GB2438794A

GB2438794A - Refrigeration plant for transcritical operation with an economiser

Info

Publication number: GB2438794A
Application number: GB0718655A
Authority: GB
Inventors: Dieter Mosemann; Dmytro Zaytsev
Original assignee: Grasso GmbH Refrigeration Technology
Current assignee: GEA Refrigeration Germany GmbH
Priority date: 2005-03-03
Filing date: 2005-03-03
Publication date: 2007-12-05
Anticipated expiration: 2025-03-03
Also published as: EP1893924A1; JP5090932B2; GB2438794B; WO2006092108A1; GB0718655D0; JP2008531969A

Abstract

The invention relates to a device on a compressor for application in refrigeration plants the compression pressure of which lies within the supercritical range for a refrigerant, for example, CO2. Conventionally on generation of refrigerant effect with a refrigeration process with a supercritical high pressure side, the flash gas component is high on depressurisation in an expansion machine and also in a throttled device. The ratio of refrigeration capacity to drive power, the COP, is correspondingly small. The energy requirement for refrigeration is unacceptably high. In an other embodiment a two-stage depressurisation is thus used, whereby a first flash gas component and a first liquid component are present at raised pressure. The first flash gas component is further compressed to the pressure of the high pressure side and the first liquid is depressurised to the pressure in the liquid separator, whereby the ratio of liquid component and flash gas component is significantly increased. A disadvantage is the requirement for a second compressor with an independent drive.

Description

Berlin, 24.02.2005 Applicant: Grasso GmbH Refrigeration Technology

Inventors: Dr.-lng. Dieter Mosemann Dr.-lng. Dmytro Zaytsev Refrigeration System for Transcritical Operation with Economizer This invention relates to 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. The pressure levels comprise the suction pressure prevailing on the compressor suction side and being close to the pressure in the evaporator, the intermediate pressure prevailing at the economizer connection, and the discharge pressure acting on the compressor discharge side and being close to the pressure in a gas cooler. The pertinent 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 transcritical or overcritical refrigeration process.

In refrigeration systems of this kind the compressor draws in working fluid evaporating in the evaporator and being under suction pressure and compresses it to the discharge pressure, the pressure on the high-pressure side. The working fluid (refrigerant) is cooled in a gas cooler and is expanded either in an expansion machine by giving off mechanical work or in a throttling device to the pressure in a liquid separator. The pressure lies below the pressure at the critical point of the working fluid. For this reason, liquid as well as vapour (flash gas) are formed which are separated from each other in the liquid separator. Due to the pressure reduction the temperature of the working fluid drops. The liquid evaporates due to heat input. The amount of heat required for this purpose is called refrigerating capacity both in the refrigeration and air conditioning technology.

The higher the liquid portion, the higher the refrigerating capacity.

The invention relates to an arrangement on a compressor for use in refrigeration systems the discharge pressure of which lies in the transcritical range of the refrigerant, e.g. C02.

According to prior art, during generation of cold by a refrigeration process with transcritical high-pressure side, the flash- gas portion will be very high during expansion in an expansion machine as well as in a throttling device. The remaining amount of liquid in relation to the mass flow delivered by the compressor is comparatively small. The relation between refrigerating capacity and power requirement, the COP, will be correspondingly small. The power requirement for generation of cold is unacceptably high. Therefore, due to another embodiment two-stage expansion is used, wherein a first flash-gas portion and a first liquid portion at higher pressure are formed. The first flash-gas portion is re-compressed by a second compressor to the pressure on the high-pressure side, and the first liquid is expanded to the pressure in the liquid separator with the ratio of liquid portion to flash-gas portion being remarkably increased. A disadvantage is the need for a second compressor with its own drive. The cost of such system will increase, and the operation of such system will become more complicated in comparison to a one-machine system, as the time sequence for starting and stopping the compressors as well as the swept volumes of both compressors must be adapted to each other, and hence controlled. Another known technical solution the so-called economizer coupling with a screw compressor used in a subcritical refrigeration cycle wherein the pressure on the high-pressure side of the compressor lies below the pressure at the critical point cannot be realized with known screw compressors, as there will be an intermediate pressure in the transcritical refrigeration cycle lying above the pressure at the critical point. Thus, during expansion from high pressure to intermediate pressure in the first stage there will arise no liquid which could be expanded further.

in known compressors with economizer connection port, the supply into this port, the superfeeding, starts at the earliest after the interlobe spaces have gained their maximum geometric volume, and due to the continual rotation of the rotors have no more connection to the inlet port arranged in the compressor housing. The chamber volume available will not be sufficient to accommodate the flash-gas portion at a pressure smaller than the pressure at the critical point of the refrigerant.

The flash-gas portion at intermediate pressure at the economizer connection should be smaller than the pressure at the critical point of the refrigerant.

The object of the invention is to realize a technical solution requiring but one compressor which can be operated with two-stage expansion and economizer. In this case the suction volume flow passes the suction side of the compressor at suction pressure. The superfeeding volume flow passes a second suction port at intermediate pressure being higher than the suction pressure and smaller than the pressure at the critical point of the refrigerant.

According to the features of the invention the refrigerant vapour leaving the gas cooler passes an internal heat exchanger prior to being expanded from high-pressure level to intermediate pressure level, and hence prior to entering the economizer connection on the compressor. In doing so, the refrigerant vapour being under high pressure is cooled on one side of the heat-exchanging surfaces of the heat exchanger, while the refrigerant vapour after leaving the evaporator is superheated on the other side of the heat-exchanging surfaces of the internal heat exchanger prior to being drawn in and compressed by the compressor.

Due to further cooling of the refrigerant vapour after leaving the gas cooler, there will arise less flash gas during expansion to the intermediate pressure level. Thus, a screw compressor of known construction will enable to realize intermediate pressures at the economizer connection lying below the critical point of the refrigerant, and the economic efficiency of the refrigeration cycle will be increased remarkably as compared to known processes.

The inlet process into a compression space, an interlobe space of a screw rotor under consideration, is terminated at the moment when there is no more flow connection of said interlobe space to the suction side of the compressor. In this phase, the geometric interlobe volume of the interlobe space considered approximately reached its maximum. Advantageously, due to further rotation of the rotor there will arise a flow connection of the interlobe space considered shortly before, at this moment or slightly later to the economizer connection port located in the housing of the rotors.

Depending on the wrap angle of the rotor profile at the male rotor, number of teeth of both rotors, the geometric interlobe volume of the interlobe space considered can be constant (transfer phase) or can decrease due to rotation of rotors.

According to a feature of the invention the refrigeration system for transcritical operation comprises at least the following components: a gas cooler, an internal heat exchanger, an intermediate pressure vessel, an evaporator, a compressor having geometrically controlled inlet and outlet ports, e.g. a screw compressor or a scroll compressor, throttling devices and interconnecting piping between the components mentioned. When the compressor is in operation, suction pressure prevails on its suction side, while discharge pressure prevails on its discharge side with the pressure on the discharge side being higher than the pressure at the critical point of the refrigerant. The compressor has an economizer connection port at the housing enabling a flow connection to the intermediate pressure vessel the pressure of which lies between discharge pressure and suction pressure. Upstream of the economizer connection at the compressor, there is arranged the internal heat exchanger with two flow paths featuring heat-exchanging surfaces, wherein one flow path is arranged downstream between gas cooler outlet and throttling point before the intermediate pressure vessel designed as liquid separator, while the other flow path is arranged downstream between evaporator outlet and suction side of the compressor. Due to the differing temperature levels, the refrigerant vapour leaving the evaporator is warmed up, while the refrigerant vapour leaving the gas cooler is cooled further in the internal heat exchanger. As a result of this cooling-down, the flash-gas portion is reduced during the subsequent expansion to intermediate pressure level in the intermediate pressure vessel. The relative liquid content of the expanded refrigerant is also increased. Less flash gas enables the intermediate pressure at the economizer connection of the compressor to substantially drop below the pressure at the critical point of the refrigerant, thus permitting the use of compressors of existing construction for this type of refrigeration system, and the operation of refrigeration systems with increased economic efficiency.

According to another feature of the invention the refrigeration system for transcritical operation comprises at least the following components: a gas cooler, a first and a second internal heat exchanger, an evaporator, a compressor having geometrically controlled inlet and outlet ports, e.g. a screw compressor or a scroll compressor, throttling devices and interconnecting piping between the components mentioned. When the refrigeration system is in operation, suction pressure prevails on the compressor suction side, while discharge pressure prevails on its discharge side with the pressure on the discharge side being higher than the pressure at the critical point of the refrigerant. The compressor has an economizer connection port at the housing enabling a flow connection to the first internal heat exchanger of the refrigeration or air conditioning system with the former having two flow paths featuring heat-exchanging surfaces, wherein one flow path relates to the high-pressure refrigerant flow from the gas cooler via the second internal heat exchanger to the evaporator, while the other flow path relates to a partial flow of high-pressure refrigerant passing a throttling device for expansion of the high-pressure refrigerant to intermediate pressure level, entering the first internal heat exchanger and cooling downstream the high-pressure refrigerant vapour of the other flow path. Upstream of the compressor suction side, there is arranged the second internal heat exchanger with two flow paths featuring heat-exchanging surfaces, wherein one flow path is located downstream between the liquid separator with the lowest pressure level of the refrigeration system and the compressor suction side, while the other flow path is located downstream between gas cooler outlet and throttling point upstream of the first internal heat exchanger.

Similarly, the same technical features apply to other types of compressors and compressors having geometrically controlled inlet and outlet ports.

Due to the technical solutions according to the invention, the mass flow circulating through the evaporator is reduced, as the superheat on the suction side increases, the flash-gas portion after expansion to intermediate pressure is reduced enabling the intermediate pressure of the first expansion stage of the two-stage expansion to be lowered so that the intermediate pressure lies substantially below the pressure at the critical point of the refrigerant.

Examples of embodiment Figure 1 shows a Ph diagram for a refrigeration or air conditioning system according to the invention.

Figure 2 shows a simplified schematic for arrangement of compressor and heat exchangers with pertinent interconnecting piping and control devices.

Figure 3 shows the Ph diagram for a refrigeration or air conditioning system according to the invention.

Figure 4 shows a simplified schematic for arrangement of compressors and heat exchangers with pertinent interconnecting piping and control devices for another refrigeration system according to the invention.

In the Ph diagram according to Figure 1, point 1 describes the condition at the evaporator outlet. The inlet condition of the refrigerant upstream of the compressor, point 2, is the outlet condition of the refrigerant after passing the internal heat exchanger. The refrigerant is superheated comparatively high, the mass flow drawn in by the compressor is reduced.

The chamber volume, e.g. the interlobe volume considered of a screw compressor, has its maximum at this point. With this maximum chamber size, the suction process is terminated, and there starts arising a flow connection to the compression chamber via the economizer connection port. Due to the entering flash-gas portion, the pressure in the compression chamber rises to the intermediate pressure Pz. As a result of mixing the colder flash gas, point 3, with the refrigerant already drawn in, point 2, the latter is compressed and cooled. The mixing process is terminated at point 4. At point 4, the economizer connection port also closes, and compression of the suction gas and the flash-gas portion to discharge pressure starts at point 5. The refrigerant passes a gas cooler which is fed with a cooling medium, e.g. cooling water, for cooling the refrigerant vapour. When leaving said gas cooler, the refrigerant has the condition at point 6. In a second heat exchanger, the internal heat exchanger through which two refrigerant flows of the refrigeration system are led, the refrigerant is cooled from point 6 to point 7. In doing so, the other refrigerant flow led through the internal heat exchanger for cooling from point 6 to point 7, is warmed from point 1 to point 2.

The refrigerant flow cooled so far is expanded from point 7 to point 8 to intermediate pressure. The refrigerant vapour is divided into a relatively small flash-gas portion, point 3, and into a relatively high liquid portion, point 9. Due to the additional cooling according to the invention, the flash-gas portion was reduced so that the intermediate pressure at the economizer connection is distant enough from the critical point. Two advantages are achieved as a result: -Higher economic efficiency of the refrigeration system by increasing the refrigerating capacity -lncrase of the difference in density between liquid and flash gas (separation of both phases in an intermediate pressure separator).

The refrigeration system for transcritical operation according to Figure 2 comprises a gas cooler 13, an internal heat exchanger 14, an intermediate pressure vessel 12 designed as liquid separator for separating flash gas and liquid, an evaporator system with heat exchanger 18 and liquid separator 17, a screw compressor 11 having geometrically controlled inlet and outlet ports, throttling devices 15, 16 and interconnecting piping between the components mentioned. When the screw compressor 11 is in operation, suction pressure prevails on its suction side 24, while discharge pressure prevails on its discharge side 25 with the pressure on the discharge side 25 being higher than the pressure at the critical point of the refrigerant. The screw compressor 11 has an economizer connection port 21 at the housing enabling a flow connection to the intermediate pressure vessel 12 the pressure of which lies between discharge pressure and suction pressure. Upstream of the suction side 24 at the screw compressor 11, there is arranged the internal heat exchanger 14 with two flow paths featuring heat-exchanging surfaces, wherein one flow path 10 is arranged downstream between gas cooler outlet 13 and throttling device 15 before the intermediate pressure vessel 12 designed as liquid separator, while the other flow path 23 is arranged downstream between evaporator outlet and suction side 24 of the screw compressor 11. Due to the differing temperature levels, the refrigerant vapour leaving the liquid separator 17 is warmed up, while the refrigerant vapour leaving the gas cooler 13 is cooled further in the internal heat exchanger. As a result of this cooling-down, the flash-gas portion is reduced during the subsequent expansion to intermediate pressure level in the intermediate pressure vessel 12. The relative liquid content of the expanded refrigerant is also increased. Less flash gas enables the intermediate pressure at the economizer connection port 21 of the compressor to substantially drop, thus permitting the use of compressors of existing construction for this type of refrigeration system, and the operation of refrigeration systems with increased economic efficiency.

In the Ph diagram according to Figure 3, point 1 describes the condition at the evaporator outlet. The inlet condition of the refrigerant upstream of the screw compressor 11, point 2, is the outlet condition of the refrigerant after passing the internal heat exchanger 14 (Figure 4). The refrigerant is superheated comparatively high, the mass flow drawn in by the compressor is reduced.

The chamber volume, e.g. the interlobe volume considered of a screw compressor, has its maximum at this point. With this maximum chamber size, the suction process is terminated, and a flow connection to the compression chamber starts to being made via the economizer connection port. Due to mixing the refrigerant flow, point 26, with the refrigerant already drawn in, point 2, the latter is compressed to the intermediate pressure Pz and cooled down. The mixing process is terminated at point 4. At point 4, the economizer connection port also closes, and compression of the suction gas and the refrigerant from the internal heat exchanger 19 to discharge pressure starts at point 27. The refrigerant passes the gas cooler 13 which is fed with a cooling medium, e.g. cooling water, for cooling the refrigerant vapour. When leaving said gas cooler 13, the refrigerant has the condition at point 28. In the internal heat exchanger 14 through which two refrigerant flows of the refrigeration system are led, the refrigerant is cooled from point 28 to point 29. In doing so, the other refrigerant flow led through the internal heat exchanger 14 for cooling from point 28 to point 29. is warmed up from point 1 to point 2.

The refrigerant flow cooled so far is expanded in two partial flows by the throttling device 20 to intermediate pressure, point 33, and by the throttling device 34. from point 3Oto point 31.

At the throttling device 20, there arises a refrigeration effect serving for cooling down the partial refrigerant flow, flow path 35, from point 29 to point 30. The temperature difference between saturation temperature at point 32 and outlet temperature at point 30 at the internal heat exchanger 19 depends on dimensioning of the internal heat exchanger 19 and might amount to approx. 5 K. Due to cooling down the refrigerant from point 29 to point 30 in the internal heat exchanger 19, the flashgas portion is reduced during expansion of the refrigerant from high pressure to suction pressure. As a result of cooling down the refrigerant mass flow in the internal heat exchanger 14 from point 28 to point 29, the volumetric refrigerating capacity of the partial refrigerant flow increases which is expanded from point 29 to point 33 so that for cooling down the high-pressure gas from point 29 to point 30 less refrigerant vapour is formed, and hence the intermediate pressure at the economizer connection is distant enough from the critical point. Two advantages are achieved as a result: -Higher economic efficiency of the refrigeration system by increasing the refrigerating capacity -Use of known screw compressors with economizer connection The refrigeration system for transcritical operation according to Figure 4 comprises a gas cooler 13, an evaporator system with heat exchanger 18 and liquid separator 17, a screw compressor 11, throttling devices 20 and 34, and interconnecting piping between the components mentioned with the compressor having an economizer connection port 21 at the housing. The first internal heat exchanger 19 and the second internal heat exchanger 14 are arranged in a manner enabling a flow connection to the first internal heat exchanger 19 of the refrigeration or air conditioning system with the former having two flow paths 22 and 35 featuring heat-exchanging surfaces, wherein one flow path 35 relates to the high-pressure refrigerant flow from the gas cooler 13 via the second internal heat exchanger 14 to the evaporator system, while the other flow path 22 relates to a partial flow of high- pressure refrigerant passing the throttling device 20 for expansion of the high-pressure refrigerant to intermediate pressure level, and having a flow connection to the first internal heat exchanger 19, and cooling downstream the high-pressure refrigerant vapour of the other flow path 35. Upstream of the compressor suction side, there is arranged the second internal heat exchanger 14 with two flow paths featuring heat-exchanging surfaces, wherein one flow path 23 is located downstream between the liquid separator 17 with the lowest pressure level of the refrigeration system and the compressor suction side 11, while the other flow path 22 is located downstream between gas cooler outlet and throttling device 20 with flow path 35 being located upstream of the first internal heat exchanger 19.

Claims

What we claim is: 1. Refrigeration system for transcritical operation

comprising at least a gas cooler, an internal heat exchanger, an intermediate pressure vessel, an evaporator, a compressor having geometrically controlled inlet and outlet ports, e.g. a screw compressor or a scroll compressor, throttling devices and interconnecting piping between the components mentioned, and when in operation, suction pressure prevails on the suction side of the compressor, while discharge pressure prevails on its discharge side with the pressure on the discharge side being higher than the pressure at the critical point of the refrigerant, the compressor having an economizer connection port at the housing enabling a flow connection to the intermediate pressure vessel of the refrigeration or air conditioning system the pressure of which lies between discharge pressure and suction pressure, wherein an internal heat exchanger is arranged with two flow paths featuring heat-exchanging surfaces, with one flow path being located downstream between gas cooler outlet and throttling point before the liquid separator, and the other flow path being located downstream between evaporator outlet and suction side of the compressor.

2. Refrigeration system for transcritical operation comprising at least a gas cooler, an evaporator, a compressor having geometrically controlled inlet and outlet ports, e.g. a screw compressor or a scroll compressor, throttling devices and interconnecting piping between the components mentioned, with the compressor having an economizer connection port at the housing, wherein in addition to the components mentioned above a first and a second internal heat exchanger featuring heat-exchanging surfaces are arranged in a manner enabling a flow connection to the first internal heat exchanger of the refrigeration or air conditioning system with the former having two flow paths, with one flow path relating to the high-pressure refrigerant flow from the gas cooler via the second internal heat exchanger to the evaporator, and the other flow path relating to a partial flow of high-pressure refrigerant passing the throttling device for expansion of the high-pressure refrigerant to intermediate pressure level, and having a flow connection to the ii economizer connection of the compressor, and cooling downstream the high-pressure refrigerant vapour of the other flow path, with the second internal heat exchanger being arranged upstream of the compressor suction side with two flow paths and featuring heat-exchanging surfaces, wherein one flow path is located downstream between the liquid separator with the lowest pressure level of the refrigeration system and the compressor suction side, while the other flow path is located downstream between gas cooler outlet and throttling device upstream of the first internal heat exchanger.