Compressed air with saturated water vapor enters the dryer’s pre-cooling device, in which, the entered hot air is cooled down by the outgoing cold air partly, thus reducing the load on the system (this process is pre-cooling). Pre-cooled air enters the evaporator. Here, heat released by the air is absorbed by media and the air reaches the pre-set temperature (2 ~ 4℃). After being cooled, the water vapor in the air condenses into droplets and is separated from the air by steam separator and then flows out of the dryer through automatic drainer. Leaving the separator, cold air enters into the pre-cooling device’s shell side for heat exchange with hot air in the inlet and leaves the dryer after being heated to a temperature about 10 ℃ lower than that of the inlet air. This reheating prevents the second moisture condensation in the outside exit air pipeline and downstream pipeline, increases the energy of the air and improves the effectiveness. In addition, after passing through the dryer, solid dust of over 3μ in the compressed air is all filtered, and the air source quality reaches the clean and dry requirements.