A transcritical CO2 heat pump heating performance improvement system and method

Abstract

The application relates to a transcritical CO2 heat pump heating performance improving system and method, wherein the system comprises a refrigerant side circuit, which comprises that the output end of a gas-liquid separator is communicated with the gas input end of a gas cooler through a compressor; the gas output end of the gas cooler is communicated with the first input end of an intermediate heat exchanger; the first output end of the intermediate heat exchanger is communicated with the input end of an evaporator, and the second output end is communicated with the input end of the gas-liquid separator; the output end of the evaporator is communicated with the second input end of the intermediate heat exchanger; the refrigerant side circuit comprises that the cooling liquid input end of the gas cooler is communicated with a return water end, and the cooling liquid output end of the gas cooler is communicated with a water supply end. According to the technical scheme, the return water temperature can be reduced through evaporation heat absorption, the refrigerant temperature before throttling is reduced, the refrigerant flow is increased, the evaporation temperature is increased, the energy utilization efficiency is further improved, and the heating stability is ensured.

Description

Technical Field

[0001] This application relates to the field of building energy conservation technology, and in particular to a transcritical CO2 heat pump heating performance improvement system and method. Background Technology

[0002] In the field of building energy conservation, transcritical CO2 heat pump heating systems have attracted widespread attention due to their environmental friendliness and high efficiency. Utilizing CO2 as a refrigerant, these systems convert lower-grade heat energy into higher-grade heat energy through a compressor, meeting the winter heating needs of buildings. However, the system's operating efficiency and stability are significantly affected by factors such as ambient temperature and return water temperature. Optimizing the circulation process to improve performance has become a key direction for the development of this technology. Among these technologies, heat pipe heat exchangers, with their high-efficiency phase-change heat transfer capability and compact structure, provide a feasible solution to the problems of system energy loss and frosting, promoting the coupled application research of transcritical CO2 heat pumps and heat pipe technology.

[0003] In existing technologies, the high return water temperature in traditional transcritical CO2 heat pump heating systems leads to increased throttling losses and reduced heating efficiency. When the return water temperature is high, the CO2 temperature at the gas cooler outlet rises. During the throttling and pressure reduction process through the electronic expansion valve, entropy increases significantly and flash vapor increases. This not only consumes additional energy but also reduces the system's coefficient of performance (COP). For example, when the return water temperature rises from 20°C to 50°C, throttling losses increase significantly, and the COP decreases drastically. Furthermore, the evaporator is prone to frosting at low temperatures, affecting system stability. When the outdoor temperature is low, the frost layer on the evaporator surface deteriorates the heat transfer effect, leading to a decrease in refrigerant flow and a reduction in heating capacity. At the same time, the compressor compression ratio increases, exacerbating component wear and raising the exhaust temperature, which in severe cases can even affect the normal operation of the unit.

[0004] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] To address the aforementioned issues, this application provides a transcritical CO2 heat pump heating performance improvement system and method, which can reduce the return water temperature through evaporative heat absorption, thereby reducing the refrigerant temperature before throttling, increasing the refrigerant flow rate and raising the evaporation temperature, while ensuring heating stability and further improving energy utilization efficiency.

[0006] To achieve the objectives of this application, the following technical solution is provided:

[0007] In a first aspect, this application provides a transcritical CO2 heat pump heating performance enhancement system, comprising:

[0008] The refrigerant-side circuit includes: a compressor, a gas cooler, an intermediate heat exchanger, an evaporator, and a gas-liquid separator;

[0009] The output end of the gas-liquid separator is connected to the gas input end of the gas cooler through the compressor;

[0010] The gas output end of the gas cooler is connected to the first input end of the intermediate heat exchanger;

[0011] The first output end of the intermediate heat exchanger is connected to the input end of the evaporator, and the second output end is connected to the input end of the gas-liquid separator.

[0012] The output end of the evaporator is connected to the second input end of the intermediate heat exchanger;

[0013] The water-side circuit includes: a water supply end and a water return end; the coolant input end of the gas cooler is connected to the water return end, and the coolant output end of the gas cooler is connected to the water supply end.

[0014] In one possible implementation, the return water end has a first temperature sensor for detecting the return water temperature.

[0015] In one possible implementation, a drying filter is provided on the connecting pipe between the gas outlet of the gas cooler and the first input of the intermediate heat exchanger.

[0016] In one possible implementation, an electronic expansion valve is provided on the connecting pipe between the first output end of the intermediate heat exchanger and the input end of the evaporator.

[0017] In one possible implementation, a first solenoid valve is provided on the connecting pipe between the return water end and the coolant inlet end of the gas cooler.

[0018] In one possible implementation, the transcritical CO2 heat pump heating performance enhancement system further includes:

[0019] The heat pipe heat exchanger circuit includes a heat pipe heat exchanger, wherein the return water end is connected to the coolant inlet end of the gas cooler via a spiral coil, and the spiral coil is connected to the cold end of the heat pipe heat exchanger for heat exchange.

[0020] In one possible implementation, the spiral coil has a second solenoid valve.

[0021] In one possible implementation, the output of the gas-liquid separator is also connected to the input of the evaporator via the compressor.

[0022] In one possible implementation, a third solenoid valve is provided on the connecting pipe between the compressor and the evaporator, and a second temperature sensor is also provided on the coil of the evaporator.

[0023] Secondly, this application also provides a method for improving the performance of a transcritical CO2 heat pump heating system, used to control the aforementioned transcritical CO2 heat pump heating performance improvement system, the method comprising:

[0024] When the temperature at the return water end is lower than the preset value, the first solenoid valve is opened and the second and third solenoid valves are closed.

[0025] When the temperature at the return water end is greater than or equal to the preset value, the second solenoid valve is opened and the first and third solenoid valves are closed.

[0026] When the temperature of the evaporator coil is lower than the second preset value, a timer is started. When the accumulated time exceeds the preset cycle and the temperature of the evaporator coil is lower than the third preset value, the second and third solenoid valves are opened and the first solenoid valve is closed.

[0027] The technical solution provided in this application may include the following beneficial effects:

[0028] The transcritical CO2 heat pump heating performance improvement system and method provided in this application can utilize the heat exchange between the cold end of a heat pipe heat exchanger and the return water to reduce the return water temperature through evaporation and heat absorption, thereby reducing the refrigerant temperature before throttling. Furthermore, the hot end of the heat pipe heat exchanger is combined with a gas-liquid separator to heat the internal refrigerant through heat release, causing some of the liquid refrigerant to vaporize and participate in the circulation, increasing the refrigerant flow and raising the evaporation temperature. At the same time, through the coordinated control of solenoid valves and temperature sensors, the system can automatically switch between three modes: conventional heating, heat recovery, and defrosting. The process can be dynamically adjusted according to the return water temperature and coil temperature, further improving energy utilization efficiency while ensuring heating stability.

[0029] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0030] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. Obviously, the drawings described below are merely some embodiments of this disclosure, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0031] Figure 1 A schematic diagram of a transcritical CO2 heat pump heating performance enhancement system provided in this application embodiment;

[0032] Figure 2 A flowchart illustrating a method for improving the heating performance of a transcritical CO2 heat pump, provided as an embodiment of this application;

[0033] Figure 3 A control logic diagram of a method for improving the heating performance of a transcritical CO2 heat pump provided in this application embodiment;

[0034] Figure 4 A schematic diagram comparing the pressure-enthalpy diagrams of a conventional heating mode and a heat recovery mode for a transcritical CO2 heat pump heating performance improvement method provided in this application embodiment;

[0035] Figure 5 A comparative schematic diagram showing the temperature change of the evaporator fins of a transcritical CO2 heat pump heating performance improvement system over time, provided as an embodiment of this application.

[0036] Figure 6 This is a comparative schematic diagram showing the change of COP over time in a transcritical CO2 heat pump heating performance improvement system provided in this application embodiment.

[0037] Figure label:

[0038] 1. Compressor; 2. Gas cooler; 3. Intermediate heat exchanger; 4. Evaporator; 5. Gas-liquid separator; 6. Heat pipe heat exchanger; 7. Water supply end; 8. Water return end; 9. Dryer filter; 10. Electronic expansion valve; 11. First solenoid valve; 12. First temperature sensor; 13. Second solenoid valve; 14. Third solenoid valve; 15. Second temperature sensor. Detailed Implementation

[0039] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, they are provided so that this disclosure will be more comprehensive and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0040] This example implementation first provides a transcritical CO2 heat pump heating performance improvement system under dynamic disturbance power management conditions of sparse feedback and missing observations. (Reference) Figure 1 As shown, the transcritical CO2 heat pump heating performance enhancement system includes a refrigerant-side loop and a refrigerant-side circuit; wherein:

[0041] The refrigerant-side circuit includes: compressor 1, gas cooler 2, intermediate heat exchanger 3, evaporator 4, and gas-liquid separator 5;

[0042] The output end of the gas-liquid separator 5 is connected to the gas input end of the gas cooler 2 through the compressor 1;

[0043] The gas output end of the gas cooler 2 is connected to the first input end of the intermediate heat exchanger 3;

[0044] The first output end of the intermediate heat exchanger 3 is connected to the input end of the evaporator 4, and the second output end is connected to the input end of the gas-liquid separator 5.

[0045] The output end of the evaporator 4 is connected to the second input end of the intermediate heat exchanger 3;

[0046] The water-side circuit includes: a water supply end 7 and a water return end 8; the coolant input end of the gas cooler 2 is connected to the water return end 8, and the coolant output end of the gas cooler 2 is connected to the water supply end 7.

[0047] In one possible implementation, the return water end 8 has a first temperature sensor 12 for detecting the return water temperature.

[0048] Understandably, by monitoring the return water temperature in real time, the first temperature sensor 12 can accurately determine whether the current operating conditions meet the start-up conditions of the conventional heating or heat recovery mode, ensuring that the conventional heating mode is used first when the return water temperature is lower than the preset value, and the heat recovery mode is switched when the return water temperature is higher than or equal to the preset value, thereby realizing dynamic control of the system's operating status and improving the targeting of energy utilization.

[0049] In one possible implementation, a drying filter 9 is provided on the connecting pipe between the gas output end of the gas cooler 2 and the first input end of the intermediate heat exchanger 3.

[0050] Understandably, the primary function of the dryer filter 9 is to remove moisture and impurities from the refrigerant. During refrigerant circulation, moisture may enter due to system leaks or environmental factors. Moisture easily freezes at low temperatures, clogging pipes or electronic expansion valves, while impurities may wear down components such as the compressor. The dryer filter 9 effectively prevents these problems, ensuring the smooth flow of the refrigerant side circuit and the long-term stable operation of the equipment.

[0051] In one possible implementation, an electronic expansion valve 10 is provided on the connecting pipe between the first output end of the intermediate heat exchanger 3 and the input end of the evaporator 4.

[0052] Understandably, the electronic expansion valve 10 controls the refrigerant flow and pressure drop by adjusting its opening, causing the high-pressure refrigerant flowing from the intermediate heat exchanger 3 to be throttled and converted into a low-pressure gas-liquid mixture, which facilitates the absorption of heat from the air in the evaporator 4. Its precise adjustment capability can adapt to the refrigerant requirements under different operating conditions, further optimizing the system's heating efficiency and stability.

[0053] In one possible implementation, a first solenoid valve 11 is provided on the connecting pipe between the return water end 8 and the coolant input end of the gas cooler 2.

[0054] Understandably, when the system is in normal heating mode, the first solenoid valve 11 is opened, allowing the return water to directly enter the gas cooler 2 to exchange heat with the refrigerant, ensuring a rapid supply of hot water; while in other modes it is closed to prevent the return water from entering the gas cooler directly without being cooled by the heat pipe heat exchanger, thus ensuring the energy-saving effect of heat recovery and defrosting modes.

[0055] Furthermore, in one possible implementation, the transcritical CO2 heat pump heating performance enhancement system further includes:

[0056] The heat pipe heat exchanger circuit includes a heat pipe heat exchanger 6. The return water end 8 is connected to the coolant input end of the gas cooler 2 through a spiral coil. The spiral coil is connected to the cold end of the heat pipe heat exchanger 6 for heat exchange.

[0057] Furthermore, the spiral coil has a second solenoid valve 13.

[0058] In one possible implementation, the output of the gas-liquid separator 5 is also connected to the input of the evaporator 4 via the compressor 1.

[0059] Understandably, this connection path provides an additional refrigerant circulation channel for the defrosting mode. During defrosting, some of the high-temperature, high-pressure refrigerant can bypass the gas cooler 2 and be directly delivered to the evaporator 4 through the compressor 1. The high temperature of the refrigerant melts the frost layer on the evaporator surface, significantly improving defrosting efficiency while avoiding indoor heating interruptions caused by defrosting.

[0060] In one possible implementation, the connecting pipe between the compressor 1 and the evaporator 4 has a third solenoid valve 14, and the coil of the evaporator 4 also has a second temperature sensor 15.

[0061] Understandably, the third solenoid valve 14 is used to control the refrigerant distribution path in defrost mode, and it only opens during defrost to ensure that the high-temperature refrigerant flows precisely to the evaporator 4 to achieve rapid defrosting; the second temperature sensor 15 monitors the evaporator coil temperature to provide a basis for defrost timing and mode switching. When the coil temperature is lower than the set value and the duration condition is met, defrost is triggered, and it stops when the temperature rises back to the threshold, ensuring the intelligence and energy efficiency of the defrost process.

[0062] Furthermore, this example embodiment also provides a method for improving the performance of a transcritical CO2 heat pump heating system, used to control the aforementioned transcritical CO2 heat pump heating performance improvement system. (Reference) Figure 2 As shown, the method may include:

[0063] In step S100, when the temperature of the return water end 8 is less than the preset value, the first solenoid valve 11 is opened and the second solenoid valve 13 and the third solenoid valve 14 are closed.

[0064] Step S200: When the temperature of the return water end 8 is greater than or equal to the preset value, the second solenoid valve 13 is opened and the first solenoid valve 11 and the third solenoid valve 14 are closed.

[0065] Step S300: When the temperature of the evaporator coil 4 is lower than the second preset value, a timer is started. When the accumulated time exceeds the preset cycle and the temperature of the evaporator coil is lower than the third preset value, the second solenoid valve 13 and the third solenoid valve 14 are opened and the first solenoid valve 11 is closed.

[0066] It should be noted that when the temperature at the return water end 8 is lower than the first preset value (e.g., 30℃), it is in normal heating mode, with the first solenoid valve 11 open and the second solenoid valve 13 and the third solenoid valve 14 closed. In this mode:

[0067] Refrigerant side loop: CO2 refrigerant is compressed into high temperature and high pressure by compressor 1, and then exchanges heat with water through shell-and-tube gas cooler 2. After passing through dryer filter 9 and intermediate heat exchanger 3, it enters evaporator 4 after being throttled and depressurized by electronic expansion valve 10. It absorbs heat from the air, and then enters gas-liquid separator 5 after passing through intermediate heat exchanger 3. The gas returns to compressor 1.

[0068] Water-side loop: The hot water discharged from the supply end 7 of the gas cooler 2 decreases in temperature after the user releases heat, and then enters the return water from the return end 8. The return water passes through the first solenoid valve 11 and enters the gas cooler 2 to exchange heat with CO2, and the temperature rises, resulting in the hot water discharged from the supply end 7.

[0069] Furthermore, when the temperature at the return water end 8 is greater than or equal to the first preset value (e.g., 30°C), it enters heat recovery mode, the second solenoid valve 13 opens, and the first solenoid valve 11 and the third solenoid valve 14 close. In this mode:

[0070] Refrigerant side loop: CO2 refrigerant is compressed into high temperature and high pressure by compressor 1, and then exchanges heat with water through shell-and-tube gas cooler 2. After passing through dryer filter 9 and intermediate heat exchanger 3, it enters evaporator 4 after being throttled and depressurized by electronic expansion valve 10. It absorbs heat from the air, and then enters gas-liquid separator 5 after passing through intermediate heat exchanger 3. The gas returns to compressor 1.

[0071] Water-side loop: The hot water discharged from the supply end 7 of the gas cooler 2 is cooled down after the user releases heat, and then enters the return water from the return end 8. The return water passes through the second solenoid valve 13 and the cold end of the U-shaped heat pipe heat exchanger to further reduce its temperature. Then it enters the gas cooler 2 to exchange heat with CO2, and its temperature rises, which is the hot water discharged from the supply end 7.

[0072] The heat pipe heat exchanger has 6 loops: one end of the heat exchanger exchanges heat with the return water through a spiral coil, absorbing heat. The hot end is connected to the gas-liquid separator, releasing heat through the hot end to heat the refrigerant in the gas-liquid separator, causing some of the liquid to vaporize. This allows more refrigerant to participate in the system circulation, thereby increasing the evaporation temperature, delaying frost formation, and improving energy efficiency.

[0073] At the same time, such as Figure 3 As shown, when the temperature (te) of the evaporator coil 4 is less than the second preset value (e.g., 3℃), the unit starts defrosting timing. When the accumulated time exceeds the set defrosting cycle and the coil temperature is lower than the third preset value (e.g., -3℃), the unit enters defrosting mode. During defrosting, when the coil temperature is not lower than the fourth preset value (e.g., 20℃), the unit stops defrosting and resumes normal heating mode or heat recovery mode, using temperature-time controlled defrosting. The second solenoid valve 13 and the third solenoid valve 14 are open, and the first solenoid valve 11 is closed. In this mode:

[0074] Refrigerant-side loop: CO2 refrigerant is compressed into high temperature and high pressure by compressor 1, and then divided into two paths. One path passes through a coaxial gas cooler 2 for heat exchange with water, then through a dryer filter 9, an intermediate heat exchanger 3, and then through an electronic expansion valve 10 for throttling and pressure reduction before entering the evaporator 4 to absorb heat from the air. After passing through the intermediate heat exchanger 3 again, it enters the gas-liquid separator 5, and the gas returns to compressor 1. The other path passes through a third solenoid valve 14 and mixes with the refrigerant throttled by the electronic expansion valve 10 before entering the evaporator 4.

[0075] Water-side loop: The hot water discharged from the supply end 7 of the gas cooler 2 is cooled down after the user releases heat, and then enters the return water from the return end 8. The return water passes through the second solenoid valve 13 and the cold end of the U-shaped heat pipe heat exchanger to further reduce its temperature. Then it enters the gas cooler 2 to exchange heat with CO2, and its temperature rises, which is the hot water discharged from the supply end 7.

[0076] The heat pipe heat exchanger has 6 loops: one end of the heat exchanger exchanges heat with the return water through a spiral coil, absorbing heat. The hot end is connected to the gas-liquid separator, and heat is released through the hot end to heat the refrigerant in the gas-liquid separator, causing some of the liquid to vaporize. This allows more refrigerant to participate in the defrosting cycle, thereby increasing the evaporation temperature and the defrosting rate.

[0077] Furthermore, such as Figure 4 As shown, the heating capacity Q1 in the conventional heating mode is: Q1 = h2 - h3

[0078] Compressor specific work w1: w1 = h2 - h1;

[0079] Heating capacity Q2 in heat recovery mode: Q2 = h 2′ -h 3′ ;

[0080] Compressor specific work w2: w2 = h 2′ -h 1′ ;

[0081] As can be seen from the figure:

[0082] Q1 <Q2

[0083] w1 <w2

[0084] therefore:

[0085]

[0086] The pressure-enthalpy diagram for defrosting mode is similar to that for heat recovery mode, so it will not be described in detail here.

[0087] Furthermore, a comparison was made between the existing technology unit and the unit of this application, under conditions of dry-bulb temperature of -5°C and relative humidity of 90%, to examine the change in evaporator fin temperature over time during the frosting process.

[0088] like Figure 5As shown, with the extension of frosting time, the surface temperature of the evaporator fins generally shows a downward trend. In existing technology units, the fin temperature drops from -10.9℃ to -13.5℃. Within the first 31 minutes of frosting, the rate of temperature decrease is relatively small. However, as the frosting time increases, the frost layer grows from granular ice crystals to needle-like frost crystals, growing along a direction perpendicular to the fin surface, and then from needle-like frost crystals to feather-like frost crystals, with a faster growth rate, thus accelerating the rate of temperature decrease. In the unit of this application, the fin temperature drops from -6.6℃ to -8.2℃. Within the first 60 minutes of frosting, the rate of temperature decrease is relatively slow. As time increases, the fin temperature drops to -8.2℃, the unit's heating performance decreases, and the system activates intelligent defrosting. Comparing the two figures, it can be seen that the fin temperature of the unit of this application is higher than that of the existing technology units, which is beneficial for delaying the growth of the frost layer.

[0089] Furthermore, a comparison is made between the existing technology units and the units of this application, under the conditions of a dry-bulb temperature of 5°C and a relative humidity of 90%, regarding the changes in the theoretical cycle performance coefficient (COP) in heating mode.

[0090] like Figure 6 As shown, under the conditions of dry bulb temperature of 5°C and relative humidity of 90%, the COP increased from 3.185 to 3.209, and the evaporation temperature increased to about 4-5°C.

[0091] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of this disclosure are indicated by the appended claims.

[0092] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit it. This application is not limited to the exact structures described above and illustrated in the accompanying drawings, and it should not be considered that the specific implementation of this application is limited to these descriptions. For those skilled in the art, various changes and modifications made without departing from the concept of this application should be considered to fall within the protection scope of this application.

Claims

1. A transcritical CO2 heat pump heating performance enhancement system, characterized in that, include: The refrigerant-side circuit includes: a compressor, a gas cooler, an intermediate heat exchanger, an evaporator, and a gas-liquid separator; The output end of the gas-liquid separator is connected to the gas input end of the gas cooler through the compressor; The gas output end of the gas cooler is connected to the first input end of the intermediate heat exchanger; The first output end of the intermediate heat exchanger is connected to the input end of the evaporator, and the second output end is connected to the input end of the gas-liquid separator. The output end of the evaporator is connected to the second input end of the intermediate heat exchanger; The water-side circuit includes: a water supply end and a water return end; the coolant input end of the gas cooler is connected to the water return end, and the coolant output end of the gas cooler is connected to the water supply end; The heat pipe heat exchanger circuit includes a heat pipe heat exchanger, wherein the return water end is connected to the coolant inlet end of the gas cooler via a spiral coil, and the spiral coil is connected to the cold end of the heat pipe heat exchanger for heat exchange. The output end of the gas-liquid separator is also connected to the input end of the evaporator through the compressor; A first solenoid valve is provided on the connecting pipe between the return water end and the coolant input end of the gas cooler; a second solenoid valve is provided on the spiral coil; and a third solenoid valve is provided on the connecting pipe between the compressor and the evaporator.

2. The transcritical CO2 heat pump heating performance enhancement system according to claim 1, characterized in that, The return water end is equipped with a first temperature sensor for detecting the return water temperature.

3. The transcritical CO2 heat pump heating performance enhancement system according to claim 1, characterized in that, A drying filter is provided on the connecting pipe between the gas output end of the gas cooler and the first input end of the intermediate heat exchanger.

4. The transcritical CO2 heat pump heating performance enhancement system according to claim 1, characterized in that, An electronic expansion valve is provided on the connecting pipe between the first output end of the intermediate heat exchanger and the input end of the evaporator.

5. The transcritical CO2 heat pump heating performance enhancement system according to claim 1, characterized in that, The evaporator coil also has a second temperature sensor.

6. A method for improving the performance of a transcritical CO2 heat pump heating system, characterized in that, The method is used to control the transcritical CO2 heat pump heating performance improvement system as described in claim 5, the method comprising: When the temperature at the return water end is lower than the first preset value, the first solenoid valve is opened and the second and third solenoid valves are closed. When the temperature at the return water end is greater than or equal to the first preset value, the second solenoid valve is opened and the first solenoid valve and the third solenoid valve are closed. When the temperature of the evaporator coil is lower than the second preset value, a timer is started. When the accumulated time exceeds the preset cycle and the temperature of the evaporator coil is lower than the third preset value, the second and third solenoid valves are opened and the first solenoid valve is closed.

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