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Frosting and Defrosting of air-cooled heat pump

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Author : Sherry
Update time : 2021-10-12 10:55:48
Causes of frosting of air-cooled heat pump;
Air-cooled heat pump takes the ambient air as the cold source, so it has strong environmental dependence. If the ambient temperature is lower, the unit will provide less heat. When the surface temperature of outdoor evaporator fins is lower than the dew point temperature of water vapor, condensed water will be produced on the surface, and if the ambient temperature is lower than 0℃, frost will be formed on the evaporator surface.
Main factors affecting frosting:
Outdoor temperature, humidity and airflow speed.
Spacing of fins, number of tube rows along airflow direction, surface roughness of fins.
In addition, the longer the unit runs, the easier it is to frost, and the shorter the last startup and shutdown time, the more likely it is to aggravate the tendency of frosting. Running at night, frosting is more prominent.
The general structure of frost is divided into icicles, ice balls, ice layers and air bubbles.
Initial frosting stage:
The frosted surface is rough, which increases the heat exchange area.
With the decrease of air flow cross section, the air flow rate increases, and the convective heat transfer coefficient on the air side increases, thus enhancing heat transfer.
Late frosting:
Frost deposition on evaporator surface will increase thermal resistance.
At the same time, the accumulated frost will increase the air resistance flowing through the evaporator, which will lead to the bias of the working point of the fan, thus reducing the air flow of the system and the heat exchange capacity of the evaporator.
End result:
On the one hand, frosting leads to the increase of heat transfer resistance between air and refrigerant; on the other hand, the increase of frost thickness leads to the increase of air flow resistance, the decrease of air flow rate and the decrease of wind side heat transfer capacity. Overall, the heat transfer effect and operational performance are reduced. When the frost increases to a certain thickness, the heat exchange rate drops rapidly.

According to the experimental data, when the air flow rate of outdoor heat exchanger is reduced from 74m3/min in non-frosting state to 20m3/min, the heat exchange rate on the wind side decreases by 20%.
About defrosting methods;
* electric heating method.
* reverse circulation defrosting method.
* hot gas bypass method.
* auxiliary defrosting of phase change heat storage device.
Reverse cycle defrosting:
When defrosting, the compressor stops, the four-way reversing valve changes from heating mode to cooling mode, the compressor starts, and the heat pump unit changes from heating to cooling. The outdoor unit, which was originally an evaporator, becomes a condenser, and the high-temperature and high-pressure gas output by the compressor directly enters the outdoor unit, releasing heat into the frost layer, and then passes through the throttle valve to the indoor unit, which will produce refrigeration effect. After defrosting, change the mode and continue to supply heat to the room.
Defrosting by hot air bypass method:
Hot gas bypass method is an improved method based on reverse circulation defrosting. It can shorten defrosting time, improve defrosting efficiency and achieve better defrosting effec
Defrosting principle: Under defrosting conditions, the high-temperature and high-pressure gas output by the compressor is directly defrosted by the electromagnetic defrosting valve to the outdoor unit, then directly returned to the gas-liquid separator and then to the compressor, forming a small cycle. Differential defrosting temperature controller is used to control defrosting. Two sensors sense coil temperature and ambient temperature respectively, and their temperature interpolation controls defrosting start temperature.

After the air conditioner goes into defrosting, it must remove the frost once. Once it is not removed, it will enter the heating process again. The frost and defrosting water that have not been removed will quickly freeze into frost or ice with higher density. When the system goes into defrosting again, it is difficult to remove the frost. In order to avoid this situation, the evaporator temperature, compressor return pipe temperature and outdoor temperature should be monitored, and the evaporation temperature should be greater than 3 degrees, and the return pipe temperature should be greater than 0 degrees.

Phase change heat storage device assisted defrosting: Using phase change heat storage technology, the new heat storage device filled with DX40 phase change material plate is connected with air source heat pump unit, and the heat needed for defrosting is compensated by using the heat storage and release characteristics of phase change heat storage device. While the heat pump unit supplies heat, the phase change heat storage device stores heat. When the heat pump unit is switched to defrost mode, the heat storage device releases heat to the air-conditioned room and assists in defrosting.

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