GB2449519A

GB2449519A - Controller for a carbon dioxide refrigeration system with compressors in a two-stage arrangement

Info

Publication number: GB2449519A
Application number: GB0804497A
Authority: GB
Inventors: Dieter Mosemann
Original assignee: Grasso GmbH Refrigeration Technology
Current assignee: GEA Refrigeration Germany GmbH
Priority date: 2007-03-21
Filing date: 2008-03-11
Publication date: 2008-11-26
Anticipated expiration: 2028-03-11
Also published as: DE102007013485A1; DE102007013485B4; GB2449519B; JP2008232613A; GB0804497D0; ITRM20080044A1

Abstract

A procedure for controlling a refrigeration system with a refrigerant flowing in a cycle, where the system comprises first and second compressors 1, 2 in a series arrangement. An intermediate-pressure connection 40 is provided between the first and second compressors in flow communication with the outlet of a first throttling device 35. The inlet of the first throttling device is arranged downstream of a gas cooler 4 which is situated downstream of the outlet of the second compressor. The inlet of a second throttling device 41 is arranged in flow communication with the intermediate-pressure connection. The outlet of the second throttling device is arranged in flow communication with an evaporator system which is in flow communication with the inlet of the first compressor. A heat exchanger 32 comprising first and second flow paths is provided in the flow path from the gas cooler. The refrigeration capacity is controlled by varying, in combination or singly: the subcooling upstream of the evaporator, the superheat at the outlet of the heat exchanger, the pressure at the intermediate pressure connection and/or the pressure downstream of the second compressor. Preferably, the first and second compressors are oil-flooded screw compressors, each comprising an economiser connection 25, 26.

Description

Controller for a Refrigeration System with Two-stage Compression The

invention relates to a procedure for controlling a two-stage C02 refrigeration system with oil-flooded screw compressors arranged directly in flow direction one after another for two-stage compression.

According to prior art for the purpose of capacity control, both screw compressors are fitted with means for changing the flow rate, e.g. a control slide each, partly arranged in the housing section enclosing the rotors and requiring further hydraulic and control elements for their activation, or they are controlled by changing the drive speed.

A disadvantage of this solution is the expenditure required. The object of the invention is to eliminate said disadvantages and to generate an efficient and cost-effective control.

According to the present invention, the screw compressors of both pressure stages preferably have an intermediate-pressure port each, the so-called economizer connection, adjoining the rotor interlobes in the housing in such a manner that there is neither a flow connection to the suction side nor to the discharge side as long as the economizer connection is connected to the interlobes. Preferably, the compressor of the upper pressure stage is designed as described in the German publicly distributed printed copy of the application papers (Offenlegungsschrift) OS 10334947.2. The arrangement of an intermediate-pressure connection in the connecting line between the compressors of both pressure stages and the existence of economizer connections on the compressors render possible a four-stage expansion of the refrigerant between gas cooler and evaporator as described in German patent application AZ: 10 2005 018 602.5.

The flash gas cited in German patent application AZ: 10 2007 003 989.3 is delivered to the intermediate-pressure connection and to the economizer connections on the compressors preferably via a controllable valve means, while the liquid is expanded via a lower" throttling point into the lower auxiliary liquid separator where flash gas and liquid are separated from each other.

Thence the liquid gets via the "lowermost" throttling point into the evaporator-liquid separator system from which the flash gas together with the refrigerant vapour generated due to heat absorption in the evaporators will be delivered to the suction side of the compressor of the lower pressure stage.

In a second solution according to the cited patent application AZ: 10 2007 003 989.3 the refrigerant flow delivered to the evaporator system will be expanded in one stage after having passed several subcooling sections. For cooling the refrigerant flow in an "upper" auxiliary liquid subcooler, a "middle" liquid subcooler and a "lower" auxiliary liquid subcooler arranged in a flow path for the main refrigerant flow, a partial flow of the refrigerant liquid is branched off ahead of the corresponding liquid subcooler section and evaporated in the corresponding liquid subcooler. Said refrigerant vapour will be admitted to the corresponding economizer connection of the compressor of the upper stage, to the intermediate-pressure connection between the compressors of the lower and upper pressure stages, and to the economizer connection of the compressor of the lower pressure stage.

To begin with, the compressor of the lower pressure stage of the two-stage compressor unit has known means, e.g. a control slide, which depending on its relative position to the rotors shortens the effective length of the rotors taking part in compression, or means for controlling the speed, to keep an operating parameter on the consumer side, e.g. the pressure at the evaporator outlet or on the suction side of the lower stage, near a specified value, e.g. to keep constant the evaporating temperature, and hence to control the temperature on the "consumer side". This is the purpose of capacity control of the lower pressure stage.

Due to change of the swept volume of the lower pressure stage, the pressure after the lower pressure stage will change.

For this reason, the refrigerating capacity of both versions according to the invention will be controlled by combining several functions: a) By the degree of subcooling the main flow in the liquid subcoolers arranged one after another along its flow path.

To this end, liquid subcoolers are cut off one after another. Stop valves in the refrigerant supply lines are closed to reduce the capacity or opened to increase the capacity.

b) By changing the superheat at the intercooler outlets.

The superheat will be increased or decreased at the intercooler outlets depending on the parameter to be controlled. To this end, valves at the intercooler outlets will be opened or closed modulating to change the degree of subcooling.

c) By changing the intermediate pressure.

Ahead of the economizer connections of both pressure stages, regulating valves are arranged preferably to control the volume flow to the economizer connection, and hence the refrigerating capacity of the refrigeration system. With this control mode according to the invention, the valves arranged between intercooler outlet and the corresponding intermediate-pressure feed into the economizer connections or into the intermediate-pressure connection will be opened or closed modulating.

d) By changing the discharge pressure of the upper pressure stage.

This arrangement for controlling the refrigerating capacity makes use of the fact that the refrigerating capacity of the upper pressure stage on cooling the refrigerant vapour in a gas cooler above the critical point will change together with the temperature of C02 at the gas cooler outlet and the discharge temperature of the upper stage. The refrigerating capacity will be varied by changing the discharge pressure of the upper stage by means of a throttle valve arranged downstream after the gas cooler. For the purpose of reducing the refrigerating capacity said pressure will be lowered by opening said valve, and for increasing the refrigerating capacity up to the optimum between refrigerating capacity and input power it will be closed further, and hence the discharge pressure will be increased. The optimum will be determined by a separate control algorithm independent of the part-load control.

The refrigerating capacity of the refrigeration system can be controlled over a wide range by combining the described control methods a) through d) or by using them singly on a compressor in the upper pressure stage without separate bypass valve means or speed control.

According to a first aspect of the invention there is provided a controller for controlling a two-stage refrigeration system, the refrigeration system comprises a refrigerant flowing in a cycle, the refrigeration system further comprises a first compressor and a second compressor in a series arrangement, the second compressor being arranged after the first compressor in the flow direction; wherein, an intermediate-pressure connection is provided between the first and second compressors such that the outlet of a first throttling device is in flow communication with both the outlet of the first compressor and the inlet of the second compressor, and where the inlet of the first throttling device is arranged, via piping, downstream of a gas cooler and the gas cooler is downstream of the outlet of the second compressor; and wherein, the inlet of a second throttling device is in flow communication with both the outlet of the first compressor and the inlet of the second compressor via the intermediate-pressure connection, and the outlet of the second throttling device is in flow communication with an evaporator system, which is in turn in flow communication with the inlet of the first compressor; the refrigeration system further comprises a first heat exchanger wherein the first heat exchanger is provided in the flow path from the gas cooler to the second throttling device and the intemiediate-pressure connection, the first heat exchanger compnsing, first and second flow paths in thermal communication with each other, the first flow path providing a flow path from the gas cooler to the second throttling device, the second flow path providing a flow path from the first throttling device to the intermediate-pressure connection, such that flow from the gas cooler divides and flows into both the inlet of the first throttling device and the inlet of the first flow path within the first heat exchanger; wherein, the controller controls the refrigeration capacity of the refrigeration system by varying in any combination or singly: (a) the subcooling of the flow upstream of the evaporator system; (b) the superheat of the flow at the outlet of the second flow path of the first heat exchanger; (c) the pressure of the flow at the intermediate-pressure connection; and/or, (d) the pressure of the flow downstream of the second compressor.

The first compressor may be a screw compressor comprising an economiser connection, and wherein a third throttling device may be provided in a flow path from the gas cooler via the first flow path of the first heat exchanger to the economiser connection of the first compressor; wherein a second heat exchanger may be provided in the flow path from the gas cooler to the second throttling device and the economiser connection of the first compressor, the second heat exchanger may comprise first and second flow paths in thermal communication with each other, the first flow path may provide a flow path from the outlet of the first flow path of the first heat exchanger to the second throttling device, and the second flow path may provide a flow path from the third throttling device to the economiser connection of the first compressor, such that flow from the outlet of the first flow path of the first heat exchanger may divide and flow into both the inlet of the third throttling device and the inlet of the first flow path within the second heat exchanger.

The second compressor may be a screw compressor comprising an economiser connection, and wherein a fourth throttling device may be provided in a flow path from the gas cooler to the economiser connection of the second compressor; wherein a third heat exchanger may be provided in the flow path from the gas cooler to the first throttling device and the economiser connection of the second compressor, the third heat exchanger may comprise first and second flow paths in thermal communication with each other, the first flow path may provide a flow path from the gas cooler to the first throttling device, and the second flow path may provide a flow path from the fourth throttling device to the economiser connection of the second compressor, such that flow from the gas cooler may divide and flow into both the inlet of the fourth throttling device and the inlet of the first flow path within the third heat exchanger.

A first controllable valve may be provided in the flow path from the outlet of the second flow path of the first heat exchanger to the intermediate pressure connection. A second controllable valve may be provided in the flow path from the outlet of the second flow path of the second heat exchanger to the economiser connection of the first compressor. A third controllable valve may be provided in the flow path from the outlet of the second flow path of the third heat exchanger to the economiser connection of the second compressor.

The first throttling device may comprise a controllable throttle such that the flow rate therethrough can be controlled. The second throttling device may comprise a controllable throttle such that the flow rate therethrough can be controlled. The third throttling device may comprise a controllable throttle such that the flow rate therethrough can be controlled. The fourth throttling device may comprise a controllable throttle such that the flow rate therethrough can be controlled.

The controller may change the refrigeration capacity of the refrigeration system by controlling the flow rate through the first controllable valve to vary the superheat of flow at the outlet of the second flow path of the first heat exchanger.

The controller may change the refrigeration capacity of the refrigeration system by controlling the flow rate through the second controllable valve to vary the superheat of flow at the outlet of the second flow path of the second heat exchanger.

The controller may change the refrigeration capacity of the refrigeration system by controlling the flow rate through the third controllable valve to vary the superheat of flow at the outlet of the second flow path of the third heat exchanger.

The controller may change the refrigeration capacity of the refrigeration system by first controlling the second controllable valve, then the third controllable valve, followed by the first controllable valve.

The controller may change the refrigeration capacity of the refrigeration system by controlling the flow rate through one or more of the throttling devices to vary the subcooling of the flow upstream of the evaporator system.

The controller may first control the flow rate through the third throttling device, then the fourth throttling device, followed by the first throttling device.

The controller may change the refrigeration capacity of the refrigeration system by controlling any one of the first, second or third controllable valves to vary the pressure of the flow at the intermediate-pressure connection.

The controller may change the refrigeration capacity of the refrigeration system by controlling the flow rate through the second throttling device to vary the pressure of the flow downstream of the second compressor.

The refrigerant may be CO2. The first or second compressor may be an oil-flooded screw compressor.

A pressure sensor may be arranged at the discharge connection after the second compressor and a temperature sensor may be arranged downstream of the outlet from the gas cooler.

The pressure of the flow downstream of the second compressor may be decreased by the controller to reduce the refrigerating capacity and may be raised by the controller to increase the refrigerating capacity up to an optimum between maximum refrigerating capacity and maximum input power, and wherein a control algorithm for the optimum pressure may be provided where the control algorithm may be independent of the part-load control. The optimum pressure control algorithm may be a function of the temperature at the gas cooler outlet and may be independent of the part-load control.

According to a second aspect of the invention there is provided control of a two-stage C02 refrigeration system with a two-stage screw compressor unit having oil-flooded screw compressors arranged directly in flow direction one after another, and both screw compressors of the lower and upper pressure stages have an intermediate-pressure port each, the so-called economizer connection, adjoining the rotor interlobes in the housing in such a manner that there is neither a flow connection to the suction side nor to the discharge side as long as the economizer connection is connected to the interlobes, and in the connecting line between the screw compressors of both pressure stages an intermediate-pressure connection is provided, and between the outlet of C02 from the gas cooler and the inlet of C02 into the evaporator system a primary flow path with throttling means is provided for the main refrigerant flow between the gas cooler outlet and the evaporator system, in which at least three heat exchangers are arranged in series with relation to said primary flow path, with the heat exchangers designed as "upper auxiliary subcooler, "middle" subcooler and "lower" auxiliary subcooler, and on one side of the heat-exchanging surfaces of the subcoolers portions of said primary flow path adjoin, and on the other side of the heat-exchanging surfaces portions of secondary flow paths adjoin forming part of a flow connection from the primary flow path via throttle valves, subcoolers and piping to the economizer connection of the screw compressor of the upper stage, to the intermediate-pressure connection between the screw compressors of the lower and upper stages and to the economizer connection of the screw compressor of the lower stage, comprising piping and controllable valves, wherein the four functions a) through d) can be found in combination or singly: (a) by the degree of subcooling the main flow in the subcoolers for which the partial refrigerant flow through the subcoolers will be increased or decreased by modulating opening or closing the throttle valves, or the "upper auxiliary subcooler, the "middle" subcooler and/or the "lower" auxiliary subcooter will be cut off one after another, (b) by the degree of superheating to be adjusted at the sensing elements of controllable valves at the subcooler outlets, (c) by changing the intermediate pressure in the secondary flow paths by modulating opening or closing control valves arranged after the corresponding subcooler, (d) by changing the discharge pressure of the upper stage, for which by means of a throttle valve arranged downstream after the gas cooler said pressure will be decreased to reduce the refrigerating capacity and be raised to increase the refrigerating capacity up to the optimum between refrigerating capacity and input power, and for the optimum pressure a control algorithm is provided independent of the part-load control.

A pressure sensor may be arranged after the compressor of the upper stage at the discharge connection, and a temperature sensor may be arranged downstream at the outlet of C02 from the gas cooler.

For the optimum pressure a control algorithm may be provided as a function of the temperature at the gas cooler outlet independent of the part-load control.

According to a third aspect of the invention there is provided a controller for a refrigeration system, the refrigeration system comprising a first compressor and a second compressor arranged in series with an intermediate-pressure connection therebetween, wherein the controller controls the refrigeration capacity of the refrigeration system by varying in any combination or singly: (a) the subcooling of flow upstream of an evaporator system; (b) the superheat of flow at the outlet of a heat exchanger arranged between a throttling device and the intermediate-pressure connection; (c) the pressure of the flow at the intermediate-pressure connection; and/or, (d) the pressure of the flow downstream of the second compressor.

For the purpose of changing the swept volume to match the vapour volume flow from the evaporator, the compressor of the lower pressure stage features known means for capacity control, e.g. a control slide or a drive with variable speed, with the control slide forming part of the cylindrical wall portions of the housing enclosing the rotors and opening a bypass in the housing for changing the swept volume of the compressor through which a portion of the drawn in volume is returned to the compressor suction side. Thus, the mass flow delivered by the compressor is changed, which multiplied by the enthalpy difference, gives the refrigerating capacity.

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 piping schematic of a refrigeration system according to a first embodiment of the invention with a two-stage screw compressor unit with fourfold expansion.

The individual control methods of the two-stage C02 refrigeration system with a two-stage screw compressor unit are described in the following. The reference numerals used refer to the figure.

The screw compressor 1 of the lower and the screw compressor 2 of the upper pressure stage are arranged downstream in series. They feature an economizer connection 25, 26 each. In the connecting line 13 between the screw compressors 1, 2 an intermediate-pressure connection 40 is provided. Between the outlet of C02 from the gas cooler 4 and the inlet into the evaporator system with the liquid separator 8, the refrigerant-leading connecting pipe 30 is arranged forming the flow path for the main reFrigerant flow between the outlet of the gas cooler 4 and the evaporator system with throttling means 41 and having partial surfaces of the heat exchangers 31, 32, 33. With reference to said flow path, the three heat exchangers 31, 32, 33 are arranged in series with the heat exchanger 31 designed as "upper" auxihary subcooler, the heat exchanger 32 as "middle" subcooler, and the heat exchanger 33 as "lower" auxiliary subcooler. On one side of the heat-exchanging surfaces of the heat exchangers 31, 32, 33, there are portions of the flow path as already mentioned, and liquid branches from the flow path via throttle valves 34, 35, 36 to the other side of the corresponding heat-exchanging surfaces. The outlets on this side of the heat exchangers 31, 32, 33 are connected to the economizer connections 25, 26 of the screw compressors 1, 2 and to the intermediate-pressure connection 40 between the screw compressors 1, 2 of the lower and upper stages.

The four functions a) through d) described will be explained below in their combination or individually.

a) The degree of subcooling the main flow in the subcoolers is changed. To this end, the partial refrigerant flow through the liquid subcoolers 31, 32, 33 will be increased or decreased by modulating opening or closing the throttle valves 34, 35, 36. Thus, the degree of cooling the working fluid prior to entering the liquid separator will change, and hence the volumetric refrigerating capacity.

b) The throttling points 34, 35 and 36 are designed, for example, as electrically operated throttle valves the superheat of which is variable at the refrigerant outlet. The degree of superheat to be adjusted at the sensing elements of said valves at the outlets of the heat exchangers 31, 32, 33 depending on the parameter to be controlled, e.g. the pressure in the liquid separator, will increase or decrease the refrigerating capacity.

c) For the purpose of changing the intermediate pressure at the connecting points 25, 26, 40, pressure regulating valves 42, 43, 44 are arranged to change the volume flow to the economizer connections 25, 26 and to the connecting point 40 between both compressors 1, 2, and hence control the refrigerating capacity of the refrigeration system, for which the valves 42, 43, 44 arranged between the outlets of the intercoolers 31, 32, 33 and the corresponding intermediate-pressure feed into the economizer connections or into the intermediate-pressure connection are opened or closed modulating.

d) Changing the discharge pressure of the upper stage of the screw compressor 2 by means of the throttle valve 45 arranged downstream after the gas cooler 4 will influence the refrigerating capacity. The pressure will be decreased to reduce the refrigerating capacity, and increased to raise the refrigerating capacity up to the optimum between refrigerating capacity and input power. The maximum pressure will be limited by a control algorithm as function of the temperature of C02 leaving the gas cooler. The pressure is optimal to make best use of the energy.

Claims

Claims: 1. A controller for controlling a two-stage refrigeration

system, the refrigeration system comprises a refrigerant flowing in a cycle, the refrigeration system further comprises a first compressor and a second compressor in a series arrangement1 the second compressor being arranged after the first compressor in the flow direction; wherein, an intermediate-pressure connection is provided between the first and second compressors such that the outlet of a first throttling device is in flow communication with both the outlet of the first compressor and the inlet of the second compressor, and where the inlet of the first throttling device is arranged, via piping, downstream of a gas cooler and the gas cooler is downstream of the outlet of the second compressor; and wherein.

the jnlet of a second throttling device is in flow communication with both the outlet of the first compressor and the inlet of the second compressor via the intermediate-pressure connection, and the outlet of the second throttling device is in flow communication with an evaporator system, which is in turn in flow communication with the inlet of the first compressor; the refrigeration system further comprises a first heat exchanger wherein the first heat exchanger is provided in the flow path from the gas cooler to the second throttling device and the intermediate-pressure connection, the first heat exchanger comprising, first and second flow paths in thermal communication with each other, the first flow path providing a flow path from the gas cooler to the second throttling device, the second flow path providing a flow path from the first throttling device to the intermediate-pressure connection, such that flow from the gas cooler divides and flows into both the inlet of the first throttling device and the inlet of the first flow path within the first heat exchanger; wherein, the controller controls the refrigeration capacity of the refrigeration system by varying in any combination or singly: (a) the subcooling of the flow upstream of the evaporator system; (b) the superheat of the flow at the outlet of the second flow path of the first heat exchanger; (c) the pressure of the flow at the intermediate-pressure connection; and/or, (d) the pressure of the flow downstream of the second compressor.
2. The controller as claimed in claim 1, wherein the first compressor is a screw compressor comprising an economiser connection, and wherein a third throttling device is provided in a flow path from the gas cooler via the first flow path of the first heat exchanger to the economiser connection of the first compressor; wherein a second heat exchanger is provided in the flow path from the gas cooler to the second throttling device and the economiser connection of the first compressor, the second heat exchanger comprising first and second flow paths in thermal communication with each other, the first flow path providing a flow path from the outlet of the first flow path of the first heat exchanger to the second throttling device, and the second flow path providing a flow path from the third throttling device to the economiser connection of the first compressor, such that flow from the outlet of the first flow path of the first heat exchanger divides and flows into both the inlet of the third throttling device and the inlet of the first flow path within the second heat exchanger.
3. The controller as claimed in claim I or 2, wherein the second compressor is a screw compressor comprising an economiser connection, and wherein a fourth throttling device is provided in a flow path from the gas cooler to the economiser connection of the second compressor; wherein a third heat exchanger is provided in the flow path from the gas cooler to the first throttling device and the economiser connection of the second compressor, the third heat exchanger comprising first and second flow paths in thermal communication with each other, the first flow path providing a flow path from the gas cooler to the first throttling device, and the second flow path providing a flow path from the fourth throttling device to the economiser connection of the second compressor, such that flow from the gas cooler divides and flows into both the inlet of the fourth throttling device and the inlet of the first flow path within the third heat exchanger.
4. The controller accordiAg to any preceding claim, wherein a first controllable valve is provided in the flow path from the outlet of the second flow path of the first heat exchanger to the intermediate pressure connection.
5. The controller according to any one of claims 2 to 4, when dependent on claim 2, wherein a second controllable valve is provided in the flow path from the outlet of the second flow path of the second heat exchanger to the economiser connection of the first compressor.
6. The controller according to any one of claims 3 to 5, when dependent on claim 3, wherein a third controllable valve is provided in the flow path from the outlet of the second flow path of the third heat exchanger to the economiser connection of the second compressor.
7. The controller according to any preceding claim, wherein the first throttling device comprises a controllable throttle such that the flow rate therethrough can be controlled.
8. The controller according to any preceding claim, wherein the second throttling device comprises a controllable throttle such that the flow rate therethrough can be controlled.
9. The controller according to any one of claims 2 to 8, when dependent on claim 2, wherein the third throttling device comprises a controllable throttle such that the flow rate therethrough can be controlled.
10. The controller according to any one of claims 3 to 9, when dependent on claim 3, wherein the fourth throttling device comprises a controllable throttle such that the flow rate therethrough can be controlled.
11. The controller according to any one of claims 4 to 10, when dependent on claim 4, wherein the controller changes the refrigeration capacity of the refrigeration system by controlling the flow rate through the first controllable valve to vary the superheat of flow at the outlet of the second flow path of the first heat exchanger.
12. The controller according to any one of claims 5 to 11, when dependent on claim 5, wherein the controller changes the refrigeration capacity of the refrigeration system by controlling the flow rate through the second controllable valve to vary the superheat of flow at the outlet of the second flow path of the second heat exchanger.
13. The controller according to any one of claims 6 to 12, when dependent on claim 6, wherein the controller changes the refrigeration capacity of the refrigeration system by controlling the flow rate through the third controllable valve to vary the superheat of flow at the outlet of the second flow path of the third heat exchanger.
14. The controller according to claims 11, 12 and 13, wherein the controller changes the refrigeration capacity of the refrigeration system by first controlling the second controllable valve, then the third controllable valve, followed by the first controllable valve.
15. The controller according to any one of claims 7 to 10, wherein the controller changes the refrigeration capacity of the refrigeration system by controlling the flow rate through one or more of the throttling devices to vary the subcooling of the flow upstream of the evaporator system.
16. The controller according to claim 15, wherein the controller first controls the flow rate through the third throttling device, then the fourth throttling device, followed by the first throttling device.
17. The controller according to any one of claims 4 to 6, wherein the controller changes the refrigeration capacity of the refrigeration system by controlling any one of the first, second or third controllable valves to vary the pressure of the flow at the intermediate-pressure connection.
18. The controller according to any one of claims 8 to 17, when dependent on claim 8, wherein the controller changes the refrigeration capacity of the refrigeration system by controlling the flow rate through the second throtthng device to vary the pressure of the flow downstream of the second compressor.
19. The refrigeration system according to any preceding claim, wherein the refrigerant is CO2.
20. The refrigeration system according to any one of claims 2 to 19 when dependent on claim 2 or 3, wherein said compressor is an oil-flooded screw compressor.
21. The controller as claimed in any preceding claim, wherein a pressure sensor is arranged at the discharge connection after the second compressor, and a temperature sensor is arranged downstream of the outlet from the gas cooler.
22. The controller as claimed in any preceding claim, wherein the pressure of the flow downstream of the second compressor is decreased by the controller to reduce the refrigerating capacity and is raised by the controller to increase the refrigerating capacity up to an optimum between maximum refrigerating capacity and maximum input power, and wherein a control algorithm for the optimum pressure is provided where the control algorithm is independent of the part-load control.
23. The controller as claimed in claim 22 wherein the optimum pressure control algorithm is a function of the temperature at the gas cooler outlet and is independent of the part-load control.
24. A controller, substantially as described herein with reference to and as shown in the accompanying drawing.