How to size a refrigerated air dryer for general purpose compressed air systems

Posted by Heath Jones on

Do you have water in your airlines? Is condensate from compressed air fouling valves on your manufacturing equipment or cnc machines? Are your employees complaining about water spraying from air guns instead of compressed air? If so, you need an air dryer. Or some service on the one you have.
wet tank drain failure
Let’s assume you don’t have one but need one. For most compressed air applications, a refrigerated dryer is sufficient. Sizing a refrigerated air dryer is simple but usually overlooked, even by people in the air compressor industry.

For reasons I honestly don’t know, most refrigerated air dryer specs are published based on 100 psi system pressure, 100 F ambient temperature and 100 F compressed air inlet temperature. In 15 years, I’ve only sold one compressor geared for 100 psi. All others have been 125 psi with the rare exception of 150 psi for specialty applications.

So, let’s assume you have a compressor that produces 40 cfm at 125 psi. The compressor I’m referencing is an Atlas Copco, G7-125. This would equate to a 10hp rotary screw compressor. A quick glance at an Atlas Copco brochure will show an FX4 dryer rated for 49 cfm with a 41 F dew point. That may look good at first but keep in mind the 100 / 100 / 100 conditions from above.

Atlas Copco G7-125 rotary screw compressor

There are three multipliers (also called correction factors) to consider when sizing a refrigerated air dryer. Ambient temperature, inlet temperature, and inlet pressure. Atlas Copco refers to these as K1, K2, and K3, respectively.

Ambient temperature is easy. You just need to know the highest temperature for your environment. For us on the Texas gulf coast, we can see temperatures approaching 100 F in the shade. If the dryer is indoors or surrounded by other equipment it would be best to takes some temperature readings and determine the actual ambient.

Inlet temperature is the most difficult. For an oil lubricated rotary screw compressor, the compressed air temperature is usually in the range of ambient plus 20 F. This is called approach temperature if you’re wondering. For hot climates, this factor will get you if neglected. Reciprocating (piston) compressors have a much higher discharge temperature. If a piston compressor doesn’t have an integrated or stand-alone aftercooler, then a high inlet temperature dryer should be used.

Pressure is simple. Just determine the pressure your system will generally operate at and use the appropriate factor. Use the lower end of your pressure band since an increase in pressure equates to an increase in dryer capacity.

Here are the multipliers for the Atlas Copco FX dryers I referenced for this article.

Ambient temperature: 100 F = 1, 104 F = 0.8, and 110 F = 0.74

Inlet temperature: 100 F = 1, 104 F = 0.82, 113 F = 0.69, 122 F = 0.58, and 131 F = 0.45

Inlet pressure: 102 psi = 1, 116 psi = 1.03, 131 psi = 1.06, 145 psi = 1.08, and 160 psi = 1.1

The formula for actual capacity: Qactual = K1 x K2 x K3 x Qnominal, where Qnominal is the cfm capacity from the brochure.

Let’s run the calculation for the above referenced FX4 refrigerated air dryer and see if it can actually handle our 40cfm, G7-125 compressor.

For the formula I’ll use these conditions based on the Texas gulf coast area: 100 F ambient, 120 F inlet air temperature, and 116 psi.

Qactual = 1 (ambient factor) x 0.58 (inlet temp factor) x 1.03 (inlet psi factor) x 49 Qnominal

Qactual = 29.3 cfm for a 41 F dew point

What this equates to is water in your air lines even though you have a dryer installed. Let’s work backward and get the correct dryer.

Qnominal = 40 (compressor flow / Qactual needed) / 1 / .58 / 1.03

Qnominal = 67 cfm

This bumps us to an FX5 dryer with a nominal capacity of 74 cfm for a 41 F dew point. You would have some excess capacity in cool weather but that’s just part of the game.

Atlas Copco FX5 non-cycling refrigerated air dryer

This is a brief look at refrigerated air dryer sizing but it’s more than enough for general shop air. No need to overcomplicate things.

If you have a large compressed air system with critical dew point requirements or need instrument grade air from a desiccant dryer, drop us a line for help. If you have questions or need help with the above info for general shop air, feel free to drop us a line.



Share this post

← Older Post

Leave a comment

Please note, comments must be approved before they are published.