Although it seems obvious, sizing a compressed air dryer is not as simple as merely matching the size of the air compressor. If you want your air dried to proper specifications under all conditions, then some research is required that may require you to adjust to a larger size.
Air dryer ratings are affected by various factors, the most important of which are the inlet air pressure, the temperature of the inlet air, and the temperature of the ambient air in which the unit must operate. If these are beyond the dryer design specifications, then some sizing adjustment is required.
In North America, the typical dryer specifications follow the “three 100” rule—that is, the dryer is designed to adequately condition the air to rated dew point with 100 psi inlet air pressure, 100° F inlet air temperature, and 100° F ambient. In other countries there are different ratings, especially in tropical environments.
Operating the dryer at lower than rated pressure causes higher than normal air velocities inside the dryer, making it harder to remove the moisture. Higher than rated inlet air temperatures introduces more moisture loading to the dryer, because for every 20° F increase in air temperature, the amount of water vapor roughly doubles, making the refrigerant circuit work harder or loading the desiccant more. And because the moisture inside the dryer releases heat when it condenses, the heat exchanger circuit inside the air dryer (refrigerant types) needs adequate ambient air (or water) temperatures to remove the heat.
A typical worst-case scenario might be during the hottest, most humid day, when the compressors are fully loaded during a system peak. In order to prevent moisture from overwhelming the dryer, its capacity must be adequate to compensate for any excessive operating conditions.
Most dryer manufacturers have sizing tables in their sales literature—be sure to carefully examine these and run the proper correction of your dryer size to prevent future water contamination problems.
For example, suppose your air dryer is in a hot compressor room and must operate with an air cooled compressor that produces 500 cfm of 120° F compressed air at 90 psi in summer and that the ambient conditions near the dryer are 110° F. If you wanted a properly sized dryer from one example supplier, you should add 51% for the high inlet temperature, about 5% for the lower pressure and another 16% for high ambient temperatures. This would result in a dryer rated at about 500 x 1.51 x 1.05 x 1.16 = 920 cfm.
You may ask, if the inlet air temperature is cool, my pressure is high, and ambient conditions are much lower than specifications, can I reduce the size of the dryer? Well, the answer is yes, but this causes another problem: pressure differential. Passing more that rated air flow through a dryer causes significantly higher pressure differential than rated to develop across the dryer. This pressure differential causes the compressor to consume more energy due to higher than desired cycle frequency and higher discharge pressure.
Oversizing the dryer, on the other hand, causes less pressure differential, and if the dryer you select is a cycling style, its power consumption will reduce during normal conditions saving electricity costs.
The dryer in the photo above is an example of the problems created due to under-sizing. It was passing cool and already dried air, but it was rated for only half of the main compressor flow. As a result, when the compressor went to full load, a pressure differential of 20 psi developed across it, causing pressure problems. The fix was to increase the compressor discharge pressure by 20 psi, adding 10% to the compressor power consumption.
Nazim says
How much capacity of dryer is required on 90kw compressor?