Explain the air source heat pump winter frost conditions and defrost methods?

When the air source heat pump unit is operating under normal conditions, the evaporator absorbs heat from the surrounding air, causing the surface temperature of the evaporator fins to decrease. As the cycle progresses, the evaporator fin surface temperature continues to decrease until it is below the dew point temperature of the surrounding air. The water vapor in the air will condense on the fin surface. If the fin temperature is lower than 0°C, the surface will Frosting phenomenon.

Explain the air source heat pump winter frost conditions and defrost methods?

As the cycle continues, the frost layer will further thicken and gradually cover the entire evaporator. The appearance of the frost layer increases the thermal resistance between the air and the working fluid, which seriously hinders the heat transfer performance of the evaporator. Not only that, the thickening of the frost layer also increases the resistance of the air flowing through the fins, reduces the air flow, and causes the evaporator performance to decay. All these problems will cause the heat pump products to not work or even damage. Therefore, using a reasonable and effective defrosting method is particularly important.

1, thermoelectric defrosting

By installing an appropriate power resistor on the heat exchanger, when the frost layer on the evaporator has accumulated to a certain degree, the switch is turned on and the resistance wire is energized and defrosted. This method is simple and easy, but it is not desirable from the perspective of energy conservation.

2, reverse cycle defrosting

One is to install a temperature sensor on the evaporator coil and determine whether the frost is formed by detecting the temperature of the outdoor coil. The other method is to judge whether the outdoor evaporator is frosted by detecting the difference between the condenser coil temperature and the room temperature (or water temperature), that is, when the evaporator frosts, the heat exchange efficiency is reduced, resulting in the heat exchange capacity of the condenser. When the temperature drops, the coil temperature decreases. When the difference between the temperature of the condenser coil and the room temperature (or the water temperature) is detected to be lower than a certain value, it can be judged that the outdoor heat exchanger is more severely frosted.

When the defrosting is started, the commutation defrosting program is started, the four-way reversing valve acts to change the flow of the refrigerant, and the unit is switched from the heating operation state to the cooling operation state. The high-temperature gas discharged from the compressor is switched to the outdoor one through the four-way valve. Defrosting is performed in the heat exchanger and defrosting is completed when the outdoor coil temperature rises to a certain temperature value.

3, the refrigerant is too cold exothermic defrosting

In this method, the refrigerant from the condenser is overcooled and throttled, and then enters the evaporator to melt the frost layer on the evaporator.

In the defrosting state under heating conditions, four solenoid valves open only one, and the liquid refrigerant from the condenser enters the fin heat exchanger to perform superheating and defrosting due to overcooling and then enters Open the corresponding gas-liquid separator for the solenoid valve. The refrigerant coming out of the outlet of the gas-liquid separator enters the collector tube, enters the distributor through the throttle valve, enters the remaining three pipelines through the check valve, enters the evaporator to evaporate, and the gaseous refrigerant enters the corresponding gas-liquid. The separator is then collected from the gas outlet to the gas gathering pipe and then passed through the Stone valve to enter the compressor to complete the cycle. When the frost is set, the frost is removed so that the unit runs in a frost-free state.

4, fan reversal defrosting

The method is improved on the basis of directional defrosting, that is, the fan reversal is enabled during the defrosting process, so that the wind is sent in the opposite direction, and the forced air enters the wind-side heat exchanger from the non-frost side to the junction. The frost side flows and blows heated air to the frost layer to defrost it. This defrosting method makes full use of the heat of the wind-side heat exchanger and relies on convection, heat conduction, and radiation to defrost at the same time. The efficiency is obviously better than the traditional defrosting method. At the same time, a certain pressure can also promote the frost shell to disintegrate away from the surface of the heat exchanger. The addition of convection heat transfer makes the defrosting process proceed quickly and thoroughly. However, due to the addition of intermediate relays and pressure switches, increased production costs.

5, hydraulic defrosting

For large heat pump systems, hydraulic defrosts are often used. The purpose of defrosting is achieved by showering the outdoor evaporator with hot water. This kind of defrosting method is simple in equipment, but it causes the moisture content of the air around the evaporator to be too high after defrosting and it is easy to frost again. It is not suitable for use in areas with low temperatures such as the north. And the waste of water resources is large and an independent water system is needed.

6, pneumatic defrosting

This method utilizes compressed air to generate a high-speed jet to directly blow the defrost layer, removing the tiny frost on the surface of the evaporator at any time, so that the evaporator surface always maintains a frost-free state. Its greatest advantage lies in the uninterrupted heating of the interior, and the weak fluctuations in the indoor thermal environment ensure comfort. However, compressed air requires additional power consumption, and the cost of the entire machine is also high.

At present, most domestic air source heat pump water heater manufacturers mainly adopt hot gas defrosting methods, specifically, reverse cycle defrosting and hot gas bypass defrosting. The reverse cycle defrosting will affect the water supply of the air source heat pump water heater, that is, during the defrosting, the user can not provide the user with the effective water temperature of hot water. At the same time, after the defrosting, the original hot water temperature will decrease, from the energy point of view The loss of this defrosting process is equivalent to twice the shutdown time of the defrosting time. After calculation, the heat supply of the unit will be reduced by about 10%.

And, the frequent commutation of the four-way valve will affect its reliability and life. The hot gas bypass defrosting is due to the heat of the high pressure side refrigerant or the heat absorbed in the evaporator. When the temperature is low and the defrosting is not fast enough, there will not be enough heat to absorb the host and the host will enter a protective shutdown state. If a simple bypass path is used, the liquid hammering phenomenon of the compressor is likely to occur. At the same time, in the defrosting process, due to the reduction of the compressor displacement, the effect of heating the hot water will be affected, and the demand for normal hot water amount cannot be satisfied.