Compressed air systems can account for a significant portion of a facility’s overall operating costs, so it goes without saying that air should be used wisely. One important factor is selecting the right type of compressed air dryer. Matching the dryer to a facility’s specific processes can save thousands of dollars in up-front costs and yield major dividends over the operating life of the system. The key is knowing how dry is “dry enough.”

Pressure dew point is the term generally used to indicate the amount of moisture the air contains at line pressure. Pressure dew point is the temperature below which moisture will condense out of compressed air. For example, if a compressed air system has a pressure dew point of 40°F, moisture in the air will not condense as long as the air temperature remains above 40°F. If the temperature drops to 35°F, however, moisture in the air will condense, and liquid water will appear in the system, potentially damaging tools, pneumatic cylinders, and downstream processes.

Air dryers remove water before it can contaminate a system, but different dryers have different capabilities. Identifying the pressure dew point an application needs lets engineers determine the appropriate dryer. They fall into three general categories for most industrial applications: refrigerated, desiccant, and heat-of-compression (HOC) dryers.

Refrigerated dryers
Refrigerated air dryers reduce compressed air temperature through contact with a cold medium. Because cold air cannot hold as much moisture as hot air, water condenses out of saturated air as its temperature decreases — drying the air. A moisture separator in the dryer subsequently removes the water, which empties through a drain.

Although there are several variations of refrigerated dryers, they generally fall into two categories: non-cycling and cycling. The fundamental difference between the two is, as the name implies, the non-cycling dryer’s refrigeration system runs continuously — regardless of demand. Most non-cycling dryers include a hot gas bypass valve that keeps the dryer from freezing. Without this valve, compressed-air temperature can fall below the freezing point. Ice could form in the air circuit and block the entire system.

Cycling dryers help facilities conserve energy. Cycling dryers use the refrigeration system to cool a glycol-water mixture. This thermal mass exchanges heat with the warm air entering the system, thereby cooling the air and warming the fluid. The refrigeration system activates when the thermal-mass temperature rises above a set point. It cools the fluid to the desired low temperature, at which time the refrigeration system turns off. This type of operation uses only the energy required to address the incoming air load on the dryer, conserving energy.

Refrigerated dryers are widely used because they are cost-effective to purchase, operate, and maintain. For example, if an industrial facility only requires a dryer to run for 15 minutes every two hours, a cycling dryer can reduce electricity and energy costs when the system is not in use.

Refrigerated dryers yield air with a pressure dew point between 38° and 50°F. Although well-suited for general manufacturing and some specialty processes, they may not be appropriate for critical applications that demand extremely dry air.