REFRIGERATION SYSTEM

Abstract

Disclosed is a CO.sub.2 based refrigeration system including a condenser for transferring heat from a CO.sub.2 refrigerant of the refrigeration system to an air stream. The system further includes a metering device downstream of the condenser and a bypass arrangement. The metering device is configured to create a pressure drop so that part of the refrigerant liquifies, when received in a supercritical state, from the condenser such that a liquid component and a flash gas component are generated. The bypass arrangement includes a valve and a bypass line to allow the refrigerant to bypass the metering device.

Claims

1. A CO.sub.2 based refrigeration system comprising: a condenser configured to transfer heat from a CO.sub.2 refrigerant of the refrigeration system to an air stream; a metering device downstream of the condenser, the metering device configured to create a pressure drop so that part of the refrigerant liquifies, when received in a supercritical state, from the condenser such that a liquid component and a flash gas component are generated; and a bypass arrangement configured to allow the refrigerant to bypass the metering device, wherein the bypass arrangement comprises a valve and a bypass line.

2. The refrigeration system according to claim 1, wherein the valve is configurable between: a first position in which refrigerant bypasses the metering device; and a second position in which the refrigerant passes through the metering device.

3. The refrigeration system according to claim 2, wherein the valve is disposed on the bypass line.

4. The refrigeration system according to claim 1, further comprising an indirect evaporative cooler arranged to cool an ambient air stream without changing its moisture content, the indirect evaporative cooler being arranged to supply the cooled ambient air to the condenser to facilitate the transfer of heat from the CO.sub.2 refrigerant, wherein the indirect evaporative cooler comprises: a first channel for receipt of a first ambient air stream from an air source; a second channel separate to the first channel, the second channel for receipt of a second air stream and comprising a wetted surface for supplying water to the second air stream by way of evaporation; a heat exchanger for exchanging heat between the first and second channels; and a diverter to divert at least a portion of the first ambient air stream into the second channel, whereby the second air stream comprises the diverted portion of the first ambient air stream, said diverter being located at an output end of the first channel such that the portion of the first ambient air stream diverted into the second channel is diverted into the input end of the second channel, the first and second channels being configured such that the first ambient air stream flows through the first channel in a direction opposite to the flow of the second air stream in the second channel; wherein the second channel is separated from the first channel by a water-impermeable wall configured so that the first ambient air stream received in the first channel is separated from the second air stream in the second channel.

5. The refrigeration system according to claim 4, wherein the first air stream of the indirect evaporative cooler comprises the cooled ambient air supplied to the condenser for condensing of the CO.sub.2 refrigerant.

6. The refrigeration system according to claim 4, comprising a controller arranged to control the supply of cooled ambient air to the condenser.

7. The refrigeration system according to claim 6, comprising a fan to move the ambient air through the indirect evaporative cooler.

8. The refrigeration system according to claim 7, wherein the controller is configured to control the fan to control movement of the ambient air through the indirect evaporative cooler.

9. The refrigeration system according to claim 6, wherein the controller is configured to control a condenser fan for moving cooled ambient air across coils of the condenser.

10. The refrigeration system according to claim 6 wherein the controller is configured to control the supply of cooler ambient air to the condenser based on the relative humidity and temperature of the air source.

11. The refrigeration system according to claim 6 wherein the controller is configured to maintain the temperature of the refrigerant in the condenser below a predetermined threshold temperature.

12. The refrigeration system according to claim 4 comprising one or more sensors for measuring the temperature and relative humidity of the air source.

13. A method of operating a CO.sub.2 based refrigeration system, the method comprising: determining whether CO.sub.2 refrigerant being discharged from the condenser of the refrigeration system is in a supercritical state; and controlling the system so as to: create a pressure drop so that part of the refrigerant liquifies by way of a throttling process when the CO.sub.2 refrigerant is determined to be in the supercritical state, and bypass the throttling process when the CO.sub.2 is determined to not be in the supercritical state.

14. The method of claim 13, further comprising: supplying a first ambient air stream from an air source; diverting at least a portion of the first ambient air stream so that a second air stream comprises the diverted portion of the first ambient air stream; cooling the second air stream by moving the second air stream across a wetted surface in a direction opposite to a flow of the first ambient air stream; transferring heat between the first ambient air stream and the second air stream to cool the first ambient air stream without changing its moisture content; and transferring heat between at least a portion of the first ambient air stream and CO.sub.2 refrigerant in a condenser of the refrigeration system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments will now be described by way of example only, with reference to the accompanying drawings in which:

[0045] FIG. 1 is a schematic showing a refrigeration system as disclosed herein;

[0046] FIG. 2 schematic illustrates the operation of an indirect evaporative cooler;

[0047] FIG. 3 is a schematic showing a method of operating the refrigeration system shown in FIG. 1A; and

[0048] FIGS. 4A, 4B and 4C are top, side and perspective views of a condenser/indirect evaporative cooler assembly.

DETAILED DESCRIPTION

[0049] In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure.

[0050] FIG. 1 shows a CO.sub.2 based refrigeration system (i.e. that uses CO.sub.2 as a refrigerant or working fluid) comprising, among other components, a compressor 102, condenser 104, expansion valve 106 and evaporator 108. In general, these components operate in the same manner as in known refrigeration systems.

[0051] In operation, CO.sub.2 refrigerant is compressed in the compressor 102, which increases the pressure and temperature of the refrigerant. The refrigerant subsequently flows from the discharge side of the compressor, via a discharge line 110, to the condenser 104 for condensing. In the presently described embodiment the condenser 104 is an air cooled condenser (e.g. comprising a coil block and fans 112 that draw air through the coil block to transfer heat from the coil block).

[0052] Condensers generally operate by way of heat transfer between a cooling medium and the refrigerant (in this case CO.sub.2). In an air cooled condenser, such as the condenser 104 illustrated in FIG. 1, the cooling medium is an air stream (or a plurality of air streams) that flows across conduits (e.g. tubes or coils) containing flowing refrigerant.

[0053] The heat exchange is driven by a difference in temperature between the cooling medium (in this case, air) and the refrigerant. As a result, during operation, the temperature of the refrigerant in the condenser 104 is higher (e.g. by 3-8K) than the temperature of the air stream. Thus, even when the ambient temperature is below the critical temperature of CO.sub.2, the refrigerant temperature can be above the critical temperature (31° C. for CO.sub.2), such that the refrigerant exists in a supercritical state. As is set forth above, if the refrigerant is unable to be condensed from a supercritical state to a subcritical state, it can be detrimental to the operation and efficiency of the refrigeration cycle.

[0054] To avoid this, or to at least reduce the possibility of this occurring, the presently described embodiment further includes an indirect evaporative cooler 114 that supplies an air stream (or a plurality of air streams) to the condenser 104 for the purpose of transferring heat from the refrigerant. As will be described in further detail below, the indirect evaporative cooler 114 is able to receive an air source (i.e. ambient or outside air) and reduce the temperature of the air, before supplying the cooled air to the condenser 104. In this way, the system 100 is no longer reliant on the ambient temperatures remaining below a particular temperature and, as a result, the refrigerant is able to be maintained in a subcritical state even under high ambient temperatures (e.g. up to 40° C.). Thus, the present system 100 may be operated in locations that would otherwise be unsuitable for CO.sub.2 based refrigeration systems.

[0055] The indirect evaporative cooler 114 can take various forms, but in general it operates by transferring heat between at least one first air stream that is cooled by an evaporative process, and at least one separate second air stream.

[0056] Operation of the indirect evaporative cooler 114 is best described by reference to FIG. 2. The indirect evaporative cooler 114 of the present embodiment comprises first and second sets of channels that are formed in pads through which the air is drawn. For illustrative purposes, only one first channel 216 and one second channel 218 is shown in FIG. 2 and described below, but one should be aware that in practice a plurality of first channels and a plurality of second channels may be present. The first channel 216 (or the “dry” channel) is separated from the second channel 218 (or the “wet” channel) by a wall 226 that is water impermeable, but that allows heat transfer between the first 216 and second 218 channels.

[0057] In operation, the first channel 216 receives a first air stream 220 from an ambient air source (i.e. at ambient temperature). The second channel comprises wetted surfaces 222 and receives a second air stream 224 that causes water on the wetted surfaces 222 to evaporate. The evaporation process results in sensible heat in the air and water becoming latent heat in the vapour, which causes a reduction in temperature of the air and of the water on the wetted surfaces 222. The difference in temperature between the first 216 and second 218 channels drives heat exchange from the first channel 216 to the associated second channel 218 via a heat exchanger in the form of a channel wall 226 (separating the first 216 and second 218 channels). In this way, the first air stream 220 in the first channel 216 is cooled as it flows along the first channel 216. This cooled air stream 228 is then supplied to the condenser (such as the condenser 104 shown in FIG. 1 and described above) for the purpose of transferring heat away from refrigerant in coils, tubes, conduits, etc. of the condenser.

[0058] The indirect evaporative cooler also comprises a diverter (not shown) that diverts a portion 230 of the first air stream 220 in each first channel 216 into the second channel 218. In this respect, the diverted portion 230 of the first air stream 220 becomes the second air stream 224 that flows over the wetted surfaces 222 in the second channel 218 (and cools the first air stream 220 via heat exchange across the channel wall 226). The cooled air stream 228 (i.e. that is not diverted) is supplied to the condenser, and the remaining diverted portion (the second air stream 224) is exhausted to the atmosphere, subsequent to it flowing over the wetted surfaces 222.

[0059] Such an arrangement means that, in practice, the cooled air stream 228 supplied from the evaporative cooler 114 (e.g. to the condenser) can be at a temperature that is below the wet bulb temperature of the ambient air received by the evaporative cooler 114 (this is not the case with direct evaporative cooling). This is because, as the first air stream 220 is cooled, both the dry and wet bulb temperatures of the first air stream 220 are lowered. Thus, the wet bulb temperature of the second air stream 224 (which is a redirected portion of the first air stream 220) is lower than the ambient wet bulb temperature.

[0060] Although not shown, the indirect evaporative cooler 114 further comprises a water supply system that supplies water (e.g. via a pump and spray nozzles) to the second channel 218 (or set of second channels). In some cases, second channels of the indirect evaporative cooler 114 may be oriented to facilitate wetting of the wetted surfaces 222.

[0061] Returning back to FIG. 1, the indirect evaporative condenser 114 comprises fans 132 that move air through the channels (216, 218), exhausts humid air (224) and supplies cooled air (228) to the condenser 104. The condenser 104 also comprises a centrifugal fan 112 that moves the supplied air across the coils to transfer heat from the coils to the air. Each of the evaporative cooler 132 and condenser 112 fans can be controlled (e.g. by a PLC) to maintain the condenser pressure at a desired level.

[0062] As is set forth above, because the indirect evaporative cooler 114 is capable of supplying air to the condenser 104 at temperatures below the wet bulb temperature of the air, it is possible to maintain the CO.sub.2 refrigerant in a subcritical state (i.e. in environments where this would otherwise not be possible due to ambient air temperatures). In this way, inefficiencies associated with supercritical CO.sub.2 refrigerant can be avoided.

[0063] Under normal operation, the subcritical CO.sub.2 is condensed to a liquid in the condenser 104, and flows via a number of components (discussed further below) to the expansion valve 106, via a receiver vessel 134. At the expansion valve 106, the CO.sub.2 refrigerant undergoes a pressure drop and lowers in temperature. The refrigerant subsequently passes through the evaporator 108 and heat is transferred to the refrigerant from the surrounding air or process fluid (i.e. so as to cool the surrounding air or process fluid such as milk, wine, water, etc.). Finally, the refrigerant returns to the compressor 102 via a suction line 136 and the cycle is repeated.

[0064] The present system 100 also provides means for handling the CO.sub.2 when in a supercritical state (i.e. under non-normal operation). For this purpose, the system 100 further includes a high pressure expansion valve 138 connected between the condenser 104 and the receiver vessel 134. As is described above, when the CO.sub.2 is supercritical it does not condense into a liquid in the condenser 104. The high pressure expansion valve 138 is configured to create a pressure drop that liquefies the refrigerant, which then flows to the receiver vessel 134 (and subsequently to the expansion valve 106 and evaporator 108).

[0065] One consequence of the throttling in the high pressure expansion valve 138 is that it forms a flash gas component, which also flows to the receiver vessel 134 (where it separates from the liquid component). To accommodate the flash gas, the present system further comprises a bypass line 140 connecting the receiver vessel 134 (in which the flash gas is separated from the liquid refrigerant) to the compressor 102. As should be apparent, the flash gas is a portion of the refrigerant that is not useful to the cooling function of the refrigeration system 100 and therefore is representative of an efficiency loss. This loss in efficiency means that the system 102 is more efficient when the CO.sub.2 is maintained in a subcritical state.

[0066] Nevertheless, even when in a subcritical state, the throttling effect of the high pressure expansion valve 138 results in a reduction in efficiency of the system 100. To avoid this, the present system 100 further includes a first bypass valve 142 that allows the refrigerant to bypass the high pressure expansion valve 138. The first bypass valve 142 may avoid (unnecessary) efficiency losses that would otherwise be present due to the CO.sub.2 refrigerant passing through the high pressure expansion valve 138. The refrigeration system 100 may comprise fittings and components able to withstand the high pressure of the CO.sub.2 bypassing the high pressure expansion valve 138 (e.g. the fitting and components may comprise a copper or steel alloy).

[0067] The system 100 also includes a second bypass valve 144 positioned on the bypass line 140 (between the receiver vessel 134 and the compressor 102) that provides control of refrigerant flow on the bypass line 140.

[0068] In this way, when the refrigerant is subcritical (e.g. because the indirect evaporative cooler 114 is operating to maintain it in this state), the first bypass valve 142 can be opened, and the second bypass valve 144 can be closed. This avoids efficiency losses due to throttling in the high pressure expansion valve 138, and closes the bypass line 140 (which is not required, because no flash gases are produced). Conversely, when the refrigerant is supercritical, the first bypass valve 142 can be closed, and the second bypass valve 144 can be opened such that the supercritical refrigerant flows through the high pressure expansion valve 138, and the flash gases (created by the throttling effect) are able to flow from the receiver 134 to the compressor 102 via the bypass line 140.

[0069] This operation of the valves is depicted in FIG. 3, which is a schematic showing exemplary operation of the refrigeration system 100. The control methodology 346 includes detecting ambient air conditions 348 (i.e. the conditions of the air received by the indirect evaporative cooler 114 for cooling). These conditions may be, for example, the humidity and the temperature of the ambient air and may be detected by suitable sensors (these sensors are discussed in more detail with reference to FIGS. 4A, B and C).

[0070] The detected conditions can then be used to determine the conditions at the condenser inlet 350, which can in turn be used to determine whether the CO.sub.2 refrigerant is in a supercritical state 352.

[0071] If the CO.sub.2 refrigerant is supercritical, then the first bypass valve 142 on the high pressure expansion valve bypass (which is normally configured in an open position) is closed 354, which causes refrigerant to flow through the high pressure expansion valve 138. This allows the supercritical CO.sub.2 refrigerant to be liquefied by the high pressure expansion valve 138. At the same time, the second bypass valve 144 on the bypass line 140 is opened 356, which allows the flash gas component (formed at the high pressure expansion valve 138) to bypass the expansion valve 106 and the evaporator 108. That is, the flash gas flows, via the bypass line 140, directly to the compressor 102.

[0072] An alert may also be raised 358 to notify an operator that the system 100 is operating in a supercritical state. The operator can then correct any issues that might be causing the system 100 to run in this state (i.e. apart from extreme climate conditions).

[0073] If, on the other hand, the CO.sub.2 refrigerant is determined to be in a subcritical state, the indirect evaporative cooler fan 132 and the condenser fan 112 may be controlled 360, 362 (depending on the detected ambient air conditions) to achieve a desired condenser pressure for maximising efficiency of the system 100.

[0074] FIG. 4 depicts an exemplary indirect evaporative cooler 414 and condenser 404 assembly 464 that may be used in a refrigeration system, such as the refrigeration system 100 described above. The assembly 464 comprises a plurality of sensors that can communicate data to a controller (not shown) so that the condenser 404 and indirect evaporative cooler 414 can be controlled in a manner that e.g. maximises efficiency of the system.

[0075] The assembly 464 comprises a humidity and temperature sensor 466 disposed at an inlet of the indirect evaporative cooler 414. This sensor 466 measures the humidity and temperature of the ambient air that is supplied to the indirect evaporative cooler 414. The assembly 464 also comprises condenser outlet 468 and condenser inlet 470 temperature sensors that detect the temperature of air entering and leaving the condenser 404. Also provided is an indirect evaporative cooler inlet pressure sensor 472, an indirect evaporative cooler exhaust pressure sensor 474, a condenser pressure sensor 476 and a condenser fan pressure sensor 478.

[0076] The data from these sensors is transmitted (e.g. wirelessly or via wired connection) to a controller. The controller makes use of this data to control aspects of the assembly, such as the condenser fans 412 and/or indirect evaporative cooler fans 432 to maximise efficiency.

[0077] Variations and modifications may be made to the parts previously described without departing from the spirit or ambit of the disclosure.

[0078] For example, the system may include additional components not discussed above, or may be configured in an alternative manner.

[0079] The indirect evaporative cooler may be arranged alternatively to that described above. For illustrative purposes, FIG. 2 shows a single dry (first) channel and a single wet (second) channel, but it should be understood that the indirect evaporative cooler can comprise a plurality of dry channels and a plurality of wet channels. For example, each dry channel may be adjacent to a plurality of wet channels (and vice versa).

[0080] Similarly, it may not be necessary that a portion of the first air stream be diverted to form the second air stream. Instead, the first (dry) air stream and second (wet) air stream may remain separate so as to be in a cross-flow arrangement. The first and second air streams may not be parallel to one another, and may instead be e.g. perpendicular to one another.

[0081] Further, and as would be appreciated by the skilled person, the means for providing water to the channels may be other than via spray nozzles.

[0082] In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the CO.sub.2 based refrigeration system.