WO2022258989A1

WO2022258989A1 - Improved apparatus and method for refrigeration systems

Info

Publication number: WO2022258989A1
Application number: PCT/GB2022/051466
Authority: WO
Inventors: James Thornton STENHOUSE
Original assignee: Stenhouse James Thornton
Priority date: 2021-06-10
Filing date: 2022-06-10
Publication date: 2022-12-15
Also published as: GB2609731A; GB202208528D0; GB202108306D0

Abstract

The invention relates to a refrigeration system which can be a cascade or auto cascade system, and which allows a blend of two or more refrigerants to be utilised within the system and at least one flow metering device is provided with an outlet which leads into the suction side inlet of at least one of the heat exchangers to form an ejector system. In one embodiment, the refrigeration system is provided so as to be capable of acting as a cryogenic refrigeration system.

Description

Improved apparatus and method for refrigeration systems

The invention which is the subject of this application relates to improvements in refrigeration systems and particularly, although not necessarily exclusively, in relation to auto cascade refrigeration systems.

Refrigeration systems are well known and there are numerous commercial and domestic uses to which the same can be put. Commercial processes include, for example; storage of medical, biological or food materials, or processes involving the production or testing of electronic and microelectronic circuits, chemical synthesis, material processing, vacuum coating. Typically all require, or are improved by, temperatures lower than maybe achieved with conventional refrigeration, which has a practical lower limit of -60 °C. Alternative options such as using a physical cryogen e.g. dry ice are in many cases not practical; consequently specialised mechanical refrigeration systems are often the most practical solution. To achieve low temperatures within these refrigeration systems it is either necessary to operate the evaporator of the system at lower pressures or to use gases which have low boiling points. In practice low boiling point refrigerants are used since evaporating refrigerants at pressures less than atmospheric pressure may cause electrical or contamination problems especially where the system is large. Low boiling point refrigerants, however have low critical temperatures and require high compressor discharge pressures to effect condensation from vapour into the liquid phase, adiabatic heating causes higher gas discharge temperatures at the compressor which is generally undesirable.

To achieve lower temperatures one form of known refrigeration system are cascade refrigeration systems (Figure 1.) in which there is provided a high stage compressor and a low stage compressor(s) and a liquid/ gas receiver. A solenoid valve is provided and discharge and suction pressure points are provided for the high pressure stage and discharge pressure points are provided for the shutoff and solenoid valve for the low pressure stage. Cascade refrigeration systems employ two or more refrigeration cycles typically operating at different pressure levels and or temperature levels. The duty of the lower temperature cycle is to provide the desired refrigeration effect at a relatively low temperature and the condenser in the lower temperature cycle is thermally coupled to the evaporator in the higher temperature cycle such that the evaporator in the higher cycle only serves to extract the heat released by the condenser in the lower cycle and this heat is turned into the ambient air or water stream in the condenser of the higher cycle. The apparatus is also known as the Kleemenko cycle or one-flow or mixed gas cascade. (Figure 2.) Typically, it is employed commercially as a single-stream mixed-refrigerant technique to cool or liquefy gases during production of industrial gases, particularly in the petrochemical industry. When applied to a closed loop refrigeration system, an auto-cascade process may achieve temperatures of less than 123°K

Smaller, efficient cryo-coolers serve growing numbers of advanced sensor and viewing systems, whilst larger systems provide cooling for medical applications e.g. MRI and PET scanners. In the field of vacuum coating, water vapour cryopumps are an essential enabling technology in the production of many thin film devices and components, from capacitors and photo-voltaic solar cells to the modern smart phone. Other emerging applications of commercial interest include cooling of high temperature superconductors and the abatement and recycling of condensable gases from exhaust and waste streams using cryo-condensation.

The apparatus typically includes plate heat exchangers which are characterised by a relatively large surface area, close approach temperatures and higher thermal efficiencies with a lower pressure drop than traditional types employed in the auto cascade process. The auto-cascade process is a multi-component refrigeration process which achieves a very low temperature in a single compressive step. However, plate heat exchangers have proven difficult to optimise in order to exploit their full potential. The system as described inevitably leads to a mixed phase flow of variable composition. Therefore, mixed flow of vapour and condensate requires a longer residency time of the fluid within the individual heat exchanger. This, in turn, leads the design of the plate heat exchangers towards longer path lengths, which become economically less attractive, especially at smaller scales. Furthermore, it can be found that plate heat exchanger-based auto cascade refrigeration processes, if they are subjected to overloading at the evaporator, can become decoupled in that each refrigeration cycle can be viewed as a separate element which is dependent upon the temperature pressure relationship existing within the whole and each other process. Typical over-load conditions are caused in batch processes where there is a large heat load — for example batch chemical processes, or during initial cooling of a large thermal mass. Furthermore, if the path length of the plate heat exchanger is shortened the tolerance of over-load is reduced as the residency time within the heat exchanger is a function of vapour velocity. The applicant has invented an auto cascade refrigeration apparatus which may also be used at a relatively low capacity

<1 kW.

A further problem with refrigeration systems generally is the environmental impact of the refrigerant material used. This problem is exacerbated where a requirement for ultra-low temperature (lower than -60 °C) cooling, which does not use an open cycle / total loss cooling process using cryogens. Typically but not exclusively liquified Nitrogen or solid Carbon Dioxide are cryogens of choice whose use has been increasing with advances in biotechnology and materials processing. A total loss cooling process is known to be both wasteful in terms of energy, dangerous outside of certain situations/ Such processes are intrinsically constrained by the size of cryogen reservoir. Overall, the carbon footprint of this form of cooling is much higher than the proposed vapour cooling cycle simply as a function of Carnot efficiency h'(%) = 1 - (G coid/T_Hot) X 100%.

To achieve low temperatures within refrigeration systems it is either necessary to operate the evaporator of the system at lower pressures and, or to use gases with low boiling points. In the first instance low boiling point refrigerants are used by preference, since evaporating refrigerants at pressures less than atmospheric pressure may causes reliability issues especially where the system is large. Low boiling point refrigerants, however, have low critical temperatures and therefore require higher compressor discharge pressures to effect condensation from vapour into the liquid phase. The adiabatic heating this causes results in higher gas discharge temperatures at the compressor which is generally undesirable. To achieve efficient cooling at temperatures lower than -60°C in a single compressive step is not possible within the constraints of commercial refrigeration plant. To achieve lower temperature two or more compressive steps in series are undertaken in a cascaded system of the type shown in Figure 1.

An aim of the present invention is to improve the handling and design of elements of the auto cascade refrigeration apparatus. A further aim is to provide improvements to auto cascade apparatus which utilise plate-type heat exchangers so that a practical working apparatus is developed which allows the exploitation of both higher efficiency and greater process stability. A further aim is to provide and facilitate the use of improved refrigerants for use in these systems.

In a first aspect of the invention there is provided a refrigeration system including a compressor to compress a heterogeneous cooling fluid, a heat exchanger to condense at least a portion of the fluid and at least one further heat exchanger, a phase separator and metering devices and an evaporator to evaporate the fluid into a return or suction stream, which re-enters the heat exchangers at a reduced pressure and wherein at least one flow metering device is provided with an outlet which leads into the suction side inlet of at least one of the heat exchangers to form an ejector system and wherein a blend of two or more refrigerants are used.

In one embodiment said refrigerants having different boiling points are non- zeotropic.

In one embodiment the said refrigerants undergo a single compressive step to achieve the required low temperature.

In one embodiment a reduction in Global Warming is 49% for a first blend and - 78% for a second blend.

In one embodiment a lowered compressor discharge temperature is achieved. In one embodiment increased heat rejection is achieved from the system condenser.

In one embodiment an improved refrigeration efficiency is achieved.

In one embodiment an extension of lowest achievable temperature is achieved.

In one embodiment the invention permits the use of refrigerants which would normally be outside of the compressors conventional operating pressure temperature range or have critical temperatures lower than -253 °K.

In one embodiment the apparatus and method enables use of standard mechanical refrigeration components without modification.

In one embodiment a lowered compression ratio is achieved.

In one embodiment one or more hydrocarbons are incorporated at concentrations less than 4% and the resultant blend is non-flammable.

In one embodiment the materials are universally compatible with all refrigeration oils including alkyl-benzene, Polyolester and mineral (synthetic and natural) with hydrocarbon incorporation.

In one embodiment zero Ozone Depletion is caused.

In one embodiment the GWP of the combined refrigerant blends is lower than that achieved within a conventional two stage cascade whilst remaining non-flammable. In one embodiment improved heat transfer is achieved through greater cycle efficiency and recovery of oil within oil separation devices, prior to the commencement of the refrigeration process described.

In one embodiment the material is a blend of refrigerants in a category which are non-flammable and carry an A1 ASHRE (American Society of Heating and Refrigeration Engineers) safety rating.

In another embodiment the material is a blend which of a second category which is non- toxic and reduced flammability classed as A2L

In another embodiment the material is a blend of second category classed as A3 non-toxic, flammable.

In a further aspect of the invention there is provided a refrigeration system using mixed refrigerants with one or more direct heat exchange surfaces for cooling ambient air to temperatures colder than -180 °K and typically colder than -163 °K for the purposes of cryo-preservation.

In one embodiment the refrigerant blend has a reduced global warming impact and includes any or any combination of hydro fluoroole fins (HFO) or hydrohaloalkenes (HHO / HFAe) as refrigerants e.g. Trans-l-chloro-3,3,3-trifluoropropene.

In one embodiment there is provided a refrigerant mixture of reduced global warming impact using any or any combination of hydro fluoroole fins (HFO) or hydrohaloalkenes (HHO / HFAe) as refrigerants e.g Trans-l-chloro-3,3,3- trifluoropropene, and or Carbon Dioxide. In one embodiment there is provided a refrigerant mixture for cryogenic cooling containing difluoromethane where it is present at or below its lower flammability limit in order to achieve to an A1 classification.

In one embodiment the heat exchanger is a plate heat exchanger.

In one embodiment the flow of fluid returning to the compressor passes through the heat exchanger and is introduced into a volume in the heat exchanger via an inlet and leaves via an outlet. Typically the outiet from the said flow metering device is of a smaller diameter than the said inlet into the heat exchanger.

In one embodiment the outlet from the flow metering device is placed at an angle <42 ° to the principle gas flow through the heat exchanger inlet to create a Venturi effect.

In one embodiment the outlet from the flow metering device is parallel to the flow from the heat exchanger inlet to the heat exchanger outlet.

In one embodiment the flow metering device outlet is inclined by an angle 42° so that the fluid stream leaving the flow metering device outlet is imparted to the main flow.

The system is so designed as to be stable whilst cooling an external evaporator and also whilst circulating internally

In one embodiment the system is able to achieved controlled temperatures and or controlled cooling rates.

In one embodiment the energy consumption and minimum achievable temperature possible are enhanced.

In one embodiment the system includes a sub-cooler which is located below a heat exchanger to which the same is connected such that the sub-cooler collects any condensate formed in the refrigeration process. Typically, all gasses condensed in the said heat exchanger pass through a metering device connected to the sub cooler.

In one embodiment a capillary line is provided to carry the return from the suction side of the sub-cooler through a swan neck configuration.

In one embodiment a sub-cooler is provided, and a flow of condensed liquid refrigerant passes from a high pressure side of the sub cooler to a low pressure side of the sub-cooler via a flow metering device.

Typically the passage of fluid from the high pressure side of the sub-cooler is via two outlets with a first outlet passing to the flow metering device and provided such that the liquid fed to the low pressure side is maintained in preference to the second outlet which is a feed to an evaporator.

In one embodiment the sub-cooler acts as both a heat exchanger and a liquid reservoir for cryogenically cooled refrigerant.

In one embodiment the control of suction pressure is regulated by a variable capacity metering device located at the out flow from the sub-cooler.

In one embodiment the system includes a phase change separator which includes an inlet to receive a mixed flow of liquid and gas, a separation element, to allow separated out gas/ vapour to exit through an outlet and the liquid to exit through an outlet and wherein the separator has a void space in which the received mixed flow of fluid is held and the diameter of the void is greater than 4 times the diameter of the inlet and the volume of the void is greater than 2% of the swept volume of the compressor.

In one embodiment the outlet is located in the base of the separator and the walls adjacent the outiet are convex in shape.

In one embodiment the refrigeration system is a cascade or an auto cascade system. In a further aspect there is provided a refrigeration system including a compressor to compress cooling fluid, a heat exchanger to condense at least some of the fluid and further heat exchangers in combination with phase separator and metering devices and an evaporator to evaporate the fluid into a return, or suction, stream and which re-enters the heat exchangers and wherein one of the heat exchangers acts as a sub-cooler ,said sub-cooler located below a heat exchanger to which the same is connected such that the sub-cooler collects any condensate formed in the refrigeration process from the heat exchanger.

In a further aspect there is provided a refrigeration system including a compressor to compress cooling fluid, a heat exchanger to condense at least some of the fluid and further heat exchangers in combination with phase separator and metering devices and an evaporator to evaporate the fluid into a return, or suction, stream and which re-enters the heat exchangers and wherein a sub-cooler is provided and a flow of condensed liquid refrigerant passes from a high pressure side of the sub cooler to a low pressure side of the sub-cooler via a flow metering device with the passage of the fluid from the high pressure side of the sub-cooler is via two outlets with a first outlet passing to the flow metering device and provided such that the liquid fed to the low pressure side is maintained in preference to the second outlet which is a feed to an evaporator.

A specific embodiment of the invention is now described with reference to the accompanying Figures; wherein;

Figure 1 illustrates a two stage cascade refrigeration system; and

Figure 2 illustrates an auto cascade refrigeration system with hot gas injection to evaporator

Figure 3 illustrates the control of suction pressure at the sub-cooler in accordance with one embodiment of the invention in order to facilitate use of novel refrigerants and reduce energy consumption Figure 4 examples of blends incorporating novel compounds which have been shown experimentally in accordance with the invention to be effective; and

Figures 5 and 6 illustrate the manner in which the current invention is an enhancement of the apparatus of GB2867596.

The invention proposes novel combinations of materials in gas blends to achieve efficient cooling at less than -60 °C. The invention combines high and low boiling point fluoro-carbon (unsaturated derivative alkane, alkene and ketone) compounds, with or without hydrocarbons to achieve cooling with compressor discharge temperatures typically less than 120°C which is seen as desirable. Within the refrigeration cycle some components may not change state as per classical vapour compression cooling described. By virtue of being incorporated into systems where they have a much very boiling point than the design cooling temperature the invention proposes mixtures which maybe described a zeotropic in nature.

Materials proposed in accordance with the invention have not previously been considered suitable for ultra-low temperature refrigeration applications since high boiling points mean they never fully leave the liquid state raising fears of liquid returning directly to the compressor. This is known to cause mechanical damage through liquid incompressibility. Experimentation with the proposed materials if used in conjunction with appropriate system design has been shown them to be highly effective in containing the discharge pressures and temperature of compressors within acceptable limit. The materials proposed are so effective that Tetrafluoromethane and tri-fluoro-methane have been successfully used in a low GWP multi component system despite critical pressures of 37.4 and 48.6 bar which are far in excess of the 22 bar considered as the normal maximum in conventional, cascade and auto-cascade refrigeration systems. Careful control of system design and refrigerant component proportions has been shown to be critical. An important criteria is that the critical temperature of the add compound is above the condensation temperature for the primary refrigerant medium. By reducing the temperature and pressure of the discharge from a compressor/ s is improved lubrication and mechanical parts are subjected to less stress and therefore maybe expected to have a longer life time. Ideal thermal materials have the following properties:

• Low viscosity

• High latent heat of vaporisation

• Low freezing point

• Good solvation for refrigerants in their liquid phase

• Zero ozone depletion potential

• Low global warming potential

• High critical temperatures relative to their boiling points.

• Chemically inert

• Non or low flammability

• Good miscibility compressor oil

The materials all meet the above criteria and have been shown in test situations with the appropriate conditions to be effective in both cascade and auto-cascade refrigeration systems.

When used as a blend within an autocascade system of the type shown in Figure 2 to achieve ultra-low temperatures there is simultaneously a reduction in pressure and discharge temperature coupled with an increase in heat transfer at the condenser.

The mechanism by which suppression of discharge pressures and temperatures is achieved is the low boiling point components solubility in its gas phase into the high boiling point material is a direct function of pressure. At the higher pressure found at discharge (200 psi of greater) components form a non- azeotropic mixture where the physical properties of the mixture are very strongly influenced by the pressure of the system. Cooling of the liquid / gas / vapour mixture at the condenser results in a liquid stream, free from vapour phase of the low boiling point component. Condensed material however is thought not to be fully miscible. This maybe contrasted with commercial refrigerant blends where the aim is to achieve a mixture with properties as close as possible to a single component (a zeotropic) system.

Once the mixture passes through the metering device the reduction in pressure causes the balance within the zeotropic mixture to shift strongly in favour of a two- component system. The Low boiling point material therefore boils off at a temperature close to that expected for the pure material. The higher boiling point component at this point must have a sufficiently low freezing point and viscosity at low temperatures to be effectively carried out of the evaporator to be returned to the compressor.

Most commercially available compressors types are however sensitive to liquid returning through the suction port therefore the proportion of the high liquid component must be matched to the systems characteristics including; refrigerant type, compressor mechanism type, evaporating and condensing conditions. In experimentation ratios between 5 and 40% were found to be effective, higher concentrations of the high boiling point component however caused damage to compressors and also problems in adjusting the flow rates through the metering devices. Hydrocarbons with similar thermo-physical properties were also investigated. Which whilst flammable did perform exceptionally well in terms of suppression of the discharge temperature, they also have the advantage of complete miscibility with all commonly used compressor lubricants and therefore help with good oil management. The patent suggests at concentrations lower than their flammability limits hydrocarbons make a useful contribution to lowered global warming of the blend and slightly increase energy conversion.

To effectively suppress the higher discharge temperatures encountered within a mixed gas (auto cascade) or multi-compression (cascade) refrigeration system. The refrigerants may or may not form miscible mixtures by virtue of chemical or thermo physical properties. The compounds proposed have zero ozone depletion and relatively small global warming impacts. The patent proposes the use of compounds in concentrations between 3 — 45% w:w within an auto-cascade refrigeration systems.

The invention describes the use of novel partially or fully fluorinated ketone's with carbon chains greater with more than 2 carbon atoms for example nonafluoro-4- (trifluoromethyl)-3-pentanone .

In one embodiment hydrocarbons are incorporated at levels less than their flammability limited to improve efficiency and oil compatibility when used in combination with the fluoro - compounds:

In one embodiment the invention makes use within the gas blend of unsaturated alkene organic compounds composed of hydrogen, one or more halogen atoms and carbon, with a double bond between one or more carbon atoms. Such molecules are also include at least one Halogen atom which supresses the flammability and in most cases usefully improves other desirable characteristics e.g. flammability. An example of such a compound is Trans-l-chloro-3,3,3-trifluoropropene.

The invention successfully utilises several mixed refrigerant gas blends which have been shown to be successful with the autocascade test system. All of the blends have shown significantiy reduced global warming potential than prior art blends. The invention proposes blends which in addition to Trifluorome thane and Argon, include, at least two or more of the following materials: Figure 4 Examples of blends incorporating novel compounds.

Trans-l-chloro-3,3,3-trifluoropropene, 2,3,3,3-tetrafluoropropene; cis-1, 1,1, 4,4,4- hexafluoro-2-butene; 1,1, 1,2, 2, 3, 4, 5, 5, 5- decafluoropentane; trans-1,1,1,4,4,4- hexafluoro-2-butene; Trans-l,3,3,3-Tetrafluoropropene; Cis 2, 3,3,3- tetrafluoroprop-l-ene; Cis -1,1, 1,4, 4, 4 - hexafluorobut— 2-ene; Trans -1, 1,1, 4,4,4- hexafluorobut-2-ene 2, 3, 3, 3; Tetrafluoro-1- chloroprop-l-ene; 2-bromo-3,3,3- trifluoropropene; 1-Difluoroethylene; Difluoromethane; Tetrafluoromethane; Fluoroethylene; Carbon Diode; Propene; Butane; Ethane; Methane; Krypton; and/ or Nitrogen,

In one embodiment there is a single step compression of a mixture of gases and cryogens.

In one embodiment, the blend includes any combination, of the above with two or more of the following: Tetrafluoromethane, Methane, Argon, Nitrogen, Ethane ,T rifluorome thane .

In one embodiment, the refrigerant used to cool the heat exchanger contains hydrofluoroolefin (HFO) and, or any combination, of Tetrafluoromethane, Methane, Argon, Nitrogen, Ethane and/ or Trifluoromethane.

In one embodiment, the heat exchanger is operated at a pressure of or above 0.5 barg.

In one embodiment, the apparatus is air or water cooled.

In one embodiment, water leaving the system is warmed, most preferably to +60°C or above in order to allow for the recovery of heat for non-portable apparatus.

Figures 5 and 6 illustrate additional features incorporated into the apparatus of this application which allows the advantages as described herein to be obtained. Referring to figure 5, the efficiency of the system is enhanced when the liquid fraction, having been separated from the non(-condensed gaseous portion on the discharge (high pressure side) by a phase separator (PhS figure 2), is discharged via a capillary line (Cap_n figure 2) into the low pressure return side of the flow either directly in line with the returning flow or at an acute angle. The purpose of this is to maintain and increase momentum and velocity of the returning stream. The benefits are greater heat transfer due to increased turbulence within the counter flow heat exchanger arrangement. A further benefit is a reduction in the possibility of fouling of the capillary lines.

Referring to figure 6, this shows the final stage of the counter flow heat exchangers of the autocascade process illustrated in figure 2. High pressure gas at, or close to, its condensation point is fed into heat exchanger A through line L. Assisted by gravity, condensed liquid accumulates in a counter flow heat exchanger C. There is a limited return path from high to low pressure side from point D to H through Cap4. as represented in figures 2 & 3. The heat exchangers A and C form a continuous path on the high-pressure side from L to D. The flow through Cap4 ensures the high-pressure gas stream is cooled and the accumulated refrigerant in C is held at a temperature significantly below its condensation point.

The return path on the low-pressure side H to M is discontinuous with gas exiting the sub-cooler C at point G being joined by gas returning from the evaporator (E figure 2) at point K in a tangential manner indicated in Figure 5. The gas from the sub-cooler C and the evaporator E exit and return to the compressor from point M. The arrangement allows for material to be both stored in a condensed state and for residual heat form the discharge to be transferred to the return stream without compromising the system’s overall stability.

Claims

1 A refrigeration system including a compressor to compress a heterogeneous cooling fluid, a heat exchanger to condense at least a portion of the fluid and at least one further heat exchanger, a phase separator and metering devices and an evaporator to evaporate the fluid into a return or suction stream which re-enters the heat exchangers at a reduced pressure and wherein at least one flow metering device is provided with an outlet which leads into the suction side inlet of at least one of the heat exchangers to form an ejector system and wherein a blend of two or more refrigerants are used to pass through the refrigeration system.

2 A system according to claim 1 wherein the refrigerants which are used have different boiling points.

3 A system according to claim 1 wherein the said refrigerants undergo a single compressive step to achieve the required temperature.

4 A system according to any of the preceding claims wherein one or more hydrocarbons are incorporated at concentrations of less than 4% in one or more of the refrigerants and the resultant blend is non-flammable.

5 A system according to any of the preceding claims wherein at least one oil separation device is provided to recover oil from the refrigerant blend prior to the commencement of the refrigeration process.

6 A system according to any of the preceding claims wherein the said blend is in a category which is non-flammable and carries an A1 ASHRE safety rating.

7 A system according to any of the preceding claims wherein the system uses said blend with one or more direct heat exchange surfaces for cooling ambient air to temperatures colder than -180 °K for the purposes of cryopreservation. 8 A system according to claim 7 wherein the refrigerant blend contains difluoromethane where it is present at or below its lower flammability limit in order to achieve to an A 1 classification.

9 A system according to any of the preceding claims wherein the heat exchanger is a plate heat exchanger.

10 A system according to any of the preceding claims wherein the flow of fluid returning to the compressor passes through the heat exchanger and is introduced into a volume in the heat exchanger via an inlet that leaves via an outlet which has a smaller diameter than the said inlet into the heat exchanger.

11 A system according to claim 10 wherein the outiet from the flow metering device is parallel to the flow from the heat exchanger inlet to the heat exchanger outlet.

12 Apparatus according to any of the preceding claims wherein the apparatus includes a sub-cooler positioned below a heat exchanger to which the same is connected to collect condensate formed in the refrigeration process and all gasses condensed pass through a metering device connected to the sub-cooler.

13 Apparatus according to claim 12 wherein the passage of fluid from the high- pressure side of the sub-cooler is via two outiet with a first outlet passing through a flow metering device and provided such that the liquid fed to the low pressure side is maintained in preference to the second outlet which is a feed to an evaporator.

14 A system according to claims 12 or 13 wherein the sub-cooler acts as both a heat exchanger and a liquid reservoir for cryogenically cooled refrigerant.

15 A system according to any of the preceding claims wherein the control of suction pressure is regulated by a variable capacity metering device located at the out flow from the sub-cooler.

16 A system according to any of the preceding claims wherein the system includes a phase change separator which includes an inlet to receive a mixed flow of liquid and gas, a separation element to allow separated out gas/ vapour to exit through an outlet and the liquid to exit through an outlet and wherein the separator has a void space in which the received mixed flow of fluid is held and the diameter of the void is greater than 4 times the diameter of the inlet and the volume of the void is greater than 2% of the swept volume of the compressor.

17 A system according to any of the preceding claims wherein the refrigeration system is a cascade or an auto cascade system.

18 A refrigeration system including a compressor to compress cooling fluid, a heat exchanger to condense at least some of the fluid and further heat exchangers in combination with phase separator and metering devices and an evaporator to evaporate the fluid into a return, or suction, stream and which re-enters the heat exchangers and wherein one of the heat exchangers acts as a sub-cooler.