How to size compressed air dryers

Sizing recommendations for compressed air dryers

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. 

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 weather. Here are some correction factors that Kaeser supplies for the SECOTEC dryer series, and below we’ll use them to size a refrigerated dryer.

Capacity correction factors for operating conditions

Pressure (psig) Temperature (°F)
75 80 85 90 95 100 105 110 115 120 125 130
60 0.95 0.85 0.76 0.67 0.59 0.52 0.46 0.41 0.36
80 1.10 0.98 0.88 0.77 0.68 0.60 0.53 0.48 0.42
100 1.25 1.12 1.00 0.88 0.78 0.69 0.61 0.54 0.48
120 1.32 1.18 1.06 0.93 0.82 0.73 0.64 0.57 0.50
140 1.38 1.24 1.11 0.97 0.86 0.76 0.67 0.60 0.53
160 1.45 1.29 1.16 1.02 0.90 0.80 0.70 0.62 0.55
180 1.50 1.34 1.20 1.05 0.93 0.82 0.73 0.65 0.57
200 1.54 1.38 1.23 1.08 0.96 0.85 0.75 0.67 0.59
230 1.58 1.42 1.26 1.11 0.99 0.87 0.77 0.69 0.60

Capacity correction factors for ambient temperature

Factor Ambient Air Temperature (°F)
75 80 85 90 95 100 105 110
1.09 1.05 1.00 0.96 0.92

For this example, we have a total flow of 200 cfm of air that needs drying. Using the size chart on the product literature for the SECOTEC series (and shown 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 multiply 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 (excerpt shown below) 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.

Model Rated Capacity (scfm)*
TC 31 115
TC 36 135
TC 44 170
TD 51 200
TD 61 240
TD 76 285
TE 102 370
TE 122 410
TE 142 490
* Rated Capacity: Based on compressed air saturated at 100°F and 100 psig and operation in a 100°F ambient.
 
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 and consequently, the dryer will have more capacity. 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 a grossly oversized dryer can waste electricity and will cost more to buy. It’s best to make sure you’ve got one sized correctly for your installation. If you aren’t sure your dryer is sized correctly, contact a system expert for help.