The effects of the outdoor air temperature and humidity on the frosting characteristics of the fin - tube evaporator coils of an air source heat pump were investigated experimentally. The variations of the evaporating temperature and the air flow rate due to the frost accumulation on the coils were considered in the experiments. The frost thickness was measured by using micro- photograph

and the frost accumulation mass was estimated by using the difference in measured humidity before and after the test evaporator. The experimental results indicate that the frost accumulation mass increases linearly with the frosting time

but the growth rate of the frost thickness could be divided into three stages

initial stage

constant speed stage

and sharply increasing stage. In the sharply increasing stage

which occurred in the last period with 20% -30% total frosting time

the growth rate of the frost thickness was 2.4-3.3 times as big as that in the constant speed stage. In addition

it is found that the frost formation rate was maximized when the outdoor air temperature was about 0 - 3% with different outdoor air relative humidity

and the effect of the relative humidity on the frost thickness growth was more than on the frost accumulation mass rate.