An air compressor operating continuously when atmospheric conditions are 50% humidity and 70° F will take in 10 gallons of water per 100 SCFM, per 24 hour period.  If atmospheric conditions are 90° F and 70% humidity this figure nearly doubles to 20 gallons of water per 100 SCFM per day.  That’s a lot of water!

Approximately 65% of this moisture will condense if the air is compressed to 100 PSIG and cooled then cooled to within 10° F of the ambient temperature.

This means 6.5 gallons of water per day will condense in our first example above and 13 gallons of water per day will be condensing in our second example above.

Depending on the design of your air system, most of this condensation will collect in the wet receiver tank and bulk moisture separator.  The liquid needs to be automatically and reliably drained.

Now, there is  a strong case for using zero air loss demand drains on all draining points.  Yet highly reliable zero loss drains are quite expensive.  And inexpensive demand drains are seldom reliable.  So despite the protests and admonitions of energy auditors everywhere, solenoid type auto drains persist in many compressed air systems, especially small horse power systems.

If you’re going to use a solenoid type auto drain, the least you can do is adjust the timer to minimize the loss of valuable compressed air while not flooding downstream equipment with condensate.

A typical solenoid timer drain will drain 1.25 gallons per minute and is normally designed for open times ranging from1 to 60 seconds and close times ranging from 1 to 60 minutes.

In our example above we are condensing 13 gallons per day of liquid, or 0.54 gallons per hour.  If we elect to open the solenoid valve once per hour it will need to drain 0.54 gallons per opening.  A typical solenoid timer need 26 seconds to purge .54 gallons.  (0.54 gal/1.25 gpm = 0.43 minutes = 26 seconds)

An alternate would be to open the valve for 13 seconds every 30 minutes.

Ideally you’ll never use a solenoid drain in a system with so much condensate that the drain needs to be open for more than a minute per hour.

Also, always protect the solenoid with a y-strainer to prevent clogging.

When installing auto drain valves it is important to provide a block and bypass valve for performing maintenance and dialing in proper timing. Once the drain is initially set it can be checked with the block and bypass by simply opening the bypass from time to time to see if there is any condensate.  If liquid flows for a few seconds or more, the timer’s open time needs to be increased.  If there is no condensate the time opened can be decreased slightly.  Over time the optimum settings can be dialed in for the application and tracked from season to season.

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