The Art of Dryer Sizing

By: Michael Camber

Compressed air dryers are commonly rated to achieve a specific moisture level (e.g. 40°F pressure dew point) for a certain volume of air flow (cfm). This nominal flow rating is typically based on a set of standard conditions (100 psig, 100°F inlet temperature, and 100°F ambient temperature). In practice, your actual conditions can change from day to day and are rarely the standard conditions, and the dryer may be over or undersized depending on how it is selected.

When sizing a dryer, it’s important to understand how temperature and pressure affect water content in the air. The water vapor content of air varies directly with temperature—if temperature increases, the air’s ability to hold water increases. As a rule of thumb, every 20°F rise in inlet air temperature may double the water load on a dryer. Pressure is the opposite. The water vapor content of air varies inversely with pressure—if pressure increases, it squeezes moisture out. Because of these two relationships, compressed air dryers have correction factors (supplied by the manufacturer) to help determine how much air a dryer can actually handle for specific conditions.

We have updated our website to include sizing correction factors and provide a sizing example using one our SECOTEC refrigerated dryer series: How to size compressed air dryers