Air Dryer Questions & Answers


Q. How does Water get in your compressed air? 
A. Compressor inlet

 Water vapor (humidity-moisture) enters the air system through the air compressor inlet air filter. The air compressor sucks in approximately 7 cubic feet of atmospheric air at 0 psig, and that volume of air is compressed into 1 cubic feet of air at 100 psig. The water vapor (humidity-moisture) that was in the 7 cubic feet of atmospheric air is now compressed into 1 cubic feet of compressed air. 

There are 3 forms of water in compressed air:

  • Liquid water
  • Aerosol (mist)
  • Vapor (gas) 

    Liquid water is easily removed by air-water separators. They remove 99% of the liquid water and 0% water vapor. 

    Water in Aerosol or Vapor form is more difficult to remove and requires the use of a Compressed Air Dryer. 

    For every 50°F drop in compressed air temperature, the moisture holding capacity of air is reduced by 50%. Drying prevents liquid water forming downstream where it can contaminate or damage the system causing operating problems, costly maintenance, and repairs.

Q. How do you select the Compressed Air Dryer you need for your equipment application?

A. The KEY to selecting the correct COMPRESSED AIR DRYER is to know the DEW POINT suitable for your equipment's application.

Q.  What is "DEW POINT"?

A. The measurement of air dryness is dew point.  

Dew point is the temperature below which water vapor will condense to liquid water at given conditions. Lowering the dew point effectively means the system can endure much lower temperatures before water droplets begin to condense.  

Q. How do you determine the dew point suitable for your application when selecting a compressed air dryer?

A. The best and easiest way is to ask the Manufacturer what the pressure dew point (PDP)requirements are for your equipment.
Another method is to calculate the dew point temperature.


A.  To calculate your DEW POINT TEMPERATURE...

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) 
This will give the DEW POINT TEMPERATURE needed to prevents liquid water forming downstream.

Determining the 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 ).


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.  

Q. How does a Refrigerated Air Dryer work?

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.
  • 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.


Have more questions?


Phone: 336-229-9999 or 888-229-9999
FAX: 336-229-9998 is owned and operated by
McGuire Air Compressors, Inc.,
Authorized Distributor for DELTECH:

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            P.O. Box 1100, Graham NC  27253
            Physical Location: 729 East Elm St, Graham,  NC  27253

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