WO2022157492A1

WO2022157492A1 - Improved material and method for refrigeration systems

Info

Publication number: WO2022157492A1
Application number: PCT/GB2022/050146
Authority: WO
Inventors: James Thornton STENHOUSE
Original assignee: Stenhouse James Thornton
Priority date: 2021-01-19
Filing date: 2022-01-19
Publication date: 2022-07-28
Also published as: GB202100708D0

Abstract

The invention relates o a system and refrigerant fluid for the system which hallows improved cooling effect and reduced environmental impact to be achieved. The fluid used in the system is a blend of at least two components having different boiling points.

Description

Improved material and method for refrigeration systems

The invention which is the subject of this application relates to improvements in refrigeration systems and particularly in relation to cascade refrigeration systems.

The provision of refrigeration systems is well known and there are numerous commercial and domestic uses to which the same can be put. Many commercial processes 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, require or are improved by temperatures lower than maybe achieved with conventional refrigeration, which has a practical lower limit of -60 °C. Alternatives using a physical cryogens e.g. dry ice are in many cases not practical; consequendy specialised mechanical refrigeration systems are often the most practical solution. Figure 1 illustrates a standard refrigeration circuit including a condenser, evaporator connected to a compressor and with discharge pressure and suction pressure points. 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. However, low boiling point refrigerants 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.

Another form of known refrigeration system is a cascade refrigeration system in which there is provided a high stage compressor and a low stage compressor 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 stages 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.

Auto cascade refrigeration systems can be 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 relatively low temepratures.

Smaller, efficient cryo-coolers are serving 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 than traditional types employed in the auto-cascade process. The autocascade process is a multi-component refrigeration process which achieves a very low temperature in a single compressive step. Plate heat exchangers are difficult to optimise in order to exploit their full potential, because the mixed flow of vapour and condensate means a requirement for a longer residency time of the fluid within the heat exchanger. This, in turn, leads the design of the plate heat exchangers towards longer path lengths, which are 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 when used as a vacuum pump. Furthermore, if the path length of the plate heat exchanger is shortened it means that the tolerance of overload is reduced. The applicant has developed an auto-cascade refrigeration apparatus which can be used on a relatively small scale basis.

A further problem with refrigeration systems generally is the environmental impact of the refrigerant material used. This problem is exacerbated by the demand for ultra-low temperature (lower than -60 °C) cooling, which does not rely upon an open cycle / total loss cooling process, where cryogens typically but not exclusively liquified Nitrogen or solid Carbon Dioxide change state has been increasing with advances in biotechnology and materials processing. A total loss cooling process is both wasteful in terms of energy, dangerous outside of certain situations, but it also intrinsically has a duration limited to the size of cryogen applied. Overall, the carbon footprint of this form of cooling is typically much higher than classical (closed) vapour cycle cooling as a function of Carnot efficiency.

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 practice low boiling point refrigerants are used since evaporating refrigerants at pressures less than atmospheric pressure may cause 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 of the type shown in Figure 1.

To achieve lower temperature two or more compressive steps in series are undertaken in a cascaded system of the type shown in Figure 2.

The aim of the present invention is to provide improvements to the handling and design of elements of the auto cascade refrigeration apparatus. A further aim is to provide improvement to auto cascade apparatus in conjunction with plate-type heat exchangers so that practical working apparatus are obtained to exploit both higher efficiency and greater process stability. A further aim is to provide 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 cooling fluid, a heat exchanger to condense at least some of the fluid and one or more 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 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 refrigerant fluid formed as a blend of two or more refrigerant material components is used within the system, said components having different boiling points.

In one embodiment the component with the higher boiling point is provided in a range of between 5 and 40% of the refrigerant fluid.

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

In one embodiment a lowered compressor discharge temperature is achieved.

In one embodiment increased heat rejection is achieved from the system condenser at a temperature higher than + 40 ° Celsius.

In one embodiment an improved refrigeration capacity 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 range or have critical temperatures lower than -20 °C.

In one embodiment the reduction in compressor discharge temperature is 49% less for a first blend and 78% less for a second blend

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 improved heat transfer is achieved through greater cycle efficiency and recovery of oil within oil separation devices.

In one embodiment the material is a blend of a first category which are nonflammable and carry an Al ASHRE (American Society of Heating and Refrigeration Engineers) safety rating.

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 -90 °C and typically colder than -110 °C 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 hydrofluoroolefins (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 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 Al 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 outlet 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 ⁰ 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.

Typically the system can be operated in Standby, Cooling and defrost modes.

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 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 oudet are convex in shape.

In one embodiment the refrigeration system is a cascade or an auto cascade system.

In one embodiment there is provided a refrigeration apparatus 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 one embodiment 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.

In a further aspect of the invention there is provided a refrigerant fluid for a refrigeration system, said fluid formed as a blend of two or more refrigerant material components is used within the system, said components having different boiling points.

In one embodiment one or more hydrocarbons are incorporated in the refrigerant at a concentration of less than 4% and the resultant blend is non-flammable.

In one embodiment the blend components include one or more partially or fully fluorinated ketones with carbon chains greater with more than 2 carbon atoms.

In one embodiment the blend includes at least one component formed by unsaturated alkene organic compounds composed of hydrogen, one or more halogen atoms and carbon, with a double bond between one or more carbon atoms.

In one embodiment the components includes at least one Halogen atom.

In one embodiment the blend includes Trifluoromethane and Argon components.

In one embodiment the blend includes one or more components selected from 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-1,3,3,3-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- 1-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 the blend includes two or more of the following: etrafluoromethane, Methane, Argon, Nitrogen, Ethane , rifluoromethane.

In one embodiment the blend includes hydrofluoroolefin (HFO) and, or any combination, of Tetrafluoromethane, Methane, Argon, Nitrogen, Ethane and/or Trifluoromethane and is used to cool the heat exchanger of the refrigeration system.

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

Figure 1 illustrates a conventional refrigeration cycle;

Figure 2 illustrates a two-stage cascade refrigeration system; and

Figure 3 illustrates an auto cascade refrigeration system.

Referring to the Figures there are illustrated three types of refrigeration system. In Figure 1 there is illustrated a conventional refrigeration cycle in which there is provided a condenser 2, high stage compressor 4 and evaporator 6. On opposing sides of the compressor 4 there is provided a discharge pressure high stage controller 8 and a suction pressure high stage controller 10 and a capillary (Cap) line 11.

In Figure 2 there is illustrated a two stage cascade refrigeration system which includes a high stage part 22 and allow stage part 24. The High stage part 22 includes a condenser 12, a discharge pressure high stage controller 14, a high stage compressor 16, a suction pressure high stage controller 18 and a cap line 20. The Low stage part includes a low stage compressor 26, an evaporator 28, cap line 30, a discharge pressure low stage shut off controller 32, a discharge pressure solenoid valve controller 34 and a liquid gas receiver 36. At the interface between the high and low stages 22,24, an interstage condenser 38 is provided.

In Figure 3 there is illustrated an auto cascade refrigeration system in which there is provided a series of plate heat exchangers 40, 42,44,46 which are interconnected to act in series and connected to evaporator 48. A compressor 50 is provided along with a buffer tank 52, condenser 54 and sub-cooler 56 which can also act as a heat exchanger.

In accordance with the invention there are provided novel combinations of materials in gas blends which can be used in the systems as described above in order 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 higher boiling point than the design cooling temperature the invention proposes mixtures which maybe described a zeotropic in nature.

The materials proposed have not previously been considered suitable for ultra-low temperature refrigeration applications since 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 have 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 single component system despite critical pressures of 37.4 and 48.6 bar which are far in excess of the 22bar 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. The important criteria is that the critical temperature of the added compound is above the condensation temperature for the primary refrigerant medium.

By reducing the temperature and pressure of the discharge from one or more compressors in the refrigeration systems so described it is found that improved lubrication is achieved and so mechanical parts are subjected to less stress and therefore can have a longer life time. Ideal thermal materials have the following properties of low viscosity, high latent heat of vaporisation, low freezing point, good solvation for refrigerants in their gaseous phase, zero ozone depletion potential, low global warming potential, high critical temperatures relative to their boiling points, be chemically inert, non flammable and have good miscibility.

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

When used as a blend within an autocascade system of the type shown in Figure 3 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 use of components in which their solubility in its gas phase into the high boiling point material is a direct function of pressure. At the higher pressure found at discharge (14bar or greater) components form a zeotropic 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 is in contrast 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 component 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 compressor 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 on percentage basis, they also have the advantage of complete miscibility with all commonly used compressor lubricants and therefore help with good oil management. Thus at concentrations lower than their flammability limits they make a useful contribution to lowered global warming of the blend but significantly 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 thermophysical properties. The compounds proposed have zero ozone depletion and relatively small global warming impacts. The use of compounds in concentrations between 3 — 45% w:w within an auto-cascade refrigeration systems is found to be advantageous.

In one embodiment partially or fully fluorinated ketones are used as one or more components of the refrigerant material 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 limit in order 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 also include at least one Halogen atom which supresses the flammability and in most cases usefully improves other desirable characteristics. An example of such a compound is Trans-l-chloro-3,3,3-trifluoropropene.

The invention therefore allows the single step compression of a mixture of gases and cryogens .

The invention successfully utilises several mixed refrigerant gas blends which have been shown to be successful with refrigeration systems, including autocascade systems. All of the blends have shown significantly reduced global warming potential than prior art blends. The invention proposes blends which in addition to Trifluoromethane and Argon, include, at least two or more of the following materials:

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-1,3,3,3-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- 1-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, 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 Bar and the apparatus is air or water cooled. The 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.

Claims

1 A refrigeration system including a compressor to compress cooling fluid, a heat exchanger to condense at least some of the fluid and one or more 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 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 refrigerant fluid formed as a blend of two or more refrigerant material components is used within the system, said components having different boiling points.

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

3 A system according to claim 1 wherein one or more hydrocarbons are incorporated in the refrigerant at a concentration of less than 4% and the resultant blend is nonflammable.

4 A system according to any of the preceding claims wherein the component with the higher boiling point is provided in a range of between 5 and 40% of the refrigerant fluid.

5 A system according to claim 1 wherein the said refrigerant blend is compatible with all refrigeration oils including alkyl-benzene, Polyolester and/or mineral (synthetic and natural) with hydrocarbon incorporation.

6 A system according to any of the preceding claims wherein the system includes at least one oil separation device to aid in the recovery of oil from the fluid.

7 A system according to any of the preceding claims wherein the refrigeration system includes one or more direct heat exchange surfaces for cooling ambient air to temperatures colder than -90° C for the purposes of cryo-preservation.

8 A system according to claim 6 wherein the refrigerant blend includes any or any combination of hydro fluoro ole fins (HFO) or hydrohaloalkenes (I II IO/ I II Ac) components.

9 A system according to any of the preceding claims wherein the flow of refrigerant fluid returning to the compressor passes through the said at least one heat exchanger and is introduced into a volume in the heat exchanger via an inlet and leaves via an outlet.

10 A system according to claim 9 wherein the outlet is an outlet of a flow metering device connected to the heat exchanger.

11. A system according to claim 9 or 10 wherein the outlet is of a smaller diameter than said inlet into the heat exchanger.

12 A system according to claim 10 or 11 wherein the outlet from the flow metering device is placed at an angle of less than 42° to the principle gas flow through the heat exchanger inlet to create a Venturi effect.

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

14 A system according to any of the preceding claims wherein the system includes a 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 and a flow of condensed liquid refrigerant passes from a high-pressure side of the subcooler to a low pressure side of the sub-cooler via a flow metering device.

15 A system according to claim 14 wherein the gasses condensed in the said heat exchanger pass through a metering device connected to the sub-cooler.

16 A system according to any of the claims 14-15 wherein the passage of fluid from a high pressure side of the sub-cooler is via two outlets, with a first outlet passing to 16 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.

17 A system according to any of the claims 14-16 wherein sub-cooler acts as both a heat exchanger and a liquid reservoir for cryogenically cooled refrigerant fluid.

18 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 the refrigerant blend, 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.

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

20 A refrigerant fluid for a refrigeration system, said fluid formed as a blend of two or more refrigerant material components is used within the system, said components having different boiling points.

21 A refrigerant fluid according to claim 20 wherein one or more hydrocarbons are incorporated in the refrigerant at a concentration of less than 4% and the resultant blend is non-flammable.

22 A refrigerant fluid according to any of claims 20-21 wherein the component with the higher boiling point is provided in a range of between 5 and 40% of the refrigerant fluid.

23. A refrigerant fluid according to any of claims 20-22 wherein the blend components include one or more partially or fully fluorinated ketones with carbon chains greater with more than 2 carbon atoms.

24 A refrigerant fluid according to any of the claims 20-23 wherein the blend includes at least one component formed by unsaturated alkene organic compounds 17 composed of hydrogen, one or more halogen atoms and carbon, with a double bond between one or more carbon atoms.

25 A refrigerant fluid according to claim 24 wherein the components includes at least one Halogen atom.

26 A refrigerant fluid according to any of the claims 20-25 wherein the blend includes Trifluoromethane and Argon components.

27 A refrigerant fluid according to claim 26 wherein the blend includes one or more components selected from Trans-l-chloro-3,3,3-trifluoropropene, 2, 3,3,3- tetrafluoropropene; cis-l,l,l,4,4,4-hexafluoro-2-butene; 1, 1,1, 2, 2, 3, 4, 5,5,5- decafluoropentane; trans-l,l,l,4,4,4-hexafluoro-2-butene; Trans- 1,3, 3,3-

Tetrafluoropropene; Cis 2, 3, 3, 3- tetrafluoroprop -1-ene; Cis -1,1, 1,4, 4, 4 - hexafluorobut— 2-ene; Trans -l,l,l,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,

28 A refrigerant fluid according to claim 27 wherein the blend further includes two or more of the following: Tetrafluoromethane, Methane, Argon, Nitrogen, Ethane ,T rifluorome thane .

29. A refrigerant fluid according to claim 20 wherein the fluid is a blend including hydrofluoroolefin (HFO) and, or any combination, of Tetrafluoromethane, Methane, Argon, Nitrogen, Ethane and/ or Trifluoromethane and is used to cool the heat exchanger of the refrigeration system.