Method for controlling a fan of a vapour compression system in accordance with a variable temperature setpoint

Abstract

A method for controlling a fan (6) of a vapour compression system (1) is disclosed, the fan (6) being arranged to provide a secondary fluid flow across a heat rejecting heat exchanger (3). A temperature difference, T=T.sub.outT.sub.amb, between a temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger (3) and a temperature, T.sub.amb, of ambient air of the heat rejecting heat exchanger (3) is established. A setpoint value, T.sub.setp, for the temperature difference, T, is obtained, the setpoint value, T.sub.setp, being dependent on the fan speed of the fan (6) in such a manner that the setpoint value, T.sub.setp, increases as the fan speed increases. The fan speed of the fan (6) is controlled in order to control the temperature difference, T, in accordance with the obtained setpoint value, T.sub.setp.

Claims

1. A method for controlling a fan of a vapour compression system, the vapour compression system comprising a compressor, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant circuit, the fan and a controller, said fan being arranged to provide a secondary fluid flow across the heat rejecting heat exchanger, the method comprising the steps of: establishing, by the controller, a temperature difference, T=T.sub.out-T.sub.amb, between a temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger and a temperature, T.sub.amb, of ambient air of the heat rejecting heat exchanger, establishing, by the controller, a fan speed of the fan, obtaining, by the controller, a setpoint value, T.sub.setp, for the temperature difference, T, based on the established fan speed of the fan, said setpoint value, T.sub.setp, being dependent on the fan speed in such a manner that the setpoint value, T.sub.setp, increases as the fan speed increases, and controlling, by the controller, the fan speed of the fan in order to control the temperature difference, T, by comparing the established temperature difference, T, to the obtained setpoint value, T.sub.setp, and controlling the fan speed on the basis of the comparison.

2. The method according to claim 1, wherein the step of controlling the fan speed of the fan comprises controlling the fan speed in order to obtain that the temperature difference, T, is larger than or equal to the obtained setpoint value, T.sub.setp.

3. The method according to claim 2, wherein the step of obtaining the setpoint value, T.sub.setp, comprises consulting a look-up table and/or determining the setpoint value, T.sub.setp, from a function defining the setpoint value, T.sub.setp, as a function of fan speed.

4. The method according to claim 2, wherein the setpoint value, T.sub.setp, varies as a linear or piecewise linear function of the fan speed.

5. The method according to claim 2, wherein the step of controlling the fan speed of the fan comprises the steps of: comparing the established temperature difference, T, to the obtained setpoint value, T.sub.setp, and decreasing the fan speed of the fan in the case that T<T.sub.setp.

6. The method according to claim 2, wherein the step of obtaining a setpoint value, T.sub.setp, comprises the steps of: obtaining a minimum setpoint value, T.sub.setp,min, being dependent on the fan speed in such a manner that the minimum setpoint value, T.sub.setp,min, increases as the fan speed increases, obtaining a system defined setpoint value, T.sub.setp,sys, and selecting the setpoint value, T.sub.setp, as the largest of the minimum setpoint value, T.sub.setp,min, and the system defined setpoint value, T.sub.setp,sys, T.sub.setp=max{T.sub.setp,min;T.sub.setp,sys}.

7. The method according to claim 1, wherein the step of obtaining the setpoint value, T.sub.setp, comprises consulting a look-up table and/or determining the setpoint value, T.sub.setp, from a function defining the setpoint value, T.sub.setp, as a function of fan speed.

8. The method according to claim 7, wherein the setpoint value, T.sub.setp, varies as a linear or piecewise linear function of the fan speed.

9. The method according to claim 7, wherein the step of controlling the fan speed of the fan comprises the steps of: comparing the established temperature difference, T, to the obtained setpoint value, T.sub.setp, and decreasing the fan speed of the fan in the case that T<T.sub.setp.

10. The method according to claim 1, wherein the setpoint value, T.sub.setp, varies as a linear or piecewise linear function of the fan speed.

11. The method according to claim 10, wherein the step of controlling the fan speed of the fan comprises the steps of: comparing the established temperature difference, T, to the obtained setpoint value, T.sub.setp, and decreasing the fan speed of the fan in the case that T<T.sub.setp.

12. The method according to claim 1, wherein the step of controlling the fan speed of the fan comprises the steps of: comparing the established temperature difference, T, to the obtained setpoint value, T.sub.setp, and decreasing the fan speed of the fan in the case that T<T.sub.setp.

13. The method according to claim 12, wherein the step of controlling the fan speed of the fan further comprises the step of increasing the fan speed of the fan in the case that T>T.sub.setp.

14. The method according to claim 1, wherein the step of obtaining the setpoint value, T.sub.setp, comprises the steps of: obtaining a minimum setpoint value, T.sub.setp,min, being dependent on the fan speed in such a manner that the minimum setpoint value, T.sub.setp,min, increases as the fan speed increases, obtaining a system defined setpoint value, T.sub.setp,sys, and selecting the setpoint value, T.sub.setp, as the largest of the minimum setpoint value, T.sub.setp,min, and the system defined setpoint value, T.sub.setp,sys, T.sub.setp=max{T.sub.setp,min;T.sub.setp,sys}.

15. The method according to claim 1, wherein the step of establishing the temperature difference, T, comprises obtaining the temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger and obtaining the temperature, T.sub.amb, of ambient air of the heat rejecting heat exchanger.

16. A method for adjusting a setpoint value for a temperature difference, T, related to a vapour compression system, the vapour compression system comprising a compressor, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant circuit, the vapour compression system further comprising a fan arranged to provide a secondary fluid flow across the heat rejecting heat exchanger and a controller controlling the fan, the temperature difference, T=T.sub.outT.sub.amb, being a temperature difference between a temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger and a temperature, T.sub.amb, of ambient air of the heat rejecting heat exchanger, the method comprising the steps of: establishing, by the controller, a fan speed of the fan, obtaining, by the controller, a setpoint value, T.sub.setp, for the temperature difference, T, based on the established fan speed of the fan, said setpoint value, T.sub.setp, being dependent on the fan speed in such a manner that the setpoint value, T.sub.setp, increases as the fan speed increases, and adjusting, by the controller, the setpoint value for the temperature difference, T, to the obtained setpoint value, T.sub.setp.

17. The method according to claim 16, wherein the step of obtaining the setpoint value, T.sub.setp, comprises consulting a look-up table and/or determining the setpoint value, T.sub.setp, from a function defining the setpoint value, T.sub.setp, as a function of fan speed.

18. The method according to claim 16, wherein the setpoint value, T.sub.setp, varies as a linear or piecewise linear function of the fan speed.

19. The method according to claim 16, wherein the step of obtaining a setpoint value, T.sub.setp, comprises the steps of: obtaining a minimum setpoint value, T.sub.setp,min, being dependent on the fan speed in such a manner that the minimum setpoint value, T.sub.setp,min, increases as the fan speed increases, obtaining a system defined setpoint value, T.sub.setp,sys, being a substantially constant setpoint value, and selecting the setpoint value, T.sub.setp, as the largest of the minimum setpoint value, T.sub.setp,min, and the system defined setpoint value, T.sub.setp,sys, T.sub.setp=max{T.sub.setp,min;T.sub.setp,sys}.

20. The method according to claim 16, wherein the step of establishing the temperature difference, T, comprises obtaining the temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger and obtaining the temperature, T.sub.amb, of ambient air of the heat rejecting heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a diagrammatic view of a vapour compression system comprising a fan being operated in accordance with a method according to an embodiment of the invention,

(3) FIG. 2 illustrates obtaining a setpoint value in accordance with a method according to an embodiment of the invention, and

(4) FIG. 3 is a block diagram illustrating a method for controlling a fan according to an embodiment of the invention.

DETAILED DESCRIPTION

(5) FIG. 1 is a diagrammatic view of a vapour compression system 1 comprising a compressor 2, a heat rejecting heat exchanger 3, an expansion valve 4 and an evaporator 5 arranged in a refrigerant circuit. A fan 6 is arranged to provide a secondary fluid flow across the heat rejecting heat exchanger 3.

(6) In the heat rejecting heat exchanger 3 heat exchange takes place between refrigerant passing through the heat rejecting heat exchanger 3 and the fluid of the secondary fluid flow, in such a manner that heat is rejected from the refrigerant and absorbed by the fluid of the secondary fluid flow. The heat transfer from the refrigerant to the fluid of the secondary fluid flow is, among other things, determined by the flow rate of the secondary fluid flow across the heat rejecting heat exchanger 3. Thus, an increase in the flow rate of the secondary fluid flow will cause an increase in the heat transfer, and a decrease in the flow rate of the secondary fluid flow will cause a decrease in the heat transfer.

(7) The flow rate of the secondary fluid flow across the heat rejecting heat exchanger 3 is determined by the fan speed of the fan 6. Thereby the heat transfer from the refrigerant to the fluid of the secondary fluid flow across the heat rejecting heat exchanger 3 is also dependent on the fan speed of the fan 6. Thus, by appropriately controlling the fan speed of the fan 6, the heat transfer taking place in the heat rejecting heat exchanger 3 can be controlled, and thereby the temperature of refrigerant leaving the heat rejecting heat exchanger 3 can be controlled.

(8) As described above, it is often desirable to control the temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger 3 in such a manner that this temperature is close to an ambient temperature, T.sub.amb, such as a temperature of the fluid of the secondary fluid flow across the heat rejecting heat exchanger 3 or an outdoor temperature, i.e. in such a manner that a temperature difference, T=T.sub.outT.sub.amb is small. However, at small temperature differences, uncertainties of the temperature sensors may lead to incorrect measured values of the temperature difference, T. In this case the measured temperature values may indicate that the temperature difference, T, is above a desired level, while the actual temperature difference is at or below this level, and that it is not possible to reduce the temperature difference further. In this case, the fan speed may be continuously increased in an attempt to decrease the temperature difference, but the increase in fan speed will have no effect in this regard, because the actual temperature difference is already at a minimum level. However, according to the method of the invention, this situation is avoided by obtaining a setpoint value, T.sub.setp, for the temperature difference, T, which increases as the fan speed increases.

(9) FIG. 2 is a graph illustrating the step of obtaining a setpoint value, using a method according to an embodiment of the invention. The graph illustrates temperature as a function of fan speed.

(10) In the graph, a constant ambient temperature, T.sub.amb, is shown as a dashed line. Thus, in the example illustrated in FIG. 2 it is assumed that the ambient temperature, T.sub.amb, is constant. It should, however, be noted that the ambient temperature, T.sub.amb, could be variable, but the principles described below will still be valid in this case.

(11) A setpoint value, T.sub.setp, for a temperature difference between a temperature, T.sub.out, of refrigerant leaving a heat rejecting heat exchanger and the ambient temperature, T.sub.amb, is dependent on the fan speed in such a manner that the setpoint value, T.sub.setp, increases as the fan speed increases. The setpoint value, T.sub.setp, is indicated at a specific fan speed 7.

(12) For a given fan speed, a temperature setpoint, T.sub.setp, is calculated as the sum of the ambient temperature, T.sub.amb, and the fan speed dependent setpoint value, T.sub.setp. In FIG. 2, T.sub.setp is illustrated by a solid line.

(13) It can be seen that the setpoint value, T.sub.setp, is a piecewise linear function of the fan speed. At fan speeds below fan speed 8 the setpoint value, T.sub.setp, is a constant value, and at fan speeds above fan speed 8, T.sub.setp increases linearly as a function of fan speed.

(14) The temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger may be controlled in accordance with the temperature setpoint, T.sub.setp.

(15) FIG. 3 is a block diagram illustrating a method for controlling a fan according to an embodiment of the invention. The ambient temperature, T.sub.amb, and the temperature, T.sub.out, of refrigerant leaving the heat rejecting heat exchanger are supplied to a fan speed controller 9. Based thereon, the fan speed controller 9 can derive the temperature difference, T=T.sub.outT.sub.amb, and use this as a control parameter for controlling the fan speed.

(16) The fan speed controller 9 further supplies the fan speed to a setpoint calculating unit 10. In the setpoint calculating unit 10 a setpoint value, T.sub.setp, is obtained, based on the fan speed received from the fan speed controller 9. The setpoint value, T.sub.setp, depends on the fan speed in such a manner that the setpoint value, T.sub.setp, increases as the fan speed increases. The setpoint value, T.sub.setp, could, e.g., be derived in the manner described above with reference to FIG. 2.

(17) The obtained setpoint value, T.sub.setp, is supplied to a selecting unit 11. Furthermore, one or more further setpoint values, T.sub.setp,1, T.sub.setp,2, is/are supplied to the selecting unit 11. For instance, one of the further setpoint values, T.sub.setp,1, could be a user defined setpoint value, and one of the further setpoint values, T.sub.setp,2, could be a setpoint value dictated by other parts of the vapour compression system, such as a heat recovery system.

(18) In the selecting unit 11, the largest of the three available setpoint values, T.sub.setp, T.sub.setp, and T.sub.setp,2, is selected as the setpoint value for the temperature difference, T, and the selected setpoint value is supplied to the fan speed controller 9. The fan speed controller 9 then controls the fan speed in order to obtain that the derived temperature difference, T, is substantially equal to the setpoint value received from the selecting unit.

(19) Since T.sub.setp, increases as the fan speed increases, T.sub.setp will be selected as the setpoint value by the selecting unit 11 at high fan speeds.

(20) It is noted that, even though the fan speed controller 9, the setpoint calculating unit 10 and the selecting unit 11 are shown as separate units in FIG. 3, it is not ruled out that two or more of the illustrated units 9, 10, 11 could form part of the same physical unit or component. Furthermore, one or more of the units 9, 10, 11 could be implemented in software and executed on one or more microprocessors.

(21) 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.