Co2 Refrigeration Circuit with Sub-Cooling of the Liquid Refrigerant Aganist the Receiver Flash Gas and Method for Operating the Same

Abstract

CO2 refrigeration circuit (2) for circulating a refrigerant in a predetermined flow direction, comprising in flow direction a hear-rejecting heat exchange, (4), a receiver (10) having a liquid portion (12) and a flash gas portion (14), and subsequent to the receiver (10) a medium temperature loop (20) and a low temperature loop (24), wherein the medium and low temperature loops (24) each comprise in flow direction an expansion device (26, 28), an evaporator (30, 32) and a compressor (46, 38), the refrigeration circuit (2) further comprising a liquid line (16) connecting the liquid portion (12) of the receiver (10) with at least one of the medium and low temperature loops (20, 24) and having an internal heat exchanger (54), and a flash gas line (50) connecting the flash gas portion (14) of the receiver (10) via the internal heat exchanger (54) with the inlet of the low temperature compressor (46), wherein the internal heat exchanger (54) transfers in use heat from the liquid flowing through the liquid line (16) to the flash gas flowing through the flash gas line (50).

Description

Embodiments of the present invention are described in greater detail below with reference to the Figures, wherein the only FIG. 1 shows a refrigeration circuit in accordance with an embodiment of the present invention.

FIG. 1 shows a CO₂ refrigeration circuit 2 for circulating a CO₂ refrigerant in a predetermined flow direction. The refrigeration circuit 2 comprises a hear-rejecting heat exchanger 4 which is with a CO₂ refrigerant a gascooler in the supercritical operational mode and a condensor in the subcritical mode. A heat exchanger outlet line 6 connects the hear-rejecting heat exchanger 4 via an intermediate expansion device 8 to a receiver 10. While the pressure of the refrigerant can be up to 120 bar and is typically approximately 85 bar in “summer mode” and approximately 45 bar in “winter mode” in the hear-rejecting heat exchanger 10 and its outlet line 6, the intermediate expansion device 8 reduces the pressure to between 30 and 40 bar and preferably 36 bar with such intermediate pressure being typically independent from “winter mode” and “summer mode”. The receiver 10 collects and separates liquid and gaseous refrigerant in a liquid and a gaseous receiver portion 12 and 14, respectively.

A liquid line 16 connects the liquid portion 12 of the receiver 10 with the refrigeration consumers 18 and 22 of the medium temperature loop 20 and the low temperature loop 24. Particularly, the liquid line 16 bifurcates into a low temperature branch line 17 and a medium temperature branch line 19. The low and medium temperature loops 20 and 24 each comprise at least one low temperature and medium temperature, respectively, refrigeration consumer 18, 22. The refrigeration consumers 18 and 22 each comprise an expansion device 26, 28 and an evaporator 30, 32.

The medium temperature loop 20 closes through the suction line 34 leading to inlets of compressors 38 of a compressor set 36 of the medium temperature loop 20 and a high-pressure line 40 which connects the outlet of the compressors 38 with the inlet of the hear-rejecting heat exchanger 4. The pressure at the inlet of the medium temperature loop compressors 38 is typically between 20 and 30 bar and approximately 26 bar which results in a temperature of the refrigerant of approximately −10° C. in the refrigeration consumer(s) of the medium temperature loop 20.

In the low temperature loop 24 the low temperature suction line 42 connects the low temperature refrigeration consumer(s) 22 with the inlets of compressors 46 of the low temperature loop compressor set 44. A return line 48 returns the low temperature loop refrigerant to the inlet of the medium temperature loop compressor set 36. While the pressure at the inlet of the low temperature loop compressor set 44 is typically between 8 and 20 bar, and preferably approximately 12 bar which results in a temperature of the refrigerant of approximately −37° C. in the refrigeration consumer(s) of the low temperature loop 24, the pressure at the outlet thereof is approximately at about the same level as the inlet pressure of the medium temperature loop compressor set. The low temperature loop 24 subsequently closes through the common loop portion with the medium-temperature loop 20, i.e. medium temperature loop compressor set 36, high-pressure line 40, hear-rejecting heat exchanger 4, intermediate expansion device 8, receiver 10 and liquid line 16.

A flash gas line 50 is connected with the gaseous portion 14 of the receiver 10. The flash gas line 50 taps flash gas which is substantially the saturation pressure, i.e. at least near the 2-phase state thereof. The flash gas line 50 leads the flash gas via a flash gas expansion device, for example a flash gas valve 52, and an internal heat exchanger 54 which is connected to the liquid line 16 in heat exchange relationship with liquid refrigerant and returns it to the inlet or suction of the low temperature loop compressor set 44. Accordingly, the flash gas which is at the intermediate pressure of approximately 36 bar in the receiver is expanded to approximately 12 bar at the inlet to the low temperature loop compressor 46. The respective cooling capacity, i.e. heat from the liquid refrigerant, will substantially be transferred to the liquid refrigerant in the internal heat exchanger 54 and increases the cooling or refrigeration capacity thereof. This transfer of heat to the flash gas refrigerant increases the temperature thereof and insures that the initially 2-phase state flash gas is fully dry and superheated before feeding into the low temperature compressor suction or inlet. The internal heat exchanger 54 can be in the liquid line 16 resulting in an increase of the refrigeration capacity of the liquid for the medium temperature and the low temperature loops 20 and 24, but can also be in any of the branch lines 17 and 19 so that the refrigeration capacity merely for this loop 20 or 24 will be increased. It is also possible to provide a switch-over valve (not shown) in the flash gas line 50 subsequent to the internal heat exchanger 54, and an alternative flash gas line (not shown) which connects the switch-over valve and thus the internal heat exchanger 54 to the inlet or suction of the medium temperature compressor set 36. By switching over between flowing the flash gas to the inlet of the low temperature compressor 46 and the inlet of the medium temperature compressor 38 the increase of the refrigeration capacity can be controlled in a wide range.

The flash gas valve 52 can be thermal expansion device and can be a controllable valve of the type as known to the skilled person. It can particularly be an electronically controlled valve or a mechanically controlled valve. It can be a thermal expansion valve TXV or an electronic expansion valve EXV.

A control 60 is provided for controlling the flash gas valve 52. The control can be separate or part of the overall refrigeration circuit control. The control can also be integrated with the flash gas valve 52. A monitoring device 56 which includes a temperature sensor 70 and a pressure sensor 72 is connected via line 58 to the control 60. The control 60 is adapted to control the flow of flash gas through the internal heat exchanger 54, for example dependent on the desired refrigeration capacity increase in the liquid refrigerant or dependent of the superheat condition of the flash gas. The control 60 can also be adapted to control the above mentioned switch-over valve.

Further sub-cooling is provided for the high-pressure refrigerant in the hear-rejecting heat exchanger outlet line 6. Therefore, a portion of the refrigerant is diverted through high-pressure expansion valve 64 and high-pressure heat exchanger 62 for sub-cooling the remainder of the refrigerant. Line 68 returns the diverted portion of the refrigerant to the inlet of the compressor 66. The inlet of compressor 66 can be at the same pressure level as the remaining compressors 38 of the compressor set 36 or at a different, i.e. higher or lower, level.

Claims

1. CO2 refrigeration circuit (2) for circulating a refrigerant in a predetermined flow direction, comprising in flow direction a hear-rejecting heat exchanger (4), a receiver (10) having a liquid portion (12) and a flash gas portion (14), and subsequent to the receiver (10) a medium temperature loop (20) and a low temperature loop (24), wherein the medium and low temperature loops (20, 24) each comprise in flow direction an expansion device (26, 28), an evaporator (30, 32) and a compressor (46, 38), the refrigeration circuit (2) further comprising a liquid line (16) connecting the liquid portion (12) of the receiver (10) with at least one of the medium and low temperature loops (20, 24) and having an internal heat exchanger (54), and a flash gas line (50) connecting the flash gas portion (14) of the receiver (10) via the internal heat exchanger (54) with the inlet of the low temperature compressor (46), wherein the internal heat exchanger (54) transfers in use heat from the liquid flowing through the liquid line (16) to the flash gas flowing through the flash gas line (50).

2. CO2 refrigeration circuit (2) according to claim 1 further comprising a flash gas valve (52) within the flash gas line (50).

3. CO2 refrigeration circuit (2) according to claim 2, wherein the flash gas valve (52) is a control valve.

4. CO2 refrigeration circuit (2) according to claim 1, further comprising a monitoring device (56) in the flash gas line (50) which is adapted for monitoring the condition of the flash gas.

5. CO2 refrigeration circuit (2) according to claim 4, wherein the monitoring device (56) includes a pressure sensor (72) and a temperature sensor (70).

6. CO2 refrigeration circuit (2) according to claim 4, further comprising a control (60) connected to the monitoring device (56) and the control valve (52) for regulating the control valve (52) based on the condition of the flash gas.

7. CO2 refrigeration circuit (2) according to claim 1, further comprising an intermediate expansion device (8) between the hear-rejecting heat exchanger (4) and the receiver (10).

8. CO2 refrigeration circuit (2) according to claim 1, wherein the outlet of the low temperature compressor (46) is connected with the inlet of the medium temperature compressor (38).

9. CO2 refrigeration apparatus (3) comprising a CO2 refrigeration circuit (2) in accordance with claim 1.

10. Method for operating a CO2 refrigeration circuit (2) for circulating a refrigerant in a predetermined flow direction, the CO2 refrigeration circuit (2) comprising in flow direction a hear-rejecting heat exchanger (4), a receiver (10) having a liquid portion (12) and a flash gas portion (14), and subsequent to the receiver (10) a medium temperature loop (20) and a low temperature loop (24), wherein the medium and low temperature loops (24) each comprise in flow direction an expansion device (26, 28), an evaporator (30, 32) and a compressor (46, 38), the refrigeration circuit (2) further comprising a liquid line (16) connecting the liquid portion (12) of the receiver (10) with at least one of the medium and low temperature loops (20, 24), wherein the method comprises the following steps: (a) tapping flash gas from the flash gas portion (14) of the receiver (10);(b) flowing the flash gas and flowing the liquid in the liquid line (16) in heat exchange relationship to effect a heat transfer from the liquid to the flash gas;(c) returning the flash gas into the low temperature loop (24) at a location near the inlet of the low temperature compressor (46).

11. Method according to claim 10, further including the step of adjusting the amount of flash gas which is tapped from the receiver (10) in accordance with the operational condition.

12. Method according to claim 10, further including the step of monitoring the condition of the flash gas and adjusting the amount of flash gas based on the flash gas condition.

13. Method according to claim 12, wherein the step of monitoring the flash gas condition includes the steps of sensing the pressure and the temperature of the flash gas.

14. Method according to claim 12, wherein the step of monitoring the condition of the flash gas is performed subsequent to the step of flowing the flash gas and the liquid in heat exchange relationship.