VAPOUR COMPRESSIONS SYSTEM

FIELD OF THE INVENTION

The present invention relates to a method for controlling a valve arrangement

interconnecting at least one oil separator and an oil receiver of a vapour compression system. According to the invention, the valve arrangement is controlled in accordance with a pressure difference between a pressure prevailing inside the oil separator(s) and a pressure prevailing inside the oil receiver.

BACKGROUND OF THE I NVENTI ON Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise at least one compressor, a heat rejecting heat exchanger, e.g. in the form of a condenser or a gas cooler, an expansion device, e.g. in the form of an expansion valve, and an evaporator arranged along a refrigerant path. Thereby refrigerant flowing in the refrigerant path is alternatingly compressed by the compressor(s) and expanded by the expansion device. Heat exchange takes place in the heat rejecting heat exchanger and in the evaporator, in such a manner that heat is rejected from the refrigerant flowing through the heat rejecting heat exchanger and heat is absorbed by the refrigerant flowing through the evaporator. Thereby the vapour compression system may provide cooling for a closed volume, via heat exchange taking place at the evaporator, or heating for a closed volume, via heat exchange taking place at the heat rejecting heat exchanger.

Normally, compressors need lubrication, e.g. in the form of oil, in order to operate properly. This introduces the risk that some oil leaves the compressor along with the compressed refrigerant during operation of the vapour compression system. In order to avoid that the oil is transferred to the entire vapour compression system along with the refrigerant, one or more oil separators may be arranged in the refrigerant path downstream relative to the compressor(s). In the oil separator(s) the oil is separated from the compressed refrigerant, and the refrigerant is passed on to the heat rejecting heat exchanger, while the oil is retained in the oil separator(s) .

The oil separator(s) may further be connected to an oil receiver, in which oil can be collected before being returned to the compressor(s). In this case, the flow of oil from the oil separator(s) to the oil receiver may be controlled by means of a valve arrangement. The valve arrangement interconnecting the oil separator(s) and the oil receiver may be controlled in such a manner that, when it is desired to supply oil from the oil separator(s) to the oil receiver, and open/close sequence of the valve arrangement is performed, and a duration of an open time of the open/close sequence defines an effective opening degree of the valve arrangement. In some prior art vapour compression systems, the duration of the open time of the open/close sequence is selected as a substantially fixed value, which is expected to provide a suitable operation of the valve arrangement under most operating conditions. However, the duration of the open time is not adjusted in response to changes in operating conditions. The prevailing operating conditions may have an impact on the fluid flow from the oil separator(s) to the oil receiver, at a given duration of the open time of the open/close sequence of the valve arrangement. Accordingly, a substantially fixed duration of the open time may result in inaccurate control of the fluid flow from the oil separator(s) to the oil receiver, via the valve arrangement.

DESCRI PTI ON OF THE I NVENTI ON It is an object of embodiments of the invention to provide a method for controlling a valve arrangement interconnecting at least one oil separator and an oil receiver of a vapour compression system, the method providing improved accuracy of the control of fluid flow through the valve arrangement.

It is a further object of embodiments of the invention to provide a method for controlling a valve arrangement interconnecting at least one oil separator and an oil receiver of a vapour compression system, the method providing accurate control of fluid flow through the valve arrangement, regardless of the prevailing operating conditions.

The invention provides a method for controlling a valve arrangement interconnecting at least one oil separator and an oil receiver, the valve arrangement, the oil separator(s) and the oil receiver being arranged in a vapour compression system, the vapour compression system further comprising at least one compressor, a heat rejecting heat exchanger, an expansion device and an evaporator, arranged along a refrigerant path, wherein each oil separator is connected to an outlet of the compressor(s) and the oil receiver is connected to an oil supply inlet of each of the compressor(s), the method comprising the steps of: - obtaining a pressure difference between a pressure prevailing inside the oil

separator(s) and a pressure prevailing inside the oil receiver, deriving a duration for an open time of an open/close sequence of the valve arrangement, based on the obtained pressure difference, and controlling the valve arrangement in accordance with the derived duration of an open tim e.

The method according to the invention is a method for controlling a valve arrangement. In the present context the term 'valve arrangement' should be interpreted to mean an arrangement comprising one or more valves, and fluid flow through the valve arrangement can be controlled by appropriately controlling the valve(s) of the valve arrangement.

The valve arrangement interconnects at least one oil separator and an oil receiver.

Accordingly, a fluid flow between the oil separator(s) and the oil receiver can be controlled by appropriately controlling the valve(s) of the valve arrangement. In the case that the valve arrangement interconnects two or more oil separators and the oil receiver, a separate valve may be provided for each oil separator, thereby allowing control of fluid flows between each of the oil separators and the oil receiver separately.

The valve arrangement, the oil separator(s) and the oil receiver are arranged in a vapour compression system. The vapour compression system further comprises at least one compressor, a heat rejecting heat exchanger, an expansion device and an evaporator, arranged along a refrigerant path. Each oil separator is connected to an outlet of the com pressor(s) and the oil receiver is connected to an oil supply inlet of each of the com pressor(s) . Accordingly, the oil separator(s) is/ are arranged in the refrigerant path downstream relative to the com pressor(s) , and oil which leaves the com pressor(s) along with the compressed refrigerant can be separated from the refrigerant in the oil separator(s) , in the manner described above. The oil can subsequently be returned to the compressor(s), via the oil receiver and the oil supply inlet of the com pressor(s) . From the oil separator(s) , the refrigerant is supplied to the heat rejecting heat exchanger.

In the present context the term 'vapour compression system' should be interpreted to mean any system in which a flow of fluid medium, such as refrigerant, circulates and is

alternatingly compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, the vapour compression system could, e.g., be a refrigeration system, an air condition system or a heat pump.

According to the method of the invention, a pressure difference between a pressure prevailing inside the oil separator(s) and a pressure prevailing inside the oil receiver is obtained. Since the valve arrangement interconnects the oil separator(s) and the oil receiver, the obtained pressure difference represents a pressure difference across the valve arrangement. Next, a duration for an open time of an open/close sequence of the valve arrangement is derived, based on the obtained pressure difference. In the present context the term

'open/close sequence' should be interpreted to mean a sequence in which the valve(s) of the valve arrangement is/are alternatingly opened and closed in accordance with a specific operating pattern. The open/close sequence may be initiated when it is desired to allow a fluid flow from the oil separator(s) to the oil receiver, and the open/close sequence may be stopped when such a fluid flow is no longer desired.

Accordingly, the valve(s) is/are opened for a specified open time, then closed for a specified closed time, opened for a specified open time, etc. The duration of the open time, relative to the duration of the closed time, defines an effective opening degree of the valve(s), while the open/close sequence is running. Thus, the duration of the open time of the open/close sequence has an impact on the fluid flow from the oil separator(s) to the oil receiver, in such a manner that a decrease in the duration of the open time results in a decrease in the fluid flow, and an increase in the duration of the open time results in an increase in the fluid flow. The pressure prevailing inside the oil receiver and/or the pressure prevailing inside the oil separator(s) may vary significantly in response to changes in operating conditions. For instance, seasonal changes may have a significant impact on the pressure prevailing inside the oil receiver and/or the oil separator(s) . For instance, in some regions the pressure difference may be in the order of 5 bar during the winter, but may be as high as 100 bar during the summer. Furthermore, the prevailing pressure levels may have a significant impact on the fluid flow from the oil separator(s) to the oil receiver. For instance, for a given open time of the open/close sequence of the valve arrangement, and thereby a given effective opening degree of the valve arrangement, a large pressure difference across the valve arrangement can be expected to provide a higher fluid flow from the oil separator(s) to the oil receiver than a somewhat lower pressure difference. Accordingly, under some operating conditions an open time of a specific duration may be insufficient to provide an oil level in the oil receiver, which is sufficient to ensure an appropriate oil supply to the compressor(s), while the same duration of the open time may cause overflow of the oil receiver under different operating conditions. Thus, operating the valve arrangement with a fixed open time of the open/close sequence may result in a very inaccurate control of the fluid flow through the valve arrangement, i.e. from the oil separator(s) to the oil receiver. This is very undesirable.

It is therefore an advantage of the present invention that the duration of the open time of the open/close sequence of the valve arrangement is derived based on the pressure difference between the pressure prevailing inside the oil separator(s) and the pressure prevailing inside the oil receiver. Thereby it is obtained that the duration of the open time of the open/close sequence always takes prevailing operating conditions into account, including pressure conditions arising as a consequence of seasonal changes. As a consequence, the valve arrangement can be operated to accurately obtain a fluid flow from the oil separator(s) to the oil receiver, which is appropriate, regardless of the prevailing operating conditions.

Finally, the valve arrangement is controlled in accordance with the derived duration of an open time, i.e. the valve arrangement is controlled with an open/close sequence having a duration of the open time which corresponds to the derived duration of the open time.

Accordingly, the valve arrangement is operated in such a manner that prevailing operating conditions are taken into account, and thereby the fluid flow from the oil separator(s) to the oil receiver is accurately controlled. Furthermore, due to the accurate control of the fluid flow, it can be ensured that the oil level in the oil receiver is always sufficient to ensure oil supply to the compressor(s), without risking an overflow of the oil receiver.

The step of obtaining a pressure difference may comprise measuring the pressure prevailing inside the oil separator(s) and the pressure prevailing inside the oil receiver, and deriving the pressure difference from the measured pressures. According to this embodiment, the pressures prevailing inside the oil separator(s) and inside the oil receiver, respectively, are measured directly. As an alternative, at least one of the pressures may be obtained in another manner, e.g. by deriving the pressure from another measured parameter. The step of deriving the pressure difference may comprise estimating a current pressure prevailing inside the oil receiver, based on the measured value of the pressure prevailing inside the oil receiver. I n some cases there may be a delay, e.g. in the order of 1 -2 seconds, from a measurement of a pressure prevailing inside the oil receiver is performed, by means of a pressure sensor, and until the measured pressure value is actually received at a controller. The pressure prevailing inside the oil receiver may vary at a time scale which is faster than this. In this case the pressure prevailing inside the oil receiver, at the point in time where the measured pressure value is received at the controller, may be estimated based on the measured pressure value.

As an alternative, the measured pressure value may be used directly. The step of deriving a duration for an open time may comprise selecting a duration which is decreasing as a function of increasing pressure difference. As described above, at a given duration of the open time of the open/close sequence, a high pressure difference across the valve arrangement may result in a higher fluid flow than a low pressure difference. Therefore, in order to obtain a given fluid flow through the valve, a long duration of the open time should be selected at small pressure differences, and a short duration of the open time should be selected at large pressure differences.

The step of deriving a duration for an open time may comprise consulting a graph or a lookup table. In this case, the graph or look-up table could be provided empirically, or it could be provided by theoretical calculations.

As an alternative, the step of deriving a duration for an open time may comprise calculating the duration, using a formula of the kind: where At is the duration for the open time, k is a constant, p _d is the density of fluid received in the oil receiver, p _u is the density of fluid leaving the oil separator(s) , and Ap is the obtained pressure difference.

According to this embodiment the duration of the open time varies as the inverse square root of the pressure difference.

The fluid flowing from the oil separator(s) to the oil receiver, via the valve arrangement, may be in the form of oil, in the form of gaseous refrigerant, or in the form of a mixture of oil and gaseous refrigerant. For instance, the fluid may be mainly in the form of oil when the oil level in the oil separator(s) is above a certain level, and the fluid may be mainly in gaseous form when the oil level in the oil separator(s) is low, i.e. when only a small amount of oil has been collected in the oil separator(s) . In the case that the fluid is mainly in the form of gaseous refrigerant, the density of the fluid leaving the oil separator(s) differs from the density of the fluid received in the oil receiver. Thereby the term -η= is not constant, but depends on the actual densities of the fluid in the oil separator(s) and in the oil receiver, respectively. However, in the case that the fluid is mainly in the form of oil, the densities can be assumed to be constant, and thereby the term

Thus, in this case the duration of the open

time can be derived using a formula of the kind: Assume that a user specifies a duration of an open time for an open/close sequence, corresponding to some typical operating conditions. The specified duration should now be scaled to correspond to other operating conditions, which will be marked by a prime in the equations below. In the case that the fluid flow through the valve arrangement is mainly in the form of oil, the scaled duration can be calculated as:

This may, e.g., be the case when there is still oil in the oil separator(s) being connected to the oil receiver via the valve arrangement. In the case that the fluid flow through the valve arrangement is mainly in the form of gaseous refrigerant, the scaled duration can be calculated as:

This may, e.g., be the case when there is no or almost no oil left in the oil separator(s) being connected to the oil receiver via the valve arrangement. In the case that a large amount of gas is dissolved in the oil, it is likely that it outgasses when passing the valve arrangement, due to the pressure drop as it passes the valve arrangement. In this case, oil with gas dissolved enters the valve arrangement, but a mixture of gas and oil leaves the valve arrangement. Accordingly, the formula above applies in this case, because the density change is large, i.e. p _u is much larger than p _d , and p _d should in this case be selected as the bulk density of the mixture.

According to an alternative embodiment, the duration of the open time may be calculated using another kind of formula. For instance, the open time may decrease linearly as a function of the pressure difference.

According to one embodiment, the vapour compression system may comprise at least two oil separators, in which case the valve arrangement may be arranged to separately control fluid flow from each of the oil separators to the oil receiver, and the method may further comprise the step of selecting one of the oil separators, and the step of controlling the valve arrangement may comprise controlling a valve interconnecting the selected oil separator and the oil receiver in accordance with the derived duration of an open time.

According to this embodiment, at least two oil separators are arranged in the vapour compression system downstream relative to the compressor(s), and the fluid flow from each of these oil separators to the oil receiver can be controlled separately and independently of the fluid flow from the other oil separator(s) to the oil receiver. Furthermore, when it is desired to establish a fluid flow from the oil separators to the oil receiver, it is possible to select from which of the oil separators the fluid should be delivered.

The step of selecting one of the oil separators may comprise the steps of: - obtaining an oil level in each of the oil separators, obtaining an oil level in the oil receiver, and comparing the oil level in the oil receiver to a predefined threshold value, in the case that the oil level in the oil receiver is below the threshold value, selecting the oil separator having the highest oil level, and - in the case that the oil level in the oil receiver is above the threshold value, selecting the oil separator having the lowest oil level.

In the case that the oil level in the oil receiver is low, there is a risk that the oil level becomes insufficient to ensure the oil supply to the com pressor(s) . Therefore, in this case it is desirable to provide a large amount of oil from the oil separator(s) to the oil receiver.

Accordingly, the oil separator having the highest oil level is selected, because this oil separator will most likely be capable of supplying mainly oil to the oil receiver.

On the other hand, in the case that the oil level in the oil receiver is high, there is a risk that the oil receiver overflows. Therefore, in this case it is desirable to provide a small amount of oil from the oil separator(s) to the oil receiver. Accordingly, the oil separator having the lowest oil level is selected, because this oil separator will most likely supply fluid in the form of gaseous refrigerant or in the form of a mixture of oil and gaseous refrigerant. The gaseous refrigerant supplied to the oil receiver can easily be removed from the oil receiver via an overflow system. Furthermore, the gaseous part of the fluid can be used for regulating the pressure prevailing inside the oil receiver. As an alternative, the vapour compression system may comprise only one oil separator, and the valve arrangement may be arranged to control the fluid flow from that oil separator to the oil receiver.

The method may further comprise the step of controlling a supply of oil from the oil receiver to each of the compressors. This may, e.g., be obtained by means of a suitable valve arrangement interconnecting the oil receiver and the com pressor(s) .

The oil supply from the oil receiver to the compressor(s) may, e.g., be controlled based on a control input received from one or more level switches arranged in the com pressor(s) . In this case oil may be supplied from the oil receiver to one or more compressors in the case that the level switch(es) reveal(s) that an oil level inside the com pressor(s) is approaching a minimum acceptable level.

As an alternative, the oil supply from the oil receiver to the com pressor(s) may be controlled based on a pressure difference between a pressure prevailing in a suction line connected to the com pressor(s) and a pressure prevailing inside the oil receiver. This could, e.g., be performed in a manner which is very similar to the manner in which the fluid flow from the oil separator(s) to the oil receiver is controlled. For instance, a duration of an open time of an open/close sequence of a valve arrangement interconnecting the oil receiver and the compressor(s) could be derived using the equations described above.

Accordingly, the step of controlling a supply of oil from the oil receiver to each of the compressors may comprise the steps of: obtaining a pressure difference between a pressure prevailing inside the oil receiver and a suction pressure for the com pressor(s) , deriving a duration for an open time of an open/close sequence of a valve

arrangement interconnecting the oil receiver and the com pressor(s) , based on the obtained pressure difference, and controlling the valve arrangement interconnecting the oil receiver and the

com pressor(s) in accordance with the derived duration of an open time.

This is very similar to the control of the fluid flow from the oil separator(s) to the oil receiver described above. Thus, according to this embodiment, the supply of oil to the compressor(s) from the oil receiver can be accurately controlled, regardless of the prevailing operating conditions. The method may further comprise the steps of: supplying a pulse of fluid from an oil separator to the oil receiver by opening at least one valve of the valve arrangement for a predefined duration, monitoring a pressure prevailing inside the oil separator in response to the supplied pulse of fluid, and determining whether oil or gas was supplied from the oil separator to the oil receiver, based on the monitored pressure.

According to this embodiment, a pulse of fluid is initially supplied from an oil separator to the oil receiver by opening at least one valve of the valve arrangement for a predefined duration. The predefined duration could, e.g., be an appropriate open time derived in the manner described above.

After the pulse of fluid has been supplied, a pressure prevailing inside the oil separator is monitored. In the case that the fluid being supplied from the oil separator to the oil receiver is mainly in the form of oil, the expected impact on the pressure prevailing inside the oil receiver is limited. In other words, if the fluid is mainly in the form of oil, the pressure prevailing inside the oil receiver will remain substantially constant in response to the supplied pulse of fluid.

On the other hand, in the case that the fluid being supplied from the oil separator to the oil receiver is mainly in the form of gaseous refrigerant, a significant increase of the pressure prevailing inside the oil receiver can be expected.

Thus, if the pressure prevailing inside the oil receiver remains substantially constant in response to the supplied pulse of fluid, it can be concluded that the supplied fluid was mainly in the form of oil. Similarly, if the pressure prevailing inside oil receiver increases significantly in response to the supplied pulse of fluid, it can be concluded that the supplied fluid was mainly in the form of gaseous refrigerant.

Accordingly, in this manner it can be determined whether oil or gas is being supplied from a given oil separator to the oil receiver. This may further be used for determining an oil level inside the oil separator, since an oil separator having oil collected therein will most likely supply oil to the oil receiver, while an oil separator having no or only a small amount of oil collected therein will most likely supply gas or a mixture of oil and gas to the oil receiver. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a diagrammatic view of a vapour compression system being controlled in accordance with a method according to an embodiment of the invention,

Fig. 2 illustrates a part of a vapour compression system comprising one oil separator,

Fig. 3 illustrates a part of a vapour compression system comprising two oil separators, and

Fig. 4 is a graph illustrating a duration of an open time as a function of a pressure difference, derived in accordance with a method according to an embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagrammatic view of a vapour compression system 1 being controlled in accordance with a method according to an embodiment of the invention.

The vapour compression system 1 comprises a number of compressors 2, four of which are shown, arranged in a compressor rack, a heat rejecting heat exchanger 3, a high pressure valve 4, a receiver 5, an expansion valve 6 and an evaporator 7 arranged along a refrigerant path. The vapour compression system 1 further comprises two oil separators 8, an oil receiver 9 and a valve arrangement 10 interconnecting the oil separators 8 and the oil receiver 9.

Refrigerant flowing in the refrigerant path is compressed by the compressors 2 and supplied to the oil separators 8. Some oil may leave the compressors 2 along with the compressed refrigerant, and in the oil separators 8 the refrigerant and the oil is separated. The refrigerant is supplied to the heat rejecting heat exchanger 3, while the oil is collected in the oil separators 8.

In the heat rejecting heat exchanger 3 heat exchange takes place between the refrigerant and the ambient or a secondary fluid flow across the heat rejecting heat exchanger 3, in such a manner that heat is rejected from the refrigerant. From the heat rejecting heat exchanger 3 the refrigerant is passed through the high pressure valve 4 and further on to the receiver 5. In the receiver 5 gaseous refrigerant is separated from liquid refrigerant. The liquid part of the refrigerant is passed on to the expansion valve, while the gaseous part of the refrigerant is supplied directly to one of the compressors 2b. This compressor 2b may be referred to as a receiver compressor 2b.

When passing through the expansion valve 6 the refrigerant undergoes expansion before being supplied to the evaporator 7. Thereby the refrigerant being supplied to the evaporator 7 is in a mixed gaseous and liquid state. In the evaporator 7 the gaseous part of the refrigerant is at least partly evaporated, while heat exchange takes place with the ambient or with a secondary fluid flow across the evaporator 7, in such a manner that heat is absorbed by the refrigerant.

Finally, the refrigerant leaving the evaporator 7 is supplied to the remaining three compressors 2a. These compressors 2a may be referred to as main compressors 2a.

The oil which is collected in the oil separators 8 can be supplied to the oil receiver 9 via the valve arrangement 10 in a manner which will be described in further detail below with reference to Figs. 2 and 3. Furthermore, the oil supplied to the oil receiver 9 can be returned to the compressors 2 by appropriately controlling valves 11 interconnecting the oil receiver and the compressors 2. This will also be described in further detail below with reference to Figs. 2 and 3. An overflow valve 12 interconnects the oil receiver 9 and the receiver 5. Thereby gaseous refrigerant being supplied from the oil separators 8 to the oil receiver 9 can be returned to the refrigerant path via the overflow valve 12 and the receiver 5.

Fig. 2 illustrates a part of a vapour compression system, which could, e.g., be similar to the vapour compression system 1 illustrated in Fig. 1. The vapour compression system of Fig. 2 comprises only one oil separator 8.

As described above, compressed refrigerant leaving the compressors 2 is supplied to the oil separator 8, possibly along with some oil from the compressors 2. In the oil separator 8 the refrigerant and the oil is separated, and the refrigerant is supplied to the heat rejecting heat exchanger while the oil is collected in the oil separator 8. When it is desired to supply the collected oil from the oil separator 8 to the oil receiver 9, a pressure difference between a pressure prevailing inside the oil separator 8 and a pressure prevailing inside the oil receiver 9 is obtained. This may, e.g., be performed by measuring the pressure prevailing inside the oil separator 8 and the pressure prevailing inside the oil receiver 9, using appropriate pressure sensors, and deriving the pressure difference from the measured pressure values.

Next, a duration of an open time of an open/close sequence of the valve arrangement 10 interconnecting the oil separator 8 and the oil receiver 9 is derived, based on the obtained pressure difference, and the valve arrangement 10 is controlled in accordance with the derived duration of the open time. As described above, this allows the valve arrangement 10 to be accurately controlled to obtain a desired fluid flow from the oil separator 8 to the oil receiver 9, regardless of the prevailing operating conditions. Thereby an appropriate oil level in the oil receiver 9 can be obtained, which ensures that a sufficient oil supply can be provided to the compressors 2, without risking that oil is supplied to the receiver 5 via the overflow valve 12.

Furthermore, oil can be supplied from the oil receiver 9 to the compressors 2, via valves 11 , whenever this is required. The valves 11 may, e.g., be controlled based on a control input from level sensors arranged in the com pressors 2. In this case a valve 11 can be opened when the corresponding level sensor indicates that the oil level in the compressor 2 is approaching a minimum level.

As an alternative, the valves 11 may be controlled in a manner which is similar to the control of the valve arrangement 10 interconnecting the oil separator 8 and the oil receiver 9. In this case a pressure difference between a pressure prevailing inside the oil receiver 9 and a suction pressure for the relevant compressor 2 is obtained, and a duration of an open time of the open/close sequence of the relevant valve 11 is derived, based on the obtained pressure difference. Thereby the supply of oil to the compressors 2 can be accurately controlled, regardless of the prevailing operating conditions. Fig. 3 illustrates a part of vapour compression system, which could, e.g., be similar to the vapour compression system 1 illustrated in Fig. 1. The vapour compression system in this case comprises two oil separators 8, each being connected to the oil receiver 9 via a separate valve of the valve arrangement 10. The valve arrangement 10 and the valves 11

interconnecting the oil receiver 9 and the compressors 2 may be controlled essentially in the manner described above with reference to Fig. 2.

However, since the vapour compression system illustrated in Fig. 3 comprises two oil separators 8, the method for controlling the valve arrangement 10 may further comprise selecting from which of the oil separators 8 fluid should be supplied to the oil receiver 9. This may, e.g., be done in the following manner. When it is desired to supply oil to the oil receiver 9, the oil levels in each of the oil separators 8, and the oil level in the oil receiver 9 are obtained. The oil level in the oil receiver 9 is compared to a predefined threshold value.

In the case that the oil level in the oil receiver 9 is below the threshold value, the oil separator 8 having the highest oil level is selected, and the valve arrangement 10 is subsequently operated in such a manner that fluid is allowed to flow from the selected oil separator 8 to the oil receiver 9, but not from the other oil separator 8 to the oil receiver 9. In the case that the oil level in the oil receiver 9 is low, there is a risk that a sufficient oil supply to the compressors 2 can not be ensured, and therefore it is necessary to ensure that a relatively large amount of oil is supplied to the oil receiver 9. Accordingly, the oil separator 8 having the highest oil level is selected, because this oil separator 8 is most likely to supply mainly oil to the oil receiver 9.

On the other hand, in the case that the oil level in the oil receiver 9 is above the threshold value, the oil receiver 8 having the lowest oil level is selected, and the valve arrangement 10 is subsequently operated in such a manner that fluid is allowed to flow from the selected oil separator 8 to the oil receiver 9, but not from the other oil separator 8 to the oil receiver 9. In the case that the oil level in the oil receiver 9 is high, there is a risk that oil will be supplied to the receiver via the overflow valve 12. Accordingly, the oil separator 8 having the lowest oil level is selected, because this oil separator 8 is most likely to supply mainly gaseous refrigerant or a mixture of oil and gaseous refrigerant to the oil receiver 9. Thereby the oil level in the oil receiver 9 is not significantly increased.

Fig. 4 is a graph illustrating a duration of an open time as a function of a pressure difference, derived in accordance with a method according to an embodiment of the invention. The open time decreases as the pressure difference increases. More particularly, the open time varies as the inverse square root of the pressure difference.