Method of controlling one or more fans of a heat rejecting heat exchanger

Abstract

A method of controlling a fan of a vapor compression system is disclosed. The vapor compression system includes a compressor, a heat rejecting heat exchanger, e.g. in the form of a gas cooler or a condenser, an expansion device and an evaporator arranged in a refrigerant circuit. The fan is arranged to provide a secondary fluid flow across the heat rejecting heat exchanger, e.g. in the form of an air flow. The method allows the electrical energy consumption of the fan to be reduced without risking instability of the vapor compression system.

Claims

1. A method of 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, said fan being arranged to provide a fluid flow across the heat rejecting heat exchanger, the method comprising the steps of: establishing a temperature, T.sub.1, of refrigerant leaving the heat rejecting heat exchanger, establishing a temperature, T.sub.2, of ambient air of the heat rejecting heat exchanger, deriving a temperature difference, T=T.sub.1T.sub.2, between the temperature (T.sub.1) of refrigerant leaving the heat rejecting heat exchanger and the temperature (T.sub.2) of ambient air of the heat rejecting heat exchanger, comparing the temperature difference, T, to a first threshold value and to a second threshold value, the second threshold value being smaller than or equal to the first threshold value, and controlling the rotational speed of the fan on the basis of the comparing step wherein the step of controlling the rotational speed of the fan comprises the steps of: if the temperature difference, T, is larger than the first threshold value, increasing the rotational speed of the fan, and if the temperature difference, T, is smaller than the second threshold value, decreasing the rotational speed of the fan, wherein increasing the rotational speed of the fan occurs more rapidly than decreasing the rotational speed of the fan.

2. The method according to claim 1, wherein the step of decreasing the rotational speed of the fan includes decreasing the speed by 0.1%-10.0% of the maximum rotational speed of the fan per minute.

3. The method according to claim 1, wherein the step of increasing the rotational speed of the fan includes increasing the rotational speed of the fan by 5%-100% of the maximum rotational speed of the fan.

4. The method according to claim 1, wherein the step of increasing the rotational speed of the fan and/or the step of decreasing the rotational speed of the fan is/are performed using an asymmetric scaling function.

5. The method according to claim 1, wherein the second threshold value is smaller than the first threshold value, the method further comprising the step of: if the temperature difference, T, is smaller than the first threshold value and larger than the second threshold value, maintaining the rotational speed of the fan.

6. The method according to claim 1, wherein the first threshold value is equal to the second threshold value.

7. The method according to claim 1, wherein the vapour compression system is operated with a refrigerant being in a supercritical state when flowing in the refrigerant circuit.

8. The method according to claim 1, wherein the refrigerant flowing in the refrigerant circuit is carbon-dioxide (CO.sub.2).

9. The method according to claim 2, wherein the step of increasing the rotational speed of the fan and/or the step of decreasing the rotational speed of the fan is/are performed using an asymmetric scaling function.

10. The method according to claim 3, wherein the step of increasing the rotational speed of the fan and/or the step of decreasing the rotational speed of the fan is/are performed using an asymmetric scaling function.

11. The method according to claim 2, wherein the step of increasing the rotational speed of the fan includes increasing the rotational speed of the fan by 5%-100% of the maximum rotational speed of the fan.

12. The method according to claim 2, wherein the second threshold value is smaller than the first threshold value, the method further comprising the step of: if the temperature difference T, is smaller than the first threshold value and larger than the second threshold value, maintain the rotational speed of the fan.

13. The method according to claim 2, wherein the first threshold value is equal to the second threshold value.

14. The method according to claim 2, wherein the vapour compression system is operated with a refrigerant being in a super critical state when flowing in the refrigerant circuit.

15. The method according to claim 2, wherein the refrigerant flowing in the refrigerant circuit is carbon dioxide (CO.sub.2).

16. A heat rejecting heat exchanger comprising a fan being arranged to provide a fluid flow across the heat rejecting heat exchanger, said fan being capable of being controlled by a method according to claim 2.

17. The method according to claim 3, wherein the second threshold value is smaller than the first threshold value, the method further comprising the step of: if the temperature difference T, is smaller than the first threshold value and larger than the second threshold value, maintain the rotational speed of the fan.

18. The method according to claim 3, wherein the first threshold value is equal to the second threshold value.

19. A heat rejecting heat exchanger comprising a fan being arranged to provide a fluid flow across the heat rejecting heat exchanger, said fan being configured to operate according to the method of claim 1.

20. A refrigeration system comprising a compressor, a gas cooler, an expansion device and an evaporator arranged in a refrigerant circuit, said gas cooler being a heat rejecting heat exchanger according to claim 19.

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) FIGS. 1-3 are graphs illustrating temperature of refrigerant leaving the heat rejecting heat exchanger, as a function of rotational speed of a fan,

(3) FIG. 4 is a graph illustrating control of rotational speed of a fan according to two different embodiments of the invention, and

(4) FIGS. 5-7 show three block diagrams, each illustrating an integrating controller for use in a method for controlling rotational speed of a fan.

DETAILED DESCRIPTION

(5) FIG. 1 is a graph illustrating temperature (outlet temperature) of refrigerant leaving a heat rejecting heat exchanger of a vapour compression system, said temperature being illustrated as a function of rotational speed of a fan arranged to provide a secondary fluid flow across the heat rejecting heat exchanger. FIG. 1 illustrates that at low fan speeds, the outlet temperature is relatively high, but the outlet temperature decreases rapidly when the fan speed is increased, the temperature thereby approaching the temperature of ambient air of the heat rejecting heat exchanger.

(6) It is a disadvantage if the outlet temperature is very high, i.e. much higher than the temperature of the ambient air, because the heat rejecting heat exchanger is not operating efficiently, and thereby the total energy consumption of vapour compression system is increased, and there is a risk of instability. Therefore, in prior art methods for controlling the rotational speed of the fan, the fan has been operated continuously at a relatively high rotational speed, at or near region 1, in order to avoid the outlet temperature increasing due to a too low fan speed. However, this causes a relatively high electrical energy consumption of the fan.

(7) FIG. 2 shows the graph of FIG. 1. However, in FIG. 2, an optimal operating point 2 for the fan speed is indicated. The optimal operating point 2 is the fan speed where the electrical energy consumption of the fan is minimised without risking an unacceptably high outlet temperature. Thus, at fan speeds below the optimal point 2, the outlet temperature becomes too high, and at fan speeds above the optimal point 2, the electrical energy consumption of the fan increases.

(8) An effective range 3 of fan speeds is also shown in FIG. 2. The effective range 3 is a range of fan speeds above the optimal point 2, where the electrical energy consumption of the fan is acceptable. Accordingly, it is desirable to operate the rotational speed of the fan in such a manner that the rotational speed of the fan is within the effective range 3. Under no circumstances should the rotational speed of the fan be allowed to fall below the optimal point 2, but it is acceptable to operate the rotational speed of the fan in the effective range 3 immediately above the optimal point 2. Accordingly, if the rotational speed of the fan decreases to falls below the optimal point 2, the rotational speed of the fan should be increased rapidly and significantly for ensuring that the rotational speed is immediately increased to a level above the optimal point 2. However, if the rotational speed of the fan increases to above the effective range 3, the rotational speed of the fan should be decreased slowly and gradually for ensuring that the rotational speed is not decreased to a value below the optimal point 2. Thus, FIG. 2 illustrates the asymmetry in control of the fan, said asymmetry having been described previously.

(9) FIG. 3 also shows the graph of FIGS. 1 and 2. In FIG. 3 a temperature dead zone 4 is illustrated. The dead zone 4 is a desired temperature range of the outlet temperature. Itit is desirable to control the vapour compression system, including the rotational speed of the fan, in such a manner that the outlet temperature is within the dead zone 4. Furthermore, this may advantageously be obtained while controlling the rotational speed of the fan to be within the effective range 3 illustrated in FIG. 2.

(10) The dead zone 4 is delimited by a first temperature value 5 and a second temperature value 6. The outlet temperature can be measured, and in response to the measured value, the fan speed can be adjusted in order to obtain temperature values which are within the dead zone 4. However, the graph shown in FIGS. 1-3 is offset when the ambient temperature changes. Therefore, instead of simply measuring the outlet temperature and comparing it to the first temperature value 5 and the second temperature value 6, the ambient temperature is also measured. The temperature difference is calculated, and the temperature difference is compared to a first threshold value, corresponding to the first temperature value 5, and to a second threshold value, corresponding to the second temperature value 6. In response to this comparison the rotational speed of the fan is controlled in the following manner.

(11) If the temperature difference is larger than the first threshold value, corresponding to the outlet temperature being higher than the first temperature value 5, the rotational speed of the fan is increased. This may, e.g., be done by jumping up the rotational speed or by ramping up the rotational speed. This is illustrated by zone 7 in FIG. 3.

(12) If the temperature difference is smaller than the first threshold value, but larger than the second threshold value, corresponding to the outlet temperature being within the dead zone, the rotational speed of the fan is maintained at the current speed. This is illustrated by zone 8 in FIG. 3.

(13) If the temperature difference is smaller than the second threshold value, corresponding to the outlet temperature being lower than the second temperature value 6, the rotational speed of the fan is decreased. This is preferably done by ramping down the speed in order to avoid that zone 7 is entered. This situation is illustrated by zone 9 in FIG. 3.

(14) The dead zone 4 is a range of outlet temperatures where the rotational speed of the fan is kept constant. When the outlet temperature is above the dead zone 4, the rotational speed of the fan is increased, preferably, e.g. jumped up or ramped up at a high rate, and when the outlet temperature is below the dead zone 4, the rotational speed of the fan is decreased, preferably ramped down at a low rate.

(15) FIG. 4 is a graph illustrating control of rotational speed of a fan according to two different aspects of control according to the invention, and in accordance with the method described above with reference to FIG. 3.

(16) The top graph shows outlet temperature as a function of time. The dead zone 4, the first temperature value 5 and the second temperature value 6 are shown.

(17) The middle graph and the lower graph show rotational speed of a fan as a function of time, according to two different control methods, and in response to the temperature variations shown in the top graph.

(18) Initially the outlet temperature is below the dead zone 4. Therefore, for the outlet temperature to increase and thereby enter the dead zone 4, the rotational speed of the fan is ramped down, i.e. it is gradually decreased, as in the control aspect illustrated in the middle graph as well as in the control aspect illustrated in the lower graph.

(19) At time 10 the outlet temperature reaches the second temperature value 6, and thereby enters the dead zone 4. In response to this, the rotational fan is maintained at a constant value as in the control aspect illustrated in the middle graph as well as in the control aspect illustrated in the lower graph. However, the outlet temperature continues to increase, and at time 11 the first temperature value 5 is reached, and the outlet temperature increases above the dead zone 4. In response to this, the rotational speed of the fan is increased, for causing the outlet temperature to decrease and once again enter the dead zone 4.

(20) In the control aspect illustrated in the middle graph, the rotational speed of the fan is increased by jumping up the rotational speed, i.e. by abruptly increasing the rotational speed by a significant amount. Subsequently, the rotational speed is maintained at a constant level for a time period (delay), in order to allow the system to react to the jump in rotational speed of the fan. When the time period has elapsed, the outlet temperature is established being still above the dead zone 4, and therefore the rotational speed of the fan is jumped up once again.

(21) In the control aspect illustrated in the lower graph, the rotational speed is ramped up, i.e. it is gradually increased.

(22) At time 12 the outlet temperature has decreased and reaches the first temperature value 5, thereby entering the dead zone 4. In response to this, the rotational speed of the fan is maintained constant as in the control aspect illustrated in the middle graph as well as in the control aspect illustrated in the lower graph.

(23) At time 13 the outlet temperature reaches the second temperature value 6, thereby decreasing below the dead zone 4. In response to this, the rotational speed of the fan is ramped down as in the control aspect illustrated in the middle graph as well as in the control aspect illustrated in the lower graph.

(24) At time 14 the outlet temperature once again reaches the second temperature value 6, thereby entering the dead zone 4. In response to this, the rotational speed of the fan is maintained constant as in the control aspect illustrated in the middle graph as well as in the control aspect illustrated in the lower graph.

(25) At time 15 the outlet temperature once again reaches the first temperature value 5, thereby increasing above the dead zone 4, and once again the rotational speed of the fan is increased in response to this. In the control aspect illustrated in the middle graph, the rotational speed of the fan is jumped up, and in the control aspect illustrated in the lower graph, the rotational speed of the fan is ramped up, as described above.

(26) Finally, at time 16 the outlet temperature once again reaches the first temperature value 5, thereby entering the dead zone 4. In response to this, the rotational speed of the fan is once again maintained constant.

(27) In summary, the control aspect method illustrated in the middle graph is an asymmetric control aspect, in the sense that the rotational speed of the fan is increased rapidly and significantly if it is established that the outlet temperature is above the dead zone 4, and the rotational speed of the fan is decreased carefully and gradually if it is established that the outlet temperature is below the dead zone 4. The control aspect illustrated in the lower graph is symmetrical in the sense that the rotational speed of the fan is increased or decreased gradually when the outlet temperature is outside the dead zone 4, regardless of whether the outlet temperature is above or below the dead zone 4.

(28) FIGS. 5-7 are three block diagrams, each illustrating a method for controlling a fan according to aspects of control according to the invention. In the control aspect illustrated in FIG. 5, an asymmetric, but substantially linear, function is used for controlling the rotational speed of the fan in response to a measured outlet temperature.

(29) In the control aspect illustrated in FIG. 6, an aggressive scaling function, in the form of an exponential function, is used for determining the rotational speed of the fan in response to a measured outlet temperature.

(30) In the control aspect illustrated in FIG. 7, the function is identical to the function used in the control aspect illustrated in FIG. 5. However, in the control aspect illustrated in FIG. 7, a closed loop feedback is used.

(31) Although various embodiments of the present invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.