We frequently receive calls for an air dryer. When pressed for specifics, the customer often has no idea how dry he needs the air to be. He may only need to remove liquid water and contaminants from his line. Or, he could need “instrument quality” air.
The amount of moisture in compressed air is expressed as pressure dew point (PDP), and is given in degrees Fahrenheit. The dew point is the temperature at which the moisture in the compressed air will condense and form a liquid. Since compressed air at 100 PSIG is saturated (100% relative humidity), some form of moisture removal is generally necessary in a compressed air system which means reducing the dew point, condensing the moisture, and removing the liquid water. In most systems, this is done using one or more of the following methods:
- An after-cooler is provided at the discharge of the compressor. This will generally lower the temperature of the discharge air to within 15 deg F of the cooling medium, either water or ambient air. This is coupled with a moisture separator with a drain (either mechanical or electric) to remove the condensed water.
- In some cases, the pipe run is long enough that the air is cooled, condensing the water in the piping, and only a moisture separator with drain is needed, depending on the quality of air needed from the system. This is generally not recommended as the condensed water can cause rust and scale in the pipe line.
- In most industrial applications where the equipment is located indoors and lines are not susceptible to freezing temperatures, a refrigerated air dryer will be sufficient. This type of dryer will typically reduce the dew point to 35-50 deg F. As long as the temperature does not drop below the dew point, the moisture will stay as a vapor and not condense as water in the lines. A point-of-use filter with drain is generally recommended downstream of the dryer and located immediately before the air-using equipment.
- In situations where air quality is critical, or where the air lines are subjected to sub-freezing temperatures, a pressure swing twin-tower desiccant dryer is recommended. This type of dryer will reduce the pressure dew point to -40 deg F. They are also available to provide -100 deg F dew point. These dryers can be heatless regenerative (require up to 15% of their capacity for purging the towers), heated regenerative (internal or external heaters), or blower purge heated regenerative dryers. Heat-of-compression dryers are also available.
- Between the extremes, are “dew point suppression” dryers. These dryers use a single desiccant tower and suppress the dew point 15-20 deg F below the entering air temperature. These consume the desiccant which must be replaced/replenished periodically.
Pye-Barker Supply Co. represents several manufacturers of premium quality air drying and cooling equipment. These can be specified to meet most any cooling/drying requirement.