Why can't the evaporation temperature of the air dryer can't be very low?

Drying machine under normal conditions, the evaporator surface layer of film-like condensation exists, if the evaporator temperature drops very low, so that the heat transfer brass surface temperature below zero, the water vapor will condense into a copper tube surface frost, with The problems arising from the sublimation of water vapor to frost than condensing to the dew to absorb about 15% more cold, so to reduce the handling capacity of the machine; the cream of the thermal conductivity of water only 1/4, the tube can not air Fully cooling, while at the same time, the evaporation temperature in the tube tends to decrease further. As a result of such a cycle, the refrigeration system will inevitably have many undesirable consequences such as liquid compression; moreover, the presence of frost will increase the air resistance and reduce the supply pressure, Serious or even make the gas blocked.