The air dryers available today can be segregated into two major

Refrigerated
Heatless

Before discussion about dryers, it is very essential to define Dew point temperature,which is the unit of measurement for dryness of compressed air.
DEW POINT temperature is the temperature at which the moisture, which is in the compressed air (in vapour form). condenses to liquid form.
PRESSURE DEW POINT termperature is the dew point temperature of the compressed air,when under pressure.
ATMOSPHERIC DEW POINT temperature is the dew point temperature of the air when it is at atmospheric pressure.
The conversion chart from pressure depoint temperature to atmospheric dew point temperature is provided at the end of this text.

.