EP2260250A1

EP2260250A1 - Refrigerating system

Info

Publication number: EP2260250A1
Application number: EP09713268A
Authority: EP
Inventors: Klaus Koch
Original assignee: Carrier Corp
Current assignee: KOCH, KLAUS
Priority date: 2008-02-21
Filing date: 2009-02-16
Publication date: 2010-12-15

Abstract

Refrigerating system (10) comprising a cooling unit, a compressor unit (22, 24) having a suction side (22a, 24a) and a pressure side (22b, 24b), a first heat exchanging unit (14) connected to said compressor unit (22, 24), said first heat exchanging unit (14) having a gas side (14a) and a liquid side (14b), said refrigerating system (10) further comprising a second heat exchanging unit (3S) having a gas side (36a) and a liquid side (36b), said refrigerating system (10) adapted to allow, in a heat pump assisted cooling mode, said refrigerant to flow from said pressure side (22b, 24b) of said compressor unit (24, 26) towards said gas side (36b) of said second heat exchanging unit (36), said liquid side (14b) of said first heat exchanging unit (14) being disconnected from said pressure side (22b, 24b) of said compressor unit (22, 24) and being connected to said suction side (22a, 24a) of said compressor unit (22, 24), such that direction of flow through said first heat exchanging unit (14) reverses.

Description

K 74416/8

REFRIGERATING SYSTEM

The present invention relates to a refrigerating system.

Refrigerating systems circulate a refrigerant within a cooling circuit comprising at least one cooling unit for cooling at least one cooling load consuming enthalpy of heat for cooling the environment thereof, at least one compressor unit connected to the cooling unit, and at least one one heat exchanging unit connected to the compressor unit. The cooling unit comprises at least one evaporator stage which evaporates refrigerant thereby depriving the the evaporator stage's environment of heat. The heat exchanging unit comprises a condenser stage condensing the evaporated refrigerant, and thus produces heat which is discharged to the environment of the heat exchanging unit. Flow of refrigerant within the cooling circuit is driven by the compressor unit having a suction side connected to the cooling unit and a pressure side connected to the heat exchanging unit.

Cooling units may e.g. comprise refrigerating sales furnitures placed at various locations in a supermarket for presenting goods at cooling temperatures, i.e. at temperatures below ambient temperature. A plurality of such cooling units may be connected in parallel within a cooling circuit, each cooling unit providing a respective cooling temperature. In such installations the cooling units may be arranged in several groups of cooling units with each group including a single or a plurality of evaporator stages. Each group of cooling units is connected in series to a respective compressor unit comprising a single or a plurality of compressor stages connected in series or in parallel. Thus the respective groups of cooling units with the compressor units assigned thereto are connected in par- allel, and refrigerant is delivered from the respective pressure sides of the compressor units to the heat exchanging unit where it is transformed again into a liquid state and supplied back in liquid form to the cooling circuit.

Typical refrigerating systems, as are used e.g. in supermarkets, only comprise a central heat exchanging unit to which is supplied refrigerant in at least partly

|l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 evaporated form from the respective pressure sides of compressor units. To effectively discharge heat produced by condensation of refrigerant the heat exchanging unit is typically located outside the building, e.g. on the roof of the building.

In a refrigerating system used exclusively for cooling purposes, the heat produced in the heat exchanging unit is simply waste heat that is released to ambient. However, during considerable time of the year, besides producing low temperatures for storing and presenting perishable goods of various type, it is also necessary to produce heat for keeping sales rooms at comfortable temperatures. Therefore the desire arises to use the heat produced by the central heat exchanging unit of the refrigerating system for heating purposes.

It is known for refrigerating systems to recover heat produced in heat exchan- ging units providing condensation of a refrigerant, and to use the recovered heat in a heating system of the building. However, a drawback of such technologies is that during those seasons of the year requiring most heating, i.e. in winter and in the transition periods between spring and winter and between autumn and winter, the refrigerating system will produce only small amounts of waste heat, since the enthalpy of heat required to be consumed in the cooling units for keeping temperatures in the cooling units at a desired level is also low, and thus the compressor stages of the cooling circuit will only run with low power. In contrast, most power for heating will be available in summer when heating of the building is typically unnecessary.

In winter it is not uncommon that the waste heat produced by a refrigerating system is not sufficient for heating the building, and hence additional heating sources are required. Installation of such additional heating sources, e.g. heat pumps, in addition to the refrigerating system will, however, increase costs, not only for providing the additional heat pump device itself, but to an even larger extent for the additional components necessary to provide a heat pump circuit operable, at least in winter, in parallel to and largely independent of the cooling circuit.

[I \8\74\74416\090216_apphcatιon_text odt] 2009-02-16 10 54 It would therefore be beneficial to provide a refrigerating system having the capability of producing, throughout the year, sufficient heat for heating a building, without having to add to the refrigerating system components necessary to form an additional heating circuit.

Exemplary embodiments of the invention comprise a refrigerating system adapted to circulate, in operation, a refrigerant within a cooling circuit, said refrigerating system comprising at least one cooling unit, at least one compressor unit connected to said cooling unit, said compressor unit having a suction side and a pressure side, and at least one first heat exchanging unit connected to said compressor unit, said first heat exchanging unit having a gas side and a liquid side, said refrigerating system further comprising at least one second heat exchanging unit having a gas side and a liquid side. Further said refrigerating system is operable in different modes of operation, according to a demand of heating and an enthalpy of heat actually consumed by said cooling unit. In a normal cooling mode, said refrigerating system is adapted to allow said refrigerant to flow within said cooling circuit from said pressure side of said compressor unit towards said gas side of said first heat exchanging unit. In a heat pump assisted cooling mode, said refrigerating system is further adapted to allow said refrigerant to flow from said pressure side of said compressor unit towards said gas side of said second heat exchanging unit, said liquid side of said first heat exchanging unit being disconnected from said pressure side of said compressor unit and being connected in fluid communication to said suction side of said compressor unit, such that direction of flow of said refrigerant through said first heat exchanging unit reverses.

Exemplary embodiments of the invention will be described in greater detail below taking reference to the accompanying drawings.

Fig. 1 shows in schematic and simplified form a refrigerating system according to an exemplary embodiment; and

Fig. 2 shows more detail a balancing assembly as is used in the refrigerating system of fig. 1.

[I \8\74\74416\090216_apphcation_text odt) 2009-02-16 10 54 Fig. 1 shows in schematic and simplified form a refrigerating system, generally designated by 10, according to an exemplary embodiment. The refrigerating system includes a cooling circuit circulating a refrigerant from a supply of substantially liquid refrigerant (not shown) via a number of cooling units (not shown, located in line 12 of the cooling circuit, note that line 12 extends beyond the end position indicated in figure 1 to the cooling units and further to the supply of refrigerant) in which said refrigerant is at least partly evaporated to a first heat exchanging unit 14 formed by a first condenser unit 16 connected in series to a subcooler unit 18, and back via a drain 20 to the supply of refrigerant. The- first condenser 16 and first subcooler 18 both work against air as medium to which heat released from the refrigerant is transferred. Flow of refrigerant within the cooling circuit is driven by a plurality of first compressor units 22, 24 connected in parallel to each other between line 12 leading to the cooling units and line 30 leading to the first heat exchanging unit 14. Each of the compressor units 22, 24 has a suction side 22a, 24a and a pressure side 22b, 24b. The suction sides 22a, 24a of the first compressor units 22, 24 (in the following also called cooling circuit compressor units) are connected to the cooling units. Further a second compressor unit 26 (in the following also called heat pump compressor unit) is connected in parallel to the cooling circuit compressor units 22, 24. The heat pump compressor unit 26 has a suction side 26a connected via line 68 to a suitable supply of refrigerant. This supply of refrigerant may be any suitable load consuming enthalpy of heat for cooling, e.g. an evaporator stage of an air conditioning system or a dryer stage, thus not requiring the heat pump compressor unit 26 to be an extra device solely for purposes of driving a refrigerant circuit under highest heating demands, but may also be used during warmer seasons of the year, e.g. to drive an air conditioning refrigerant circuit.

The pressure sides 22b, 24b, 26c of all compressor units 22, 24, 26 are connected to a gas side 14a of the first heat exchanging unit 14. A first valve (main val- ve) 28 is provided in a line 30 connecting the pressure sides 22b, 24b, 26b of the compressor units 22, 24, 26 to the gas side 14a of the first heat exchanging unit 14. For reasons outlined below, the first valve 28 is a proportional valve the opening of which can be controlled such as to provide a desired amount of flow of refrigerant to the first heat exchanging unit 14. Further a check valve 32

[l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 is provided in a line 34 connecting a liquid side 14b of the first heat exchanging unit 14 to the drain 20.

The refrigerating system 10 further comprises a second heat exchanging unit 36 comprising a heat pump circuit having a gas side 36a and a liquid side 36b. The second heat exchanging unit 36 is adapted to transfer heat between a heat pump circuit circulating the refrigerant between the gas side 36a and the liquid side 36b, and a heating circuit (schematically depicted at 38) of a building circulating a heating fluid via a heating fluid pump 40.

The gas side 36a of the second heat exchanging unit 36 is connected to the pressure sides 22b, 24b, 26b of the compressor units 22, 24, 26 via a second valve 42. Also the second valve 42 is a proportional valve the opening of which can be controlled such as to provide a desired amount of flow of refrigerant to the second heat exchanging unit 36. The liquid side 36b of the second heat exchanging unit 36 is connected to the drain 20 via a balancing assembly generally designated by 44. A balancing assembly 44 corresponding to figure 1 is depicted in more detail in figure 2, and thus the following description also refers to figure 2.

The balancing assembly 44 is formed by two riser tubes 46, 48, namely an outer riser tube 46 and an inner riser tube 48 inserted into the outer riser tube substantially coaxial thereto. Both the inner and the outer riser tube 46, 48 thus extend substantially in vertical direction, with the bottom end of the inner riser tube 46 being open and facing the bottom end of the outer riser tube 48 which is closed. An inlet port 50 of refrigerant to the balancing assembly 44 formed in an upper side of the inner riser tube 48 is connected to the liquid side 36b of the second heat exchanging unit 36. A first output port 52 of refrigerant from the balancing assembly 44 is connected to the central drain 20 of the refrigera- ting system via a line opening into line 34 at a position downstream of the check valve 32 (i.e. an a side thereof facing away from the first heat exchanging unit 14). The first outlet port 52 is arranged in substantial height above the bottom of the first and second riser tubes 46, 48. Thus refrigerant being supplied to the inlet port 50 of the inner riser tube 48 will flow through the open bottom end of the inner riser tube 48 into the outer riser tube 46 and accumulate in the

[l:\8\74\74416\090216_application_text.odt) 2009-02-16 10:54 volume of the outer riser tube 46 surrounding the inner riser tube 48, this volume thus forming a dead volume in which, in operation of the refrigerating system in the heat pump assisted cooling mode, flow of refrigerant is stagnant. With increasing amount of refrigerant supplied to the inlet port 50 the level of refrigerant standing in the outer riser tube 46 will increase. After the level of refrigerant has reached a predetermined level at which the first outlet port 52 is formed refrigerant will be supplied via the outlet port 52 to drain 20.

In the outer riser tube 46 are formed a plurality of (in this example 3) second outlet ports 54a, 54b, 54c. Each of these second outlet ports 54a, 54b, 54c is provided with a respective regulating valve 56a, 56b, 56c, these regulating valves 56a, 56b, 56c thus forming a regulating valve unit, and opens into a line 58. Line 58 opens into line 34 connecting the liquid side 14b of the first heat exchanging unit 14 to the check valve 32 at a position, in the normal cooling mode, upstream of the check valve 32 (i.e. at a side thereof towards the first heat exchanging unit 14). Thus refrigerant being discharged from the liquid side 36b of the second heat exchanging unit 36 can be recirculated to the liquid side 14b of the first heat exchanging unit 14 via the second outlet ports 54a, 54b, 54c of the balancing assembly 44.

Further, the outer riser tube 46 is provided with a detecting means 60 for detecting a level of refrigerant stagnant in the outer riser tube 46, the detecting means 60 being located at a higher level than the first outlet port 52 and second outlet ports 54a, 54b, 54c. The detecting means 60 detects whether the level of refrigerant in the dead volume formed by the riser tubes 46, 48 has reached the level of the detecting means 60, and provides a corresponding signal to each of the regulating valves 56a, 56b, 56c of the regulating valve unit 56. In response to a signal from the detecting means 60 that the refrigerant in the dead volume has reached the level of the detecting means 60, the regulating valves 56a, 56b, 56c are opened according to a prescribed control scheme. This prescribed control scheme might e.g. be a feedback control by which opening of the regulating valves 56a, 56b, 56c is controlled such as keep the level of refrigerant in the dead volume equal to the height of the level detecting means 60, or may alternatively be a control scheme according to which, in case the level of refrige- rant in the dead volume is equal to or higher than the level of the level detec-

[I \8\74\74416\090216_applιcatιon_text odt) 2009-02-16 10 54 ting means 60, the regulating valves 56a, 56b, 56c are opened in accordance with power consumption of the compressor units 22, 24, 26 (e.g. for each of the compressor units 22, 24, 26 running at a prescribed power a corresponding one of the valves 56a, 56b, ,56c will be opened).

Further the refrigerating system 10 comprises a line 62 connecting the gas side 14a of the first heat exchanging unit 14 to the suction sides 22a, 24a, 26a of the compressor units 22, 24, 26. Line 64 opens into line 30 at a position between the gas side 14a of the first heat exchanging unit 14 and the first valve 28. A third valve 64 is provided in line 62. The third valve 64 may simply be an

ON/OFF valve, such as to disconnect, in the closed state thereof, the gas side 14a of the first heat exchanging unit 14 from the suction sides 22a, 24a, 26a of the compressor units 22, 24, 26. Preferably, however, the third valve 64 is a proportional valve allowing to adjust its opening according to an amount of ref- rigerant to be supplied, in the heat pump assisted cooling mode, from the gas side 14 of the first heat exchanging unit 14 to the suction sides 22a, 24a, 26a of the compressor units 22, 24, 26.

Further a fifth valve 66 is provided in line 12 at a position between the suction side 26a of the heat pump compressor unit 26 and the suction sides 22a, 24a of the cooling circuit compressor units 22, 24. The fifth valve 66 is a proportional valve the opening of which can be controlled such as to allow flow of a predetermined amount of refrigerant between the suction sides 22a, 24a of the cooling circuit compressor units 22, 24 and the suction side 26a of the heat pump compressor unit 26.

The refrigerating system 10 can be operated in a plurality of different modes of operation, depending on the position of valves 28, 42, 64, 66 which can be controlled according to enthalpy of heat consumed in the cooling units of the coo- ling circuit and the demand of heating for the building, and the position of the check valve 32 which will adjust to an open or closed position according to the selected mode of operation.

In a normal cooling mode, only a cooling circuit is realized with no additional heating. In this normal cooling mode the first valve 28 is open and the third val-

(l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 ve 64 and the fourth valve 66 are closed. The heat pump compressor unit 26 does not have any function in the cooling circuit and thus will generally be out of operation. Hence, the the pressure sides of the compressor units 22, 24 will be in fluid connection to the gas side 14a of the first heat exchanging unit 14. The second valve 42 is kept closed, such that the second heat exchanging unit 36 will not be supplied with refrigerant, and thus be disconnected from the cooling circuit. No significant pressure differential will arise across the check valve 32, and thus the check valve 32 is open. Therefore, in this normal cooling mode refrigerant is transported by the compressor units 22, 24 from the cooling units to the first heat exchanging unit 14 and passes the first heat exchanging unit 14 from the gas side 14a to the liquid side 14b . The first heat exchanging unit 14 operates as a condenser unit, comprising a condenser 14 and subcooler 16 connected in series. Heat produced by condensation of refrigerant in the first heat exchanging unit 14 is discharged to the environment.

Should a demand of heating arise, the refrigerating system 10 will be switched to a further mode of operation, namely a cooling circuit assisted heating mode, in which refrigerant supplied by the compressor units 22, 24 is delivered in part to the second heat exchanging unit 36 for transferring heat to the heating cir- cuit 38. In this cooling circuit assisted heating mode the second valve 42 will be opened with the opening thereof being controlled to allow flow of refrigerant, as required to satisfy the demand of heating. Refrigerant not being supplied to the second heat exchanging unit 36 will still be supplied to the first heat exchanging unit 14. Thus, also the first valve 28 is still open, with the opening the- reof being controlled according to the demand of heating, namely such as to supply to the first heat exchanging unit 14 the remainder amount of refrigerant not required for heating. Position of the other valves is identical to the normal cooling mode. Also flow of refrigerant through the first heat exchanging unit 14 is identical to the normal cooling mode, the first heat exchanging unit 14 thus working as a condenser stage of the cooling circuit. Also the second heat exchanging unit 36 is working as a condenser stage of the cooling circuit connected in parallel to the first heat exchanging unit 14.

With further increasing demand of heating the amount of refrigerant to be sup- plied to the second heat exchanging unit 36 will increase. As typically also out-

[l:\8\74\74416\090216_application_text.odt) 2009-02-16 10:54 side temperatures will fall in cold seasons, simultaneously the enthalpy of heat consumed by the the cooling units will decrease. Therefore, the remaining amount of refrigerant to be supplied to the first heat exchanging unit 14 will decrease and finally become zero. In this situation condensation of all refrigerant circulating in the cooling circuit will be effected by the second heat exchanging unit 36, and thus the first heat exchanging unit 14 will become more or less superfluous.

Should demand of heating still increase, evaporation of refrigerant in the coo- ling circuit will not consume sufficient enthalpy of heat to satisfy the demand of heating. To some extent it will be possible to operate the evaporator stages of the cooling units in a less efficient manner, with the consequence that, to maintain a desired cooling temperature at the cooling units, a larger amount of refrigerant is necessary and thus more enthalpy of heat is consumed by the cooling units.

Should, however, demand of heating increase to such an extent that less efficient operation of the evaporator stages, as described above, will not be sufficient any more to provide for the enthalpy of heat required to be released in the second heat exchanging unit 36, it will be required to supply refrigerant to the second heat exchanging unit 36 independently of the cooling circuit formed by passing refrigerant through the cooling units.

In the refrigerating system 10 such additional supply of refrigerant to the se- cond heat exchanging unit 36 is possible without having to install any further components to the refrigerating system 10. Rather, by changing position of the valves the refrigerating system 10 is switchable into a further mode of operation, namely into a heat pump assisted cooling mode, in which the first heat exchanging unit 14 is working as an evaporator stage consuming enthalpy of heat and providing the second heat exchanging unit 36 with the necessary amount of refrigerant in substantially vaporous form.

To switch the refrigerating system 10 into the heat pump assisted cooling mode, first the first valve 28 will be closed, and in a state in which the first valve 28 is closed and the second valve 42 is opened, the third valve 64 will be ope-

U:\8\74\74416\090216_application_text.odtl 2009-02-16 10:54 ned. Thereby, the gas sides 22a, 24a (26a) of the compressor units 22, 24 (26), instead of the pressure sides 22b, 24b (26c) thereof, will be connected to the gas side 14a of the first heat exchanging unit 14. Therefore the compressor units 22, 24 (the heat pump compressor unit 26 will, at least in the initial phase after switching to the heat pump assisted cooling mode, still be out of operation) will suck refrigerant, substantially in vaporous form, from the first heat exchanging unit 14 via the gas side 14a thereof, and supply this refrigerant to the second heat exchanging unit 36 where it will condense. From the liquid side 36b of the second heat exchanging unit 36 the refrigerant will be delivered to the balancing assembly 44 and to the drain 20. This will further lead to a sufficient pressure differential across the check valve 32 such that the check valve 32 will close. Now a heat pump circuit driven by the compressor units 22, 24 is established, this heat pump circuit including the first heat exchanging unit 14 and the cooling units as evaporator stages connected in parallel. The cooling units and the first heat exchanging unit 14 consume enthalpy of heat which is released in the second heat exchanging unit 36 to transfer heat to the heating circuit 38 according to the actual demand of heating. This heat pump circuit, besides heating the heating fluid in the heating circuit 38, also provides the function of keeping the cooling units at desired temperatures.

In the heat pump circuit established in the heat pump assisted cooling mode, a significantly larger amount of refrigerant will have to be circulated than in the cooling circuit established in the normal cooling mode and in the cooling circuit assisted heating mode. This amount of refrigerant is supplied from the liquid side 36b of the second heat exchanging unit 36 via the second outlet ports 54a, 54b, 54c of the balancing assembly 44 to the liquid side 14b of the first heat exchanging unit 14. Thus, the refrigerant necessary for heating purposes does not have to pass the cooling units, nor is it necessary to provide any buffer or storage devices for supplying refrigerant in case of large demands of heating. The balancing assembly 44, as explained above, allows to adjust the amount of refrigerant supplied to the liquid side 14b of the first heat exchanging unit 14 to the amount of refrigerant liquefied by the second heat exchanging unit 36, and thus avoids accumulation of liquid refrigerant at the liquid side 36b. This is important for efficient operation of a heat exchanging unit working as a heat pump.

[l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 For moderately increasing demands of heating it will be sufficient to drive the flow of refrigerant in the heat pump circuit established in the heat pump assisted cooling mode by the compressor units 22, 24 assigned to the cooling units. Should, however, the demand of heating increase even more, it may become necessary to additionally provide a separate compressor unit for supplying refrigerant to the second heat exchanging unit 36. For this case, it is preferable if a further heat pump compressor unit 26 is connected in parallel to the cooling circuit compressor units 22, 24. Preferably, operation of this heat pump compres- sor unit 26 is started when the actual demand of heating requires a total power of the compressor units larger than the total power rating of the cooling circuit compressor units 22, 24. Once the additional heat pump compressor unit 26 has gone into operation, it is preferable to operate the cooling circuit compressor units 22, 24 at a predetermined operation point of maximum efficiency and to operate the additional heat pump compressor unit 26 at variable power according to the additional power required to supply sufficient refrigerant to the second heat exchanging unit 36. This is achieved by providing the fifth valve 64 in a line connecting the suction sides of the heat pump compressor unit 26 and the cooling circuit compressor units 22, 24. By controlling the opening of this valve 64, the operating point of the cooling circuit compressor units 22, 24 can be held stably at a same operating point, regardless of the actual power requirement necessary to keep the respective cooling units at their desired cooling temperature.

The embodiments described before provide a refrigerating system having the capability of producing sufficient heat to heat a building throughout the year without having to add to the refrigerating system components necessary to form an additional heating circuit.

The refrigerating system is adapted to circulate, in operation, a refrigerant within a cooling circuit. The refrigerating system comprises at least one cooling unit, at least one compressor unit in fluid connection to the cooling unit, the compressor unit having a suction side and a pressure side, and at least one first heat exchanging unit in fluid connection to the compressor unit, the first heat exchanging unit having a gas side and a liquid side. The refrigerating system

(l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 further comprises at least one second heat exchanging unit having a gas side and a liquid side. Preferably the second heat exchanging unit is thermally connected with a heating circuit of a building.

Further the refrigerating system is operable in different modes of operation, according to a demand of heating and an amount of enthalpy of heat actually consumed by the cooling units of the cooling circuit.

In a normal cooling mode, the cooling circuit is adapted to keep at least one cooling load at a predetermined temperature below ambient temperature, the cooling load thus consuming enthalpy of heat. In the normal cooling mode, the refrigerating system is adapted to allow the refrigerant to flow within the cooling circuit from the pressure side of the compressor unit towards the gas side of the first heat exchanging unit. The refrigerant, after being transformed into a substantially liquid state, exits from the first heat exchanging unit on the liquid side thereof. The cooling circuit is separated from the second heat exchanging unit, and thus the refrigerating system does not have any heat pump circuit.

The refrigerating system is switchable into a further heat pump assisted cooling mode. In the heat pump assisted cooling mode, the refrigerating system is adapted to allow the refrigerant to flow from the pressure side of the compressor unit towards the gas side of the second heat exchanging unit, and the liquid side of the first heat exchanging unit is disconnected from the pressure side of the compressor unit and is connected in fluid communication to the suction side of the compressor unit, such that direction of flow of refrigerant through the first heat exchanging unit reverses.

As is usual with cooling circuits, preferably the cooling unit may comprise one or a plurality of evaporator stages. Preferably each of such evaporator stages will be assigned to a respective cooling site (e.g. a refrigerated sales furniture). The compressor unit may be connected in series to the cooling unit, and the first heat exchanging unit may be connected in in series to the compressor unit. The cooling circuit may comprise a plurality of cooling groups, each cooling group comprising a respective cooling unit and compressor unit, the cooling groups being connected in parallel to each other.

[l:\8\74\74416\090216_application_text.odt) 2009-02-16 10:54 Further the first heat exchanging unit may comprise a condenser stage and a subcooler stage connected in series, as is typical for refrigerating systems. The- first heat exchanging unit may be constructed such as to work against air as medium to which heat released from the refrigerant is transferred. E.g. in typical installations the first heat exchanger unit is installed on the roof of a building, and outside air is passed through the first heat exchanger unit such as to come into thermal contact with the refrigerant.

In such refrigerating systems, in the normal cooling mode, the first heat exchanging unit will comprise the condenser stage adapted to liquefy the refrigerant passing it. In the heating pump assisted cooling mode, the second heat exchanging unit will comprise a condenser stage adapted to liquefy the refrigerant passing it, whereas the first heat exchanging unit will comprise an evapo- rator stage evaporating refrigerant passing it. The first heat exchanging unit thus is provided with bi-functionality, namely adapted such as to work as a condenser stage or as an evaporator stage, depending on the mode of operation of the refrigerating system, i.e. depending on the direction of flow of refrigerant through the first heat exchanging unit. In a preferable embodiment of the first heat exchanging unit the condenser stage and the evaporator stage will be realized by the same device. Specifically, such a bi-functionality can be realized using a condenser having a gas compensation unit as is described in EP 1 406 050 A2, the disclosure of which is hereby incorporated by reference.

When, as is preferable, the first heat exchanging unit comprises a condenser stage connected in series with a subcooler stage, the subcooler stage may be adapted to work, in the the heat pump assisted cooling mode, as an evaporator stage, and the condenser stage may be adapted to work, in the heat pump assisted cooling mode, as a further evaporator stage and/or as a strainer stage. Thus, the first heat exchanging unit can be used effectively throughout the year, in the warmer periods working as a condenser stage in the cooling circuit formed in the normal cooling mode, and in the colder periods working as an evaporator stage in a heat pump circuit formed in the heat pump assisted cooling mode. There are essentially no periods during which the first heat exchan- ging unit is out of service. It is further not necessary to provide the first heat ex-

|l \8\74\74416\090216_apphcatιon_text odt] 2009-02-16 10 54 changing unit with separate condenser and evaporator stages for the cooling circuit and the heat pump circuit, respectively.

The refrigerating system may be operable in further modes of operation, accor- ding to a demand of heating and an amount of enthalpy of heat actually consumed by the evaporator stages of the cooling circuit. E.g. in case the demand of heating is less than corresponding to the enthalpy of heat consumed by evaporating refrigerant in the evaporator stages of the cooling unit(s), the refrigerating system may be operable in a cooling circuit assisted heating mode in which the cooling circuit is connected to the second heat exchanging unit in such a way that at least part of the refrigerant is allowed to flow from the pressure side of the compressor unit towards the gas side of the second heat exchanging unit, and that at least part of the refrigerant is allowed to flow from the pressure side of the compressor unit towards the gas side of the first heat exchanging unit.

Preferably, the gas side of the first heat exchanging unit is connected to the pressure side of the compressor unit via a first valve, and the gas side of the second heat exchanging unit is connected to the pressure side of the compressor unit via a second valve. In this case, it is advisable that both the first valve and the second valve be controllable in coordination to each other, such as to allow that in each situation the total amount of refrigerant delivered from the pressure side of the compressor unit is condensed by the first and second heat exchanging units.

In a preferred example the second valve is a proportional valve. This allows the second valve to be controlled in accordance with a demand of refrigerant to be supplied to the second heat exchanging unit, i.e. an actual demand of heating. In this case, the first valve is preferably adapted to be opened according to the remaining amount of refrigerant supplied from said pressure side of said compressor unit. The first valve may also be a proportional valve. In this case the first valve may be controlled inversely to the second valve, such that the amount of refrigerant passing the first and second valves in total corresponds to the enthalpy of heat consumed by the evaporator stages of the cooling unit(s). Alternatively the first valve may be of a differential pressure type ope-

(l:\8\74\74416\090216_application_text.odt) 2009-02-16 10:54 ning in case a pressure differential across the first valve exceeds a first predetermined threshold, and closing in case the pressure differential across the first valve falls below a second predetermined threshold. In this embodiment the first valve will open and close intermittently according to the pressure differen- tial, this pressure differential being adjustable such that in average the remaining amount of refrigerant not required for heating will be delivered to the first heat exchanging unit.

It may even be conceivable that both the first and the second valves comprise ON/OFF valves, each of the first and second valves being controlled intermittently in such a way that, at each point of time, one of the first and second valves is opened and the other is closed. In such an arrangement, the time for which the second valve is opened may be adjusted according to the amount of refrigerant to be supplied to the gas side of the second heat exchanging unit, i.e. a demand of heating. Intermittent control of the first and second valves, as described, requires a specific adaption of the control circuitry to overcome instability problems.

In a further preferred embodiment the refrigerating system comprises a line connecting the gas side of the first heat exchanging unit to the suction side of the compressor unit. This line may be provided with a third valve. When the refrigerating system is to be operated in the heat pump assisted cooling mode, the third valve will be opened and the first valve will be closed, such as to allow refrigerant to be sucked from the gas side of the first heat exchanging unit to- wards the suction side of the compressor unit. Therefore it will basically be sufficient if the third valve is of an ON/OFF type, however preferably the third valve will be controlled in coordination to said first valve such as to be open when said first valve is closed. Providing the third valve as a proportional valve may be advantageous, since allowing to adjust an amount of refrigerant supplied from the first heat exchanging unit to the compressor unit in the heat pump assisted cooling mode.

Further the refrigerating system preferably comprises a fourth valve connected in series with the first heat exchanging unit on the liquid side thereof, i.e. in the normal cooling mode of said refrigerating system on a downstream side the-

[l:\8\74\74416\090216_application_text.odt) 2009-02-16 10:54 reof. The fourth valve will be controlled such as to allow, in the normal cooling mode, flow of refrigerant from the liquid side of the first heat exchanging unit to a drain of refrigerant, and, in the heat pump assisted cooling mode, to be closed such as to block any flow of refrigerant from the liquid side of the first heat exchanging unit towards the drain or vice versa.

The drain of refrigerant will commonly be connected to a main reservoir of refrigerant from which refrigerant, in essentially liquid form, is supplied to the respective cooling units of the refrigerating system. Preferably the liquid side of the second heat exchanging unit is also connected to the drain of refrigerant at a position being, in the normal cooling mode, downstream from the fourth valve.

Further a supply of at least partly liquid refrigerant might be connected to the Ii- quid side of the first heat exchanging unit at a position located, in the normal cooling mode, upstream from the fourth valve. This supply of refrigerant might e.g. open into a line connecting the liquid side of the first heat exchanging unit and the fourth valve. In a state in which the fourth valve is closed, via this supply of refrigerant liquid refrigerant can flow to the first heat exchanging unit to be evaporated therein and taking up heat from the environment. The evaporated refrigerant will further be transported via the compressor unit to the second heat exchanging unit where it will condense and transfer heat to a heating circuit. Thus, a heat pump circuit will be formed in the heat pump assisted cooling mode.

In a simple arrangement the fourth valve can be a check valve switching into a closed condition in case a pressure differential across the check valve exceeds a predetermined threshold. Preferably the check valve is adapted to switch to a closed condition in case pressure on its side connected to the drain of refrige- rant is higher than pressure on its side connected to the liquid side of the first heat exchanging unit.

Preferably the supply of refrigerant is, at least in the heat pump assisted cooling mode, in fluid connection to the liquid side of the second heat exchanging unit, i.e. in fluid connection to that heat exchanging unit operating as a heat pump in

[l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 the heat pump assisted cooling mode. Thus, the second heat exchanging unit (working as a heat pump), the first heat exchanging unit (working as an evaporator stage) and the compressor unit will form the heat pump circuit for heating up the building. The cooling units of the cooling unit will be connected to this heat pump circuit, working as further evaporator stages, and thus will be connected in parallel to the first heat exchanging unit. The cooler outside temperatures will be, and thus the less cooling power is consumed by the cooling units of the cooling circuit, the more important for operation of the the heat pump circuit according to a given demand of heating will be the enthalpy of heat consumed by the first heat exchanging unit.

To obtain sufficient heating efficiency of the second heat exchanging unit working as a heat pump, it is important to prevent accumulation of refrigerant at the liquid side of the second heat exchanging unit. Such accumulation of refri- gerant can be avoided by connecting the liquid side of the second heat exchanging unit in series to a balancing assembly for refrigerant. This balancing assembly might further be in fluid connection to a drain of refrigerant, such as to be able to discharge refrigerant. The primary function of such balancing assembly is to provide transport of essentially all of the refrigerant produced in the se- cond heat exchanging unit away from the liquid side of the second heat exchanging unit. In a rather simple form this could be achieved by providing the balancing assembly with a buffer in which a predetermined amount of refrigerant can be stored, and connection to a drain to transport away a predetermined amount of refrigerant. Since provision of a buffer will inevitably lead to re- duced efficiency, it is preferable to keep the size of such buffer small and provide drain of refrigerant from the buffer, which drain has a spillover function, of a capacity as large as possible within the limitations imposed by the cooling circuit of the refrigerating system.

In a currently preferred embodiment, which allows to reduce the size of buffer to almost zero, the balancing assembly comprises, besides a first port (inlet port) in fluid connection to the liquid side of the second heat exchanging unit, a second port (outlet port) in fluid connection to a suitable drain of refrigerant, and detecting means for detecting an amount of refrigerant supplied through the first port. The second port is adapted such as to adjust passage of a variable

[l:\8\74\74416\090216_ap_Plιcatιon_text.odt] 2009-02-16 10:54 amount of refrigerant, this variable amount being adjusted according to the amount of refrigerant detected by the detecting means. Provision of a balancing assembly, as disclosed herein and in the following preferred embodiments thereof, is considered to be of significant technical value. Thus applicant reser- ves the right to seek protection for such balancing assembly independent of the particular application in a refrigerating system as disclosed herein.

Preferably, at least when the refrigerating system in the heat pump assisted cooling mode, the balancing assembly, more precisely at least one outlet port thereof, is in fluid connection to the liquid side of the first heat exchanging unit. This allows refrigerant to be supplied to the first heat exchanging unit operating as an evaporator stage in the heat pump assisted cooling mode. With such construction, refrigerant being discharged from the second heat exchanging unit can be supplied to the evaporator stage of the heat pump circuit directly, ins- tead of being supplied to the cooling circuit via the cooling unit(s). The refrigerating system will operate in the heat pump assisted cooling mode preferably under such conditions in which the cooling circuit will circulate via the cooling unit(s) only a small amount of refrigerant. Therefore, supply of liquid refrigerant from the second heat exchanging unit via the balancing assembly to the first heat exchanging unit, bypassing the cooling unit(s), will allow to circulate a significantly larger amount of refrigerant within the heat pump circuit, and thus allow to a adjust the amount of refrigerant circulating in the heat pump circuit to any desired demand of heating.

In a currently preferred embodiment the balancing assembly comprises a regulating valve unit allowing to regulate an amount of refrigerant supplied from the balancing assembly to the liquid side of the first heat exchanging unit. E.g. the regulating valve unit may be located between the second port of the balancing assembly and the position where the supply of refrigerant opens into a line connecting the liquid side of the first heat exchanging unit and the check valve. The regulating valve unit may e.g. include a plurality of solenoid valves, wherein the opening of each of these solenoid valves is controlled such that a level of liquid refrigerant in the balancing assembly does not exceed a predetermined level.

[I \8\74\74416\090216_applιcatιon_text odt] 2009-02-16 10 54 Further, to detect an amount of refrigerant in the balancing assembly, the balancing assembly may comprise a dead volume, and level detection means adapted to detect a level of refrigerant in the dead volume or at least adapted to detect whether a level of refrigerant in the dead volume has reached a prede- termined level. In this case, the regulating valve unit may be controlled based on a detection signal from the level detection means. A dead volume is considered to be a volume in which, in operation of the refrigerating system in the heat pump assisted cooling mode, such that refrigerant is provided to the dead volume from the liquid side of the second heat exchanging unit and refrigerant is discharged from the dead volume via the supply to the drain, flow of refrigerant is essentially stagnant (i.e. flow of refrigerant is essentially suppressed). It is not required that this dead volume be sufficiently large to buffer a significant amount of refrigerant, rather any small dead volume will be sufficient, provided the refrigerant is stagnant within that dead volume.

Since the level of refrigerant within the dead volume will increase in case supply of refrigerant is larger than drain, and will decrease in case supply is smaller than drain, detecting the level of refrigerant in the dead volume will be an excellent measure of the amount of refrigerant to be discharged through the se- cond port of the balancing assembly.

As an example, the level detection means may comprise a level sensor supplying a signal in case the level of refrigerant within the dead volume is at a predetermined level higher than the level of the outlet port(s) for discharging refrige- rant from the balancing assembly. In case such signal is received from the level sensor, the regulating valve unit may be controlled by a feedback loop in such a way that the level of refrigerant in the dead volume is held at the predetermined level. Alternatively the regulating valve unit may be controlled in such a way that, in case the level of refrigerant in the dead volume is detected to be higher than the predetermined level, the regulating valve unit is controlled according to the power consumed by the compressor unit. Detection of a predetermined level of refrigerant in the dead volume is a threshold condition that more liquid refrigerant is produced by the second heat exchanging unit than is discharged to the drain and/or the first heat exchanging unit. In that situation, power actually consumed by the compressor unit is used as measure for the

[l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 amount of refrigerant that is to be discharged additionally, and opening of the regulating valve unit is controlled accordingly. E.g. in case the regulating valve unit comprises a plurality of regulating valves connected in parallel and the compressor unit comprises a plurality of compressor stages connected in paral- IeI, a simple control scheme may be set such that a respective valve is opened for each of the compressor stages running at a predeterminend power level.

To realize a dead volume as described herein before, the balancing assembly may e.g. comprise an inner riser tube inserted into an outer riser tube, the inner riser tube being connected to the liquid side of the second heat exchanging unit, the outer riser tube being connected to any suitable drain. Such suitable drain might be e.g. a central drain of refrigerant to which all refrigerant circulating in the refrigerant system is supplied after having been liquefied and from which it is supplied to the cooling unit(s). Alternatively or additionally, at least when the refrigerating system operates in the heat pump assisted cooling mode, preferably such drain will be the liquid side of the first heat exchanging unit. The inner and the outer riser tube are preferably arranged such as to extend in substantially vertical direction or at least arranged with such inclination as to have a significant component extending in vertical direction.

For allowing a gas phase of the refrigerant entering the balancing assembly from the liquid side of the second heat exchanger unit to be separated from the refrigerant being supplied to drain, preferably a bore is formed in an upper part of the balancing assembly, preferably at the inner riser tube, at a position hig- her than the outlet port of the balancing assembly.

In a preferred embodiment of a refrigerating system a plurality of first compressor units are connected in parallel to each other, i.e. are connected such that the suction sides of each of the compressor units are in fluid connection to each other, and that the pressure sides of each of the compressor units are in fluid connection to each other.

Preferably each of the first compressor units is assigned to a respective cooling unit, e.g. a standard cooling unit including a standard evaporator stage or a deep freezing unit including a deep freezing evaporator stage, each of the stan-

(l:\8\74\74416\090216_application_text.odt] 2009-02-16 10:54 dard cooling units and deep freezing units being connected in parallel such that a pressure side of each of the compressors of the standard cooling units and the deep freezing units is connectable to the gas side of the first and said second heat exchanging units.

The refrigerating system preferably comprises at least one heat pump compressor unit having a suction side and a pressure side. At least when the refrigerating system is operated in the heat pump assisted cooling mode, the suction side of the heat pump compressor unit is connectable to the gas side of the first heat exchanging unit, and the pressure side of the heat pump compressor unit is connectable to the gas side of the second heat exchanging unit. Further, at least when the refrigerating system is operated in the heat pump assisted cooling mode, the heat pump compressor unit is not assigned to any of the cooling units of the cooling circuit. The heat pump compressor unit thus provi- des an additional compressor unit for driving flow of refrigerant from the gas side of the first heat exchanging unit to the gas side of the second heat exchanging unit. Operation of the heat pump compressor unit is preferably started when power of the fist compressor units (that are assigned to respective cooling units of the cooling circuit) is not sufficient to deliver to the second heat ex- changing unit an amount of refrigerant as required by an actual demand of heating. It is not absolutely necessary that the heat pump compressor unit be an extra device provided solely for purposes of driving the heat pump circuit under high load conditions. It is conceivable to connect the gas side of the heat pump compressor unit to suitable other devices consuming cooling energy. E.g. the gas side of the heat pump compressor unit may be connected to an evaporator stage of of an air conditioning system. Then the heat pump compressor unit can be used for driving an air conditioning system in the warmer seasons of the year when it is usually not required to drive the heat pump circuit by an additional heat pump compressor unit. There are further possibilities to connect the gas side of the heat pump exchanger unit with additional devices consuming cooling enthalpy, e.g. a dryer stage.

In a further preferred embodiment the suction sides of the heat pump compressor unit and the suction sides of the first compressor units (that are assigned to respective cooling units) are connected via a line provided with a fifth valve,

[l:\8\74\74416\090216_application_textodtl 2009-02-16 10:54 such that depending on power requirements refrigerant supplied from the first heat exchanging unit can be transported via the first compressor units and/or the additional heat pump compressor unit. Preferably the fifth valve is a proportional valve. This allows to control the opening of the fifth valve in such a way that the amount of refrigerant supplied from the first heat exchanging unit is transported partly via the heat compressor unit and partly via the first compressor units. Thereby, it is possible to operate the first compressor units, once the heat pump compressor unit has been started, at an operating point with maximum efficiency.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt the particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

[l:\8\74\74416\090216_application_text.odt) 2009-02-16 10:54

Claims

1. Refrigerating system (10) adapted to circulate, in operation, a refrigerant within a cooling circuit, said refrigerating system (10) comprising at least one cooling unit, at least one compressor unit (22, 24) connected to said cooling unit, said compressor unit (22, 24) having a suction side (22a, 24a) and a pressure side (22b, 24b), and at least one first heat exchanging unit (14) connected to said compressor unit (22, 24), said first heat exchanging unit (14) having a gas side (14a) and a liquid side (14b), said refrigerating system (10) further comprising at least one second heat exchanging unit (36) having a gas side (36a) and a liquid side (36b), said refrigerating system (10) adapted to allow, in a normal cooling mode, said refrigerant to flow within said cooling circuit from said pressure side

(22b, 24b) of said compressor unit (22, 24) towards said gas side (14a) of said first heat exchanging unit (14), and said refrigerating system (10), in a heat pump assisted cooling mode, further being adapted to allow said refrigerant to flow from said pressure side (22b, 24b) of said compressor unit (24, 26) towards said gas side

(36a) of said second heat exchanging unit (36), said liquid side (14b) of said first heat exchanging unit (14) being disconnected from said pressure side (22b, 24b) of said compressor unit (22, 24) and being connected in fluid communication to said suction side (22a, 24a) of said compressor unit (22, 24), such that direction of flow of refrigerant through said first heat exchanging unit (14) reverses.

2. Refrigerating system (10) according to claim 1, wherein said refrigerating system (10), in a cooling circuit assisted heating mode, is adapted to al- low at least part of said refrigerant to flow from said pressure side (22b,

24b) of said compressor unit (22, 24) towards said gas side (36a) of said second heat exchanging unit (36), and at least part of said refrigerant to flow from said pressure side (22b, 24b) of said compressor unit (22, 24) towards said gas side (14a) of said first heat exchanging unit (14).

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3. Refrigerating system (10) according to claim 1 or 2, wherein said gas side (14a) of said first heat exchanging unit (14) is connected to said pressure side (22b, 24b) of said compressor unit (22, 24) via a first valve (28), and said gas side (36a) of said second heat ex- changing unit (36) is connected to said pressure side (22b, 24b) of said compressor unit (22, 24) via a second valve (42), both said first and said second valves (28, 42) being controllable in coordination to each other.

4. Refrigerating system (10) according to claim 3, wherein said second valve (42) is a proportional valve adapted to be opened according to an amount of refrigerant to be supplied to said gas side (36a) of said second heat exchanging unit (36), said first valve (28) being adapted to be opened according to the remaining amount of refrigerant supplied from said pressure side (22b, 24b) of said compressor unit (22, 24).

5. Refrigerating system (10) according to any of claims 1 to 4, further comprising a line (62) connecting said gas side (14a) of said first heat exchanging unit (14) to said suction side (22a, 24a) of said compres- sor unit (22, 24), said line (62) being provided with a third valve (64).

6. Refrigerating system (10) according to any of claims 1 to 5, further comprising a fourth valve (32) connected in series with said first heat exchanging unit (14) on said liquid side (14b) thereof, said fourth val- ve (32) allowing, in said normal cooling mode, flow of refrigerant from said liquid side (14b) of said first heat exchanging unit (14) to a drain (20) of refrigerant, a supply (58) of at least partly liquid refrigerant being connected to said liquid side (14b) of said first heat exchanging unit (14) at a position located, in said normal cooling mode, upstream from said fourth valve (32).

7. Refrigerating system (10) according to claim 6, wherein said fourth valve (32) is a check valve.

8. Refrigerating system (10) according to claim 6 or 7,

[l:\8\74\74416\090216_application_textodt] 2009-02-16 10:54 wherein said supply (58) of refrigerant, at least in said heat pump assisted cooling mode, is in fluid connection to said liquid side (36b) of said second heat exchanging unit (36).

9. Refrigerating system (10) according to any of claims 1 to 8, wherein said liquid side (36b) of said second heat exchanging unit (36) is connected in series with a balancing assembly (44) for refrigerant, said balancing assembly (44) being further in fluid connection to a drain (20, 58) of refrigerant.

10. Refrigerating system (10) according to claim 9, wherein said balancing assembly (44), at least in said heat pump assisted cooling mode, is in fluid connection to said liquid side (14b) of said first heat exchanging unit (14).

1 1. Refrigerating system (10) according to claim 10, wherein said balancing assembly (44) comprises a regulating valve unit (56) allowing to regulate an amount of refrigerant supplied from said balancing assembly (44) to said liquid side (14b) of said first heat exchan- ging unit (14).

12. Refrigerating system (10) according to any of claims 9 to 1 1, wherein said balancing assembly (44) comprises a dead volume in which said refrigerant is essentially stagnant, and level detection means (60) ad- apted to detect a level of refrigerant in said dead volume or whether a level of refrigerant in said dead volume has reached a predetermined level, said regulating valve unit (56) being controlled based on a detection signal from said level detection means (60).

13. Refrigerating system (10) according to claim 12, wherein said regulating valve unit (56) is controlled in such a way that said level of refrigerant in said dead volume of said balancing assembly (44) is held at said predetermined level.

14. Refrigerating system (10) according to claim 12,

|l \8\74\74416\090216_apphcatιon_text odt) 2009-02-16 10 54 wherein said regulating valve unit (56) is controlled in such a way that, in case said level of refrigerant in said dead volume of said balancing assembly (44) is detected to be higher than said predetermined level, said regulating valve unit (56) is controlled according to the power consumed by said compressor unit (22, 24).

15. Refrigerating system (10) according to any of claims 9 to 14, wherein said balancing assembly (44) comprises an inner riser tube (48) inserted into an outer riser tube (46), said inner riser tube (48) being in fluid connection to said liquid side (36b) of said second heat exchanging unit (36), said outer riser tube (46) being in fluid connection to a drain of refrigerant (20, 58).

16. Refrigerating system (10) according to any of claims 1 to 15, comprising a plurality of compressor units (22, 24, 26) connected in parallel to each other.

17. Refrigerating system (10) according to any of claims 1 to 16, further comprising at least one heat pump compressor unit (26) having a suction side (26a) and a pressure side (26b), wherein at least in said heat pump assisted cooling mode said heat pump compressor unit (26) is not assigned to any of said cooling units of said cooling circuit, said suction side (26a) of said heat pump compressor unit (26) is connectable to said gas side (14a) of said first heat exchanging unit (14), and said pressure side (26b) of said heat pump compressor unit (26) is connectable to said gas side (36a) of said second heat exchanging unit (36).

18. Refrigerating system according to claim 17, wherein a fifth valve (66) is provided in a line connecting said suction side (26a) of said heat pump compressor unit (26) to the suction sides (22a,

24a) of said compressor units (22, 24) assigned to respective cooling units, said fifth valve (66) being a proportional valve.

[I \8\74\74416\090216_apphcatιon_text odt) 2009-02-16 10 54