You have water in your air compressor’s receiver tank?  You have water in your compressed air lines too?

Don’t worry.   There is nothing wrong with your compressor.  In fact liquid water is a normal result of air compression.  The compressor draws in naturally occurring atmospheric humidity, i.e. water vapor.  The compressor then concentrates this vapor, raising the humidity level.  In fact, air leaving the compressor is almost always 100% humidity.

If you’ve ever lived in a humid climate you know what happens when humid air suddenly cools.  The evidence is morning dew on the grass.  The same thing happens within a compressed air system: humid air cools after leaving the air compressor and water condenses.

Often I get a call from a customer who’s got an air dyer yet still has moisture forming downstream of the dryer.  “Why is there water in the compressed air lines?” This is a question I’ve been answering again and again over my 20 year career with Van Air Systems.

To answer this question you need to ask some questions.

Does your compressed air system have a moisture separator, an after-cooler, and an automatic drain installed downstream of the compressor?  A good cooler with a separator and auto drain are the first tools in the compressed air treatment process.  In fact, an efficient cooler alone condenses about 70% of the water.

Many new compressors have an integral after-cooler, separator, and auto drain.

How efficient is the cooler?  To answer this measure the temperature of the air leaving the cooler.  How much warmer is the compressed air than the ambient temperature?  Ten degrees warmer?  Fifteen degrees warmer?  Twenty-five?  A low differential between the compressed air temperature and the ambient air temperature is good.  This differential is called the “approach temperature.”   A low approach temperature get more water out of the compressed air.

Ideally the discharge temperature from the aftercooler should be 10-20F above ambient temperature.  This should be verified with an infrared temperature gun.

The water condensed within the cooler needs to be removed from the piping system.  This is the job of a water separator.   If there’s an automatic drain on the moisture separator, make sure it’s working properly.  Auto drains are notorious for clogging, especially low cost drains.  If the drain is not functioning properly the condensed water moves downstream to the next component in the system.

The next component in the system is usually a filter and then a dryer.  Make sure these components are properly sized.  Do the capacity of the filter and dryer match the capacity of the compressor?

Filters are used to remove compressor lubricant or particulates.  A filter will also remove small amounts of liquid water.   But again, if a filter’s auto drain fails the water moves on down the road to the next component , which is the dryer.

To this point, your air treatment components have been removing liquid water only.  The role of the dryer is to remove humidity.  A compressed air dryer removes water vapor, not liquid.

This bears repeating: Dryers are designed to remove moisture in vaporous form only they do not like free water!!

There are many types of compressed air dryers.  Which type of compressed air dryer is the best?  There is no single answer.  A nuclear power plant requires a different type of air dryer than, say, a tire shop.   But for general purpose manufacturing and shop air it’s safe to say that the correct dryer type is one that produces a dew point 10F below the lowest ambient temperature to which the compressed air lines will be exposed.

Are your air lines entirely indoors in warm rooms?  Do your air lines run outdoors where they are exposed to sub-freezing temperatures?  Do your air lines run through refrigerated rooms?

If the compressed air lines are exposed to ambient temperatures that are below the outlet dew point provided by the dryer then you will have water in your compressed air lines.  For example, a refrigerated air dryer typically produces a 35-38F pressure dew point.  If you operate a refrigerated air dryer but run outdoor air lines that see ambient temperatures below 35F, guess what?  You will see liquid in that line.

Needless to say, there are many reasons why you might have water in your compressed air system.  If need help understanding the root causes of water in compressed air, call us.  1-800-840-9906 .   Or shoot us an e-mail at   We will gladly help you understand why there’s water in your air system.

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