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 two temperature sensors (306, 307) monitor an evaporator inlet temperature, T.sub.e,in, at a hot gas inlet (304) of the evaporator (104) and an evaporator outlet temperature, T.sub.e,out, at a hot gas outlet (305) of the evaporator (104). A difference between T.sub.e,in and T.sub.e,out, is monitored and defrosting is terminated when the rate of change of the difference between T.sub.e,in and T.sub.e,out 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 two 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.
2. The method according to claim 1, 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 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 the hot gas inlet of the evaporator and through refrigerant passages of the evaporator.
4. The method according to claim 3, wherein the hot gas gradually 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 1, wherein the evaporator is in a flooded state.
7. The method according to claim 1, wherein the evaporator is in a non-flooded state.
8. The method according to claim 2, wherein during the defrosting mode a hot gas from the compressor unit is supplied to the hot gas inlet of the evaporator and through refrigerant passages of the evaporator.
9. The method according to claim 4, wherein air in the evaporator and the air surrounding the evaporator are heated by means of convection.
10. The method according to claim 2, wherein the evaporator is in a flooded state.
11. The method according to claim 3, wherein the evaporator is in a flooded state.
12. The method according to claim 4, wherein the evaporator is in a flooded state.
13. The method according to claim 5, wherein the evaporator is in a flooded state.
14. The method according to claim 2, wherein the evaporator is in a non-flooded state.
15. The method according to claim 3, wherein the evaporator is in a non-flooded state.
16. The method according to claim 4, wherein the evaporator is in a non-flooded state.
17. The method according to claim 5, wherein the evaporator is in a non-flooded state.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The invention will now be described in further detail with reference to the accompanying drawings in which
[0035] FIG. 1 shows a simplified diagram of a vapour compression system,
[0036] FIG. 2 shows a perspective view of an evaporator (a), (b) and an air flow through the evaporator in a cooling mode (c),
[0037] FIG. 3 shows natural air flow in an evaporator operating in a defrosting mode,
[0038] FIG. 4 shows an evaporator tube without (a) and with (b) ice buildup,
[0039] FIG. 5 shows diagrams of surface temperature changes over time of an evaporator tube when there is no ice buildup on the tube, and
[0040] FIG. 6 shows diagrams of surface temperature changes over time of an evaporator tube when there is ice buildup on the tube.
DETAILED DESCRIPTION
[0041] 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.
[0042] 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.
[0043] 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.
[0044] FIG. 3(a) 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 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 inlet side 303 of the evaporator 104.
[0045] FIGS. 3(b) and 3(c) show perspective view of two opposite sides of the evaporator 104 with horizontally connected tubes 201 such that there is a continuous flow path from the top to the bottom of the evaporator 104. A central feed pipe 304 at the top of the evaporator 104 is configured to feed each of the top tubes 201-t. Refrigerant flows through the entire evaporator 104 until it exits at the bottom of the evaporator 104 through a central suction pipe 305. The hot gas may also enter the central feed pipe 304, at the top of the evaporator 104, heat it, and then leaves the tubes at the central suction pipe 305. The central suction pipe 305 at the bottom of the evaporator 104 is fed by the bottom tubes 201-b.
[0046] During defrosting, at least two temperature sensors 306 and 307 monitor temperatures at the hot gas inlet 304, T.sub.e,in, where the hot gas enters the evaporator 104 and at the hot gas outlet 305, T.sub.e,out, where the hot gas leaves the evaporator 104. The temperature sensors 306 and 307 may be placed on the outer side of the central feed pipe 304 and the central suction pipe 305, respectively. Alternatively, the temperature sensor 306 may be placed on one of the top tubes 201-t, and the temperature sensor 307 may be placed on one of the bottom tubes 201-b. in yet one alternative, the sensors 306 and 307 may be placed on the bends of the tubes at the end of the evaporator (not shown). In this way, the temperature of the surface near the hot gas inlet 304 of the evaporator 104 is measured as well as the surface near the hot gas outlet 305 of the evaporator 104.
[0047] FIG. 4(a) shows a simplified evaporator tube 201 without any buildup of ice. FIG. 4(a) shows the fins 400 of the evaporator tube 201. FIG. 4(b) shows the evaporator tube 201 with ice buildup 401. The tube 201 has an inlet to which the central feed pipe 304 is merged and an outlet to which the central suction pipe 305 is merged, where the hot gas may enter and exit the evaporator. In the defrosting mode, the hot gas is introduced into the central feed pipe 304 of the tube 201. Normally, the inlet of the tube 201 will have higher temperature than the outlet of the tube.
[0048] FIG. 5 shows diagrams of surface temperature changes over time of an evaporator tube when there is no ice buildup on the tube. Curves 501 and 502 represent temperature of a tube inlet and outlet over time respectively and curve 503 is the difference between the inlet and outlet temperatures. Even when there is no ice on the evaporator, some time is needed to reach stable condition, i.e., to reach a point when there a rate of change if a difference between the two temperatures approaches zero. As shown in FIG. 5(b), in the zone 504, heating of the evaporator itself happens and that requires certain amount of energy. As soon as the evaporator starts to be heated, convection of the hot air to ambient starts, as indicated by curve 505. Convection of the hot air dominates when the evaporator stable conditions are reached, and this is indicated by dashed line 506.
[0049] FIG. 6(a) shows diagrams of surface temperature changes over time of an evaporator tube when there is ice buildup on the tube. Curve 601 represents temperature of a tube inlet over time, T.sub.e,in, and curve 602 represents a temperature of a tube outlet over time, T.sub.e,out. The difference between T.sub.e,in and T.sub.e,out is represented by curve 603. A derivative over time of the difference between T.sub.e,in and T.sub.e,out, i.e., a derivative of curve 603 represents a rate of change of the difference between T.sub.e,in and T.sub.e,out. Typically, at the beginning of defrosting, both temperature at the inlet and outlet will be the same, as can be seen from the curves 601 and 602. Then, the temperature at the evaporator inlet may begin to rise faster than the temperature at the evaporator outlet. This is expected, as the hot gas will heat the structural support of the evaporator and melt frost and ice at the areas closer to the hot gas inlet. Depending on the amount of frost or ice, a time period during which the temperatures at the inlet and outlet of the evaporator are different and rise in different manner may vary.
[0050] As frost and ice melt from the evaporator, temperatures of the inlet and outlet of the evaporator may stabilize and reach constant values, as shown by the last portion of the curves 601 and 602. When both temperatures have the constant values, their difference becomes constant and therefore the rate of change of the difference approaches zero, as represented by the last portion of the curve 603. When the rate of change of the difference between T.sub.e,in and T.sub.e,out approaches zero the evaporator operates as when there is no ice or frost on its surface, i.e. as illustrated in FIG. 5. Therefore, no change in the difference between the two temperatures indicates that all the ice or frost has been removed and there is no need for further defrosting. At this point, defrosting may be terminated as all frost or ice has been removed from the evaporator. Time required for full defrosting to happen, may depend on various factors such as size of the vapour compression system, temperature of the hot gas, amount of ice to be melted, temperature of the system at the beginning of defrosting, ambient conditions such as temperature, pressure, humidity, and the like.
[0051] FIG. 6(b) shows a comparison between an evaporator without ice and an evaporator with 1.5 mm ice buildup. It can be seen from the FIG. 6(b) that ice influences transient behaviour of the evaporator. It can be seen from the graphs that in a zone 1, the heating of the evaporator is mainly happening. In a zone 2, most of the ice is melted and extra heat capacity is required to elevate temperature of the evaporator tubes and melt ice. The area below the curves represents the energy needed to heat up the evaporator and melt ice buildups. The stable condition is reached in a zone 3, when the evaporator is ice-free and normal thermal convection happens. Naturally, this occurs later than in case when there is no ice buildup on the evaporator.
[0052] 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.
