The air source heat pump (ASHP) system has excellent energy saving and emission reduction effects. However

under environmental conditions of low temperature and high humidity in winter

frost will occur on the surface of the evaporator easily

which affects the heat transfer performance and reliability of the system. The experimental study of frost growth characteristics on the surface of the finned-tube heat exchangers was carried out in this paper. The frost surface temperature was measured by the infrared thermal imager

and calibrated by using the direct measurement with micrometer and thermocouple device. The effects of the ambient temperature

relative humidity and face velocity on the frost thickness

frost mass and the frost-wet air interface conditions are analyzed. The difference between the frost surface temperature and the ambient temperature is regarded as the heat transfer driving force of frosting

and the difference between the water vapor partial pressure in wet air and the saturated water vapor partial pressure at the interface

i.e.

the interface water vapor pressure difference

is regarded as the mass transfer driving force of frosting. The frosting mechanism of the finned-tube heat exchangers is analyzed

which lay a foundation for the optimum design and anti-frosting and defrosting of ASHP system.