus, in accordance with one of its aspects, the present invention provides a ULT freezer having a cooling chamber accessible via a door, the cooling chamber being cooled by a refrigeration circuit, the refrigeration circuit comprising at least a first stage vapour compression refrigeration circuit comprising a first stage variable speed compressor and a first stage expansion valve with the first stage compressor, when in operation, circulating a first stage refrigerant through the first stage refrigeration circuit, wherein the ULT freezer is configured to operate in a fast recovery cooling mode and in an energy efficient cooling mode, wherein, in the fast recovery cooling mode the first stage expansion valve provides a fast recovery throttling effect to the first stage refrigerant, wherein, in the energy efficient cooling mode the first stage expansion valve provides an energy efficient throttling effect to the first stage refrigerant, and wherein the energy efficient throttling effect of the first stage expansion valve is greater than the fast recovery throttling effect of the first stage expansion valve.

[0010] The refrigeration circuit may be a single stage refrigeration circuit. This is particularly appropriate for a ULT freezer which is configured to maintain the cooling space at one or more set-point temperatures with each set-point temperature being >-50°C and <-15°, preferably >-50°C and <-30°C. Alternatively, the refrigeration circuit may be a cascade refrigeration circuit; this is particularly preferred for a ULT freezer which is configured to maintain the cooling space at one or more set-point temperatures with each set-point temperature being >-95°C and <-15°C , preferably >-95°C and <-40°C, more preferably >-95°C and <-70°C and with at least one of the set-point temperatures being <-50°C, preferably <-55°C.

[0011] Thus, in accordance with a further aspect, the present invention provides a ULT freezer having a cooling chamber accessible via a door, the cooling chamber being cooled by a cascade refrigeration circuit which comprises a first stage vapour compression refrigeration circuit, a second stage vapour compression refrigeration circuit and an interstage heat exchanger configured to transfer heat from the second stage refrigeration circuit to the first stage refrigeration circuit the first stage refrigeration circuit comprises a first stage variable speed compressor and a first stage expansion valve with the first stage compressor, when in operation, circulating a first stage refrigerant through the first stage refrigeration circuit, the second stage refrigeration circuit comprises a second stage variable speed compressor and a second stage expansion valve with the second stage compressor, when in operation, circulating a second stage refrigerant through the second stage refrigeration circuit wherein the ULT freezer is configured to operate in a fast recovery cooling mode and in an energy efficient cooling mode; wherein, in the fast recovery cooling mode i) the first stage expansion valve provides a fast recovery throttling effect to the first stage refrigerant and ii) the second stage expansion valve provides a fast recovery throttling effect to the second stage refrigerant; wherein, in the energy efficient cooling mode i) the first stage expansion valve provides an energy efficient throttling effect to the first stage refrigerant and ii) the second stage expansion valve provides an energy efficient throttling effect to the second stage refrigerant; wherein the energy efficient throttling effect of the first stage expansion valve is greater than the fast recovery throttling effect of the first stage expansion valve; and wherein the energy efficient throttling effect of the second stage expansion valve is greater than the fast recovery throttling effect of the second stage expansion valve.

[0012] The expansion valve of each stage of the refrigeration circuit is switchable between a fast recovery mode in which it provides a fast recovery throttling effect and an energy efficient throttling effect, the energy efficient throttling effect being greater than the fast recovery throttling effect. As used herein the term “throttling effect” is the resistance to flow of the refrigerant provided by the expansion valve so that, for a refrigerant provided at an inlet to the expansion valve at the same pressure and temperature, the greater the throttling effect of the expansion valve, the lower the mass flow rate of refrigerant through the expansion value will be. Thus, increasing the throttling effect of the expansion valve (with the refrigerant provided at the inlet to the expansion valve at the same pressure and temperature) decreases the evaporation temperature of the refrigerant. In ideal (i.e. theoretical) systems, throttling effect is provided without any change in the enthalpy of the refrigerant; in the present invention with real (non-ideal) refrigerants and inevitable energy losses, changes in enthalpy during throttling of the refrigerant(s) are inevitable. During the fast recovery cooling mode the expansion valve is configured to provide the fast recovery throttling effect; in combination with a high compressor speed, this can be used to provides a high mass flow rate of refrigerant through the expansion valve and a high cooling capacity to rapidly remove heat from the ULT freezer. During the energy efficient cooling mode the expansion valve is configured to provide the energy efficient throttling effect; in combination with a low compressor speed, this can be used to provides a low mass flow rate of refrigerant through the expansion valve (which, thanks to the greater throttling effect, nevertheless provides a suitable evaporation temperature for the refrigerant) and provides good energy efficiency of the ULT freezer during the energy efficient cooling mode. In addition, rather than using a single, fixed throttling effect for both the fast recovery cooling mode and the energy efficient cooling mode, the fast recovery throttling effect of the expansion valve is preferably optimised for the fast recovery mode and the energy efficient throttling effect is preferably optimised for the energy efficient cooling mode; this can be used to optimise the cooling capacity in each cooling mode.

[0013] Each refrigerant is preferably a hydrocarbon refrigerant; this avoids the use of CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons) and HFC (hydrofluorocarbon) whose use is undesirable, and in some cases banned, due to their greenhouse gas effect. The first stage refrigerant is preferably R290 (propane). Where the refrigeration circuit is a cascade refrigeration circuit, the first stage refrigerant is preferably R290 (propane) and the second stage refrigerant is preferably R170 (ethane); this provides for advantageous operating conditions. As used herein, reference to a refrigerant by its “R” number, for example R290 (propane) and R170 (ethane), means “refrigerant grades” of the materials i.e., grades (which are generally less than 100% pure) which are intended for and used for refrigeration applications. For example, each of R290 (propane) and R170 (ethane) preferably and generally has a purity of at least 97.5 %. Preferably, each refrigerant is a hydrocarbon refrigerant and each of the first stage refrigeration circuits and the second stage refrigeration circuit comprises less than 400g, more preferably less than 300g and even more preferably less than 200g of refrigerant. It is notable that good performance for the ULT freezers can be obtained using such small quantities of hydrocarbon refrigerants which both reduce the use of resources and mitigate against flammability risks of using greater quantities of hydrocarbons.

[0014] For the first stage expansion valve: - the fast recovery throttling effect is preferably a throttling effect which, when the first stage refrigerant is R290 (propane) which is provided to the second stage expansion valve at an entry pressure in the range 18 bars to 10 bar and an entry temperature in the range 40°C to 30°C, provides an exit pressure of about 1 bar and an exit temperature in the range -50 to -43 °C; and -the steady-state throttling effect is preferably a throttling effect which, when the first stage refrigerant is R290 (propane) which is provided to the first stage expansion valve at an entry pressure in the range 10 to 15 bars and an entry temperature in the range 33 to 28°C, provides an exit pressure of about 1 bar and an exit temperature in the range -46 °C to -40 °C. This combination provides a particularly suitable arrangement for the first stage expansion valve, particularly when part of a cascade refrigeration system.

[0015] For the second stage expansion valve: -the fast recovery throttling effect is preferably a throttling effect which, when the second stage refrigerant is R170 (ethane) which is provide to the second stage expansion valve at an entry pressure in the range 8 bar to 15 bars and an entry temperature in the range -20 to -35 °C, provides an exit pressure in the range 2 bars to 1 bar and an exit temperature in the range -70 to -94 °C; and - the steady-state throttling effect is preferably a throttling effect which, when the second stage refrigerant is R170 (ethane) which is provided to the second stage expansion valve at an entry pressure of 8 bar and an entry temperature of -35 °C, provides an exit pressure in the range 2 bars to 1 bar and an exit temperature in the range -94 to -90 °C. This combination provides a particularly suitable arrangement for the second stage expansion valve.

[0016] One particularly advantageous way of configuring the or each expansion valve is using a combination of a high throttling capillary tube and a low throttling capillary tube and a solenoid valve to control flow through the capillary tubes. This allows the expansion valve(s) to be implemented using simple, reliable and readily available components. It is particularly preferred for one or more of the high throttling capillary tube(s) and the low throttling capillary tube(s) to have the same internal diameter and for the length of the low throttling capillary tube to be less than the length of the high throttling capillary tube; this simplifies the components required. In a preferred arrangement, the low throttling (fast recovery) capillary and high throttling (energy efficient) capillary are arranged in series with a solenoid valve controlling a bypass such that: - in the fast recovery cooling mode the solenoid valve is opened so that i) a portion of the refrigerant flows in series through both the energy efficient capillary and the fast recovery capillary and ii) another portion of the refrigerant flows through the bypass opened by the solenoid valve and through the fast recovery capillary (without passing through the energy efficient capillary). - in the energy efficient cooling mode the solenoid valve is closed so that the entire flow of the refrigerant thorough the expansion valve passes in series through both the energy efficient capillary and the fast recovery capillary. This arrangement provides a) a fast recovery throttling effect to the refrigerant in the fast recovery cooling mode as, due to the bypass, a significant portion of the refrigerant (with a high mass flow) bypasses the energy efficient (high throttling) capillary and b) an energy efficient throttling effect to the refrigerant in the energy efficient cooling mode (which is greater that the fast recovery throttling effect) as the entire flow of refrigerant passes through the high throttling capillary tube (as well as also passing through the low throttling capillary tube).

[0017] In the fast recovery mode, each expansion valve and each compressor is operated in its fast recovery mode configuration. Each time the fast recovery cooling mode is initiated, for example upon the door of the ULT freezer being opened, the variable speed compressors are preferably operated at a maximum fast recovery mode compressor speed; this is intended to return the temperature of the cooling chamber to its set-point temperature as quickly as possible. Preferably, during the fast recovery cooling mode the speed of each variable speed compressors is progressively reduced from a maximum fast recovery mode compressor speed to a minimum fast recovery mode compressor speed. Once a predetermined cooling chamber temperature and / or temperature stability has been reestablished, which can be determined for example as a function of stability of the temperature of the cooling chamber around a desired temperature and / or the operation of the compressors, the ULT switches from its fast recovery mode to its energy efficient mode. In the energy efficient mode, each expansion valve and each compressor is operated in its energy efficient mode configuration. During the energy efficient mode each variable speed compressor is operated at at least one energy efficient cooling mode (low) compressor speed. Upon initiation of the energy efficient mode, each variable speed compressor is preferably operated at a speed which is less than the maximum fast recovery mode compressor speed, more preferably less than 75% of the maximum fast recovery mode compressor speed and most preferably less than the minimum fast recovery mode compressor speed. Preferably, during the energy efficient cooling mode, the speed of each compressor is progressively reduced from a maximum energy efficient cooling mode compressor speed to a minimum energy efficient cooling mode compressor speed; the maximum energy efficient cooling mode compressor speed is preferably less than or equal to, more preferably less than, the minimum fast recovery mode compressor speed.

[0018] During operation, the ULT when functioning correctly will operate for most of time in the energy efficient cooling mode. This provides particularly good energy efficiency as it involves low speed operation of each compressor. The fast recovery cooling mode is preferably initiated automatically upon detection of any one of a number of predefined triggers. Such triggers preferably include: the ULT being initially switched on, correct mains power being reestablished (eg after a brown out), manual triggering by an authorised user, selection of a new setpoint temperature for the cooling chamber and opening of the ULT door when the ULT is in operation.

[0019] The ULT freezer is preferably configured to operate at over a range of ambient temperatures; preferably over the entire range of 10°C to 43°C.

[0020] Embodiments of the inventions will now be described, by way of example only, with reference to the accompanying drawings, of which: Fig 1 is a schematic representation of a ULT freezer; Fig 2 is a schematic representation of a cascade refrigeration circuit of the ULT freezer; Fig 3 is a schematic representation of an expansion valve of the cascade refrigeration circuit; and Fig 4 is a schematic representation of an alternative expansion valve.

[0021] Fig 1 is a perspective view of a ULT freezer 1 having a cooling chamber 2 accessible via a door 3. Compartment doors 4 provide access to compartments of the cooling chamber 2. The cooling chamber is cooled by a refrigeration circuit 5 (illustrated in Fig 2) which is partially housed in a cooling circuit compartment 6.

[0022] The refrigeration circuit 5 illustrated in Fig 2 is a two-stage cascade refrigeration circuit comprising a first stage vapour compression refrigeration circuit 10 in which R290 (propane) circulates as the first stage refrigerant, a second stage vapour compression refrigeration circuit 20 in which R170 (propane) circulates as the second stage refrigerant and an interstage heat exchanger 7. Cooling of the cooling chamber 2 of the ULT freezer is carried out by transfer of heat from the cooling chamber 2 of the ULT freezer to the second stage refrigerant as the second stage refrigerant passes through an evaporator 25 which is thermally connected to the cooling chamber 2, subsequent transfer of heat from the second stage refrigerant to the first stage refrigerant as each refrigerant passes through the interstage heat exchanger 7 and transfer of heat from the first stage refrigerant to an atmosphere surrounding a condenser 12 of the first stage refrigeration circuit (assisted by a condenser fan).

[0023] The second stage refrigeration circuit 20 comprises a variable speed second stage compressor 21 which, when in operation, circulates the second stage refrigerant (R170 (ethane)) sequentially through: a desuperheater 22, the inter-stage heat exchanger 7, a filter drier 23, a second stage expansion valve 24 (which comprises a second stage fast recovery capillary 241, a second stage energy efficient capillary 242 and a solenoid valve 243), through the evaporator 25 and back to the second stage compressor 21. The second stage refrigeration circuit also incorporates in sequence between the second stage compressor 21 and the desuperheater 22 i) a second stage pressostat 211 and ii) a second stage oil separator 212.

[0024] The first stage refrigeration circuit 10 comprises a variable speed compressor 11 which, when in operation, circulates the first stage refrigerant (R290 (propane)) sequentially through a first stage condenser 12, a first stage filter drier 13, a first stage expansion valve 14 (which comprises a first stage fast recovery capillary 141, a first stage energy efficient capillary 142 and a solenoid valve 143), the inter-stage heat exchanger 7 and back to the first stage compressor 11. The first stage refrigeration circuit also incorporates a first stage pressostat 111 arranged between the first stage compressor 11 and the first stage condenser 12.

[0025] The refrigeration circuit also comprises: i) a capillary tube heat exchanger 31 arranged to transfer heat from the first stage cooling circuit energy efficient capillary 142 to the first stage suction line (i.e. the portion of the first stage refrigeration circuit arranged between the interstage heat exchanger 7 and the first stage compressor 11)- this is preferably arranged by simple physical contact between the first stage cooling circuit energy efficient capillary 142 and the first stage suction line, for example by wrapping the first stage cooling circuit energy efficient capillary 142 around a portion of the first stage suction line, and improves the functioning of the first stage cooling circuit energy efficient capillary 142 by evacuating heat; ii) a capillary tube heat exchanger 32 arranged to transfer heat from the second stage cooling circuit energy efficient capillary 242 to a portion of the second stage refrigeration circuit at an outlet of the evaporator 25 - this is preferably arranged by simple physical contact between the second stage cooling circuit energy efficient capillary 242 and this portion of the second stage refrigeration circuit, for example by wrapping the second stage cooling circuit energy efficient capillary 242 around a tube, notably a copper tube, forming this portion of the second stage refrigeration circuit, and improves the functioning of the second stage cooling circuit energy efficient capillary 242 by evacuating heat; iii) a heat exchanger 33 which transfers heat between a portion of the first stage refrigeration circuit at an outlet of the condenser 12 to the suction line of the first stage refrigeration compressor, for example by simple physical contact between these portions of the refrigeration circuit and increases efficiency of the refrigeration circuit; iv) a heat exchanger 34 which transfers heat between a portion of the first stage refrigeration circuit at an outlet of the condenser 12 to the suction line of the second stage refrigeration compressor (i.e. the portion of the second stage refrigeration circuit arranged between the evaporator 25 and the second stage compressor 21), for example by simple physical contact between these portions of the refrigeration circuit and increases efficiency of the refrigeration circuit; and v) an anti-frost heat exchanger 35 which transfers heat from a portion of the second stage refrigeration circuit at an outlet of the superdeheater 22 to the suction line of the second stage refrigeration compressor, for example by simple physical contact between these portions of the refrigeration circuit, and helps to ensure that the temperature of the second stage refrigerant which is sucked into the second stage compressor 21 is at suitable temperature (i.e. not too cold) for correct functioning of the second stage compressor 21. The functioning of each of the aforementioned heat exchangers (31,32,33,34,35) is independent of the others; nevertheless, it is preferred to include all five of these heat exchangers into the cooling circuit 5.

[0026] In the illustrated embodiment, the expansion valves are configured as shown in Table 1 using copper capillary tubes: Table 1: First stage expansion valve (R290 (propane)) Second stage expansion valve (R170 (ethane)) Fast recovery capillary tube Internal diameter: 1.24 mm Length: 9 m Fast recovery capillary tube Internal diameter: 0.8 mm Length: 1.5m Energy efficient capillary tube Internal diameter: 0.8 mm Length: 1.9 m Energy efficient capillary tube Internal diameter: 0.7mm Length: 1.4 m

[0027] A further pre-evaporator capillary tube (not shown) may be provided in the refrigeration circuit of the evaporator 25, between the expansion valve 24 and the evaporator 25, notably by using a pre-evaporator capillary tube in the refrigeration circuit between the expansion valve and the evaporator. This provides a convenient way of configuring the refrigeration circuit; when used, the pre-evaporator capillary tube will contribute to the total throttling effect to which the second stage refrigerant is subjected.

[0028] The inter-stage heat exchanger 7, the first stage expansion valve and the second stage expansion valve 24 are arranged within a thermally insulated compartment 36, for example surrounded by or filled with thermally insulated foam; this reduces undesirable heat losses.

[0029] The arrangement of the first stage expansion valve 14 is shown in more detail in Fig 3. The fast recovery (low throttling) capillary 141 and energy efficient (high throttling) capillary 142 are arranged in series in the first stage refrigeration circuit with the solenoid valve 143 controlling a bypass. In the fast recovery cooling mode the solenoid valve 142 is opened so that i) a portion of the refrigerant flows in series through both the energy efficient capillary 142 and the fast recovery capillary 141 and ii) another portion of the refrigerant flows through the bypass opened by the solenoid valve 142 and through the fast recovery capillary 141 (without passing through the energy efficient capillary 142). This provided a fast recovery throttling effect to the first stage refrigerant. In the energy efficient cooling mode the solenoid valve 142 is closed so that the entire flow of the refrigerant thorough the expansion valve 14 passes in series through both the energy efficient capillary 142 and the fast recovery capillary 141. This provides an energy efficient throttling effect to the first stage refrigerant which is greater that the fast recovery throttling effect. An equivalent arrangement is used for the second stage expansion valve 24.

[0030] An alternative possible arrangement for the first stage expansion valve 14 is shown in more detail in Fig 4. In this arrangement, the fast recovery capillary 141 and energy efficient capillary 142 are arranged in parallel in the first stage refrigeration circuit with a solenoid valve 143, 144 arranged in each of the branches. In this arrangement: - in the fast recovery cooling mode, the solenoid valve 143 in the branch containing the fast recovery capillary 141 is opened and the solenoid valve in the branch containing the energy efficient capillary 142 is closed. The entire flow of the refrigerant thorough the expansion valve 14 thus passes through the fast recovery capillary 141 providing a fast recovery throttling effect; - in the energy efficient cooling mode, the solenoid valve 143 in the branch containing the fast recovery capillary 141 is closed and the solenoid valve in the branch containing the energy efficient capillary 142 is opened. The entire flow of the refrigerant thorough the expansion valve 14 thus passes through the energy efficient capillary 142 providing an energy efficient throttling effect which is greater than the fast recovery throttling effect. An equivalent arrangement may be used for the second stage expansion valve 24.

[0031] A further possible arrangement for each expansion valve is similar to that of Fig 4 but dispenses with the solenoid valve 144 arranged in the branch comprising the energy efficient capillary. In this arrangement, in the fast recovery cooling mode the solenoid valve 143 is opened and in the energy efficient mode the solenoid valve is closed.

[0032] The ULT freezer is configured to operate as follows: i) upon detection of a fast recovery trigger (for example opening of the ULT door 3) the ULT freezer enters a fast recovery mode. During the entire fast recovery mode, each of the expansion valves (14, 24) is arrange so as to prove a fast recovery throttling effect to its respective refrigerant. ii) in an initial phase of the fast recovery mode, each compressor 11, 21 is run an at initial phase fast recovery speed, notably a maximum fast recovery mode compressor speed, for example 4400 RPM and is subsequently turned off at an appropriate time (for example once a desired set point temperature of the cooling chamber 2 has been achieved); iii) the ULT subsequently operates in a series of successive additional fast recovery mode phases during which the compressors 11,21 are operated at a fast recovery speed (initiated, for example, by the temperature in the cooling circuit 2 having risen to a preset threshold) and subsequently turned off at an appropriate time (for example once a desired set point temperature of the cooling chamber 2 has again been achieved). During the successive additional fast recover mode phases, the speeds of the compressors 11, 12 are reduced in a stepwise fashion for successive phases, for example in step reductions of 100 RPM until a final, minimum fast recovery mode compressor speed is achieved, for example 3100 RPM; the speeds of the compressors in each additional fast recover mode phase are preferably controlled as a function of the compressor speed and time during which the compressors were run during the previous fast recovery mode phase. iv) upon detection of an energy efficient cooling mode trigger, notably that the compressor speed during the fast recovery mode has been reduced to a predetermined minimum fast recovery mode compressor speed, the ULT enters an energy efficient cooling mode. During the entire energy efficient cooling mode each of the expansion valves (14, 24) is arranged so as to provide an energy efficient throttling effect to its respective refrigerant. v) in an initial phase of the energy efficient cooling mode, each compressor 11, 21 is run an at initial phase energy efficient speed, notably a maximum energy efficient compressor speed, for example 3000 RPM and is subsequently turned off at an appropriate time (for example once a desired set point temperature of the cooling chamber 2 has been achieved); vi) the ULT subsequently operates in a series of successive additional energy efficient cooling mode phases during which the compressors 11,21 are operated at an energy efficient recovery speed (initiated, for example, by the temperature in the cooling circuit 2 having risen to a preset threshold) and subsequently turned off at an appropriate time (for example once a desired set point temperature of the cooling chamber 2 has again been achieved). During the successive additional energy efficient cooling mode phases, the speeds of the compressors 11, 21 are reduced in a stepwise fashion for successive phases, for example in step reduction of 100 RPM until a final, minimum energy efficient cooling mode compressor speed is achieved, for example 2200 RPM; the speeds of the compressors in each additional energy efficient cooling mode phase are preferably controlled as a function of the compressor speed and time during which the compressors were run during the previous energy efficient cooling mode phase. vii) the ULT then continues to operate in its energy efficient cooling mode (using the minimum energy efficient cooling mode compressor speed each time the compressors are operated) until another fast recovery trigger is detected.

[0033] Reference number 1 ULT freezer 2 cooling chamber 3 door 4 compartment door 5 refrigeration circuit 6 cooling circuit compartment 7 inter-stage heat exchanger 10 first stage vapour compression refrigeration circuit 11 first stage refrigeration circuit compressor 12 condenser 13 filter drier 14 expansion valve 111 pressostat 141 fast recovery capillary 142 energy efficient capillary 143 solenoid valve 144 solenoid valve 20 second stage vapour compression refrigeration circuit 21 second stage refrigeration circuit compressor 22 desuperheater 23 filter dryer 24 expansion valve 25 evaporator 211 pressostat 212 oil separator 241 fast recovery capillary 242 energy efficient capillary 243 solenoid valve 31 Capillary tube heat exchanger 32 Capillary tube heat exchanger 33 Heat exchanger 34 Heat exchanger 35 Anti-frost heat exchanger 36 Thermally insulated compartment

Claims

1. A ULT freezer having a cooling chamber accessible via a door, the cooling chamber being cooled by a refrigeration circuit, the refrigeration circuit comprising at least a first stage vapour compression refrigeration circuit comprising a first stage variable speed compressor and a first stage expansion valve with the first stage compressor, when in operation, circulating a first stage refrigerant through the first stage refrigeration circuit, wherein the ULT freezer is configured to operate in a fast recovery cooling mode and in an energy efficient cooling mode, wherein, in the fast recovery cooling mode the first stage expansion valve provides a fast recovery throttling effect to the first stage refrigerant,wherein, in the energy efficient cooling mode the first stage expansion valve provides an energy efficient throttling effect to the first stage refrigerant, andwherein the energy efficient throttling effect of the first stage expansion valve is greater than the fast recovery throttling effect of the first stage expansion valve.

2. A ULT freezer in accordance with claim 1, wherein:- the refrigeration circuit is a single stage refrigeration circuit.- notably, in which the ULT freezer is configured to maintain the cooling space at one or more set-point temperatures, each set-point temperature being >-50°C and <-15°, preferably >-50°C and <-30°C.

3. A ULT freezer in accordance with any of claim 1, wherein:- the refrigeration circuit is a cascade refrigeration circuit;- notably in which the ULT freezer is configured to maintain the cooling space at one or more set-point temperatures, each set-point temperature being >-95°C and <-15°C , preferably >-95°C and <-40°C, more preferably >-95°C and <-70°C;- the cascade refrigeration circuit comprises the first stage vapour compression refrigeration circuit, a second stage vapour compression refrigeration circuit and an inter-stage heat exchanger configured to transfer heat from the second stage refrigeration circuit to the first stage refrigeration circuit;-wherein the second stage refrigeration circuit comprises a second stage variable speed compressor and a second stage expansion valve with the second stage compressor, when in operation, circulating a second stage refrigerant through the second stage refrigeration circuit;wherein, in the fast recovery cooling mode the second stage expansion valve provides a fast recovery throttling effect to the second stage refrigerant,wherein, in the energy efficient cooling mode the second stage expansion valve provides an energy efficient throttling effect to the second stage refrigerant, and wherein the energy efficient throttling effect of the second stage expansion valve is greater than the fast recovery throttling effect of the second stage expansion valve.

4. A ULT freezer in accordance with claim 3, wherein:- the first stage refrigerant is a hydrocarbon refrigerant, notably R290 (propane); and - the second stage refrigerant is a hydrocarbon refrigerant, notably R170 (ethane).

5. A ULT freezer in accordance with claim 3 or claim 4, wherein, for the first stage expansion valve:-the fast recovery throttling effect is a throttling effect which when the first stage refrigerant is R290 (propane) which is provided to the second stage expansion valve at an entry pressure of 18 bars to 10 bar and an entry temperature of 40°C to 30°C, provides an exit pressure of about 1 bar, notably 0.8 bar to 1.2 bars, and an exit temperature in the range -50 to -43 °C; and-the steady-state throttling effect is a throttling effect which, when the first stage refrigerant is R290 (propane) which is provided to the first stage expansion valve at an entry pressure of 10 to 15 bars and an entry temperature of 33 to 28°C, provides an exit pressure in the range of about 1 bar, notably 0.8 bars to 1.2 bars, and an exit temperature in the range -46 °C to -40 °C.

6. A ULT freezer in accordance with any of claims 3 to 5, wherein, for the second stage expansion valve:-the fast recovery throttling effect is a throttling effect which, when the second stage refrigerant is R170 (ethane) which is provided to the second stage expansion valve at an entry pressure of 8 bar to 15 bars and an entry temperature of -20 to -35 °C, provides an exit pressure in the range 2 to 1 bars and an exit temperature in the range -70 to -94 °C; and- the steady-state throttling effect is a throttling effect which, when the second stage refrigerant is R170 (ethane) which is provided to the second stage expansion valve at an entry pressure of 8 bars and an entry temperature of -35 °C, provides an exit pressure in the range 2 to 1 bar and an exit temperature in the range -94 to -90 °C.

7. A ULT freezer in accordance with any preceding claim, whereineach expansion valve comprises a fast recovery capillary tube which provides the fast recovery mode throttling effect for its respective cooling circuit and an energy efficientcapillary tube which provides the energy efficient cooling mode throttling effect for its respective cooling circuit.

8. A ULT freezer in accordance with claim 7, whereineach expansion valve further comprises a switching valve, and in which the switching valve is configured such that i) in the fast recovery cooling mode the switching valve causes the fast recovery capillary tube to be connected to its respective refrigeration circuit and causes the energy efficient capillary tube be bypassed in its respective refrigeration circuit and ii) in the energy efficient cooling mode the switching valve causes the energy efficient capillary tube and the fast recovery capillary tube to be connected in series in their respective refrigeration circuit.

9. A ULT freezer in accordance with claim 7, whereineach expansion valve further comprises a switching valve, and in which the switching valve is configured such that i) in the fast recovery cooling mode the switching valve causes the fast recovery capillary tube to be connected to its respective refrigeration circuit and ii) in the energy efficient cooling mode the switching valve causes the energy efficient capillary tube to be connected to its respective refrigeration circuit and causes the fast recovery capillary tube to be disconnected from its respective the refrigeration circuit.

10. A ULT freezer in accordance with any preceding claim, whereinthe ULT freezer is configured to operate such that i) during the fast recovery cooling mode the speed of each variable speed compressors is progressively reduced from a maximum fast recovery mode compressor speed to a minimum fast recovery mode compressor speed and ii) during the energy efficient cooling mode each variable speed compressor is operated at at least one energy efficient cooling mode compressor speed which is less than the maximum fast recovery mode compressor speed, preferably less than 75% of the maximum fast recovery mode compressor speed and more preferably less than the minimum fast recovery mode compressor speed.

11. A ULT freezer in accordance with claim 10, whereinduring the energy efficient cooling mode, the speed of each compressor is progressively reduced from a maximum energy efficient cooling mode compressor speed to a minimum energy efficient cooling mode compressor speed, preferably in which the maximum energy efficient cooling mode compressor speed is less than orequal to, more preferably less than, the minimum fast recovery mode compressor speed.

12. A ULT freezer in accordance with any preceding claim, whereinthe ULT freezer is configured toa) monitor the occurrence of one of more fast recovery mode triggers, notably one or more fast recovery mode triggers selected from: the refrigeration circuit being reconnected to a power supply sufficient to power each compressor; a manual reset; the ULT freezer exiting the energy efficient cooling mode; opening of the ULT freezer door; and selection of a new temperature set-point for the cooling chamber; andb) initiate the fast recovery mode upon detection of any of the fast recovery mode triggers;c) monitor the occurrence of an energy efficient cooling mode trigger, notably an energy efficient cooling mode trigger selected from a compressor speed during the fast recovery mode having been reduced to a predetermined minimum fast recovery mode compressor speed and an indication of a predefined level of stability of the temperature of the cooling chamber at the set-point temperature having been achieved; andd) initiate the energy efficient cooling mode upon detection of the energy efficient cooling mode trigger.

13. A ULT freezer, notably a ULT freezer in accordance with any preceding claim, wherein- the ULT freezer comprises an electrically heatable cooling chamber pressure equalisation valve configured to allow air to pass from an exterior of the ULT freezer through the pressure equalisation valve into the cooling chamber;- the ULT freezer being configured to activate electrical heating of the electrically heatable pressure equalisation valve upon detection of opening of the door; and- the ULT freezer preferably being configured to initiate deactivation of the electrical heating of the electrically heatable pressure equalisation valve upon detection of closing of the door, notably to deactivate the electrical heating of the electrically heatable pressure equalisation valve after a pre-set time period from detection of closing of the door.

14. A ULT freezer having a cascade refrigeration circuit, notably a ULT freezer in accordance with claim 3 or in accordance with any preceding claim as dependent upon claim 3, wherein the refrigeration system comprises a subcooling heat exchanger configured totransfer heat from a first stage condenser outlet line to at least one and preferably each of a first stage compressor suction line and a second stage compressor suction line.

15. A ULT freezer having a cascade refrigeration circuit, notably a ULT freezer in accordance with claim 3 or in accordance with any preceding claim as dependent upon claim 3, wherein the refrigeration system comprises a first stage energy efficient capillary tube heat exchanger configured to transfer heat from the first stage energy efficient capillary tube to a first stage compressor suction line.

16. A ULT freezer having a cascade refrigeration circuit, notably a ULT freezer in accordance with claim 3 or in accordance with any preceding claim as dependent upon claim 3, wherein the refrigeration system comprises a second stage energy efficient capillary tube heat exchanger configured to transfer heat from the second stage energy efficient capillary tube to a second stage compressor suction line, notable at an outlet of the second stage evaporator.

17. A method of operating a ULT freezer having a cooling chamber accessible via a door, the cooling chamber being cooled by a refrigeration circuit, the refrigeration circuit comprising at least a first stage vapour compression refrigeration circuit comprising a first stage variable speed compressor which, when in operation, circulates a first stage refrigerant around the first stage refrigeration circuit and a first stage expansion valve, notably a ULT freezer in accordance with any preceding claim, the method comprising: a) periodically operating the refrigeration circuit in a fast recovery mode in which the first stage compressor is operated at at least one fast recovery speed and the first stage expansion valve is operated in a fast recovery mode in which it provides a fast recovery throttling effect to the first stage refrigerant; andb) periodically operating the refrigeration circuit in an energy efficient mode in which the first stage compressor is operated at at least one energy efficient speed and the first stage expansion valve is operated in an energy efficient mode in which it provides an energy efficient throttling effect to the first stage refrigerant;and in which- the energy efficient speed of the first stage compressor is lower than the fast recovery speed of the first stage compressor; and;- the energy efficient throttling effect of the first stage expansion valve is greater than the fast recovery throttling effect of the first stage expansion valve.

18. A method of operating a ULT freezer in accordance with claim 17,wherein:- the refrigeration circuit is a cascade refrigeration circuit;- the cascade refrigeration circuit comprises the first stage refrigeration circuit, a second stage vapour compression refrigeration circuit and an inter-stage heat exchanger configured to transfer heat from the second stage refrigeration circuit to the first stage refrigeration circuit;- the second stage vapour compression refrigeration circuit comprising a second stage variable speed compressor which, when in operation, circulates a second stage refrigerant around the second stage refrigeration circuit and a second stage expansion valve,notably a ULT freezer in accordance with claim 3 or any of claims 4 to 14 as dependent upon claim 3, whereina) operating the refrigeration circuit in the fast recovery mode further comprises operating the second stage compressor at least one fast recovery speed and operating the second stage expansion valve in a fast recovery mode in which it provides a fast recovery throttling effect to the second stage refrigerant;b) operating the refrigeration circuit in the energy efficient mode further comprises operating the second stage compressor at least one energy efficient speed and operating the second stage expansion valve in an energy efficient mode in which it provides an energy efficient throttling effect to the second stage refrigerant;and in which- the energy efficient speed of the second stage compressor is lower than the fast recovery speed of the second stage compressor; and;- the energy efficient throttling effect of the second stage expansion valve is greater than the fast recovery throttling effect of the second stage expansion valve.

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