Method to delay frost formation under high relative humidity by choosing proper heat exchange temperature difference and air velocity

To delay frost formation and maintain high heat transfer capacity of an evaporator, a closed-loop wind tunnel was designed and constructed to investigate the effect of heat exchange temperature difference (ΔT) and air velocity on frost formation characteristics, refrigerating capacity and water removal of air. The closed-loop wind tunnel provided flowing air at temperature of −18 °C, relative humidity of 85% and velocity from 2 to 5 m/s. During experiment, three obvious stages of frost thickness were found when air initial velocity was 3–5 m/s. With ΔT changing from 7 to 3 °C, the water removal of air through the evaporator decreased by 3–5 times and the high efficient operation time ratio (HETR) increased by 2–3 times. Meanwhile, the average frost mass reduced by about half at any fixed air velocity. When ΔT was 7 °C and air velocity was 2 m/s, the average refrigerating capacity was almost equal to that when ΔT was 3 °C and air velocity was 5 m/s. The combined results showed that smaller ΔT and larger air velocity was useful to forbid frost formation, and to maintain stable high refrigerating capacity and relative humidity.