A METHOD FOR CONTROLLING PRESSURE AND OIL LEVEL IN AN OIL RECEIVER OF A VAPOUR COMPRESSIONS SYSTEM

Abstract

A method for controlling a valve arrangement (10) interconnecting at least one oil separator (8) and an oil receiver (9) in a vapour compression system (1) is disclosed. A pressure difference between a pressure prevailing inside the oil separator(s) (8) and a pressure prevailing inside the oil receiver (9) is obtained. Then a duration for an open time of an open/close sequence of the valve arrangement (10) is derived, based on the obtained pressure difference, and the valve arrangement (10) is controlled in accordance with the derived duration of an open time. The supply of oil to the oil receiver (9) can be accurately controlled, regardless of the operating conditions.

Claims

1. 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 time.

2. The method according to claim 1, wherein the step of obtaining a pressure difference comprises 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.

3. The method according to claim 2, wherein the step of deriving the pressure difference comprises estimating a current pressure prevailing inside the oil receiver, based on the measured value of the pressure prevailing inside the oil receiver.

4. The method according to claim 1, wherein the step of deriving a duration for an open time comprises selecting a duration which is decreasing as a function of increasing pressure difference.

5. The method according to claim 1, wherein the step of deriving a duration for an open time comprises consulting a graph or a look-up table.

6. The method according to claim 1, wherein the step of deriving a duration for an open time comprises calculating the duration, using a formula of the kind: $.Math..Math.t=\frac{k.Math.{}_d}{\sqrt{{}_u}}.Math.\frac{1}{\sqrt{.Math..Math.p}},$ where t is the duration for the open time, k is a constant, .sub.d is the density of fluid received in the oil receiver, .sub.u is the density of fluid leaving the oil separator(s), and p is the obtained pressure difference.

7. The method according to claim 1, wherein the vapour compression system comprises at least two oil separators, wherein the valve arrangement is arranged to separately control fluid flow from each of the oil separators to the oil receiver, and wherein the method further comprises the step of selecting one of the oil separators, and the step of controlling the valve arrangement comprises controlling a valve interconnecting the selected oil separator and the oil receiver in accordance with the derived duration of an open time.

8. The method according to claim 7, wherein the step of selecting one of the oil separators comprises 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.

9. The method according to claim 1, further comprising the step of controlling a supply of oil from the oil receiver to each of the compressors.

10. The method according to claim 9, wherein the step of controlling a supply of oil from the oil receiver to each of the compressors comprises the steps of: obtaining a pressure difference between a pressure prevailing inside the oil receiver and a suction pressure for the compressor(s), deriving a duration for an open time of an open/close sequence of a valve arrangement interconnecting the oil receiver and the compressor(s), based on the obtained pressure difference, and controlling the valve arrangement interconnecting the oil receiver and the compressor(s) in accordance with the derived duration of an open time.

11. The method according to claim 1, further comprising 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.

12. The method according to claim 2, wherein the step of deriving a duration for an open time comprises selecting a duration which is decreasing as a function of increasing pressure difference.

13. The method according to claim 3, wherein the step of deriving a duration for an open time comprises selecting a duration which is decreasing as a function of increasing pressure difference.

14. The method according to claim 2, wherein the step of deriving a duration for an open time comprises consulting a graph or a look-up table.

15. The method according to claim 3, wherein the step of deriving a duration for an open time comprises consulting a graph or a look-up table.

16. The method according to claim 4, wherein the step of deriving a duration for an open time comprises consulting a graph or a look-up table.

17. The method according to claim 2, wherein the step of deriving a duration for an open time comprises calculating the duration, using a formula of the kind: $.Math..Math.t=\frac{k.Math.{}_d}{\sqrt{{}_u}}.Math.\frac{1}{\sqrt{.Math..Math.p}},$ where t is the duration for the open time, k is a constant, .sub.d is the density of fluid received in the oil receiver, .sub.u is the density of fluid leaving the oil separator(s), and p is the obtained pressure difference.

18. The method according to claim 3, wherein the step of deriving a duration for an open time comprises calculating the duration, using a formula of the kind: $.Math..Math.t=\frac{k.Math.{}_d}{\sqrt{{}_u}}.Math.\frac{1}{\sqrt{.Math..Math.p}},$ where t is the duration for the open time, k is a constant, .sub.d is the density of fluid received in the oil receiver, .sub.u is the density of fluid leaving the oil separator(s), and p is the obtained pressure difference.

19. The method according to claim 4, wherein the step of deriving a duration for an open time comprises calculating the duration, using a formula of the kind: $.Math..Math.t=\frac{k.Math.{}_d}{\sqrt{{}_u}}.Math.\frac{1}{\sqrt{.Math..Math.p}},$ where t is the duration for the open time, k is a constant, .sub.d is the density of fluid received in the oil receiver, .sub.u is the density of fluid leaving the oil separator(s), and p is the obtained pressure difference.

20. The method according to claim 2, wherein the vapour compression system comprises at least two oil separators, wherein the valve arrangement is arranged to separately control fluid flow from each of the oil separators to the oil receiver, and wherein the method further comprises the step of selecting one of the oil separators, and the step of controlling the valve arrangement comprises controlling a valve interconnecting the selected oil separator and the oil receiver in accordance with the derived duration of an open time.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The invention will now be described in further detail with reference to the accompanying drawings in which

[0068] 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,

[0069] FIG. 2 illustrates a part of a vapour compression system comprising one oil separator,

[0070] FIG. 3 illustrates a part of a vapour compression system comprising two oil separators, and

[0071] 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

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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 compressors 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.