How does a Refrigerated Air Dryer Work

COMPRESSED AIR CIRCUIT

  • The refrigerated air dryer cools the incoming compressed air first in an air-to-air heat exchanger where the outgoing cool dry air pre-cools the hot incoming air and condenses some moisture out.
  • Then the incoming air enters an air-to-refrigerant heat exchanger where the air is cooled to 38º F by the liquid refrigerant.
    This process causes the moisture to condense into liquid water and it is drained away.
    The out going air then enters the air-to-air heat exchanger and is warmed up to keep the outside of pipes from sweating.

REFRIGERATION CIRCUIT

  • The refrigeration compressor pumps hot hi-pressure gas refrigerant (Freon) into the condenser which transfers the heat from the refrigerant gas to the ambient air as the gas condenses into a liquid.
  • The liquid refrigerant (Freon) is then metered to a cold low pressure where it enters the air-to-refrigerant heat exchanger and the heat from the hot compressed air is adsorbed into the cold refrigerant (Freon).
  • The refrigeration compressor then sucks low pressure hot gas refrigerant (Freon) into the refrigeration compressor and the cycle starts over again.

Pressure Dew Point – For a given pressure, the temperature at which water VAPOR will begin to condense INTO liquid water.

What pressure dew point do I need?
First – here’s some information to help you understand pressure dew point: 

The lowest pressure dew point class for a refrigerated dryer is Class 4. Class 4 delivers a pressure dew point of +38°F. Refrigerated dryers should not operate below the Class 4 range because the water vapor will freeze in the dryer.
The highest pressure dew point for a refrigerated dryer is Class 6. Class 6 delivers a pressure dew point of +50°F. The highest practical pressure dew point because higher pressure dew point causes condensation in downstream piping.

Ways to Determine Pressure Dew Point:

1. Ask the Manufacturer what the pressure dew point (PDP) requirements are for your equipment.

2. You can CALCULATE the PRESSURE DEW POINT TEMPERATURE you need.

Here’s how:

1. Determine the lowest ambient temperature your compressed air piping system will be exposed to.
Check the location of air lines throughout air conditioned or unheated areas underground or between buildings.

(For example, your compressor and piping is inside your facility and the lowest air temperature it would ever be exposed to is 58ºF.)

2. Now you need to take that temperature number and lower it by 20º.

(For example, your 58ºF lowest ambient temperature -20º = (38º PDP NEEDED)

This will give the PRESSURE DEW POINT TEMPERATURE needed to prevent liquid water forming downstream. Determining the PRESSURE DEW POINT TEMPERATURE will help you determine the “dew point class” of the dryer you need.


These “classifications” are industry standards for compressed air dryers as established by the ISO (International Organization for Standardization).

ISO 8573.1 AIR QUALITY CLASSES of PRESSURE DEW POINTS THAT APPLY TO REFRIGERATED AIR DRYERS:

Class 4 maximum pressure dew point +38 º F

Class 5 maximum pressure dew point +45 º F

Class 6 maximum pressure dew point +50 º F

The lower the dew point, the dryer the air.