Refrigeration system and associated method

Abstract

A carbon dioxide (CO2) based refrigeration system including a heat exchanger that transfers heat from gas return line CO2 on a first side of the heat exchanger to CO2 delivered from a receiver vessel on a second side of the heat exchanger.

Description

REFRIGERATION SYSTEM AND ASSOCIATED METHODTechnical Field

[0001] The present disclosure relates to refrigeration systems, in particular CO2 based refrigeration systems.Background of the Disclosure

[0002] Vapour compression refrigeration systems are a common method of air- conditioning and refrigeration. These systems rely on the continuous cycling of a refrigerant undergoing as series of phase changes, through which the refrigerant absorbs heat from the environment or space to be cooled, reducing the temperature of the environment or space. Vapour compression refrigeration systems generally comprise at least an evaporator, compressor, condenser and expansion valve. At the evaporator, heat from the surrounding environment or space is absorbed by the liquid refrigerant, causing the refrigerant to vaporise and become a gas. The gaseous refrigerant is then compressed by the compressor, increasing the pressure and temperature of the refrigerant, before it is cooled by the condenser, returning back into a liquid form (typically through indirect heat transfer to a secondary fluid). The liquid refrigerant passes through an expansion valve and back into the evaporator, where the cycle repeats. Commonly, these systems also include a receiver vessel, in which the liquid refrigerant is stored between cycles. The receiver vessel is usually placed after the condenser in the system and ensures that refrigerant enters the expansion valve and evaporator in a liquid state.

[0003] A number of refrigerants have historically been used in vapour compression systems, however following the signing of the Montreal Protocol in 1996, many previously widespread refrigerants based on chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), most notably refrigerants known collectively under the brand name Freon, have been phased out due to their detrimental effects on the environment, in particular their contribution to ozone layer depletion. More recently, efforts have been made to also phase out refrigerants based on hydrofluorocarbons (HFCs), which while not contributing to ozone layer depletion, still produce a large quantity of greenhouse gases and thus also have a detrimental effect on the environment. As a result, there is a growing trend towards more environmentally friendly refrigerants.

[0004] One such environmentally friendly refrigerant is carbon dioxide (CO2). CO2 based refrigeration systems are as or more efficient than conventional Freon-based refrigeration systems in conditions where the ambient temperature is low. As the ambient temperature rises, however, a significant reduction in performance is observed. This is attributed to the critical temperature of CO2, the temperature above which the refrigerant cannot be liquefied, being a relatively low 31.1 °C. In contrast, the Freon refrigerant R-12 has a critical temperature of 111.9°C.

[0005] This low critical temperature is particularly an issue in climates where the ambient temperature is often high. For example, in Sydney, Australia, the ambient temperature can range between -5°C and 48°C, with the majority of the year falling between 13°C and 27°C. These warm ambient temperatures have an adverse effect on both the operation and efficiency of a CO2 based vapour compression refrigeration system. Once the ambient temperature exceeds approximately 18°C, the performance of the refrigeration compressor is reduced in terms of work done and of power consumed, and the CO2 system becomes less efficient that a conventional system using a Freon refrigerant.

[0006] One attempt at solving this issue is through the development of trans-critical refrigeration systems. These systems include a condenser which can operate in two modes - a sub-critical mode (below 31.1 °C) in which the condenser liquefies the refrigerant, and a supercritical mode where the condenser instead acts as a gas cooler, and will not liquefy the refrigerant as it passes through it. While this permits operation when the refrigerant is above a critical temperature, high discharge pressures are required to maintain the system efficiency and the second mode of operation requires higher motor power. As a result, these systems are less energy efficient when operating in the super-critical mode, and a user may not choose these systems owing to their high power consumption. Further, when the refrigerant is in a supercritical phase, there is increased generation of flash gas and a higher compressor capacity is required relative to the sub-critical mode of operation, in order to maintain the receiver vessel at the design pressure. The reduction in compressor volumetric efficiency in the super-critical mode also results in poorer compressor performance.

[0007] There thus exists a need to provide a CO2 based refrigeration system which can maintain sub-critical operation even in higher ambient temperatures.

[0008] A method of maintaining sub-critical operation is to add a third stage of cooling cascaded to the system, this auxiliary cooling system being operated when the ambient temperature is high. While this provides an energy saving compared to a trans-critical system, the installation costs of adding the auxiliary cooling system are high.

[0009] Accordingly, there remains a need to provide a CO2 based refrigeration system which can maintain sub-critical operation in high ambient temperatures without the installation costs of adding an auxiliary cooling system.Summary of the Invention

[0010] According to a first broad aspect, there is provided a carbon dioxide (CO2) based refrigeration system including a heat exchanger that transfers heat from gas return line CO2 on a first side of the heat exchanger to CO2 delivered from a receiver vessel on a second side of the heat exchanger.

[0011] In some embodiments, the system further includes: a temperature sensor located between the first side of the heat exchanger and the receiver vessel to measure a temperature of the gas return line CO2 exiting the first side of the heat exchanger; a discharge pressure regulating valve located proximate the temperature sensor; wherein a discharge pressure of the system is regulated by the discharge pressure regulating valve based on the temperature of the gas return line CO2 exiting the first side of the heat exchanger.

[0012] In some embodiments, a flow rate of the CO2 delivered from the receiver vessel is adjusted to control the temperature of the gas return line CO2 exiting the first side of the heat exchanger.

[0013] In some embodiments, the temperature of the gas return line CO2 exiting the first side of the heat exchanger is controlled so that the discharge pressure remains below the critical point of CO2.

[0014] In some embodiments, the temperature is controlled to maintain the discharge pressure at approximately 6500 kPa.

[0015] In some embodiments, the CO2 delivered from a receiver vessel is gravity fed into the second side of the heat exchanger. In an alternative embodiment, the CO2 delivered from areceiver vessel is delivered into the second side of the heat exchanger via a metering device. In one form the metering device is a valve.

[0016] In some embodiments, the CO2 exiting the second side of the heat exchanger is cycled back to the receiver vessel.

[0017] In some embodiments, the CO2 exiting the second side of the heat exchanger is fed into a suction line of a compressor of the refrigeration system.

[0018] In some embodiments, the heat exchanger is located at the base of the receiver vessel.

[0019] According to a second broad aspect, there is provided a carbon dioxide (CO2) based refrigeration system including: a gas cooler; a receiver vessel; an evaporator; a compressor; a first heat exchanger, including a first side which receives gas return line CO2 flowing out of the gas cooler, and a second side which receives suction line CO2 flowing out of the evaporator and passes the suction line CO2 to the compressor; and a second heat exchanger, including a first side which receives the gas return line CO2 flowing out of the first heat exchanger and passes the gas return line CO2 to the receiver vessel and a second side which delivers CO2 from the receiver vessel.

[0020] In some embodiments, the system further includes: a temperature sensor located between the second heat exchanger and the receiver vessel for measuring the temperature of the gas return line CO2 exiting the second heat exchanger; a discharge pressure regulating valve located proximate the temperature sensor; wherein a discharge pressure of the system is regulated by the discharge pressure regulating valve based on the temperature of the gas return line CO2 exiting the second heat exchanger; and the amount of CO2 delivered from the receiver vessel by the second heat exchanger the second side is adjusted to maintain a predetermined temperature.

[0021] In some embodiments, the predetermined temperature is selected to maintain the discharge pressure below the critical point of CO2.

[0022] In some embodiments, the second heat exchanger is located in series with the first heat exchanger.

[0023] In a third broad aspect, there is provided a method of maintaining carbon dioxide (CO2) refrigerant in a refrigeration system below a critical point; including: passing gas return line CO2 through a first side of a heat exchanger to cool the gas return line CO2; delivering CO2 from a receiver vessel through a second side of the heat exchanger to transfer heat from the first side of the heat exchanger.

[0024] In some embodiments, the method further includes: measuring the temperature of the gas return line CO2 exiting the heat exchanger on the first side.

[0025] In some embodiments, the method further includes: adjusting the amount of CO2 passing through the second side of the heat exchanger based on the temperature of the gas return line CO2 exiting the heat exchanger on the first side.

[0026] In some embodiments, the amount of CO2 passing through the second side of the heat exchanger is adjusted to obtain a predetermined temperature; wherein the predetermined temperature is chosen based off a desired discharge pressure of the refrigeration system.

[0027] In some embodiments, CO2 is gravity fed from the receiver vessel through the second side of the heat exchanger. In an alternative embodiment, the CO2 delivered from a receiver vessel is delivered into the second side of the heat exchanger via a metering device. In one form the metering device is a valve.

[0028] In some embodiments, some embodiments, the CO2 exiting the second side of the heat exchanger is cycled back to the receiver vessel.

[0029] In some embodiments, the CO2 exiting the second side of the heat exchanger is fed into a suction line of a compressor of the refrigeration system.

[0030] In some embodiments, the method further includes passing gas return line CO2 through a second heat exchanger prior to the first heat exchanger, the second heat exchanger transferring heat from the gas return line CO2 to a suction line of the refrigeration system.

[0031] According to a fourth aspect, there is provided a carbon dioxide (CO2) based refrigeration system using a method according to the third aspect.

[0032] Other aspects, features, and advantages will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, which are apart of this disclosure and which illustrate, by way of example, principles of the inventions disclosed.Brief Description of the Figures

[0033] The present disclosure will become better understood from the following detailed description of various non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:

[0034] FIGURE 1 shows a schematic diagram of a refrigeration circuit with an auxiliary refrigeration circuit for use when the ambient temperature is high.

[0035] FIGURE 2 shows a schematic diagram of a refrigeration circuit according to an embodiment of the present invention.

[0036] FIGURE 3 shows a schematic diagram of a refrigeration circuit according to another embodiment of the present invention.Detailed Description

[0037] The term 'CO2 based refrigeration system' has been used for readability throughout this specification. These systems may also be referred to as CO2 vapour-compression systems, CO2 refrigeration systems, or CO2 based vapour-compression refrigerators, and it will be understood by a person skilled in the art that this term generally refers to refrigeration systems which utilize CO2 as a refrigerant, relying on phase-changes in the CO2 to remove heat from the surrounding environment or space.

[0038] Throughout the description, the terms 'refrigeration', 'refrigeration system', and 'refrigeration circuit' will be understood to refer generally to any system which removes heat from a space or environment, and may be applicable to various systems including air conditioners, refrigerators, and chillers without departing from the spirit of the invention.

[0039] The inventors have found that a CO2 based refrigeration system can be kept in sub- critical operation, that is to say that that the CO2 refrigerant remains below the critical temperature throughout the operation of the system, by the addition of a heat exchanger which transfers heat from the CO2 refrigerant leaving a gas cooler to liquid CO2 refrigerant which is delivered from and back to the receiver vessel in which refrigerant is stored before being sentto an evaporator. Otherwise, stated, the CO2 refrigerant leaving the gas cooler may be thought of as gas return line CO2 or gas return line CO2 refrigerant.

[0040] In trans-critical CCh-based refrigeration systems, the gas cooler is used as a condenser in a sub-critical mode and a gas-cooler in super-critical mode. In contrast, the gas cooler of systems according to the present invention only need to function at all times as a gas de-superheater rather than a condenser, as the CO2 leaving the gas cooler will then be cooled by at least one heat exchanger prior to entering the receiver vessel and be in a predominantly condensed state by the time it enters the receiver vessel. In preferred embodiments, the CO2 refrigerant passes through two heat exchangers in series, a first heat exchanger which transfers heat from the CO2 flowing out of the gas cooler to the suction line of the system, and a second heat exchanger which cools the CO2 further by transferring heat from the CO2 leaving the first heat exchanger to a portion of stored liquid refrigerant delivered from the receiver vessel. By passing through two heat exchangers in series before entering the receiver vessel, the CO2 refrigerant can be reliably cooled to an extent that the CO2 refrigerant condenses and enters the receiver vessel in a liquid state.

[0041] In preferred embodiments of the system, the heat exchanger is in the form of a plate heat exchanger (PHX). It will be understood by the person skilled in the art, however, that in other embodiments, the heat exchanger may be a tube and shell heat exchanger, a double pipe heat exchanger, or any other heat exchanger type which can transfer heat from a fluid in a first side of the exchanger to another fluid in a second side of the exchanger.

[0042] CO2-based refrigeration systems, as with other vapour-compression refrigeration systems, commonly utilize a control system to automatically adjust the operation of the refrigeration system (such as the compressors, evaporators, and gas coolers / condensers) to maintain temperatures and pressures at the highest possible efficiency. In embodiments of the present invention, the control system uses the temperature of the refrigerant as it enters a discharge pressure regulating (DPR) valve to control the discharge pressure of the system via the DPR valve. In addition, the DPR valve may also control and maintain the liquid CO2 level in the receiver vessel. This DPR valve is positioned proximate the receiver vessel, so that CO2 refrigerant has been cooled by at least one heat exchanger before entering the valve. As a result, the temperature of the CO2 refrigerant is always at a low enough temperature to maintain the system operating in a sub-critical manner and to maintain a relatively low discharge pressure. For example, in preferred embodiments, the temperature is selected to maintain a dischargepressure of approximately 6500 kPa. The temperature of the CO2 refrigerant entering the DRP valve is controlled by adjusting the flow of liquid CO2 refrigerant from the receiver vessel through the opposite side of the heat exchanger to the CO2 refrigerant flowing from the gas cooler and being directed to the DPR valve. This may be achieved, for example, by a flow control valve between the receiver vessel and the heat exchanger, the flow control valve being controlled by the control system. This flow control valve may be referred to as a return gas temperature control valve, as the rate of flow of cycled liquid CO2 adjusts the temperature of the returning gas CO2 refrigerant from the gas cooler.

[0043] This arrangement, where the CO2 refrigerant flowing out of the gas cooler passes through the first side of a heat exchanger, and liquid CO2 refrigerant delivered from the receiver vessel passes through the second side of the heat exchanger to cool the CO2 refrigerant flowing out of the gas cooler, provides a number of benefits. Firstly, as the system never operates in the super-critical phase, the motor power absorbed by the compressors is always low, and the compressors' volumetric efficiency is maintained at the highest possible level. This also assists in removing any flash gas generated by the heat exchanger, as well as flash gas generated at the DPR valve. Secondly, the lower discharge pressure also reduces the amount of heat inputted into the discharge gas stream, and will also require less compressors to operate in times of high ambient temperature compared to existing systems. This is because the volumetric efficiency gain also affects the portion of the compressor suction capacity being used to deal with gas returned from the evaporators.

[0044] In preferred embodiments of the invention, the heat exchanger which receives CO2 refrigerant flowing out of the gas cooler on one side and CO2 refrigerant from the receiver vessel on the other side is placed at the base of the receiver vessel. As well as reducing the footprint of the system, the heat exchanger may be gravity fed CO2 refrigerant from the receiver vessel, the rate of flow being controlled by a flow control valve between the receiver vessel and the heat exchanger.

[0045] FIGURE 1 shows a schematic diagram of an existing CC -based refrigeration system which uses a third stage of cooling in order to maintain the operation of the system in a sub-critical operation. Otherwise stated, the term sub-critical operation refers to a mode of operation where the CO2 refrigerant is kept below the critical point of CO2, the CO2 refrigerant being able to undergo phase changes.

[0046] The refrigeration system includes a condenser 1, compressor 2, evaporator 3, receiver vessel 4, and an oil separator 5. Some key valves are also represented in the figure. These include a receiver pressure regulating (RPR) valve 6, a discharge pressure regulating (DPR) valve 7, a liquid supply valve 8, a suction return valve 9, and an expansion valve 10. A temperature sensor 11 is provided proximate the DPR valve 7. FIGURE 1 also shows a heat exchanger 12 between the condenser 1 and the receiver vessel 4, and a third stage of cooling 13 which acts to further condense gaseous CO2 leaving the condenser 1 when the ambient temperature is high. This third stage of cooling may also be considered an auxiliary condenser.

[0047] In operation, liquid CO2 flows from the receiver vessel 4 and passes through expansion valve 10 and evaporator 3, the amount of CO2 being controlled by the liquid supply valve 8. This section of pipe or tube is commonly referred to as the liquid line. In the evaporator, the CO2 undergoes a phase change to a gas and / or vapour, in doing so absorbing heat from the ambient environment. The gas / vapour then travels through one side of the heat exchanger 12, absorbing heat from the other side of the heat exchanger, to the compressor 2. This section of pipe or tube is commonly referred to as the suction line. In the compressor 2, the CO2 vapour undergoes an increase in pressure and temperature, before passing to the condenser 1 through oil separator 5. The oil separator removes oil from the compressor which has remained with the refrigerant, optionally for recycling back into the compressor. The condenser 1 cools the hot gas and returns the CO2 refrigerant (preferably in a liquid state) to the receiver vessel 4, passing through the other side of heat exchanger 12 along the way, to cool the refrigerant further before it is stored in the receiver vessel 4 for the cycle to begin again. The system also includes a discharge pressure regulating (DPR) valve 7, which is operated by a control system based off the measurements taken at temperature sensor 11. In this form, the DPR valve 7 is able to control the discharge pressure of the system.

[0048] In an alternative arrangement, the DPR valve 7 may be arranged to control the level of liquid CO2 contained in the receiver vessel 4. In this arrangement, a liquid level sensor in the receiver vessel 4 senses the level of the CO2 within the receiver vessel 4 and sends a signal via a control system to increase or decrease the flow of liquid CO2 through the DPR valve 7 to maintain the level of liquid CO2 in the receiver vessel 4 at a desired level.

[0049] When the ambient temperature is high, however, the condenser 1 may not sufficiently cool the CO2 refrigerant to condense it. Accordingly, the system may not function efficiently. In order to maintain the system operating in a sub-critical manner, the systemincludes a further stage of cooling 13, which can otherwise be considered as an auxiliary cooling system or cooler, which the CO2 refrigerant passes through following the condenser 1 when the ambient temperature is high. This cooler ensures that the CO2 refrigerant is condensed even in periods of high ambient temperature where the condenser 1 fails to adequately condense the refrigerant. For example, this auxiliary cooling system may take the form of a water chiller operating at ~ 12°C (saturated suction temperature) to cool water to ~20°C. The cool water is then run through the first side of a heat exchanger, such as a plate heat exchanger, near the condenser to ensure that the temperature of the refrigerant run through the second side of the heat exchanger is kept low.

[0050] There are a number of drawbacks, however, with this arrangement. The cost of adding and installing this further cooling system are high, and also may increase the size of the system.

[0051] The present disclosure will become better understood from the following example of a non-limiting embodiment.

[0052] A schematic diagram of a CO2 based refrigeration system according to the present invention is shown in FIGURE 2 and FIGUE 3. For ease of understanding, the same reference numerals are used for the same components as shown in FIGURE 1.

[0053] While the general working of the refrigeration system of FIGURE 2 is similar to the system of FIGURE 1, the system differs in that there is no third stage of cooling or auxiliary cooler present in this system. Instead, the CO2 refrigerant passes through a second heat exchanger 14 before entering the receiver vessel 4. On the other side of heat exchanger 14 is liquid CO2 which flows from the receiver vessel 4 and is cycled back into the receiver vessel after exiting the heat exchanger 14.

[0054] An alternative embodiment is shown in FIGURE 3 where the liquid CO2 exiting the heat exchanger 14 is fed into the suction return line. In this embodiment, it is possible to push the CO2 from the receiver vessel 4 through the heat exchanger 14 because of the pressure difference between the receiver vessel 4 and the suction return line leading to the compressor of the refrigeration system. For example, if the receiver vessel 4 is at a pressure of 3600 kPa and the suction return line is at 2900 kPa, then this provides a 700 kPa pressure difference to assist with driving the CO2 through the heat exchanger 14.

[0055] As a result, the CO2 refrigerant flowing from the gas cooler la and heat exchanger 12 depicted in both FIGURE 2 and FIGURE 3 is cooled sufficiently before entering the receiver vessel 4.

[0056] In the embodiment shown in FIGURE 2, the heat exchanger 14 is located at the base of receiver vessel 4. This both reduces the overall footprint of the system, and allows liquid CO2 to be gravity fed into the other side of the heat exchanger 14. A return gas temperature control valve 15 allows a control system to adjust the flow of liquid CO2 into the second heat exchanger, enabling the control of the temperature of the CO2 refrigerant flowing from the gas cooler la and heat exchanger 12.

[0057] Advantageously, the temperature sensor 11 and discharge pressure regulating (DPR) valve 7 is located after the second heat exchanger 14. The control system can thus adjust the flow of cycled liquid CO2 through return gas temperature control valve 15 and the heat exchanger 12 to maintain a suitable temperature at the temperature sensor 11 and DPR valve 7. The control system can thus choose an appropriate discharge pressure based on this temperature to keep the system operating in a sub-critical mode. For example, the desired discharge pressure may be approximately 6500 kPa, and the flow of cycled liquid CO2 may be adjusted to maintain a suitable temperature to achieve this discharge pressure.

[0058] This system features a gas cooler la. In practice, this may be similar to or built from the same components as condenser 1 in FIGURE 1, however because the CO2 refrigerant is cooled by two heat exchangers 12 and 14 before entering the receiver vessel 4, the gas cooler la does not have to condense the refrigerant, and can instead act as a de-superheater rather than a condenser. Accordingly, in the present system, it would be incorrect to refer to the gas cooler la as a condenser.

[0059] The refrigeration system of FIGURE 2 and FIGURE 3 is thus able to maintain sub- critical operation, even when the ambient temperature is high, without the need to install a third stage of cooling. This provides a number of cost benefits, and further as the discharge pressure of the system is kept low, the amount of heat inputted into the discharge gas stream is reduced, and requires less compressors in operation (in particular when the ambient temperature is high). The motor power absorbed by the compressors remains low, and the volumetric efficiency of the compressors remain at the highest possible level throughout operation. This enables thecompressors to remove any flash gas generated by the heat exchangers. The present system thus has a higher efficiency compared to existing refrigeration systems.

[0060] It will be understood that the schematic diagrams of FIGURES 1, 2 and 3 are simplified illustrations for ease of understanding, and while the principle working of the circuit will remain the same, in practice the compressor 2 may actually be a set of compressors, or multiple stages of sets of compressors. Similarly, aspects of typical refrigeration systems such as header lines and suction lines have been shown in a simplified manner in the figures. A person skilled in the art will understand that despite not being shown in the figures, these items may still be present in systems according to the present invention.

[0061] In the foregoing description of certain embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents which operate in a similar manner to accomplish a similar technical purpose.

[0062] In this specification, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of’. A corresponding meaning is to be attributed to the corresponding words “comprise”, “comprised” and “comprises” where they appear.

[0063] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0064] In addition, the foregoing describes only some embodiments of the invention(s), and alterations, modifications, additions and / or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive.

[0065] Furthermore, invention(s) have described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended tocover various modifications and equivalent arrangements included within the spirit and scope of the invention(s). Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment.

Claims

The claims:

1. A carbon dioxide (CO2) based refrigeration system including a heat exchanger that transfers heat from gas return line CO2 on a first side of the heat exchanger to CO2 delivered from a receiver vessel on a second side of the heat exchanger.

2. The system of claim 1, further including: a temperature sensor located between the first side of the heat exchanger and the receiver vessel to measure a temperature of the gas return line CO2 exiting the first side of the heat exchanger; a discharge pressure regulating valve located proximate the temperature sensor; wherein a discharge pressure of the system is regulated by the discharge pressure regulating valve based on the temperature of the gas return line CO2 exiting the first side of the heat exchanger.

3. The system of claim 2, wherein a flow rate of the CO2 delivered from the receiver vessel is adjusted to control the temperature of the gas return line CO2 exiting the first side of the heat exchanger.

4. The system of claim 3, wherein the temperature of the gas return line CO2 exiting the first side of the heat exchanger is controlled so that the discharge pressure remains below the critical point of CO2.

5. The system of claim 4, wherein the temperature is controlled to maintain the discharge pressure at approximately 6500 kPa.

6. The system of any one of claims 1 to 5, wherein the CO2 delivered from a receiver vessel is gravity fed into the second side of the heat exchanger.

7. The system of any one of claims 1 to 6, wherein the CO2 from the receiver vessel is delivered into the second side of the heat exchanger via a metering device.

8. The system of any one of claims 1 to 7, wherein the CO2 exiting the second side of the heat exchanger is cycled back to the receiver vessel.. The system of any one of claims 1 to 7, wherein the CO2 exiting the second side of the heat exchanger is fed into a suction line of a compressor of the refrigeration system.

10. The system of any one of claims 1 to 9, wherein the heat exchanger is located at the base of the receiver vessel.

11. A carbon dioxide (CO2) based refrigeration system including: a gas cooler; a receiver vessel; an evaporator; a compressor; a first heat exchanger, including a first side which receives gas return line CO2 flowing out of the gas cooler, and a second side which receives suction line CO2 flowing out of the evaporator and passes the suction line CO2 to the compressor; and a second heat exchanger, including a first side which receives the gas return line CO2 flowing out of the first heat exchanger and passes the gas return line CO2 to the receiver vessel and a second side which delivers CO2 from the receiver vessel.

12. The system of claim 11, further including: a temperature sensor located between the second heat exchanger and the receiver vessel for measuring the temperature of the gas return line CO2 exiting the second heat exchanger; a discharge pressure regulating valve located proximate the temperature sensor; wherein a discharge pressure of the system is regulated by the discharge pressure regulating valve based on the temperature of the gas return line CO2 exiting the second heat exchanger; and the amount of CO2 delivered from the receiver vessel by the second heat exchanger to the second side is adjusted to maintain a predetermined temperature.

13. The system of claim 12, wherein the predetermined temperature is selected to maintain the discharge pressure below the critical point of CO2.

14. The system of any one of claims 11 to 13, wherein the second heat exchanger is located in series with the first heat exchanger.

15. The system of any one of claims 12 to 14, wherein the CO2 from the receiver vessel is delivered into the second side of the second heat exchanger via a metering device.

16. The system of any one of claims 12 to 15 wherein the CO2 exiting the second side of the second heat exchanger is cycled back to the receiver vessel.

17. The system of any one of claims 12 to 16 wherein the CO2 exiting the second side of the second heat exchanger is fed into a suction line of the compressor.

18. A method of maintaining carbon dioxide (CO2) refrigerant in a refrigeration system below a critical point; including: passing gas return line CO2 through a first side of a heat exchanger to cool the gas return line CO2; delivering CO2 from a receiver vessel through a second side of the heat exchanger to transfer heat from the first side of the heat exchanger.

19. The method of claim 18, further including: measuring the temperature of the gas return line CO2 exiting the heat exchanger on the first side.

20. The method of claim 19, further including: adjusting the amount of CO2 passing through the second side of the heat exchanger based on the temperature of the gas return line CO2 exiting the heat exchanger on the first side.

21. The method of claim 20, wherein the amount of CO2 passing through the second side of the heat exchanger is adjusted to obtain a predetermined temperature; wherein the predetermined temperature is chosen based off a desired discharge pressure of the refrigeration system.

22. The method of any one of claims 18 to 21, wherein CO2 is gravity fed from the receiver vessel through the second side of the heat exchanger.

23. The method of any one of claims 18 to 22, wherein the CO2 from the receiver vessel is delivered into the second side of the heat exchanger via a metering device.

24. The method of any one of claims 18 to 22, wherein the CO2 exiting the second side of the heat exchanger is cycled back to the receiver vessel.

25. The method of any one of claims 18 to 23 wherein the CO2 exiting the second side of the heat exchanger is fed into a suction line of the compressor.

26. The method of any one of claims 18 to 25, further including passing gas return line CO2 through a second heat exchanger prior to the first heat exchanger, the second heat exchanger transferring heat from the gas return line CO2 to a suction line of the refrigeration system.

27. A carbon dioxide (CO2) based refrigeration system using a method according to any one of claims 18 to 26.

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