WO2013007310A1

WO2013007310A1 - Refrigeration circuit with oil compensation

Info

Publication number: WO2013007310A1
Application number: PCT/EP2011/062063
Authority: WO
Inventors: Sascha HELLMANN; Christoph Kren
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2011-07-14
Filing date: 2011-07-14
Publication date: 2013-01-17
Anticipated expiration: 2014-01-14
Also published as: EP2732222A1; CN103649653B; CN103649653A; EP2732222B1

Abstract

A refrigeration circuit (2) comprises a circulating refrigerant and in the direction of the flow of the refrigerant: at least one compressor (4a, 4b, 4c) of a first type; a heat rejecting heat exchanger (6); an expansion device (8); and an evaporator (10). The refrigeration circuit (2) further comprises at least one compressor (12) of a second type; an oil collector (26), which is selectively connectable to the compressors (4a, 4b, 4c) of the first type and/or to the compressors (12) of the second type for providing a fluid connection between the oil collector (26) and the respective compressors. The oil collector (26) is selectively connectable to a high-pressure line (36) for increasing the pressure within the oil collector (26) in order to transfer oil from the oil collector (26) to the least one compressor (12) of the second type.

Description

Refrigeration Circuit with Oil Compensation

Refrigeration circuits comprising in the direction of the flow of a circulating refrigerant at least one compressor, a heat rejecting heat exchanger, an expansion device and an evaporator are known in the state of the art. It is also known to provide an additional economizer circuit for further cooling the refrigerant leaving the heat rejecting heat exchanger before it is expanded in order to increase the efficiency of the refrigeration circuit.

Such an economizer circuit comprises at least one additional compressor operating at a higher suction pressure than the compressor(s) of the refrigeration circuit.

Due to the higher suction pressure the compressor(s) of the economizer circuit lose more oil, which is used for lubricating the compressors, to the refrigerant circulating within the refrigeration circuit than the compressor(s) of the refrigeration circuit. Over the time of operation this effect results in an accumulation of oil within the compressor(s) of the re- frigeration circuit and a loss of oil of the compressor(s) of the economizer circuit. When the oil level within the compressor(s) of the economizer circuit drops below a minimum level the compressor(s) may be seriously damaged.

Thus, there is a need for transferring oil back from the compressor(s) of the refrigeration circuit to the compressor(s) of the economizer circuit.

Accordingly, it would be beneficial to provide an refrigeration circuit and a method of operating such an refrigeration circuit allowing for oil compensation between compressors operating at different suction levels and in particular allowing a transfer of oil from com- pressors of a first group working at low suction (inlet) pressure to compressors of a second group working at a higher suction (inlet) pressure.

Exemplary embodiments of the invention include a refrigeration circuit circulating a refrigerant and comprising in the direction of the refrigerant's flow at least one compressor of a first group of compressors, a heat rejecting heat exchanger, an expansion device, and an evaporator. The refrigeration circuit further comprises at least one compressor of a second group of compressors and an oil collector, which is selectively connectable to the compressors of the first group of compressors for collecting excess oil from the at least one compressor of the first group of compressors and/or to the compressors of the second group of compressors. The oil collector is selectively connectable to a high-pressure line for increasing the pressure within the oil collector in order to transfer oil from the oil collector to the at least one compressor of the second group of compressors.

Further exemplary embodiments of the invention include a method of transferring oil from at least one compressor of a first group of compressors to at least one compressor of a second group of compressors via an oil collector, the oil collector being selectively connectable to said compressors, the method including the steps of fluidly connecting the oil col- lector to the oil sumps of the compressor(s) of the first group of compressors; decreasing the pressure in the oil collector below the suction pressure of the compressor(s) of the first group of compressors in order to suck oil from the oil sumps of the compressor(s) of the first group of compressors into the oil collector; fluidly connecting the oil collector to the compressor(s) of the second group of compressors; increasing the pressure in the oil col- lector above the suction pressure of the compressor(s) of the second group of compressors in order to supply oil from the oil collector to the compressor(s) of the second group of compressors.

Further exemplary embodiments of the invention include a method of oil compensation between at least two compressors of two different groups of compressors including the steps of monitoring the oil level in at least one compressor of the second group of compressors; and transferring oil from at least one compressor of the first group of compressors to the compressor of the second group of compressors, using a method of transferring oil from at least one compressor of a first group of compressors to at least one compressor of a second group of compressors via an oil collector, if the oil level in the compressor of a second group of compressors has dropped below a predetermined minimum value.

Further exemplary embodiments of the invention include a method of oil compensation between at least two compressors of two different groups of compressors including the steps of monitoring the oil level in at least one compressor of the first group of compressors; and transferring oil from at least one compressor of the first group of compressors to the compressor of the second group of compressors, using a method of transferring oil from at least one compressor of a first group of compressors to at least one compressor of a second group of compressors via an oil collector, if the oil level in at least one compressor of the first group of compressors has risen above a predetermined maximum value.

Exemplary embodiments of the invention are described in greater detail below with reference to the figures, wherein: Figure 1 shows a schematic view of a refrigeration circuit comprising an economizer circuit; Figure 2 shows a schematic view of an arrangement for oil compensation according to a first embodiment the invention;

Figure 3 shows a schematic view of a refrigeration circuit comprising an economizer circuit and a freezing circuit; and

Figure 4 shows a schematic view of an arrangement for oil compensation according to a second embodiment the invention.

Figure 1 shows a schematic view of an exemplary embodiment of a refrigeration circuit 2 comprising in the direction of the flow as indicated by arrows of the refrigerant circulating within the refrigeration circuit 2 a set of compressors 4a, 4b, 4c of a first group of compressors connected in parallel, a heat rejecting heat exchanger 6, an economizer heat exchanger 14, a high pressure valve or high pressure device 17, a refrigerant collector 18, an expansion device 8, and an evaporator 10 connected to the suction (inlet) side of the com- pressors 4a, 4b, 4c.

The refrigeration circuit 2 further comprises in the direction of flow as indicated by arrows an economizer circuit with an economizer expansion device 16, an economizer heat exchanger 14 and an economizer compressor 12.

For operating the economizer circuit a partial refrigerant flow coming from the heat rejecting heat exchanger 6 is branched off the main flow of the refrigerant and expanded by means of the economizer expansion device 16. The expanded refrigerant coming from the economizer expansion device 16 flows through a first portion 14a of the economizer heat exchanger 14 for cooling the main flow of refrigerant coming from the heat rejecting heat exchanger 6 and flowing to the high pressure valve or high pressure device 17 through a second portion 14b of the economizer heat exchanger 14 against the partial flow of expanded refrigerant. After leaving the first portion 14a of the economizer heat exchanger 14 the partial flow of refrigerant is compressed by the economizer compressor 12 and sup- plied back to the inlet of the heat rejecting heat exchanger 6 at the same (high) pressure as the refrigerant supplied by the compressors 4a, 4b, 4c of the first group of compressors. The refrigerant flowing through the economizer circuit is not expanded as much by the economizer expansion device 16 as the refrigerant flowing through the main portion of the refrigeration circuit 2 by the high pressure valve 17 and the expansion device 8. The economizer evaporating temperature and thus the pressure is higher than the evaporating temperature and the pressure of the compressors 4a, 4b, 4c of the first group of compressors. Therefore, the suction density is higher in the economizer suction port.

This pressure difference results in a net transfer of oil from the economizer compressor 12 to the compressors 4a, 4b, 4c of the refrigeration circuit 2, as it has been discussed before.

Figure 2 shows an exemplary embodiment of an oil transfer mechanism according to the invention, which is configured for transferring oil from the compressors 4a, 4b, 4c of a first group of compressors operating at a low suction pressure level to at least one compressor 12 of a second group of compressors operating at a higher suction pressure level than the compressors of the first group.

The compressors 4a, 4b, 4c of the first group operating at low suction pressure level may correspond to the compressors 4a, 4b, 4c of a refrigeration circuit 2 and the compressor 12 of the second group of compressors may correspond to the economizer compressor 12 of the economizer circuit shown in Figure 1.

Refrigerant is supplied at low pressure to the compressors 4a, 4b, 4c of the first group by means of a refrigerant suction line 3 and the compressed refrigerant is supplied back to the refrigeration circuit 2 at high pressure via a high pressure discharge line 5.

Each of the compressors 4a, 4b, 4c of the first group comprises an oil sump 7a, 7b, 7c, and a minimum oil level 9a, 9b, 9c is defined for each of the oil sumps 7a, 7b, 7c.

An excess oil line 22 opens into the oil sumps 7a, 7b, 7c above the respective minimum oil level 9a, 9b, 9c fluidly connecting the oil sumps 7a, 7b, 7c of the compressors 4a, 4b, 4c to each other and to an oil collector 26, which is arranged at a level below the oil compressors 4a, 4b, 4c of the first type.

If the oil level in at least one of the compressors 4a, 4b, 4c of the first type is located above the respective minimum oil level 9a, 9b, 9c, the excess oil line 22 allows oil to flow from each of the oil sumps 7a, 7b, 7c into the oil collector 26. In the embodiment shown in Fig. 2 the oil collector 26 is formed as a pipe which is arranged basically horizontally but with a slight inclination with respect to the horizontal, so that the oil collector 26 has an upper end portion 26a arranged at a higher level than a lower end portion 26b. An oil supply line 30 connected to the lower portion 26b of the oil collector 26 fluidly connects the oil collector 26 to the compressor 12 of the second group of compressors allowing to supply oil from the oil collector 26 via the oil supply line 30 to the oil sump 13 of said compressor 12 of the second group of compressors. An oil supply valve 32 is arranged in the oil supply line 30. If the refrigeration circuit 2 comprises only one compressor 12 of the second group of compressors, the oil supply valve 32 can be a one-way, non-return valve which allows oil to flow from the oil collector 26 to the compressor 12 of the second group of compressors but does not allow oil to flow in the opposite direction from the compressor 12 of the second group of compressors into the oil collector 26. If the refrigeration circuit 2 comprises more than one compressor 12 of the second group of compressors switchable oil supply valves 32, e.g. solenoid valves, may be respectively associated to each of the compressors 12 of the second group of compressors in order to selectively allow and disallow oil to flow from the oil collector 26 to the oil sump 13 of each of the compressors 12 of the second group of compressors.

The upper end portion 26b of the oil collector 26 is connected by means of a high pressure line 36 to the high pressure discharge line 5 of the compressors 4a, 4b, 4c of the first group of compressors. A switchable high pressure valve 38 is arranged within said high pressure line 36 allowing to selectively increase the pressure within the oil collector 26 by opening the high pressure valve 38 and fluidly connecting the oil collector 26 to the high pressure discharge line 5.

The oil collector 26 is further connected by a low pressure line 40 to the refrigerant suction line 3 of the compressors 4a, 4b, 4c of the first group of compressors. The low pressure line 40 opens into the refrigerant suction line 3 by means of an ejector 44 causing, in operation, the pressure in the low pressure line 40 to drop below the pressure in the refrigerant suction line 3.

A switchable low pressure valve 42 is arranged within the low pressure line 40 allowing to selectively reduce the pressure in the oil collector 26 to a value below the suction pressure of the compressors 4a, 4b, 4c of the first group of compressors by opening the low pressure valve 42 and fluidly connecting the oil collector 26 to the ejector 44 arranged at the low pressure line 3. The end walls of the oil collector 26 are respectively provided with transparent inspection glasses 28a, 28b for allowing visual inspection of the oil collected within the oil collector 26. When the refrigeration circuit 2 is operating with the economizer circuit being activated , oil is transferred from the oil sump 13 of the compressor 12 of the second group of compressors to the oil sumps of the compressors 4a, 4b, 4c of the first group of compressors due to the pressure difference between the compressors 4a, 4b, 4c, 12 of the first and second group of compressors, as it has been discussed before.

For collecting the excessive oil accumulating in the oil sumps 7a, 7b, 7c of the compressors 4a, 4b, 4c of the first group of compressors, the switchable low pressure valve 42 is opened fiuidly connecting the oil collector 26 by means of the low pressure line 40 to the ejector 44 arranged in the refrigerant suction line 3 of the compressors 4a, 4b, 4c of the first group of compressors.

The ejector 44 reduces the pressure in the low pressure line 40 and in the oil collector 26 to a value below the suction pressure of the compressors 4a, 4b, 4c of the first group of compressors. Due to this pressure difference between the oil collector 26 and the compres- sors 4a, 4b, 4c oil from an area above the minimum oil levels 9a, 9b, 9c of the oil sumps 7a, 7b, 7c of the compressors 4a, 4b, 4c of the first group of compressors is sucked through the excess oil line 22 into the oil collector 26. In this mode of operation (oil collecting mode), when oil is sucked from the oil sumps 7a, 7b, 7c of the compressors 4a, 4b, 4c of the first group of compressors into the oil collector 26, the high pressure valve 38 and the oil supply valve 32 are closed.

If the oil collector 26 is arranged at a level below the level of the compressors 4a, 4b, 4c of the first group of compressors the difference in height may be large enough so that the low pressure line 40 may not even be necessary for transferring oil from the compressors 4a, 4b, 4c of the first group of compressors to the oil collector 26. In this case a flow of excessive oil from the oil sumps 7a, 7b, 7c into the oil collector 26 may be caused by the difference of hydrostatic pressure between the compressors 4a, 4b, 4c and the oil collector 26.

In order to transfer oil, which has been collected in the oil collector 26 as it has been de- scribed before, from the oil collector 26 to the compressor 12 of the second group of compressors, the low pressure valve 42 is closed while the high pressure valve 38 and the oil supply valve 32 are opened (oil supply mode). Opening the high pressure valve 38 fluidly connects the collector 26 to the high pressure discharge line 5 of the compressors 4a, 4b, 4c of the first group of compressors increasing the pressure within the oil collector 26. An orifice or one-way-valve 24 arranged within the excess oil line 22 avoids that the increased pressure in the oil collector 26 is immediately equalized via the excess oil line 22 into the compressors 4a, 4b, 4c of the first group of compressors.

Instead, the oil which has been collected in the oil collecting mode in the lower end portion 26b of the oil collector 26 is pressed by the increased pressure within the oil collector 26 via the oil supply line 30 and the open oil supply valve 32 into the oil sump 13 of the compressor 12 of the second group of compressors in order to increase the oil level 15 within said compressor 12 of the second group of compressors.

This transfer of oil from the oil collector 26 to the compressor 12 of the second group of compressors provides an effective oil compensation between the compressors 4a, 4b, 4c of the first group of compressors operating at relatively low suction pressure and the compressor 12 of the second group of compressors operating at a higher suction pressure.

After oil has been transferred from the oil collector 26 to the compressor 12 of the second group of compressors, the high pressure valve 38 is closed in order to stop the flow of oil from the oil collector 26 to the compressor 12 of the second group of compressors.

The oil collector 26 may be refilled with oil from the compressors 4a, 4b, 4c of the first group of compressors by reopening the low pressure valve 42 and reducing the pressure within the oil collector 26 to a value below the suction pressure of the compressors 4a, 4b, 4c of the first group of compressors, again.

It is self evident to the skilled person that the embodiment shown in Figure 2 may be used similarly for supplying oil to a plurality of compressors 12 of the second group of com- pressors. The oil sumps 13 of the compressors 12 of the second group of compressors may be connected by an oil compensation line to each other. Alternatively a plurality of oil supply lines 30 may connect each of the compressors 12 individually to the oil collector 26. In this case, each of the compressors 12 of the second group of compressors may be supplied with oil from the oil collector independently of the other compressors 12 of the second group of compressors.

The described method of oil transfer may by controlled with the help of oil sensors 34 which are arranged in at least one of the oil sumps 7a, 7b, 7c, 13 and/or in the oil collector 26. In particular the oil transfer may be started when the oil level in one of the compressors 12 of the second group of compressors falls below the minimum oil level 15 and/or the oil level within the compressors 4a, 4b, 4c of the first group of compressors or the oil collector 26 exceeds a predetermined maximum oil level.

Figure 3 shows a second embodiment of a refrigeration circuit 2, which comprises an additional freezing circuit 54.

The features of the second embodiment which are identical to the features of the first em- bodiment shown in Figure 1 are denoted by the same reference signs and will not be discussed in detail again.

The freezing circuit 54 is configured for generating freezing temperatures in addition to the refrigerating temperatures provided by the refigerating circuit 2. Typical refrigerating temperatures are in the range of 0 °C to 10 °C, whereas freezing temperature are typically in the range of -20 °C to -5 °C.

The freezing circuit 54 comprises a freezing expansion device 52 which is connected to the refrigeration circuit 2 The freezing expansion device 52 is supplied with a portion of the refrigerant circulating within the refrigeration circuit 2. Said portion of the refrigerant is expanded by the freezing expansion device 52 to an even lower pressure than the refrigerant expanded by the expansion device 8 and is evaporated by the freezing evaporator 50 arranged downstream of the freezing expansion device 52 in order to generate freezing temperatures below the temperatures generated by the evaporator 10 of the refrigeration circuit 2.

After leaving the freezing evaporator 50 the refrigerant of the freezing circuit 54 is compressed by a freezing compressor 48 to the suction (inlet) pressure of the compressors 4a, 4b, 4c of the refrigeration circuit 2, i.e. to the same pressure as the refrigerant leaving the evaporator 10 of the refrigeration circuit 2. The freezing compressor 48 is a compressor of a third group of compressors having a suction (inlet) pressure level below the inlet pressure of the compressors 4a, 4b, 4c of the refrigeration circuit 2 and an outlet pressure level identical to the suction (inlet) pressure of the compressors 4a, 4b, 4c of the refrigeration circuit 2. In this non-limiting embodiment, the freezing expansion device 52, the freezing evaporator 50 and the freezing compressor 48 are connected in parallel to the expansion device 8 and the evaporator 10. Figure 4 shows a second exemplary embodiment of an oil compensation system according to the invention which may be operated in combination with a refrigeration circuit 2 comprising a freezing circuit 54 as it is shown in Figure 3. The features which correspond to the features of the first embodiment shown in Figure 2 are denoted by the same reference signs and will not be discussed in detail again.

In the second embodiment shown in Figure 4 the reduced pressure in the oil collector 26, which is provided to suck oil form the oil sumps 7a, 7, 7c of the compressors 4a, 4b, 4c of the first group of compressors into the oil collector 26, is not generated by connecting the low pressure line 40 to the refrigerant suction line 3 of the compressors 4a, 4b, 4c of the first group of compressors by means of an ejector 44 as in the first embodiment shown in Figure 1. Instead, the low pressure line 40 is fluidly connected to an inlet line 46 of a compressor 48 of a third group of compressors operating at a suction (inlet) pressure below the suction (inlet) pressure of the compressors 4a, 4b, 4c of the first group of compressors. This reduces the pressure within the oil collector 26 below the suction (inlet) pressure of the compressors 4a, 4b, 4c of the first group of compressors and causes a flow of oil from the oil sumps 7a, 7b. 7c of the compressors 4a, 4b, 4c of the first group of compressors to the oil collector 26.

The compressor 48 of the third group of compressors may be a freezing compressor 48 of a freezing circuit 54 comprised in the refrigeration circuit 2 as shown in Figure 3.

Using the freezing compressor 48 of a freezing circuit 54 operating at a lower suction pressure than the compressors 4a, 4b, 4c of the first group of compressors for reducing the pressure within the oil collector 26 avoids the need of providing an ejector 44 in the refrigerant suction line 3 of the compressors 4a, 4b, 4c of the first group of compressors. Thus, the costs for providing and maintaining the ejector 44 can be saved.

The refrigeration circuit according to exemplary embodiments as described herein allows for a compressor oil distribution in systems with different crankcase pressure, and it is particularly suited for HFC systems and for sub and transcritical C0₂ systems, although it is not limited thereto. The refrigeration circuit according to exemplary embodiments as described herein allows to transfer oil from compressors of a first group running at low suction pressure to compressors of a second group running at a higher suction pressure, and it is comparably cheap, since it saves costly oil separators on the compressors high pressure side. Moreover, for transcritical C0₂ systems it eliminates the need for and the maintenance of a coalesence filter. The refrigeration circuit according to exemplary embodiments as described herein uses the excess oil in the first group of compressors running at first suction level and forming low-pressure compressors, collects the same in an oil receiver and promotes it depending on demand in the second group of compressors running at second suction level and forming high-pressure compressors.

By the term "group of compressors" any number of compressors, including only one compressor, is to be understood. The number of compressors per group of compressors may vary.

The refrigeration circuit may comprise an economizer circuit having an economizer expansion device for expanding a partial refrigerant flow coming from the heat rejecting heat exchanger; an economizer heat exchanger arranged downstream of the heat rejecting heat exchanger for cooling a portion of the refrigerant flowing from the heat rejecting heat exchanger in the direction to the economizer expansion device and to the evaporator against the expanded partial refrigerant flow; and at least one compressor of the second group of compressors for compressing the refrigerant partial flow to the outlet pressure of the at least one compressor of the first group before feeding the refrigerant partial flow to the pressure line. An economizer circuit increases the efficiency of the refrigeration circuit.

Solenoid or non-return valves can be provided at the at least one compressor of the second group of compressors. The at least one compressor of the second group of compressors can also be called economizer compressor(s).

The refrigeration circuit may comprise a low-pressure line connected to the oil collector for reducing the pressure within the oil collector. Reducing the pressure within the oil collector supports and enhances the flow of oil from the compressors to the oil collector. The refrigeration circuit may comprise at least one excess oil line connecting the oil collector to at least one of the compressors of the first group of compressors. The excess oil line allows oil to flow from the compressors of the first group of compressors to the oil collector. The excess oil line may open into an oil sump of said compressor of the first group of compressors above a defined minimum oil level of said oil sump. Arranging the opening of the excess oil line above a minimum oil level of the oil compressors of the first group of compressors avoids that extracting oil from the oil sumps via the respective excess oil line results in reducing the oil level below the minimum oil level.

The high-pressure line may be connectable to the outlet side of at least one of the compres- sors of the first group. Connecting the high-pressure line to the outlet side of at least one of these compressors allows to increase the pressure in the high-pressure line without the need for an additional pressure generator/compressor.

The refrigeration circuit may comprise a throttle or one-way-valve arranged in the oil ex- cess line. A throttle or one-way-valve arranged in the oil excess line avoids that an increased pressure in the oil collector is equalized immediately via the low-pressure line. Instead, it allows that an increased pressure within the oil collector is maintained for some time in order to transfer oil from the oil collector to the compressors) of the second group of compressors.

The refrigeration circuit may comprise a switchable valve arranged in the oil supply line connecting the oil collector to at least one compressor of the second group of compressors in order to selectively allow and disallow a flow of oil between the oil collector and at least one compressor of the second group of compressors.

The refrigeration circuit may comprise a switchable valve arranged in the low-pressure line, and/or a switchable valve arranged in the high-pressure line in order to selectively connect the oil collector to the high-pressure line and to the low-pressure line for allowing to selectively increase and decrease the pressure within the oil collector. Increasing and decreasing the pressure within the oil collector allows to support the transfer of oil to and from the oil collector.

The low-pressure line may be connectable to the suction side of at least one compressor of the first group of compressors. Connecting the low-pressure line to the suction side of at least one of the compressors provides a simple way of reducing the pressure within the oil collector without the need for an additional pressure reducing device.

The low-pressure line may be connectable to an ejector which is arranged in the suction line of at least one compressor of the first group of compressors and configured for reduc- ing the pressure in the low-pressure line. Connecting the low-pressure line to an ejector allows the pressure in the low-pressure line and in the oil collector to be reduced below the suction pressure of said compressors of the first group of compressors in order to suck oil from the compressors of the first group of compressors into the oil collector. The refrigeration circuit may further comprise a high pressure valve or high pressure device and a refrigerant collecting container which is arranged upstream of the expansion device. This allows to provide a two-stage-expansion circuit comprising a high-pressure portion and an intermediate pressure portion.

The refrigeration circuit may comprise at least one compressor of a third group of compressors having a lower suction pressure than the compressors of the first group of compressors, the low-pressure line being connectable to the inlet side of at least one compressor of said third group of compressors. A compressor of a third group of compressors having a lower suction pressure than the compressors of the first group of compressors provides an alternative for providing a low pressure within the oil collector in order to suck oil from the compressors of the first group of compressors into the oil collector.

The compressor(s) of the first group of compressors may be compressors which are configured for normal refrigerating operation at temperatures of more than 0 °C and the compressors) of the third group of compressors may be compressor(s) configured for freezing operation at temperatures below 0 °C. This allows to provide refrigerating temperatures and freezing temperatures with a single refrigeration circuit.

The oil collector may have the form of a pipe, which is basically arranged horizontally. An oil collector formed as a pipe can be manufactured easily at low costs and does not occupy a lot of space so that it may be integrated easily into the refrigeration circuit. The pipe forming the oil collector may be arranged with a slight inclination with respect to the horizontal in order to allow the oil to collect in a lower portion of the pipe.

The high-pressure line and/or the low pressure line may open to the top of the oil collector. As liquid oil will drop to the bottom of the oil collector this will separate liquid oil from the gas phase and avoid the formation of foam when gaseous refrigerant is introduced into the oil collector via the high-pressure line in order to increase the pressure within the oil collector.

The oil collector may be arranged at a level below the compressors of the first group of compressors. Arranging the oil collector at a level below the compressors of the first group of compressors allows oil to flow from the compressors of the first group of compressors to the oil collector driven by gravity. If the vertical distance between the level of the compressors of the first group of compressors and the level of the oil collector is large enough, there may be no need to decrease the pressure within the oil collector below the suction pressure of the compressors of the first group of compressors, as the oil is transferred from the compressors of the first group of compressors to the oil collector by the force of gravity, alone.

The refrigeration circuit may comprise at least one oil sensor for determining the oil level in at least one of the compressors and/or the oil collector. An oil sensor allows to start and stop the transfer of oil based on the actual oil level within the oil compressors and/or the oil collector, respectively, allowing a very efficient and exact oil compensation between the compressors.

A method for transferring oil from a at least one compressor of a first group of compressors to at least one compressor of a second group of compressors via an oil collector comprises the steps of: fluidly connecting the oil collector to the oil sump(s) of the compressor^) of the first group of compressors; decreasing the pressure in the oil collector below the suction pressure of the compressor(s) of the first group of compressors in order to suck oil from the oil sump(s) of the compressor(s) of the first group of compressors into the oil collector; fluidly connecting the oil collector to the compressor(s) of the second group of compressors; increasing the pressure in the oil collector above the suction pressure of the compressor(s) of the second group of compressors in order to supply oil from the oil collector to the compressor(s) of the second group of compressors.

The step of increasing the pressure in the oil collector may include the step of fluidly connecting the oil collector to the outlet side of at least one of the compressors. This increases the pressure within the oil collector without the need for an additional pressure increasing device.

The step of reducing the pressure may include the step of fluidly connecting the oil collector to the inlet side of at least one of the compressors. This reduces the pressure within the oil collector without the need for an additional pressure reducing device.

The step of reducing the pressure may include the step of fluidly connecting the oil collector to an ejector arranged in the suction line of at least one of the compressors. This allows to reduce the pressure within the oil collector even below the suction pressure of said compressor(s).

A method of oil compensation between at least two compressors may include the steps of monitoring the oil level in at least one compressor of the first group of compressors and starting a transfer of oil from at least one compressor of a first group of compressors to at least one compressor of the second group of compressors when the oil level in the compressor of the first group of compressors has increased above a predetermined value.

A method of oil compensation between at least two compressors may include the steps of monitoring the oil level in at least one compressor of a second group of compressors and starting a transfer of oil from at least one compressor of a first group of compressors to the compressor of the second group of compressors when the oil level in the compressor of the second group of compressors has dropped below a predetermined value. This reliably avoids that the compressor(s) of the second group of compressors are jammed or damaged due to a loss of oil.

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 equivalence my be substitute for elements thereof without departing from the scope of the inven- tion. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention is not limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the pendent claims.

Reference Numerals

2 refrigeration circuit

3 suction line

4a, 4b, 4c compressors of a first type

5 high pressure discharge line

6 heat rejecting heat exchanger

7a, 7b, 7c oil sumps of the compressors of the first type

8 expansion device

9a, 9b, 9c minimum oil levels of the compressors of the first type

10 evaporator

12 compressor of a second type

13 oil sump of the compressor of the second type

14 economizer heat exchanger

14a first portion of the economizer heat exchanger

14b second portion of the economizer heat exchanger

15 oil level of the compressor of the second type

16 economizer expansion device

7 high pressure valve

8 refrigerant collecting container

2 excess oil line

4 one-way-valve

6 oil collector

6a upper end portion of the oil collector

6b lower end portion of the oil collector

8a, 28b inspection glasses

0 oil supply line

2 oil supply valve

4 oil sensor

6 high-pressure line

8 high pressure valve

0 low pressure line

2 switchable valve

4 ejector

6 inlet line of a compressor of a third group

8 compressor of a third group

0 freezing evaporator

2 freezing expansion device freezing circuit

Claims

1. A refrigeration circuit (2) circulating a refrigerant and comprising in the direction of flow:

at least one compressor (4a, 4b, 4c) of a first group of compressors;

a heat rejecting heat exchanger (6);

an expansion device (8); and

an evaporator (10);

the refrigeration circuit (2) further comprising

at least one compressor (12) of a second group of compressors;

an oil collector (26) being selectively connectable to the compressors (4a, 4b, 4c) of the first group of compressors and/or to the compressors (12) of the second group of compressors for collecting excess oil from the at least one compressor (4a, 4b, 4c) of the first group of compressors;

wherein the oil collector (26) is selectively connectable to a high-pressure line (36) for increasing the pressure within the oil collector (26) in order to transfer oil from the oil collector (26) to the at least one compressor (12) of the second group of compressors.

2. Refrigeration circuit (2) of claim 1, wherein the suction pressure of the compressors (12) of the second group of compressors is larger than the suction pressure of the compressors (4a, 4b, 4c) of the first group of compressors.

3. Refrigeration circuit (2) of claim 1 or 2, comprising an economizer circuit with

an economizer expansion device (16) for expanding a partial refrigerant flow coming the heat rejecting heat exchanger (6);

an economizer heat exchanger (14) arranged downstream of the heat rejecting heat exchanger (6) for cooling the refrigerant flowing from the heat rejecting heat exchanger (6) in the direction to the expansion device (8) and to the evaporator (10) against the expanded refrigerant partial flow; and

wherein the at least one compressor (12) of the second group of compressors compresses the refrigerant partial flow to the outlet pressure of the at least one compressor (4a, 4b, 4c) of a first group of compressors before feeding the refrigerant partial flow to the pressure line.

4. Refrigeration circuit (2) of any of the preceding claims, further comprising a low-pressure line (40) connected to the oil collector (26) for reducing the pressure in the oil collector (26).

5. Refrigeration circuit (2) of any of the preceding claims, further comprising at least one excess oil line (22) connecting at least one of the compressors (4a, 4b, 4c) of the first group of compressors to the oil collector (26), wherein the excess oil line (22) opens into an oil sump (7a, 7b, 7c) of said compressor (4a, 4b, 4c) above a minimum oil level (9a, 9b, 9c).

6. Refrigeration circuit (2) of any of the preceding claims, wherein the high pressure discharge line (36) is connectable to the pressure line (5) of at least one of the compressors (4a, 4b, 4c) of the first group of compressors.

7. Refrigeration circuit (2) of any of the preceding claims, further comprising a throttle or one-way-valve (24) arranged in the excess oil line (22).

8. Refrigeration circuit (2) of any of the preceding claims, further comprising an oil sup- ply valve (32) arranged in the oil supply line (30) connecting the oil collector (26) to at least one compressor (12) of the second group of compressors.

9. Refrigeration circuit (2) of any of the preceding claims, further comprising a switch- able valve (42) arranged in the low-pressure line (40) and/or a switchable valve (38) ar- ranged in the high-pressure line (36).

10. Refrigeration circuit (2) of any of the preceding claims, wherein the low-pressure line (40) is connectable to a suction line (3) of at least one compressor (4a, 4b, 4c) of the first group of compressors.

11. Refrigeration circuit (2) of claim 10, wherein the low-pressure line (40) is connectable to an ejector (44) which is arranged in the suction line (3) of at least one compressor (4a, 4b, 4c) of the first group of compressors and configured for reducing the pressure in the low-pressure line (40).

12. Refrigeration circuit (2) of any of the preceding claims, further comprising a high pressure valve (17) and a refrigerant collecting container (18) being arranged, in the direction of flow of the refrigerant, between the heat rejecting heat exchanger (6) and the expansion device (8).

13. Refrigeration circuit (2) of any of the preceding claims, further comprising at least one compressor (48) of a third group of compressors having a lower suction pressure than the compressors (4a, 4b, 4c) of the first group of compressors, wherein the low-pressure line (40) is connectable to the inlet of at least one compressor (48) of the third group of compressors.

14. Refrigeration circuit (2) of claim 13, wherein the compressors (4a, 4b, 4c) of the first group of compressors are compressors which are configured (4a, 4b, 4c) for normal refrigerating operation and the compressors of the third group of compressors (48) are compressors which are configured for freezing operation.

15. Refrigeration circuit (2) of any of the preceding claims, wherein the oil collector (26) has the form of a pipe which is arranged basically horizontally.

16. Refrigeration circuit (2) of any of the preceding claims, wherein the high-pressure line (36) and/or the low-pressure line (40) are connected to the top of the oil collector (26).

17. Refrigeration circuit (2) of any of the preceding claims, wherein the oil collector (26) is arranged at a level below the compressors (4a, 4b, 4c) of the first group of compressors.

18. Refrigeration circuit (2) of any of the preceding claims comprising at least one oil sensor (34) for sensing the oil level of at least one of the compressors (4a, 4b, 4c, 12, 48) and/or the oil level in the oil collector (26).

19. Method of transferring oil from at least one compressor (4a, 4b, 4c) of a first group of compressors to at least one compressor (12) of a second group of compressors via an oil collector (26), the oil collector (26) being selectively connectable to said compressors (4a, 4b, 4c, 12), the method including the steps of:

fluidly connecting the oil collector (26) to the oil sumps (7a, 7b, 7c) of the compressors) (4a, 4b, 4c) of the first group of compressors;

decreasing the pressure in the oil collector (26) below the suction pressure of the compressors) (4a, 4b, 4c) of the first group of compressors in order to suck oil from the oil sumps (7a, 7b, 7c) of the compressor(s) (4a, 4b, 4c) of the first group of compressors into the oil collector (26);

fluidly connecting the oil collector (26) to the compressor(s) (12) of the second group of compressors;

increasing the pressure in the oil collector (26) and/or decreasing the suction pressure of the compressor(s) (12) of the second group of compressors so that the pressure in the oil collector (26) becomes bigger than the suction pressure of the compressor(s) (12) of the second group of compressors in order to supply oil from the oil collector (26) to the compressors) (12) of the second group of compressors.

20. Method of claim 19, wherein the step of increasing the pressure in the oil collector (26) includes fluidly connecting the oil collector (26) to the pressure line of at least one of the compressors (4a, 4b, 4c, 12, 48).

21. Method of claim 19 or 20, wherein the step of reducing the pressure in the oil collector (26) includes fluidly connecting the oil collector (26) to the suction line of at least one of the compressors (4a, 4b, 4c, 12, 48).

22. Method of claim 19 or 20, wherein the step of reducing the pressure in the oil collector (26) includes fluidly connecting the oil collector (26) to an ejector (44) arranged in the suction line of at least one of the compressors (4a, 4b, 4c, 12, 48),

23. Method of oil compensation between at least two compressors (4a, 4b, 4c, 12) of two different groups of compressors including the steps of:

monitoring the oil level in at least one compressor (12) of the second group of compressors; and

transferring oil from at least one compressor (4a, 4b, 4c) of the first group of compressors to the compressor (12) of the second group of compressors, using the method of any of claims 19 to 22, if the oil level in the compressor (12) of a second group of compressors has dropped below a predetermined minimum value.

24. Method of oil compensation between at least two compressors (4a, 4b, 4c, 12) of two different groups of compressors including the steps of:

monitoring the oil level in at least one compressor (4a, 4b, 4c) of the first group of compressors; and

transferring oil from at least one compressor (4a, 4b, 4c) of the first group of compressors to the compressor (12) of the second group of compressors, using the method of any of claims 19 to 22, if the oil level in at least one compressor (4a, 4b, 4c) of the first group of compressors has risen above a predetermined maximum value.