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 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 ambient conditions.
In selecting which correction factors to use, make sure you go with the worst case scenario—this is usually during the hotter, more humid summer. Here are some correction factors that Kaeser supplies for one of our dryer lines, and below we’ll use them to size a refrigerated dryer.
For this example, we have a total flow of 200 cfm of air that needs drying. Using the size chart on the product literature (below), you might select the TD 51 dryer which is rated for 200 cfm at standard conditions. This might work quite well during cooler months but in your plant the pressure at the dryer inlet is 120 psig, and during the summer the ambient temperature where the dryer is located is 95° F, and the temperature of the compressed air at the dryer inlet is 110° F.
Take the correction factor for the operating conditions and multiple it by the factor for the ambient temperature:
0.82 X 1.05 = 0.861
This means that at the above conditions, the dryer will have only 86% (172 cfm) of its nominal capacity to dry the air to 40° F pressure dew point. To figure out the right sized dryer, divide the total flow by the total correction factor: 200 cfm / 0.861= 232 cfm (at standard conditions).
Now, you can look at the dryers’ stated capacity on the dryer literature and see which one will dry 232 cfm effectively. Based on this, the TD 61 (240 cfm) is well sized to meet the demand during the worst case periods.
Note that the location of the compressors and dryers can make a big difference in dryer performance. The cooler the air going into the compressor, the lower the dryer inlet temperature. If the compressor room is hot, consider moving the dryer to another, cooler location to boost its effectiveness. The same goes for wet tanks.
Having an undersized dryer can lead to poor air quality while an over-sized dryer can waste electricity. It’s best to make sure you’ve got one sized just right for your installation. If you aren’t sure if your dryer is sized correctly, contact a system expert for help.