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Brief introduction of defrosting of air-cooled heat pump hot

  Air-cooled heat pump cold hot water unit, as central air conditioning the source of cold and heat has been widely promoted from south to north since it was widely used in China in the 1990s. The advantage of this type of unit is that it is used in one machine and two units, which improves the utilization rate of the unit. When cooling, the condenser is air-cooled, eliminating the cooling water system required for the water-cooled chiller; the heat pump is used for heating. Energy saving and no use boiler the pollution caused by the environment, the installation and arrangement of the unit is convenient, and it can be placed on the roof without special setting of the machine room. However, from the actual operation of the air-cooled heat pump water chiller unit, it is not ideal for such units in areas with low temperature and high relative humidity, such as the Yangtze River basin heating operation. The main reason is that the evaporator heating and defrost caused the unit’s heating capacity to decline.

  Cause of frosting

  When the air-cooled heat pump unit is running in winter, its ribbed coil heat exchanger functions as an evaporator. Due to the lower evaporation temperature, the temperature of the coil surface also drops, even below 0 centigrade. When the outdoor air flows through the coil under the fan drive, the moisture contained in it will precipitate and adhere to the surface of the coil to form a frost layer. With the formation of the frost layer, the performance of the unit is degraded, the working conditions are deteriorated, and the heating capacity is also reduced. The formation and influencing factors of the frost layer are more complicated. I think the main influencing factors are the cooling surface, outdoor climatic conditions and time.

  Cooling surface factor

  The cooling surface factor refers to the temperature and shape structure of the fin heat exchanger.

  Temperature factor

  When the coil surface temperature is below 0 centigrade and below the dew point temperature of the inlet air, the air fraction will precipitate and form a frost layer on the evaporator surface. The greater the temperature difference between the surface of the coil and the inlet air, the faster the frosting rate and the more severe the frosting.

  Structural factors of the heat exchanger:

  Including rib edge effect, rib spacing, tube row number along the airflow direction, rib surface roughness.

  Climatic conditions

  1. Temperature and humidity: The greater the relative humidity of the air, the more server the frost on the coil. In the climatic conditions of the Yangtze River Basin, when the relative humidity of the outdoor air is less than 50%, the ribbed coils rarely frost. When the outdoor air temperature is lower than minus 5 degrees centigrade, as the temperature further decreases, the air tends to dry and the frosting decreases. The actual operation shows that when the outdoor air temperature is minus 2-7 degrees centigrade, and the relative humidity is greater than 50%, the frosting of the ribbed coil is the most serious. From the influence of the humidity on the evaporation temperature, it ca be seen that the evaporation temperature is higher when the humidity is higher for a period of time, but the evaporation temperature is lower after a certain time. The evaporation temperature is lower than when the humidity is low. This is because the humidity is large in the previous period, causing the evaporator to have a large load, and thus the evaporation temperature is high; as the frosting is intensified, the thickness of the frost layer will exceed the frosting thickness when the humidity is small, so the evaporation temperature is lower. The frost layer is thicker after frosting, so the air volume is always smaller than the air volume when the humidity is small.

  2. Airflow speed: In the case of force convection, the flow velocity of the airflow has a greater influence on the formation of frost. When the head wind speed is very small, the coil is frosted faster and the thickness of the frost layer is increased faster; when the head wind speed is larger, the frosting condition is obviously improved. This is mainly because the wind speed is reduced, the load is reduced, the evaporation temperature is rapidly decreased, and the frosting speed is accelerated, while the wind speed is increased, the load is increased, the evaporation temperature is increased, the temperature difference is decreased, and the tendency of frosting is lowered.

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