When the air source heat pump unit is operating under normal operating conditions, the evaporator absorbs heat from the ambient air, causing the surface temperature of the evaporator fin to decrease. As the cycle progresses, the surface temperature of the evaporator fins continues to decrease until below the dew point of the ambient air, water vapor in the air condenses on the surface of the fins, and if the fin temperature is below 0 ° C, Frosting appears. Detailed air source heat pump winter frost conditions and defrost methods are there? As the cycle continues, the frost layer is further thickened and gradually covers the entire evaporator. The appearance of the frost layer increases the thermal resistance of heat exchange between the air and the working fluid, which seriously hinders the heat exchange performance of the evaporator. Not only that, thickening of the frost layer also increased the resistance of the air flowing through the fins, reducing the air flow, resulting in evaporator performance degradation. These problems will result in heat pump products can not work or even damage. Therefore, the use of reasonable and effective method of defrosting is particularly important. 1, thermoelectric defrost through the heat exchanger installed on the appropriate power resistor, when the evaporator layer of cream accumulated to a certain extent, the switch is turned on, the resistance wire energized heat defrosting. This method is simple and easy, but from the energy point of view is not desirable. 2, reverse cycle defrost One is installed in the evaporator coil temperature sensor, outdoor temperature by detecting the coil to determine whether the frost. The other is by detecting the difference between the condenser coil temperature and room temperature (or water temperature) to determine whether the outdoor evaporator frosting, that is, when the evaporator frosted, the heat transfer efficiency decreases, resulting in condenser heat transfer When the difference between the temperature of the condenser coil and the room temperature (or water temperature) is lower than a certain value, it can be judged that the frost in the outdoor heat exchanger is more serious. Defrost start reversing defrost program, four-way reversing valve action, changing the flow of refrigerant, so that the unit from the heating state to cooling state, the compressor discharge of high temperature gas through the four-way valve to the outdoor switch Defrost in the heater, defrost ends when the outdoor coil temperature rises to a certain temperature. 3, the refrigerant cold heat release defrost This method is to condense the refrigerant out of the cold after the throttle, and then into the evaporator to melt the frost on the evaporator. In the heating condition defrosting state, the four solenoid valve opens only one, the liquid refrigerant from the condenser, from the open solenoid valve into the fin heat exchanger for cold heat release defrosting, and then enter the Open the solenoid valve corresponding to the gas-liquid separator. From the gas-liquid separator out of the liquid refrigerant into the header, and then through the throttle into the distributor, through the one-way valve into the remaining three pipes into the evaporator evaporation, gaseous refrigerant into the corresponding gas and liquid Separator, and then collected from the outlet to the manifold and then through the switch valve into the compressor to complete the cycle. By setting the cream in the frost will be removed, so that the unit in frost-free operation. 4, the fan reversing method of defrosting This method is based on the reversing defrosting improved, that is, in the defrost process to enable fan reversal, so that the opposite direction of the air, forced air from the non-frosted side of the entry Wind side heat exchanger and flow to the frosting side, the heated air blown frost layer. This defrosting method takes full advantage of the heat of the wind side heat exchanger, defrost simultaneously by means of convection, thermal conduction and radiation, and the efficiency is obviously better than that of the conventional defrosting method. At the same time, a certain pressure can also promote the collapse of the frost shell from the heat exchanger surface, the addition of convection heat makes defrosting process carried out quickly and thoroughly. However, due to an increase of intermediate relays and pressure switches and other devices, increased production costs. 5, hydraulic defrost For large heat pump systems, often using hydraulic defrost measures. The purpose of defrosting is achieved by showering the outdoor evaporator with hot water. The defrosting method has the advantages of simple equipment, but causes the moisture content around the evaporator after defrosting is too high and is easy to frosting again, so it is not suitable for use in areas with lower temperature such as the north. And the larger the waste of water resources, the need for an independent water system. 6, pneumatic defrost This method uses compressed air to produce high-speed jet directly blown defrosting layer, at any time to clear the evaporator surface of the small condensate cream, the evaporator surface is always frost-free state. Its biggest advantage is uninterrupted heating of the room, weak indoor thermal environment to ensure comfort. Compressed air, however, requires additional power consumption and a higher overall cost of ownership. At present, most domestic manufacturers of air-source heat pump water heaters use hot gas defrost methods, specifically reverse-cycle defrosting and hot gas bypass defrosting. Reverse cycle defrosting will affect the air-source heat pump water heaters, that is, during the defrost period, can not provide users with effective water temperature hot water, at the same time, after defrost, the original hot water temperature will be reduced from the energy point of view , The loss of this defrosting process is equivalent to twice the defrost time downtime, after calculation, will make the unit heat supply decreased by about l0%. And, four-way valve frequent commutation will affect its reliability and life expectancy. The hot gas by-pass defrosting heat due to the high-pressure side of the refrigerant or from the evaporator to absorb heat, when the temperature is low, defrosting fast enough, there will not be enough heat to absorb, will make the host into a protective shutdown. Such as the use of a simple by-pass road, it is easy to produce liquid crystal phenomenon of the compressor. At the same time, in the defrosting process, due to the compressor displacement reduced, will affect the effect of heating hot water, can not meet the normal demand for hot water. Asia Fluid Network () Editor: Huang Nengwen, QQ:
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