Method for terminating defrosting of an evaporator by use of air temperature measurements

Abstract

A method for terminating defrosting of an evaporator (104) is disclosed. The evaporator (104) is part of a vapour compression system (100). The vapour compression system (100) further comprises a compressor unit (101), a heat rejecting heat exchanger (102), and an expansion device (103). The compressor unit (101), the heat rejecting heat exchanger (102), the expansion device (103) and the evaporator (104) are arranged in a refrigerant path, and an air flow is flowing across the evaporator (104). When ice is accumulated on the evaporator (104), the vapour compression system (100) operates in a defrosting mode. At least one temperature sensor (305) monitors a temperature T.sub.air, of air leaving the evaporator (104). A rate of change of T.sub.air is monitored and defrosting is terminated when the rate of change of the temperature, T.sub.air, approaches zero.

Claims

1. A method for terminating defrosting of an evaporator, the evaporator being part of a vapour compression system, the vapour compression system further comprising a compressor unit, a heat rejecting heat exchanger, and an expansion device, the compressor unit, the heat rejecting heat exchanger, the expansion device and the evaporator being arranged in a refrigerant path, and an air flow flowing across the evaporator, the method comprising the steps of: operating the vapour compression system in a defrosting mode, monitoring, by at least one temperature sensor, at least one temperature, T.sub.air, of air leaving the evaporator, monitoring a rate of change of the temperature, T.sub.air, and terminating defrosting when the rate of change of the temperature, T.sub.air, approaches zero.

2. The method according to claim 1, wherein the step of terminating defrosting is performed when the rate of change of the temperature, T.sub.air, has been smaller than a predetermined threshold value for a predetermined time.

3. The method according to claim 1, wherein during the defrosting mode a hot gas from the compressor unit is supplied to refrigerant passages of the evaporator.

4. The method according to claim 3, wherein the hot gas heats the evaporator from the top to the bottom.

5. The method according to claim 3, wherein air in the evaporator and the air surrounding the evaporator are heated by means of convection.

6. The method according to claim 3, wherein the hot gas heats the evaporator from the bottom to the top.

7. The method according to claim 1, wherein the evaporator is in a flooded state.

8. The method according to claim 1, wherein the method further comprises the steps of: monitoring, by at least two additional temperature sensors, an evaporator inlet temperature, T.sub.e,in, at a hot gas inlet of the evaporator and an evaporator outlet temperature, T.sub.e,out, at a hot gas outlet of the evaporator, monitoring a rate of change of a difference between T.sub.e,in and T.sub.e,out, and terminating defrosting when the rate of change of the difference between T.sub.e,in and T.sub.e,out approaches zero.

9. The method according to claim 8, wherein the step of terminating defrosting is performed when the rate of change of the difference between T.sub.e,in and T.sub.e,out has been smaller than a predetermined threshold value for the predetermined time.

10. The method according to claim 1, wherein the step of monitoring at least one temperature, T.sub.air, comprises monitoring a first air temperature, T.sub.air,in, at an air inlet of the evaporator and a second air temperature, T.sub.air,out, at an air outlet of the evaporator.

11. The method according to claim 2, wherein during the defrosting mode a hot gas from the compressor unit is supplied to refrigerant passages of the evaporator.

12. The method according to claim 4, wherein air in the evaporator and the air surrounding the evaporator are heated by means of convection.

13. The method according to claim 4, wherein the hot gas heats the evaporator from the bottom to the top.

14. The method according to claim 5, wherein the hot gas heats the evaporator from the bottom to the top.

15. The method according to claim 2, wherein the evaporator is in a flooded state.

16. The method according to claim 3, wherein the evaporator is in a flooded state.

17. The method according to claim 4, wherein the evaporator is in a flooded state.

18. The method according to claim 5, wherein the evaporator is in a flooded state.

19. The method according to claim 6, wherein the evaporator is in a flooded state.

20. The method according to claim 2, wherein the method further comprises the steps of: monitoring, by at least two additional temperature sensors, an evaporator inlet temperature, T.sub.e,in, at a hot gas inlet of the evaporator and an evaporator outlet temperature, T.sub.e,out, at a hot gas outlet of the evaporator, monitoring a rate of change of a difference between T.sub.e,in and T.sub.e,out, and terminating defrosting when the rate of change of the difference between T.sub.e,in and T.sub.e,out approaches zero.

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 shows a simplified diagram of a vapour compression system,

(3) FIG. 2 shows a perspective view of an evaporator (a), (b) and an air flow through the evaporator in a cooling mode (c),

(4) FIG. 3 shows an evaporator operating in a defrosting mode,

(5) FIG. 4 shows diagrams of surface temperature changes over time of a simple evaporator with four rows of tubes when there is no ice buildup thereon, and

(6) FIG. 5 shows diagrams of surface temperature changes over time of a simple evaporator with four rows of tubes when there is ice buildup on the tube.

DETAILED DESCRIPTION

(7) FIG. 1 shows a simplified diagram of a vapour compression system 100 comprising a compressor unit 101, a heat rejecting heat exchanger 102, an expansion device 103 and an evaporator 104. The compressor unit 101 shown in FIG. 1 comprises two compressors. It is noted that it is within the scope of the present invention that the compressor unit 101 comprises only one compressor, e.g. a variable capacity compressor, or that the compressor unit 101 comprises three or more compressors. Refrigerant flowing through the system 100 is compressed by the compressor unit 101 before being supplied to the heat rejecting heat exchanger 102. In the heat rejecting heat exchanger 102, heat exchange takes place with a secondary fluid flow across the heat rejecting heat exchanger 102 in such a manner that heat is rejected from the refrigerant. In the case that the heat rejecting heat exchanger 102 is in the form of a condenser, the refrigerant passing through the heat rejecting heat exchanger 102 is at least partly condensed. In the case that the heat rejecting heat exchanger 102 is in the form of a gas cooler, the refrigerant passing through the heat rejecting heat exchanger 102 is cooled, but it remains in a gaseous state.

(8) The refrigerant leaving the heat rejecting heat exchanger 102 is then passed through the expansion device 103 which may, e.g., be in the form of an expansion valve. The refrigerant passing through the expansion device 103 undergoes expansion and is further supplied to the evaporator 104. In the evaporator 104, heat exchange takes place with a secondary fluid flow across the evaporator 104 in such a manner that heat is absorbed by the refrigerant, while the refrigerant is at least partly evaporated. The refrigerant leaving the evaporator 104 is then supplied to the compressor unit 101.

(9) FIGS. 2(a) and 2(b) show perspective views of a generic model of an evaporator 104. In the evaporator 104 the liquid refrigerant is evaporated into a gaseous form/vapour. The evaporator 104 of FIG. 2 comprises a plurality of tubes 201 which guide the liquid refrigerant there through and which are enclosed in an evaporator structural support 202. The tubes 201 may typically be arranged in a horizontal manner. The length of the tubes 201 may vary and that length may define one dimension of the evaporator 104. The evaporator 104 comprises a fan 203 which drives a secondary air flow across the evaporator 104 and over the evaporator tubes 201 as indicated by arrows 204 in FIG. 2(c). In case of a refrigeration system, the liquid refrigerant absorbs heat from the air passing through the evaporator 104, thereby reducing the temperature of the air and providing cooling for a closed volume being in contact with the evaporator 104. The closed volume may, e.g., be a refrigeration chamber.

(10) FIG. 3 shows a cross section of the evaporator 104 operating in a defrosting mode. During the defrosting mode, the fan 203 is turned off. In the defrosting mode, the tubes 201 may be heated from inside by a hot gas. When defrosting with the hot gas, the evaporator 104 is heated from the top part 301 and as the hot gas flows through the tubes 201 all of the metal of the evaporator 104 is gradually heated. The hot gas will gradually flow towards the bottom part 302 of the evaporator 104. Because of the mass and gradual cooling/condensing of the hot gas, the top 301 and bottom 302 of the evaporator are heated with a delay. The hot gas heats up the tubes 201, heats and melts ice accumulated on the tubes 201 and fins (not shown). While the entire evaporator 104 is heating, convection to the surrounding air is happening, i.e., the volume of air between the fins and tubes 201 is also heating. The volume of air will start natural movement, as indicated by arrows 300, due to differences in temperature. The volume of air moves into the direction of the fan 203 and towards openings on the air inlet side 303 of the evaporator 104.

(11) During defrosting, at least one temperature sensor 305 monitors a temperature of air leaving the evaporator 104. Alternatively, the sensor 305 may be positioned at the air inlet 303 of the evaporator 104, as indicated by a dashed line box 306. When measuring the air temperature by either the sensor 305 or 306 close to the inlet or outlet of the evaporator 104, the transient behaviour of the air temperature inside the evaporator can be recorded.

(12) FIG. 4(a) shows a simplified model of an evaporator 400 having only four rows of tubes 401-404. The sensor 305 is monitoring the temperature of air leaving the evaporator 400. On this simple evaporator 400 with four rows, the convective heat transfer, Q, to the surrounding air can be expressed as Q=hAΔT, where h is a heat transfer coefficient [W/(Km.sup.2)], A is the evaporator area [m.sup.2], and ΔT is T.sub.air−T.sub.e, T.sub.e is the evaporation temperature. Assuming the same size and constant temperature of the surrounding air, then the total convective heat transfer can be expressed as ΣQ=hAΣ(ΔT). The tubes 401-404 are typically heated with the hot gas one after another, i.e., with a short time delay, as represented in FIG. 4(b). The graphs in FIG. 4(b) show surface temperature for each of four tubes 401-404 of the evaporator 400 when there is no ice on the evaporator 400 and its tubes 401-404. The tubes 401-404 are slowly heated up and after a certain time their temperature reaches a constant value. That is when a steady state starts for each of the tubes 401-404. The accumulated temperature difference Σ(ΔT) is represented by curve 405 in FIG. 4(c), again, in the case when there is no ice nor frost accumulated on the evaporator 400. The accumulated temperature difference Σ(ΔT) of the surface temperatures of the tubes 401-404 reflects the heating of the surrounding air temperature inside the evaporator 400. The same temperature trend is then monitored by the sensor 305. In the first 8 minutes the evaporator 400 itself is heated and the air temperature measured by the sensor 305 is constantly rising. Once the evaporator 400 is heated, stable convection happens and air temperature at the outlet of the evaporator 400 reaches a constant value. That is when the rate of change of the air temperature approaches zero and when defrosting can be terminated.

(13) FIG. 5(a) shows diagrams 501-504 of surface temperature changes over time of the same simple evaporator 400 with four rows of tubes in case when there is frost or ice buildup on the tubes 401-404. Curve 501 corresponds to the tube 401, as the first row 401 of tubes is heated first. The effect of ice melting results in a different temperature profile compared to one shown in FIG. 4(b). The temperature change in this case is similar to the case when there is no ice in the first several minutes as, at first, it is only the evaporator itself which is heating up. When the surface temperature of the tubes reaches zero, ice starts melting and the surface temperature maintains the same temperature for a short period of time, as shown by all the curves 501-504. In this short period of time, the rate of change of the surface temperature of the tube approaches zero. This short period of time is one of the reasons why the step of terminating defrosting may be performed when the rate of change of air temperature is smaller than a predetermined threshold for a predetermined time. When ice starts melting the surface temperature of the tubes will rise again and reach a steady state later compared to the case when there is no ice. This difference may be seen in FIG. 5(b) where both cases are shown, curve 405 represents air temperature change when there is no ice, and curve 505 represents air temperature change when there is ice on the evaporator 400. It can be seen that the steady state is reached more than 2 minutes later than in the case when there is no ice on the evaporator 400. As stated above, the surrounding air temperature inside the evaporator 400 will be heated as the profile of the accumulated temperature difference. When measuring the temperature with the sensor 305, a similar profile is seen.

(14) 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.