GB2440669A

GB2440669A - A Refrigeration System with an Economizer for Transcritical Operation.

Info

Publication number: GB2440669A
Application number: GB0714959A
Authority: GB
Inventors: Dieter Mosemann; Dmytro Zaytsev
Original assignee: Grasso GmbH Refrigeration Technology
Current assignee: GEA Refrigeration Germany GmbH
Priority date: 2006-08-01
Filing date: 2007-07-31
Publication date: 2008-02-06
Anticipated expiration: 2027-07-31
Also published as: GB0714959D0; US20080302129A1; DE102006035784B4; GB2440669B; ITRM20070158A1; JP2008039383A; DE102006035784A1

Abstract

A refrigeration circuit has a compressor 21 operating at least on three pressure levels: suction pressure 11 on the compressor suction side 29; intermediate pressure 10 at an economizer connection 31; and discharge pressure 2 on a discharge side 22. The circuit also has a gas cooler 23, an evaporator 30 and a controllable throttling point 28, and an intercooler 24. The intercooler has first and second flow paths, with the first flow path arranged between the gas cooler and the evaporator. The second flow path connects the intercooler at inlet and outlet connections, with the inlet connection arranged to provide flow from piping between the economizer and the evaporator, and via a controllable throttling point 26. The outlet connection provides flow from the economizer to the compressor economizer connection. The circuit may also include an aftercooler 27 in a flow path between the intercooler and the evaporator, and a separator 25 may be connected to the aftercooler and provide a liquid coolant flow. The separator may be provided in the flow path between the evaporator and the suction side of the compressor, and the compressor may be a screw or scroll type.

Description

Refrigeration System for Transcritical Operation with Economizer and

Low-Pressure Receiver This invention relates to a refrigeration system and particularly, but not exclusively, relates to a refrigeration system for transcritical operation with compressors operating at least on three pressure levels. The various sides of the compressor are also designated as low-pressure side, intake side or suction side, and as high-pressure side or discharge side respectively. The pressure on the high-pressure side is higher than the pressure at the critical point of the refrigerant. Therefore, this process is designated as a transcritical or overcritical refrigeration process. The economizer connection is arranged between suction-and discharge sides of the compressor. The inlet process at the economizer connection starts when there is no more flow connection of this interlobe space to the compressor suction side. In this phase, the geometric interlobe volume of the interlobe space has reached its maximum. Depending on the wrap angle of the rotor profile at the male rotor, number of lobes of both rotors, the geometric interlobe volume of the interlobe space can be constant (transfer phase) or can decrease due to rotation of rotors.

The invention relates to a refrigeration system featuring a heat exchanger, a so-called aftercooler, arranged in or at the liquid separator on the low-pressure side communicating with the liquid separator, and the refrigerant (the working fluid) being under discharge pressure is subcooled nearly to evaporation temperature in the aftercooler prior to its expansion, thus changing from the vapor phase to the liquid phase, before it is expanded into the evaporators at the throttling point of the refrigeration system.

The pressure upstream of this throttling point is kept constant by opening or closing the throttling point thereby the compressor to operate at constant discharge pressure. The refrigerating capacity of the refrigeration system changes depending on the temperature to which the refrigerant was cooled down in the gas cooler. It will be reduced as a result of higher outlet temperatures at the gas cooler, as at higher temperatures for cooling-down the working fluid in the aftercooler prior to expansion more working fluid will evaporate in the low-pressure separator than will leave the gas cooler at low temperatures. Therefore, the efficiency of the refrigeration system will decrease with increasing temperature at the gas cooler.

The object of the invention is to improve the process and to increase the efficiency of the refrigeration system.

According to the feature of the invention the refrigeration system for transcritical operation comprises in addition to the components gas cooler, aftercooler, evaporator with low-pressure separator, compressor, throttling devices and interconnecting piping between the components mentioned an intercooler with heat-exchanging surfaces arranged in the same flow path in flow direction downstream of the gas cooler and upstream of the aftercooler, and through which for cooling of the hot-gas flow a partial refrigerant flow is taken either upstream or downstream of the aftercooler from the same flow path and carried to the economizer connection of the compressor via the intercooler in another flow path. According to the features of the invention a partial flow of the subcooled liquid working fluid is led to the intercooler via a controllable throttling point to cool down the working fluid leaving the gas cooler prior to entering the aftercooler. In this way, the refrigerant vapour being under high pressure is cooled down on one side of the heat-exchanging surfaces of the intercooler, while the refrigerant on the other side of the heat-exchanging surfaces of the intercooler evaporates.

According to a first aspect of the present invention, there is provided a refrigeration device with a compressor operating at least on three pressure levels: suction pressure on the compressor suction side, intermediate pressure at an economizer connection and discharge pressure on a discharge side, the refrigeration device further comprising a gas cooler, an evaporator and a controllable throttling point, wherein there is an intercooler with first and second flow paths, the first flow path arranged in flow direction between the gas cooler and the evaporator, and the second flow path on its inlet side into the intercooler having a flow connection via a controllable throttling point and piping to the flow path between the intercooler and the evaporator, and on the outlet side of the second flow path of the intercooler there is a connection to the economizer connection of the compressor.

An aftercooler may be provided in the flow path between the intercooler and the evaporator and a first liquid separator may be provided in the flow path between the evaporator and the suction side of the compressor, wherein the first liquid separator may also be in flow communication with the aftercooler such that flow from the first liquid separator is in thermal communication with the flow path between the intercooler and the evaporator.

The second flow path of the intercooler, on its inlet side into the intercooler, may have a flow connection via the controllable throttling point and piping to the flow path between the aftercooler and the evaporator. The second flow path of the intercooler, on its inlet side into the intercooler, may have a flow connection via the controllable throttling point and piping to the flow path between the intercooler and the aftercooler.

A working fluid may be transcritical during the refrigeration process.

The compressor may comprise geometrically controlled inlet and outlet ports and the compressor may comprise a screw compressor or a scroll compressor.

According to a second aspect of the present invention, there is provided a refrigeration device comprising a first heat exchanger, a second heat exchanger, a first expansion device, an evaporator and a compressor; the second heat exchanger further comprising a first flow channel and a second flow channel in thermal communication with each other and the compressor further comprising a first inlet port, a second inlet port and an outlet port, wherein; the refrigeration device is arranged so that a first working fluid flows in a cyclic fashion through the first heat exchanger, the first flow channel of the second heat exchanger, the first expansion device, the evaporator, through the first inlet port of the compressor and returning to the first heat exchanger, via the outlet port of the compressor; a manifold is further provided in the flow path between the first flow channel of the second heat exchanger and the first expansion device, wherein, a manifold inlet is in flow communication with the first flow channel of the second heat exchanger and a first manifold outlet is in flow communication with the first expansion device, wherein; a second manifold outlet is in flow communication with a second expansion device, which is in turn in flow communication with the second flow channel of the second heat exchanger, the second expansion device being arranged so that a portion of the first working fluid flows from the second manifold outlet through the second expansion device to the second flow channel of the second heat exchanger, wherein the refrigeration device is arranged so that the first working fluid exiting the second flow channel of the second heat exchanger flows into the second inlet port of the compressor.

The first working fluid in the first heat exchanger and first flow channel of the second heat exchanger may be at a first working pressure, the first working fluid in the evaporator may be at a second working pressure, and the first working fluid in the second flow channel of the second heat exchanger may be at a third working pressure.

The first working pressure may be above the critical pressure of the first working fluid.

The second working pressure may be below the critical pressure of the first working fluid. The third working pressure may be greater than the second working pressure and less than the first working pressure.

The first expansion device or second expansion device may comprise a turbine. The first expansion device or second expansion device may comprise a throttle. The second expansion device may be variable such that the third working pressure can be varied. A throat area of the second expansion device may be variable.

A third heat exchanger may be provided in the flow path between the second heat exchanger and the first expansion device and a first liquid separator may be provided in the flow path between the evaporator and the first inlet port of the compressor, wherein the first liquid separator may also be in flow communication with the third heat exchanger such that flow from the first liquid separator may be in thermal communication with the flow path between the second heat exchanger and the first expansion device.

The manifold may be provided in the flow path between the third heat exchanger and the first expansion device. The manifold may be provided in the flow path between the second heat exchanger and the third heat exchanger.

The first working fluid may evaporate in the evaporator. The first working fluid may comprise a refrigerant. The flow entering the first liquid separator from the evaporator may be a two-phase fluid and the flow leaving the separator for the first inlet port of the compressor may be a vapour.

A fourth heat exchanger may be provided in the flow path between the manifold and the second expansion device and a second liquid separator may be provided in the flow path between the second heat exchanger and the second inlet port of the compressor, wherein the second liquid separator may also be in flow communication with the fourth heat exchanger such that flow from the second liquid separator may be in thermal communication with the flow path between the manifold and the second expansion device.

The compressor may comprise a screw compressor. The second inlet port of the compressor may correspond to an economiser connection. The flow into the second inlet port of the compressor may reduce the temperature of the flow within the compressor. The compressor may comprise a first compressor device and a second compressor device and the second inlet port of the compressor may correspond to an inlet between the first compressor device and the second compressor device.

The first expansion device, second expansion device and manifold arrangement may comprise a single expansion device with an inlet, a first outlet and a second outlet, such that a flow from the inlet to the first outlet of the single expansion device may correspond to a flow through the first expansion device and that a flow from the inlet to the second outlet of the single expansion device may correspond to a flow through the second expansion device.

The first heat exchanger may be in flow communication with a second working fluid so that the first working fluid in the first heat exchanger may be in thermal communication with the second working fluid. The first working fluid in the evaporator may be in thermal communication with a medium which is to be refrigerated.

According to a third aspect of the present invention, there is provided a refrigeration device comprising a first heat exchanger, a second heat exchanger, a first expansion device, an evaporator and a compressor; the second heat exchanger further comprising a first flow path and a second flow path in thermal communication with each other; wherein, a working fluid flows through the first heat exchanger, the first flow path of the second heat exchanger, the first expansion device, the evaporator, the compressor and returning to the first heat exchanger, such that the working fluid flows in a cyclic fashion; the refrigeration system further comprising a bypass circuit arranged such that a portion of the working fluid flowing from the second heat exchanger to the first expansion device is diverted into a second expansion device, the bypass circuit is further arranged so that the working fluid from the second expansion device flows through the second flow path of the second heat exchanger and into the compressor.

According to a fourth aspect of the present invention, there is provided an arrangement at or in a refrigeration system for transcritical operation with compressors featuring geometrically controlled inlet and outlet ports, e.g. screw compressors or scroll compressors, operating at least on three pressure levels, suction pressure on the compressor suction side, intermediate pressure at the economizer connection and discharge pressure, having a gas cooler, a liquid separator on the low-pressure side, an aftercooler for cooling the refrigerant being under discharge pressure prior to its expansion into the evaporator with the aflercooler communicating with a low-pressure liquid separator, and with a controllable throttling point, wherein there is an intercooler with two flow paths with the first flow path arranged in flow direction between the gas cooler and the aflercooler, and the second flow path on its inlet side into the intercooler having a flow connection via a controllable throttling point and piping either to the outlet of the first flow path of the intercooler or to the outlet of the aftercooler, and on the outlet side of the second flow path of the intercooler there is a connection to the economizer connection of the compressor.

The refrigerant evaporated is led to the economizer connection. Due to this intermediate cooling, the cooling-down capacity will decrease during the subsequent cooling in the aftercooler. As a result of cooling the refrigerant vapour in the intercooler, there will be created less vapour in the aftercooler on the side of the liquid separator.

The refrigerating capacity of the refrigeration system will increase also with higher temperatures at the gas cooler, and the efficiency will be improved due to two-stage compression of the refrigerant.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, in which:-Figure 1 shows a simplified schematic for arrangement of compressor and heat exchangers with interconnecting piping and control devices; Figure 2 shows a Ph (Pressure-enthalpy) diagram for a refrigeration-or air conditioning system according to the invention; Figure 3 shows a simplified schematic for arrangement of compressor and heat exchangers with interconnecting piping and control devices for another arrangement according to the invention; and Figure 4 shows a Ph (Pressure-enthalpy) diagram for the arrangement according to the invention in compliance with Figure 3.

The refrigeration system for transcritical operation according to Figure 1 comprises a gas cooler 23, an intercooler 24, an evaporator 30, a liquid separator 25 communicating with an aftercooler 27, a screw compressor 21 having geometrically controlled inlet and outlet ports, throttling devices 26, 28 and interconnecting piping between the components mentioned. When compressor 21 is in operation, suction pressure 11 prevails on its suction side 29, while discharge pressure 12 prevails on its discharge side 22 with the pressure on the discharge side 22 being higher than the pressure at the critical point of the refrigerant. The compressor has an economizer connection port 31 at the housing enabling a flow connection to intercooler 24, and the pressure in this pipe section lies between discharge pressure and suction pressure. In the Ph diagram according to Figure 2, point 1 describes the condition on the suction side of compressor 21. The outlet condition of the refrigerant after compressor 21, point 2, is the inlet condition into gas cooler 23.

The refrigerant passes gas cooler 23 which is fed by a cooling medium, e.g. cooling water, for cooling the refrigerant vapour. When leaving said gas cooler 23, the refrigerant has the condition at point 3. In intercooler 24 through which two refrigerant flows of the refrigeration system are led, the refrigerant is cooled from point 3 to point 4. For this purpose, the partial refrigerant flow expanded to intermediate pressure level will be evaporated and superfed via economizer port 31 into the compressor housing without considerably influencing the suction volume flow. The refrigerant flow is further cooled from point 4 to point 5 in aftercooler 27 wherein liquid evaporates in aftercooler 27 communicating with liquid separator 25, and hence reducing the available volumetric refrigerating capacity by the enthalpy difference from point I to point 9. Point 9 corresponds to the condition of the refrigerant at the evaporator outlet characterized by a two-phase mixture. The intermediate pressure level 10 can be used for changing the refrigerating capacity by way of rising the intermediate pressure, and hence changing the intermediate cooling effect. Due to cooling the refrigerant vapour in intercooler 24, there will be created less vapour in aftercooler 27 on the side of liquid separator 25.

The refrigerating capacity of the refrigeration system will also be increased at higher temperatures at gas cooler 23, and the effectiveness will be improved due to two-stage compression of the refrigerant.

The refrigeration system for transcntical operation according to Figure 3 is configured similarly to Figure 1 with the distinguishing feature that the second flow path of intercooler 24 on its inlet side via piping and throttling device 32 is connected to an intermediate-pressure aflercooler 34 having on the inlet side into intercooler 24 a flow connection to the outlet of the first flow path of intercooler 24. The outlet side of the second flow path of intercooler 24 is connected to economizer connection 31 at compressor 21 via intermediate-pressure separator 33. Intermediate-pressure aftercooler 34 communicates with intermediate-pressure separator 33.

In the Ph diagram according to Figure 4, point 13 describes the inlet condition into intercooler 24, and point 17 the outlet condition after intercooler 24.

Claims

What we claim is: 1. A refrigeration device with a compressor operating

at least on three pressure levels: suction pressure on the compressor suction side, intermediate pressure at an economizer connection and discharge pressure on a discharge side, the refrigeration device further comprising a gas cooler, an evaporator and a controllable throttling point, wherein there is an intercooler with first and second flow paths, the first flow path arranged in flow direction between the gas cooler and the evaporator, and the second flow path on its inlet side into the intercooler having a flow connection via a controllable throttling point and piping to the flow path between the intercooler and the evaporator, and on the outlet side of the second flow path of the intercooler there is a connection to the economizer connection of the compressor.

2. The refrigeration device according to claim 1, wherein an aftercooler is provided in the flow path between the intercooler and the evaporator and a first liquid separator is provided in the flow path between the evaporator and the suction side of the compressor, wherein the first liquid separator is also in flow communication with the aflercooler such that flow from the first liquid separator is in thermal communication with the flow path between the intercooler and the evaporator.

3. The refrigeration device according to claim 2, wherein the second flow path of the intercooler, on its inlet side into the intercooler, has a flow connection via the controllable throttling point and piping to the flow path between the aflercooler and the evaporator.

4. The refrigeration device according to claim 2, wherein the second flow path of the intercooler, on its inlet side into the intercooler, has a flow connection via the controllable throttling point and piping to the flow path between the intercooler and the aftercooler.

5. The refrigeration device according to any previous claim, wherein a working fluid is transcritical during the refrigeration process.

6. The refrigeration device according to any previous claim, wherein the compressor comprises geometrically controlled inlet and outlet ports.

7. The refrigeration device according to any previous claim, wherein the compressor comprises a screw compressor or a scroll compressor.

8. The refrigeration device substantially as described herein, with reference to and as shown in the accompanying drawings.