WO2016150664A1

WO2016150664A1 - A method for controlling compressor capacity in a vapour compression system

Info

Publication number: WO2016150664A1
Application number: PCT/EP2016/054387
Authority: WO
Inventors: Claus ZEBERG-MIKKELSEN
Original assignee: Danfoss Värmepumpar Ab
Priority date: 2015-03-24
Filing date: 2016-03-02
Publication date: 2016-09-29

Abstract

A method for controlling a vapour compression system (1) comprising a variable capacity compressor unit (2) comprising one or more compressors, a condenser (3), an expansion device (4) and an evaporator (5) arranged along a refrigerant path, is disclosed. A subcooling value of refrigerant leaving the condenser (3) is obtained and compared with a previously defined threshold value. In the case that the obtained subcooling value is below the threshold value, control of the compressor unit (2) is limited by preventing a compressor capacity provided by the compressor unit (2) from being increased.

Description

A METHOD FOR CONTROLLING COMPRESSOR CAPACITY IN A VAPOUR COMPRESSION SYSTEM

FIELD OF THE INVENTION

The present invention relates to a method for controlling a vapour compression system comprising a variable capacity compressor unit. More particularly, according to the method of the invention, the variable capacity compressor unit is controlled in such a manner that the subcooling of refrigerant leaving a condenser of the vapour compression system is maintained at a level which ensures efficient operation of the vapour compression system.

BACKGROUND OF THE INVENTION Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, e.g. in the form of a condenser, an expansion device, e.g. in the form of an expansion valve, and a heat absorbing heat exchanger, e.g. in the form of an evaporator, arranged along a refrigerant path. Refrigerant flowing in the refrigerant path is thereby alternatingly compressed by means of the compressor(s) and expanded by means of the expansion device.

In the heat rejecting heat exchanger, heat exchange takes place with the refrigerant in such a manner that heat is rejected from the refrigerant to the ambient, e.g. in the form of a secondary fluid flow across the heat rejecting heat exchanger. Thereby heating is provided to the secondary fluid flow. In the case that the heat rejecting heat exchanger is a condenser, the refrigerant flowing through the heat rejecting heat exchanger is thereby at least partly condensed.

Similarly, in the heat absorbing heat exchanger, heat exchange takes place with the refrigerant in such a manner that heat is absorbed by the refrigerant from the ambient, e.g. in the form of a secondary fluid flow across the heat absorbing heat exchanger. Thereby cooling is provided to the secondary fluid flow. In the case that the heat absorbing heat exchanger is an evaporator, the refrigerant flowing through the heat absorbing heat exchanger is thereby at least partly evaporated.

Accordingly, the vapour compression system provides heating and/or cooling due to the heat exchange taking place at the heat rejecting heat exchanger and/or the heat absorbing heat exchanger. As described above, in the case that the heat rejecting heat exchanger is a condenser, the refrigerant passing through the condenser is at least partly condensed, i.e. at least a part of the refrigerant leaving the condenser is in a liquid state. In general the temperature of refrigerant leaving the condenser will be lower than the condensing temperature, or bubble point temperature, of the refrigerant, at the prevailing pressure condition . The difference between the condensing temperature and the temperature of refrigerant leaving the condenser is generally referred to as the subcooling .

If not all of the refrigerant passing through the condenser is condensed, then the refrigerant leaving the condenser is in a mixed gaseous and liquid state, in the sense that it contains some refrigerant in the gaseous phase and some refrigerant in the liquid phase. Thereby some of the refrigerant supplied to the expansion device is in the gaseous phase. This may result in flash gas or bubbles entering the expansion device, and in an insufficient supply of refrigerant to the evaporator. This can also occur, if the refrigerant is insufficiently subcooled in combination with large pressure drop in the refrigerant path from the condenser to the expansion valve. The situations described above may occur if the compressor unit is operated at a capacity level where not all of the refrigerant passing through the condenser is condensed and/or is insufficiently subcooled.

In the case that it is required to meet an increase in a heating or cooling demand, the fact that the refrigerant is insufficiently condensed can caused problems when increasing the compressor capacity, thereby increasing the refrigerant flow in the vapour compression system . In this situation, when increasing the compressor capacity, the compressor unit will most likely suck refrigerant from the evaporator to an extent where there is a risk that the evaporator runs dry, thereby decreasing the efficiency of the operation of the vapour compression system . This may in addition lead to a low pressure stop and/or the compressor exceeding its operating area.

One way of avoiding the problems described above could be to select a very slow ramp up rate of the compressor capacity, when increasing the compressor capacity. In this case, when the control of the vapour compression system requires an increase in the compressor capacity in order to meet a required cooling or heating demand, the compressor capacity is only allowed to increase at a slow rate. This has the consequence that the vapour compression system is only able to react slowly to changes in cooling or heating demand .

US 4,841,734 discloses a refrigerant control system. A thermistor senses actual refrigerant temperature from the condenser for comparison with the saturation temperature to determine subcooling. A microprocessor disables the compressor in the event of excessive or insufficient subcooling . DESCRIPTION OF THE INVENTION

It is an object of embodiments of the invention to provide a method for controlling a vapour compression system which ensures efficient operation of the vapour compression system .

It is a further object of embodiments of the invention to provide a method for controlling a vapour compression system in which it is prevented that flash gas is present in the refrigerant being supplied to the expansion device.

The invention provides a method for controlling a vapour compression system, the vapour compression system comprising a variable capacity compressor unit comprising one or more compressors, a condenser, an expansion device and an evaporator arranged along a refrigerant path, the method comprising the steps of: obtaining a subcooling value of refrigerant leaving the condenser, comparing the obtained subcooling value with a previously defined threshold value, and in the case that the obtained subcooling value is below the threshold value, limiting control of the compressor unit by preventing a compressor capacity provided by the compressor unit from being increased .

The invention relates to a method for controlling a vapour compression system. 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 or a medium. Thus, the vapour compression system may be a refrigeration system, an air condition system, a heat pump, etc.

The vapour compression system comprises a variable capacity compressor unit, a condenser, an expansion device, e.g . in the form of an expansion valve, and an evaporator arranged along a refrigerant path. The compressor unit comprises one or more compressors, and the capacity of the compressor unit may, e.g., be varied by varying a speed of one or more compressors and/or by switching one or more compressors on or off. This will be described in further detail below. Thus, refrigerant flowing in the refrigerant path is compressed by the compressor(s) of the compressor unit. The compressed refrigerant is supplied to the condenser, where it is at least partly condensed, while heat exchange takes place with the ambient, e.g. in the form of a secondary fluid flow across the condenser, in such a manner that heat is rejected from the refrigerant. The (partly) condensed refrigerant is then supplied to the expansion device, where it undergoes expansion, and refrigerant in a mixed liquid and gaseous state is supplied to the evaporator. In the evaporator, the refrigerant is at least partly evaporated, while heat exchange takes place with the ambient, e.g. in the form of a secondary fluid flow across the evaporator, in such a manner that heat is absorbed by the refrigerant, or cooling is provided to the secondary fluid flow. Finally, the refrigerant is once again supplied to the compressors of the compressor unit.

According to the method of the invention a subcooling value of refrigerant leaving the condenser is initially obtained. As described above, the subcooling value is the temperature difference between the condensing temperature of the refrigerant and the actual temperature of the refrigerant leaving the condenser, under the given circumstances and operating conditions, including the pressure of the refrigerant and the type of refrigerant used.

Accordingly, it is normally not possible to measure the subcooling value directly, but the subcooling value can be derived from one or more measured parameters, for instance the temperature of refrigerant leaving the condenser and the pressure of refrigerant leaving or entering the condenser. This will be described in further detail below.

Next, the obtained subcooling value is compared to a previously defined threshold value. The threshold value may be a fixed value, for a given vapour compression system with a given type of refrigerant. As an alternative, the threshold value may be varied during operation, depending on specific operating conditions. This will be described in further detail below. In the case that the comparison reveals that the obtained subcooling value is below the threshold value, the control of the compressor unit is limited by preventing a compressor capacity provided by the compressor unit from being increased.

If the subcooling of refrigerant leaving the condenser is low, the temperature of the refrigerant leaving the condenser is close to the condensing temperature. Thereby there is a risk that not all of the refrigerant is condensed when passing through the condenser, i .e. there is a risk that gaseous refrigerant is allowed to pass through the condenser, entering the expansion valve, and resulting in insufficient refrigerant flow to the evaporator. As described above, this is undesirable. If the obtained subcooling value is below the threshold value, this indicates that the subcooling value is 'low', and that there is therefore a risk that the situation described above may occur. It should be understood that the threshold value may advantageously be selected in such a manner that it reflects that subcooling values below the threshold value represent a risk that gaseous refrigerant is passing through the condenser.

Accordingly, when it is established that the obtained subcooling value is below the threshold value, the compressor capacity of the compressor unit is not allowed to increase. Thus, the compressor capacity is either maintained at the current level, or it may be decreased.

Thereby it is prevented that the supply of refrigerant to the condenser is increased from the current level, and consequently the suction of refrigerant from the evaporator is not increased due to reduced refrigerant flow through the expansion valve. Accordingly, the supply of refrigerant to the condenser is maintained at a level where it can be assumed that all of the refrigerant being supplied to the condenser is actually condensed when passing through the condenser, and the situation described above is thereby avoided. However, the compressor(s) of the compressor unit are still operating, thereby ensuring that the vapour compression system is operating substantially as it is supposed to. The only difference from normal operation being that, in the case that the compressor unit is requested to increase the compressor capacity, then this is not allowed, and the compressor capacity is instead maintained at the current level.

Thus, the method of the present invention ensures that the supply of refrigerant to the condenser does not exceed a level at which the condenser is no longer capable of condensing all of the refrigerant supplied thereto, and thereby efficient operation of the vapour compression system is ensured . Furthermore, it is not necessary to select a slow ramp up rate for the compressor capacity. Thereby, during normal operation, i.e. when there is no risk of gaseous refrigerant passing through the condenser, the vapour compression system is able to react fast to changes in cooling or heating demand .

The step of obtaining a subcooling value of refrigerant leaving the condenser may comprise the steps of: measuring a temperature of refrigerant leaving the condenser, measuring a pressure of refrigerant entering the condenser and/or a pressure of refrigerant leaving the condenser, and calculating the subcooling value based on the measured temperature and pressure. The condensing temperature of the refrigerant passing through the condenser is dependent on the pressure of the refrigerant. Accordingly, for a given type of refrigerant, it is possible to derive the condensing temperature from the pressure of the refrigerant passing through the condenser. It can be assumed that the pressure of the refrigerant only varies insignificantly as it passes through the condenser. Therefore an appropriate estimate for the pressure prevailing inside the condenser can be obtained by measuring the pressure of the refrigerant immediately before it enters the condenser or immediately after it leaves the condenser. As an alternative, the pressure of refrigerant entering the condenser as well as the pressure of refrigerant leaving the condenser may be measured, and an estimate for the pressure prevailing inside the condenser can be calculated as an appropriately weighted average of the two measured pressure values. As another alternative, the pressure of refrigerant entering the condenser or the pressure of refrigerant leaving the condenser can be measured at any distance, and an estimate for the pressure prevailing inside the condenser can be calculated by appropriately correlations of the pressure drop between the measured pressure point and the condenser.

Once an estimate for the pressure prevailing inside the condenser has been obtained, the condensing temperature can be derived, e.g. via a look-up table or a graph providing correlated values of pressure and condensing temperature for the particular refrigerant flowing in the refrigerant path of the vapour compression system. It should be noted that in the case that a mixed refrigerant is applied, the bubble point temperature rather than the condensing temperature will be derived in this manner.

When the condensing temperature has been derived as described above, the subcooling value can be calculated as the difference between the derived condensing temperature and the measured temperature of refrigerant leaving the condenser.

As an alternative, the subcooling value can be obtained in any other suitable manner.

The threshold value may be a substantially constant value. This should be interpreted to mean that, for a given vapour compression system having a given type of refrigerant flowing in the refrigerant path, a suitable threshold value is selected. However, the threshold value is, according to this embodiment, not changed during operation of the vapour compression system. The threshold value is selected in such a manner that, for the specific system and the specific kind of refrigerant, the threshold value represents a subcooling value, below which there is a risk that gaseous refrigerant is passing through the condenser, resulting in insufficient refrigerant flow to the evaporator as flash gas or bubbles enter the expansion valve, and thereby resulting in inefficient operation of the vapour compression system, as described above.

Design parameters of the vapour compression system which could influence on the choice of threshold value could, e.g., include dimensions of piping of the vapour compression system, refrigerant charge, evaporator and condenser size and type, presence or absence of various components, such as subcoolers, receivers, liquid-line-suction-line heat exchangers, etc. Furthermore, various control settings of the vapour compression system may be taken into account when selecting the threshold value, e.g. a set ramp up time for the compressor capacity. A slow ramp up time will allow a lower threshold value to be selected than a fast ramp up time.

As an alternative, the threshold value may be variable, depending on the current compressor capacity. According to this embodiment, apart from taking the specific vapour compression system and the specific kind of refrigerant into account when the threshold value is selected, the current compressor capacity is also taken into account. For instance, in the case that the current compressor capacity is low, a threshold value can be selected which is lower than is the case when the current compressor capacity is high .

Alternatively or additionally, other factors may be taken into account when determining the threshold value. For instance, the threshold value may be depend on the current evaporating temperature and/or the current condensing temperature. According to one embodiment the method of the present invention may be performed in response to a request for an increase in the compressor capacity. According to this embodiment, the threshold value may be selected according to how large an increase in compressor capacity is requested. For instance, if only a small capacity increase is requested, a lower threshold value can be selected than is the case if a larger capacity increase is requested.

The compressor unit may comprise at least one variable speed compressor, and the step of limiting control of the compressor unit may comprise preventing an increase in the speed of the variable speed compressor. According to this embodiment the compressor capacity is adjusted by adjusting the speed of the variable speed compressor. This allows a continuous adjustment of the compressor capacity.

Alternatively or additionally, the compressor capacity may be adjusted by switching one or more compressors of the compressor unit between an 'on' state and an 'off state. In this case the compressor capacity may be adjusted in a stepwise manner. However, a substantially continuous adjustment of the compressor capacity can be obtained by modulating the switching of the compressors.

The step of limiting control of the compressor unit may comprise operating the compressor unit to maintain the compressor capacity substantially constant at the current capacity level. According to this embodiment, the compressor capacity is not decreased when it is determined that the obtained subcooling value is below the threshold value. Instead, the current compressor capacity is simply maintained. Thereby the vapour compression system continues to operate at as high a compressor capacity as possible, thereby allowing the vapour compression system to meet other control demands, such as a request for obtaining a specific temperature of a secondary fluid flow across the condenser or the evaporator, to the greatest possible extent.

The method may further comprise the steps of: monitoring the subcooling value, and removing the limit to the control of the compressor unit in the case that the subcooling value increases above the threshold value. According to this embodiment, the limit imposed on the control of the compressor unit is only upheld as long as it is necessary, i .e. as long as there is a risk that gaseous refrigerant is passing through the condenser, resulting in inefficient operation of the vapour compression system. Thus, if the subcooling value increases above the threshold value, this risk is no longer present, and therefore there is no longer a need for limiting the control of the compressor unit. Accordingly, when this situation is detected, the limit to the control of the compressor unit is removed, and the compressor unit is once again allowed to be controlled in a normal manner, including allowing the compressor capacity to increase.

The vapour compression system may be a heat pump, e.g. for a hot water system or for a space or floor heating system. In this case the vapour compression system provides heating via heat exchange taking place at the condenser.

As an alternative, the vapour compression system could be a refrigeration system, a cooling system, an air condition system, etc.

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 adapted to be controlled according to a first embodiment of the invention, Fig. 2 is a diagrammatic view of a vapour compression system being adapted to be controlled according to a second embodiment of the invention,

Fig. 3 is a diagrammatic view of a vapour compression system being adapted to be controlled according to a third embodiment of the invention, and Fig. 4 is a flow chart illustrating 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 adapted to be controlled according to a first embodiment of the invention . The vapour compression system 1 comprises a variable capacity compressor unit 2, a condenser 3, an expansion device in the form of an expansion valve 4, and an evaporator 5, arranged along a refrigerant path .

Refrigerant flowing in the refrigerant path is compressed by the compressors of the compressor unit 2 before it is passed to the condenser 3. While passing through the condenser 3 the refrigerant is at least partly condensed, and heat exchange takes place with the ambient in such a manner that heat is rejected from the refrigerant flowing through the condenser 3.

The refrigerant is then passed to the expansion valve 4, where it undergoes expansion before being supplied to the evaporator 5 in a mixed gaseous and liquid form. In the evaporator 5 the refrigerant is at least partly evaporated, and heat exchange takes place with the ambient in such a manner that heat is absorbed by the refrigerant flowing through the evaporator 5. Finally, the refrigerant is once again supplied to the compressor unit 2.

Three temperature sensors 6 and three pressure sensors 7 are arranged along the refrigerant path for measuring the temperature and the pressure of the refrigerant at various positions along the refrigerant path. A first temperature sensor 6a and a first pressure sensor 7a are arranged in the suction line interconnecting an outlet of the evaporator 5 and an inlet of the compressor unit 5. The temperature and pressure values measured by means of the first temperature sensor 6a and the first pressure sensor 7a may, e.g., be used for determining the superheat of refrigerant leaving the evaporator 5. The superheat of refrigerant leaving the evaporator 5 is often used for controlling the supply of refrigerant to the evaporator 5 by means of the expansion valve 4. A second temperature sensor 6b and a second pressure sensor 7b are arranged in the part of the refrigerant path which interconnects an outlet of the compressor unit 2 and an inlet of the condenser 3, and a third temperature sensor 6c and a third pressure sensor 7c are arranged in the part of the refrigerant path which interconnects an outlet of the condenser 3 and the expansion valve 4.

Based on the pressure measurements performed by the second pressure sensor 7b and/or by the third pressures sensor 7c, an estimate for the pressure of the refrigerant condensing and passing through the condenser 3 can be obtained . Based thereon, and knowing which kind of refrigerant is flowing in the refrigerant path, a condensing temperature of the refrigerant flowing through the condenser 3 can be derived. Using the derived condensing temperature and the temperature measurements performed by the third temperature sensor 6c, a subcooling value for the refrigerant leaving the condenser 3 can be obtained as the difference between the condensing temperature and the actual temperature of refrigerant leaving the condenser 3.

During operation of the vapour compression system 1 the subcooling value of refrigerant leaving the condenser 3 is obtained, in the manner described above. The obtained subcooling value is then compared to a previously defined threshold value. The threshold value depends on the specific design of the vapour compression system 1, and on the specific kind of refrigerant flowing in the refrigerant path . Furthermore, the threshold value may be variable, e.g . depending on the current compressor capacity.

In the case that the comparison reveals that the obtained subcooling value is below the threshold value, the control of the compressor unit 2 is limited by preventing a compressor capacity provided by the compressor unit 2 from being increased. When the control of the compressor unit 2 is limited in this manner, and the normal control loop requires that the compressor capacity is increased in order to meet a demand for cooling or heating, then the request for an increase of the compressor capacity is simply refused, and the compressor capacity is instead maintained at the current level.

If the subcooling value of refrigerant leaving the condenser 3 is low, this is an indication that the temperature of refrigerant leaving the condenser 3 is close to the condensing temperature. When this is the case, there is a risk that not all of the refrigerant supplied to the condenser 3 is condensed, when passing through the condenser 3, and/or that the subcooling is insufficient. Thereby gaseous refrigerant is passing through the condenser 3, and refrigerant in gaseous form is supplied to the expansion valve 4. This may have the consequence that the supply of refrigerant to the evaporator 5 is reduced or insufficient, thereby leading to an inefficient operation of the vapour compression system 1. In the case that a situation is approaching where not all of the refrigerant flowing through the condenser 3 is condensed, it can be prevented that this situation is reached by preventing an increase in the supply of refrigerant to the condenser 3. According to the method of the invention, this is done by limiting control of the compressor unit 2 by preventing the compressor capacity from being increased, when the obtained subcooling value is below the threshold value.

The threshold value is selected in such a manner that it represents a subcooling value, below which there is a risk that the situation described above is reached, if the supply of refrigerant to the condenser 3 is allowed to increase. Thus, when the vapour compression system 1 is controlled in accordance with the method of the invention, the compressor unit 2 is operated at a compressor capacity which is as close to the compressor capacity required by a cooling or heating demand as possible, but the compressor capacity is limited in the case that the subcooling value decreases below the threshold value, thereby ensuring efficient operation of the vapour compression system 1. Fig. 2 is a diagrammatic view of a vapour compression system 1 being adapted to be controlled according to a second embodiment of the invention . The vapour compression system 1 of Fig . 2 is very similar to the vapour compression system 1 of Fig . 1, and it will therefore not be described in further detail here.

The vapour compression system 1 of Fig. 2 comprises a de-superheater 8 arranged in the part of the refrigerant path interconnecting the outlet of the compressor unit 2 and the inlet of the condenser 3.

The de-superheater 8 is also a heat exchanger, where the temperature of the refrigerant leaving the compressor is reduced and maybe partly condensed by rejecting heat to a secondary fluid The de-superheater 8 may, e.g., operate in a separate secondary circuit, e.g . for producing hot water, whereas the condenser produces space or floor heating. Thereby the vapour compression system 1 is capable of producing hot water and space heating simultaneously.

The second pressure sensor 7b is arranged in the part of the refrigerant path which interconnects an outlet of the compressor unit 2 and an inlet of the de-superheater 8. The measurements performed by means of the second pressure sensor 7b may be used for estimating the pressure prevailing inside the condenser 3, essentially as described above with reference to Fig. 1. However, in this case the pressure drop across the de-superheater 8 must be taken into account when estimating the pressure inside the condenser 3, based on the measurements performed by the second pressure sensor 7b. As an alternative, the second pressure sensor 7b could be located between the de-superheater and the condenser, in which case the pressure drop across the de-superheater does not have to be taken into account in the estimation of the pressure inside the condenser. Fig. 3 is a diagrammatic view of a vapour compression system 1 being adapted to be controlled according to a third embodiment of the invention . The vapour compression system 1 of Fig . 3 is very similar to the vapour compression systems 1 of Figs. 1 and 2, and it will therefore not be described in further detail here.

The vapour compression system 1 of Fig. 3 comprises a subcooler 9 arranged in the part of the refrigerant path interconnecting the outlet of the condenser 3 and the expansion valve 4. The subcooler 9 is effectively a heat exchanger which cools the refrigerant leaving the condenser 3. Thereby, if some of the refrigerant leaving the condenser 3 is in a gaseous state, it can be further condensed and subcooled in the subcooler 9. Furthermore, since the temperature of the refrigerant is decreased in the subcooler 9, the subcooling of the refrigerant is increased. Therefore, even if the subcooling of the refrigerant leaving the condenser 3 is very low, the subcooling of the refrigerant reaching the expansion valve 4 may be sufficient to ensure efficient operation of the vapour compression system 1, due to the refrigerant passing through the subcooler 9.

In the vapour compression system 1 of Fig. 3, the third temperature sensor 6c is arranged in the refrigerant path between the subcooler 9 and the expansion valve 4, and the third pressure sensor 7c is arranged in the refrigerant path between the condenser 3 and the subcooler 9. Accordingly, when the subcooling value is obtained, as described above with reference to Fig. 1, it is obtained on the basis of the temperature of the refrigerant leaving the subcooler 9 and entering the expansion valve 4, rather than on the basis of the, somewhat higher, temperature of refrigerant leaving the condenser 3. This is appropriate, since the crucial point is that gaseous refrigerant should not reach the expansion valve 4. Thus, due to the subcooler 9, efficient operation of the vapour compression system 1 can be obtained, even if the subcooling of refrigerant leaving the condenser 3 is very low. This will allow the compressor unit 2 to be operated at a compressor capacity which is even closer to the compressor capacity required by a cooling or heating demand .

Fig. 4 is a flow chart illustrating a method according to an embodiment of the invention . The process is started at step 10. At step 11 the temperature and the pressure of refrigerant leaving the condenser are measured. It should be noted, that the pressure of refrigerant entering the condenser could, alternatively or additionally, be measured. In any event, the pressure measurement(s) is/are used for estimating a pressure of refrigerant passing through the condenser.

At step 12 a subcooling value of refrigerant leaving the condenser is calculated, based on the measured temperature and pressure. As described above, the subcooling value is calculated as the difference between a condensing temperature of the refrigerant and the measured temperature, the condensing temperature being derived from the estimate for the pressure inside the condenser, and based on knowledge regarding the kind of refrigerant flowing in the refrigerant path.

At step 13 it is investigated whether or not the obtained subcooling value is below a previously defined threshold value. The previously defined threshold value depends on various design features of the vapour compression system, and on the kind of refrigerant flowing in the refrigerant path . Furthermore, the threshold value corresponds to a subcooling value, below which there is a risk that not all refrigerant supplied to the condenser is condensed, if the supply of refrigerant to the condenser is increased. In the case that step 13 reveals that the obtained subcooling value is below the previously defined threshold value, then the process is forwarded to step 14, where the control of the compressor unit is limited in such a manner that the compressor capacity provided by the compressor unit is prevented from being increased. Thereby it is prevented that the supply of refrigerant to the condenser is increased. The process is then forwarded to step 15, where the compressor unit is operated, with the limit on the compressor capacity, which was defined in step 14. Thus, the compressor capacity is operated in accordance with a cooling or heating demand of the vapour compression system . However, if a compressor capacity is requested, which exceeds the compressor capacity level at the time where the limit was set, then the compressor capacity is not increased, but instead maintained at the limited level.

During operation, the process is returned to step 11 for continued monitoring of the temperature and pressure of refrigerant leaving the condenser, calculation of the subcooling value, and comparison of the subcooling value with the previously defined threshold value.

In the case that step 13 reveals that the obtained subcooling is not below the previously defined threshold value, then the process is forwarded to step 16, where it is investigated whether or not a limit has previously been set on the control of the compressor unit, as described above. If this is not the case, the process is simply forwarded to step 15, and the compressor unit is operated without a limit. In the case that step 16 reveals that a limit has previously been set on the control of the compressor unit, then the process is forwarded to step 17, where the limit is removed, because the conditions for setting the limit no longer apply. Then the process is forwarded to step 15, and the compressor unit is operated without the limit.

Claims

1. A method for controlling a vapour compression system (1), the vapour compression system (1) comprising a variable capacity compressor unit (2) comprising one or more compressors, a condenser (3), an expansion device (4) and an evaporator (5) arranged along a refrigerant path, the method comprising the steps of: obtaining a subcooling value of refrigerant leaving the condenser (3), comparing the obtained subcooling value with a previously defined threshold value, and in the case that the obtained subcooling value is below the threshold value, limiting control of the compressor unit (2) by preventing a compressor capacity provided by the compressor unit (2) from being increased .

2. A method according to claim 1, wherein the step of obtaining a subcooling value of refrigerant leaving the condenser (3) comprises the steps of: measuring a temperature of refrigerant leaving the condenser (3), - measuring a pressure of refrigerant entering the condenser (3) and/or a pressure of refrigerant leaving the condenser (3), and calculating the subcooling value based on the measured temperature and pressure.

3. A method according to claim 1 or 2, wherein the threshold value is a substantially constant value.

4. A method according to claim 1 or 2, wherein the threshold value is variable, depending on the current compressor capacity.

5. A method according to any of the preceding claims, wherein the compressor unit (2) comprises at least one variable speed compressor, and wherein the step of limiting control of the compressor unit (2) comprises preventing an increase in the speed of the variable speed compressor.

6. A method according to any of the preceding claims, wherein the step of limiting control of the compressor unit (2) comprises operating the compressor unit (2) to maintain the compressor capacity substantially constant at the current capacity level.

7. A method according to any of the preceding claims, further comprising the steps of: - monitoring the subcooling value, and removing the limit to the control of the compressor unit (2) in the case that the subcooling value increases above the threshold value.

8. A method according to any of the preceding claims, wherein the vapour compression system (1) is a heat pump.