GB2577862A

GB2577862A - Compound heat transfer system

Info

Publication number: GB2577862A
Application number: GB1815104.3A
Authority: GB
Inventors: Knight Patrick; Grice Liam; Oddie James
Original assignee: Arctic Circle Ltd
Current assignee: Arctic Circle Ltd
Priority date: 2018-09-17
Filing date: 2018-09-17
Publication date: 2020-04-15
Also published as: GB201815104D0

Abstract

A method of operating a single or dual temperature compound (multi-stage) heat transfer system 100’’’ having: a low-pressure compressor stage 120’’’ comprising n compressors 120A,B, C…, a high-pressure compressor stage 140’’’; an interstage section 130’’’; and a controller 110 ‘’’. Outlet 124 of the low-pressure stage delivers refrigerant through the interstage section to an inlet 142 of the high-pressure stage at an interstage pressure. In one arrangement, the controller operates to set a target interstage pressure and varies operation of the system to cause the interstage pressure to approach the target interstage pressure to maximise the efficiency of the heat transfer system. In another arrangement the system comprises a flash tank 260’ and the controller sets a flash tank pressure between a first and a second target pressure. A regulating valve 262’ allows flow to the interstage section. The dual temperature operation may be delivered by passing refrigerant to the interstage section via interstage evaporator 380. In a further arrangement the interstage section is sized to suppress the rate of pressure rise therein caused by switching on the nth compressor in the low-pressure stage.

Description

TITLE: COMPOUND HEAT TRANSFER SYSTEM

DESCRIPTION

The present invention relates to heat transfer apparatus, and particularly but not exclusively, to improvements in the configuration and operation of compound (multi-stage) vapour-compression heat transfer systems. Such systems include, but are not limited to, refrigeration systems (e.g. refrigerator, freezer and air-conditioning systems) and heat pumps.

A single-stage vapour-compression heat transfer system has one low-side pressure (the evaporator pressure) and one high-side pressure (the condenser pressure). Single-stage systems provide adequate performance provided that the temperature difference between the heat source and the heat sink ("the lift") is small. However, there are many applications where the lift can be higher than can be practically and efficiently achieved in a single-stage, either due to the requirement of low evaporator temperature (e.g., for the frozen food or chemical industries) or due high condenser temperature (e.g., in heat-pump applications for drying or process control). To achieve such high lifts, multi-stage, compound systems may be used. Because of their relative complexity, multi-stage, compound systems provide design challenges that are absent from single-stage systems.

A two-stage internally or externally compounded heat transfer system has a low-pressure compression stage and a high-pressure compression stage. Typically, the displacements of the low-pressure stage and the high-pressure stage are matched to each other and act together as fixed-capacity compression steps. This means that the interstage pressure floats according to the volume ratio between the low-pressure stage and the high-pressure stage. Whilst the low-pressure stage suction pressure remains fixed, the high-pressure stage discharge pressure varies with ambient, which means that the optimum interstage pressure for overall system efficiency needs to change depending on these two parameters.

Additionally, the low-pressure stage discharge into the interstage is superheated during compression and then may be cooled with an air-cooled coil (e.g. a desuperheater operative to reject heat to ambient). It is possible that the desuperheater could cool the low-pressure stage discharge fluid flow to a temperature below its saturation point which would cause the fluid flow to partially or completely condense. This in turn could cause compressor damage to the high-pressure stage compressors.

The present applicant has identified the need for an improved compound heat transfer system capable of operating more efficiency than the prior art.

In accordance with a first aspect of the present invention, there is provided a method of operating a compound heat transfer system having: a low-pressure (e.g. lower pressure) compressor stage; a high-pressure (e.g. higher pressure) compressor stage; an interstage section; and a controller; wherein an outlet of the low-pressure stage delivers a refrigerant output through the interstage section to an inlet of the high-pressure stage at an interstage pressure; wherein the controller operates to set a target interstage pressure and to vary operation of the system to cause the interstage pressure to approach the target interstage pressure to maximise the efficiency of the heat transfer system.

Typically, the controller varies operation of high-pressure compressor stage to cause the interstage pressure to approach (e.g. rise or fall to approach) the target interstage pressure. Alternatively or additionally, the controller may conceivably vary operation of the low-25 pressure compressor stage to alter the interstage pressure.

In one embodiment one or both of the high-pressure compressor stage and the low-pressure compressor stage comprises multiple compressors (e.g. arranged in parallel). In one embodiment, the interstage pressure is altered by switching one or more compressors in a stage on or off. In one embodiment, different compressors in a single stage may be configurable to operate at different (e.g. variable) speeds.

In one embodiment, the controller monitors the condition of refrigerant in the interstage section and varies the target interstage pressure in response to the monitored condition. The controller typically operates such that, in the event that the monitored condition indicates that refrigerant in the interstage section is approaching saturation, it reduces the target interstage pressure.

The controller typically monitors one or both of the temperature and the pressure in 5 the interstage section.

In one embodiment, the interstage section includes a heat rejecter (e.g. operative to reject heat from refrigerant as it passes from the low-pressure stage to the high-pressure stage via the interstage section).

In one embodiment, the controller monitors the condition of refrigerant leaving the 10 heat rejecter.

In one embodiment, the controller operates such that, in the event that the monitored condition (e.g. condition of refrigerant in the interstage section or condition of refrigerant leaving the heat rejecter) indicates that refrigerant in the interstage section is approaching saturation, it reduces the target interstage pressure.

In one embodiment, the compound heat transfer system further comprises a flash tank operative to separate gaseous refrigerant from liquid refrigerant; and a flash tank pressure regulating valve for controlling flow of gaseous refrigerant from the flash tank to the interstage section; wherein the controller further operates to set a target flash tank pressure at which the flash tank pressure regulating valve opens to allow flow of gaseous refrigerant from the flash tank to the interstage section, the controller being operative to alter the target flash tank pressure between a first pressure target and a second pressure target.

In this way, a method of operating a compound heat transfer system is provided that reduces the frequency at which compressors are started due to flash tank egress. Advantageously, not only does the proposed method increase efficiency but the reduction in starting/stopping of compressors unnecessarily helps increase capacity and hence system stability.

In one embodiment, the second pressure target is a higher pressure than the first pressure target.

In one embodiment, the controller is operative to switch the target flash tank pressure 30 to the second pressure target in response to determining that one or more of the high-pressure compressor stage and the low-pressure compressor stage are switched off For example, in the case that the high-pressure compressor stage and low-pressure compressor stage each comprise multiple compressors, the controller may be operative to switch the target flash tank pressure to the second pressure target in response to determining that either all of the high-pressure compressors are off or all of the low-stage compressors are off, or all compressors are off.

In one embodiment, the controller is operative to switch the target flash tank pressure to the first pressure target in response to determining that one or more of the high-pressure compressor stage and the low-pressure compressor stage are switched on.

In one embodiment, the controller is operative to switch the target flash tank pressure to the first pressure target in response to determining that the flash tank pressure is greater 10 than or equal to the second pressure target.

In one embodiment, the low-pressure compressor stage comprises a plurality of n compressors.

In one embodiment, the interstage section is sized to supress the rate of pressure rise in the interstage section caused by the switching on of the nth compressor in the low-pressure 15 compressor stage In one embodiment, the n >3.

In the case of a plurality of n compressors having different expected volumetric displacement values (e.g. different maximum volumetric displacement values), the nth compressor is typically the compressor with the largest expected volumetric displacement 20 value.

In one embodiment, the interstage section defines a total interior gas-receiving volume in litres that is at least about 2 times the expected volumetric displacement of the nth compressor in m3/hour (e.g. at least about 2.5 times the expected volumetric displacement of the nth compressor in m3/hour). In other words, the interstage section defines a total interior gas-receiving volume in m3 that at least about 0.002 times (e.g. at least about 0.0025 times) the expected volumetric displacement of the nth compressor in m7hour.

In one embodiment, the interstage section defines a total interior gas-receiving volume in litres that is about 2 to about 10 times (e.g. about 2 to about 7 times, e.g. about 2.5 to about 5 times) the expected volumetric displacement of the nth compressor in ml/hour. In other words, the interstage section defines a total interior gas-receiving volume in m3 that is about 0.002 to about 0.01 times (e.g. about 0.002 to about 0.007 times, e.g. about 0.0025 to about 0.005 times) the expected volumetric displacement of the nth compressor in m3/hour.

Typically the plurality of n compressors are fixed speed compressors (rotary or reciprocating) In one embodiment, the interstage section comprises a surge drum defining a gas storage volume.

In one embodiment, the gas storage volume of the surge drum in litres is at least about 0.5 times (e.g. at least about 0.8 times, e.g. at least about 1 times) the expected volumetric displacement of the nth compressor in m3/hour. In other words, the gas storage volume of the surge drum in m3 is at least about 0.0005 times (e.g. at least about 0.0008 times, e.g. at least about 0.001 times) the expected volumetric displacement of the nth compressor in m3/hour.

In one embodiment, the gas storage volume of the surge drum in litres is about 0.5 to about 7 times (e.g. about 0.8 to about 5 times, e.g. about Ito about 3.5 times) the expected volumetric displacement of the nth compressor in m3/hour. In other words, the gas storage volume of the surge drum in in3 is about 0.0005 to about 0.007 times (e.g. about 0.0008 to about 0.005 times, e.g. about 0.001 to about 0.0035 times) the expected volumetric displacement of the nth compressor in m3/hour.

In one embodiment, the surge drum is a non-accumulator/non-filtering device (e.g. not operative to function as a suction accumulator that collects and accumulates liquid components of the refrigerant flow).

Typically, embodiments of the invention implement a method of operating a heat transfer system that is a refrigeration system (e.g. refrigerator, freezer or air-conditioning system). Alternatively, embodiments of the invention may implement a method of operating a heat pump.

Embodiments include, but are not limited to: a system with a single heat load connected to the low-pressure stage; a system with a first heat load connected to the low-pressure stage and a second heat load connected to the interstage section; a system with a single heat load connected to the low-pressure stage and a flash tank operative to return vapour to the interstage section; and a system with first heat load connected to the low-pressure stage, a second heat load connected to the interstage section, and a flash tank operative to return vapour to the interstage section.

In accordance with a second aspect of the present invention, there is provided a heat transfer system configured to operate in accordance with any embodiment of the first aspect of the invention.

In accordance with a third aspect of the present invention, there is provided a 5 computer program which, when executed by a processor, causes the processor to carry out a method in accordance with any embodiment of the first aspect of the invention.

In accordance with a fourth aspect of the present invention, there is provided a computer program product carrying instructions which, when executed by a processor, causes the processor to carry out a method in accordance with any embodiment of the first aspect of 10 the invention.

The computer program product may be provided on any suitable carrier medium.

In one embodiment the computer program product comprises a tangible carrier medium (e.g. a computer-readable storage medium).

In another embodiment the computer program product comprises a transient carrier 15 medium (e.g. data stream).

In accordance with a fifth aspect of the present invention, there is provided a method of operating a compound heat transfer system having: a low-pressure (e.g. lower pressure) compressor stage; a high-pressure (e.g. higher pressure) compressor stage; an interstage section; a flash tank operative to separate gaseous refrigerant from liquid refrigerant; a flash tank pressure regulating valve for controlling flow of gaseous refrigerant from the flash tank to the interstage section; and a controller; wherein an outlet of the low-pressure stage delivers a refrigerant output through the interstage section to an inlet of the high-pressure stage at an interstage pressure; wherein the controller operates to set a target flash tank pressure at which the flash tank pressure regulating valve opens to allow flow of gaseous refrigerant from the flash tank to the interstage section, the controller being operative to alter the target flash tank pressure between a first pressure target and a second pressure target.

In one embodiment, the controller is operative to switch the target flash tank pressure to the second pressure target in response to determining that one or more of the high-pressure 5 compressor stage and the low-pressure compressor stage are switched off In one embodiment, the controller is operative to switch the target flash tank pressure to the first pressure target in response to determining that one or more of the high-pressure compressor stage and the low-pressure compressor stage are switched on.

In one embodiment, the controller is operative to switch the target flash tank pressure 10 to the first pressure target in response to determining that the flash tank pressure is greater than or equal to the second pressure target.

In one embodiment one or both of the high-pressure compressor stage and the low-pressure compressor stage comprises multiple compressors (e.g. arranged in parallel). In one embodiment, the interstage pressure is altered by switching one or more compressors in a stage on or off In one embodiment, different compressors in a single stage may be configurable to operative at different (e.g. fixed) speeds.

In one embodiment, the interstage section is sized to supress the rate of pressure rise in the interstage section caused by the switching on of the nth compressor in the low-pressure compressor stage.

In one embodiment, the n >3.

In the case of a plurality of 71 compressors having different expected volumetric displacement values (e.g. different maximum volumetric displacement values), the nth compressor is typically the compressor with the largest expected volumetric displacement value.

In one embodiment, the interstage section defines a total interior gas-receiving volume in litres that is at least about 2 times the expected volumetric displacement of the nth compressor in ml/hour (e.g. at least about 2.5 times the expected volumetric displacement of the nth compressor in m3/hour). In other words, the interstage section defines a total interior gas-receiving volume in m3 that at least about 0.002 times (e.g. at least about 0.0025 times) the expected volumetric displacement of the nth compressor in m3/hour.

In one embodiment, the interstage section defines a total interior gas-receiving volume in litres that is about 2 to about 10 times (e.g. about 2 to about 7 times, e.g. about 2.5 to about 5 times) the expected volumetric displacement of the nth compressor in m3/hour. In other words, the interstage section defines a total interior gas-receiving volume in m3 that is about 0.002 to about 0.01 times (e.g. about 0.002 to about 0.007 times, e.g. about 0.0025 to about 0.005 times) the expected volumetric displacement of the nth compressor in m3/hour.

In one embodiment, the gas storage volume of the surge drum in litres is about 0.5 to about 7 times (e.g. about 0.8 to about 5 times, e.g. about 1 to about 3.5 times) the expected volumetric displacement of the nth compressor in m3/hour. In other words, the gas storage volume of the surge drum in m3 is about 0.0005 to about 0.007 times (e.g. about 0.0008 to about 0.005 times, e.g. about 0.001 to about 0.0035 times) the expected volumetric displacement of the nth compressor in m3/hour.

Typically, embodiments of the invention implement a method of operating a heat 30 transfer system that is a refrigeration system (e.g. refrigerator, freezer or air-conditioning system). Alternatively, embodiments of the invention may implement a method of operating a heat pump.

Embodiments include, but are not limited to: a system with a single heat load connected to the low-pressure stage and a flash tank operative to return vapour to the interstage section; and a system with first heat load connected to the low-pressure stage, a second heat load 5 connected to the interstage section, and a flash tank operative to return vapour to the interstage section.

In accordance with a sixth aspect of the present invention, there is provided a compound heat transfer system comprising: a low-pressure compressor stage comprising a plurality of n low-pressure compressors (e.g. connected in parallel); a high-pressure compressor stage; and an interstage section; wherein an outlet of the low-pressure stage is configured to deliver a refrigerant output through the interstage section to an inlet of the high-pressure stage at an interstage pressure; wherein the interstage section is sized to supress the rate of pressure rise in the interstage section caused by the switching on of the nth compressor in the low-pressure compressor stage.

In one embodiment, the n >3.

In one embodiment, the interstage section defines a total interior gas-receiving volume in litres that is at least about 2 times the expected volumetric displacement of the nth compressor in m3/hour (e.g. at least about 2.5 times the expected volumetric displacement of the nth compressor in m3/hour). In other words, the interstage section defines a total interior gas-receiving volume in m3 that at least about 0.002 times (e.g. at least about 0.0025 times) the expected volumetric displacement of the nth compressor in m3/hour.

In one embodiment, the interstage section defines a total interior gas-receiving volume in litres that is about 2 to about 10 times (e.g. about 2 to about 7 times, e.g. about 2.5 to about 5 times) the expected volumetric displacement of the nth compressor in in3/hour. In other words, the interstage section defines a total interior gas-receiving volume in ni3 that is about 0.002 to about 0.01 times (e.g. about 0.002 to about 0.007 times, e.g. about 0.0025 to about 0.005 times) the expected volumetric displacement of the nth compressor in IM/hour.

Typically the plurality of compressors are fixed speed compressors (rotary or reciprocating).

In one embodiment, the interstage section comprises a surge drum defining a gas storage volume.

In one embodiment, the gas storage volume of the surge drum in litres is about 0.5 to about 7 times (e.g. about 0.8 to about 5 times, e.g. about Ito about 3.5 times) the expected volumetric displacement of the nth compressor in m3/hour. In other words, the gas storage volume of the surge drum in m3 is about 0.0005 to about 0.007 times (e.g. about 0.0008 to about 0.005 times, e.g. about 0.001 to about 0.0035 times) the expected volumetric displacement of the nth compressor in m3/hour.

In one embodiment, the compound heat transfer system further comprises a flash tank operative to separate gaseous refrigerant from liquid refrigerant; and a flash tank pressure 25 regulating valve for controlling flow of gaseous refrigerant from the flash tank to the interstage section.

In one embodiment, the heat transfer system is a refrigeration system (e.g refrigerator, freezer or air-conditioning system) or a heat pump.

Embodiments include, but are not limited to: a system with a single heat load connected to the low-pressure stage; a system with a first heat load connected to the low-pressure stage and a second heat load connected to the interstage section; a system with a single heat load connected to the low-pressure stage and a flash tank operative to return vapour to the interstage section; and a system with first heat load connected to the low-pressure stage, a second heat load connected to the interstage section, and a flash tank operative to return vapour to the 5 interstage section.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a single-temperature 2-stage vapour-compression refrigeration system in accordance with a first embodiment of the present invention; Figure 2 is a schematic view of a single-temperature 2-stage vapour-compression refrigeration system in accordance with a second embodiment of the present invention, Figure 3 is a schematic view of a dual-temperature 2-stage vapour-compression refrigeration system in accordance with a third embodiment of the present invention; Figure 4 is a schematic view of a dual-temperature 2-stage vapour-compression 15 refrigeration system in accordance with a fourth embodiment of the present invention; Figure 5 is a schematic view of a single-temperature 2-stage vapour-compression refrigeration system in accordance with a fifth embodiment of the present invention; Figures 6 and 7 are flow diagrams illustrating operation of vapour-compression refrigeration systems embodying the invention; and Figure 8 is a graph illustrating total interstage system volume versus rate of pressure rise.

Figure 1 shows a single-temperature 2-stage refrigeration system 100 comprising a refrigeration circuit 105 and a controller 110. Refrigeration circuit 105 includes a low-pressure (LT) compressor stage 120 comprising a plurality of ii low-pressor compressors 120A-C, an interstage section 130 including an ambient rejecter 132, a high-pressure (MT) compressor stage 140 comprising a plurality of p high-pressure compressors 140A-C, a condenser 150, a receiver 160, an expansion valve 170 and an evaporator 180 connected to a load. Low-pressure compressor stage 120 includes a suction input 122 and an output 124 connected to an inlet of an ambient rejecter 132. An outlet of the ambient rejecter 132 is connected to an inlet 142 of a high-pressure compressor stage 140. An outlet 144 of the high-pressure compressor stage 140 is connected to an inlet of condenser 150. Pressure sensor 106 and temperature sensor 107 connected to controller 110 are provided in the interstage

U

section 130 (at the inlet 142 of the high-pressure compressor stage 140) to allow controller 110 to monitor respectively the interstage pressure and interstage temperature.

Figure 2 shows a single-temperature 2-stage refrigeration system 100' based on the refrigeration system 100 of Figure 1 (features in common are labelled accordingly) in which 5 receiver 160 is replaced by a flash tank 260 connected to the outlet of the ambient rejecter 132' of the interstage section 130' via a flash tank pressure regulating valve 262 connected to controller 110'. In addition, refrigeration system 100' additionally includes a condenser pressure regulating valve 250 connected between the condenser 150 and a flash tank 260. A pressure sensor 108 connected to controller 110' is provided at flash tank 260 to allow 10 controller 110 to monitor the flash tank pressure and control operation of flash tank pressure regulating valve 262 Figure 3 shows a dual-temperature 2-stage refrigeration system 100" based on the refrigeration system 100 of Figure 1 (features in common are labelled accordingly), the dual-temperature operation being achieved with the addition of an interstage evaporator 280 15 connected in series with an evaporator pressure regulating device 282 between the outlet of the ambient rejecter 132" of the interstage section 130" and the outlet of the receiver 160'. Figure 4 shows a single-temperature 2-stage refrigeration system 100-based on the refrigeration system 100' of Figure 2 (features in common are labelled accordingly), the dual temperature operation being achieved with the addition of an interstage evaporator 380 20 connected in series with an evaporator pressure regulating device 382 between the outlet of the ambient rejecter 132-of the interstage section 130-and the outlet of the flash tank 260'.

Figure 5 shows a single-temperature 2-stage refrigeration system 100-based on the refrigeration system 100' of Figure 2 with the addition of an interstage surge drum 400 that 25 will be described in more detail below with reference to Figure 8.

Operation of an embodiment of the invention will now be described with reference to Figures 6 and 7 In order to optimise the interstage pressure, controllers 110, 110', 110", 110-' continually monitor the state of the refrigerant in the interstage section based on pressure and 30 temperature readings obtained via pressure sensors 106, 106', 106", 106-and temperature sensors 107, 107', 107 107-and calculate a temperature margin Z, which represents a

H

temperature of the refrigerant above that of the saturation temperature ("the interstage superheat").

If the actual temperature of the refrigerant in the interstage section is less than Z, then the interstage target pressure 131 is set On this embodiment) as: Pt = (Ahigh-pressure compressor stage discharge pressure / low-pressure compressor stage suction pressure))*low-pressure compressor stage suction pressure and the monitoring continues.

If the actual temperature of the refrigerant in the interstage section is not less than Z, then the controller causes a reduction in the interstage pressure Pt by an amount AP. The 10 controller then waits for a predetermined time W. After that time W, the controller tests whether the state of the refrigerant in the interstage section meets the condition: Y = user-defined pressure change (bar) + X = user-defined superheat differential (K). If this condition is not met, then the procedure described in the last-preceding paragraph is repeated. If it is met, then the controller resumes monitoring of the refrigerant, 15 as described in the last-but-one preceding paragraph.

In the case of systems 100', 100" ' and 100" " including flash tanks 260, 260', 260", a further degree of system control is achieved by controllers 110', 110m, 110' m using the method illustrated in Figure 7.

Controllers 110', 110m, 110m are configured to control flash tank pressure regulating valve 262, 262', 262" to open at either a (lower pressure) value A or a (higher pressure) value B. As illustrated in Figure 7, if the system is in a condition where: a. any compressor of the high-compression stage 140', 140", 140" " is running or b. the pressure in the flash tank 260, 260', 260" is at a pressure in excess of a threshold B, then the target flash tank pressure is set at the lower pressure value A and the controllers 110', 110" ', 110-' operate the high-pressure compressor stage 140', 140", 140-such that the flash tank pressure falls towards A at a predetermined rate C. If, however, the flash tank pressure is below B and no compressor is running, the target flash tank pressure is set by the controllers 110', HO'", 110" at a higher value B thereby providing an additional buffer that prevents (or at least delays) the need for the high-pressure compression stage 140', 140", 140" " to be switched on. Advantageously, this technique helps to reduce the frequency of compressor activation and hence improves the efficiency and reliability of the system.

N

With reference to Figure 8, when low-pressure compressors start running the mass of fluid entering the interstage section increases and this in turn increases the pressure within the interstage section. This change of pressure has two associated problems: rate of change and absolute pressure reached. A high rate of change in pressure has consequences for overall system stability and safety, including problems with high-pressure compression stage staging, flash tank valve control, and flash tank pressure regulation. If an upper absolute pressure is reached this would have the negative consequence of the system shutting down under safety control and/or pressure relief valve operation venting refrigerant.

As illustrated in Figure 8, the present applicant has identified that the volume of the interstage region (pipework and components) between the low-pressure compressor stage and the high-pressure compressor stage is crucial to suppressing this rate of pressure rise. The addition of the interstage drum 400 in system 100" " allows for this rate of change to be supressed. To achieve a suitable rate of change there is an optimum volume rate for this section of the system and a vessel can be sized to achieve the optimal balance of performance.

In system 100-the low-pressure compressor stage 120" ' comprises n fixed speed low-pressure compressors 120A-C". The interstage section 130 is sized to supress the rate of pressure rise in the interstage section caused by the switching on of the nth compressor (the nth compressor is taken to be the largest of the compressors if they have different maximum volumetric displacement values, otherwise any one of the;; compressors).

The interstage section (including the surge drum 400) is sized to define a total interior gas-receiving volume in litres that is 2.5-5 times the maximum volumetric displacement of the nth compressor in m3/hour. In other words, the interstage section defines a total interior gas-receiving volume in litres in m3 that is 0.0025-0.005 times the maximum volumetric displacement of the nth compressor in m3/hour.

Of the interstage section volume, the surge drum 400 contributes a gas storage volume that is 1-3.5 times the maximum volumetric displacement of the nth compressor in m3/hour. In other words, gas storage volume defines a volume m3 that is 0.001-0.0035 times the maximum volumetric displacement of the nth compressor in m3/hour.

In one embodiment, the surge drum 400 is a non-accumulator/non-filtering device 30 (e.g. not operative to function as a suction accumulator that collects and accumulates liquid components of the refrigerant flow).

Claims

Claims: I. A method of operating a compound heat transfer system having: a low-pressure compressor stage; a high-pressure compressor stage; an interstage section; and a controller; wherein an outlet of the low-pressure stage delivers a refrigerant output through the interstage section to an inlet of the high-pressure stage at an interstage pressure; wherein the controller operates to set a target interstage pressure and to vary operation of the system to cause the interstage pressure to approach the target interstage pressure to maximise the efficiency of the heat transfer system.
2 A method according to claim 1, wherein the controller varies operation of high-pressure compressor stage to cause the interstage pressure to approach the target interstage pressure.
3. A method according to claim 1 or claim 2, wherein the controller monitors the condition of refrigerant in the interstage section and varies the target interstage pressure in 20 response to the monitored condition.
4. A method according to any of the preceding claims, wherein the controller monitors one or both of the temperature and the pressure in the interstage section.
5. A method according to any the preceding claims, wherein: the interstage section includes a heat rejecter; and the controller monitors the condition of refrigerant leaving the heat rej ecter.
6. A method according to any of the preceding claims, wherein the controller operates such that, in the event that the monitored condition indicates that refrigerant in the interstage section is approaching saturation, it reduces the target interstage pressure.
7. A method according to any of the preceding claims, wherein: the compound heat transfer system further comprises a flash tank operative to separate gaseous refrigerant from liquid refrigerant; and a flash tank pressure regulating valve for controlling flow of gaseous refrigerant from the flash tank to the interstage section; and the controller further operates to set a target flash tank pressure at which the flash tank pressure regulating valve opens to allow flow of gaseous refrigerant from the flash tank to the interstage section, the controller being operative to alter the target flash tank pressure between a first pressure target and a second pressure target.
8. A method according to claim 7, wherein the second pressure target is a higher pressure than the first pressure target.
9. A method according to claim 8, wherein the controller is operative to switch the target flash tank pressure to the second pressure target in response to determining that one or more 15 of the high-pressure compressor stage and the low-pressure compressor stage are switched off.
A method according to claim 8 or claim 9, wherein the controller is operative to switch the target flash tank pressure to the first pressure target in response to determining 20 that one or more of the high-pressure compressor stage and the low-pressure compressor stage are switched on.
11. A method according to any of claims 8-10, wherein the controller is operative to switch the target flash tank pressure to the first pressure target in response to determining 25 that the flash tank pressure is greater than or equal to the second pressure target.
12. A method of operating a compound heat transfer system having: a low-pressure compressor stage; a high-pressure compressor stage; an interstage section; a flash tank operative to separate gaseous refrigerant from liquid refrigerant; a flash tank pressure regulating valve for controlling flow of gaseous refrigerant from the flash tank to the interstage section; and a controller; wherein an outlet of the low-pressure stage delivers a refrigerant output through the 5 interstage section to an inlet of the high-pressure stage at an interstage pressure; wherein the controller operates to set a target flash tank pressure at which the flash tank pressure regulating valve opens to allow flow of gaseous refrigerant from the flash tank to the interstage section, the controller being operative to alter the target flash tank pressure between a first pressure target and a second pressure target.
13. A method according to claim 12, wherein the second pressure target is a higher pressure than the first pressure target.
14 A method according to claim 13, wherein the controller is operative to switch the target flash tank pressure to the second pressure target in response to determining that all of the high-pressure compressor stage and the low-pressure compressor stage are switched off A method according to claim 13 or claim 14, wherein the controller is operative to switch the target flash tank pressure to the first pressure target in response to determining 20 that one or more of the high-pressure compressor stage and the low-pressure compressor stage are switched on.16. A method according to any of claims 13-15, wherein the controller is operative to switch the target flash tank pressure to the first pressure target in response to determining 25 that the flash tank pressure is greater than or equal to the second pressure target.17. A heat transfer system configured to operate in accordance with the method of any of claims 1-16.18. A computer program which, when executed by a processor, causes the processor to carry out the method of any of claims 1 -1 6.19. A computer program product carrying instructions which, when executed by a processor, causes the processor to carry out the method of any of claims 1-16.20. A compound heat transfer system comprising: a low-pressure compressor stage comprising a plurality of n low-pressure compressors; a high-pressure compressor stage; and an interstage section; wherein an outlet of the low-pressure stage is configured to deliver a refrigerant 10 output through the interstage section to an inlet of the high-pressure stage at an interstage pressure; wherein the interstage section is sized to supress the rate of pressure rise in the interstage section caused by the switching on of the nth compressor in the low-pressure compressor stage.21. A compound heat transfer system according to claim 20, wherein the interstage section defines a total interior gas-receiving volume in litres that is at least about 2 times the expected volumetric displacement of the nth compressor in m3/hour.22. A compound heat transfer system according to claim 21, wherein the interstage section defines a total interior gas-receiving volume in litres that is about 2 to about 10 times the expected volumetric displacement of the nth compressor in ml/hour.23. A compound heat transfer system according to claim 22, wherein the interstage 25 section defines a total interior gas-receiving volume in litres that is about 2 to about 7 times the expected volumetric displacement of the nth compressor in m3/hour, 24. A compound heat transfer system according to claim 23, wherein the interstage section defines a total interior gas-receiving volume in litres that is about 2.5 to about 5 times 30 the expected volumetric displacement of the nth compressor compressors in m3/hour.25. A compound heat transfer system according to any of claims 20-24, wherein the interstage section comprises a surge drum defining a gas storage volume.26. A compound heat transfer system according to claim 25, wherein the gas storage volume of the surge drum in litres is at least about 0.5 times the expected volumetric displacement of the nth compressor in Mil/hour.27. A compound heat transfer system according to claim 26, wherein the gas storage volume of the surge drum in litres is about 0.5 to about 7 times the expected volumetric 10 displacement of the nth compressor in nil/hour.28. A compound heat transfer system according to claim 27, wherein the gas storage volume of the surge drum in litres is about 0.8 to about 5 times the expected volumetric displacement of the nth compressor in m3/hour.29. A compound heat transfer system according to claim 28, wherein the gas storage volume of the surge drum in litres is about 2.5 to about 3.5 times the expected volumetric displacement of the nth compressor in m3/hour.