Question: On our air compressor, we have a drain at the bottom of what looks like a filter, located after the air cooler. The drain failed so we turned it off … how important is it anyway?
Answer: Here is the data you need to know about condensed water flow in compressed air systems (taken from the CAGI Handbook Section 4). On a day when it is 80° F with 60% humidity, every 1000 cubic feet of inlet air contains 0.1210 USG of water vapor. Compressor output of a fully loaded 200 hp is 1,000 cfm, so 60,000 cubic feet per hour.
But the air coming out of compressor discharge, if cooled to 100° F by the aftercooler, can only contain 0.0478 gallons per 1000 of water vapor. The difference is 0.0732 gallons per 1000, so 4.3 gallons per hour of condensed water needs to come out at the compressor water separator drain (that filter looking thing) or about 105 gallons per day. Turning off the drain will flood the air dryer with excess water. Dryers are not designed to process free water.
If the dryer is working properly, its output air contains 0.0050 gallons per 1000 at 35° F dewpoint and 100 psi; the difference from the aftercooler is 0.0478 – 0.0050 = 0.0428 gallons per 1000 — or 2.6 gallons per hour, 62 gallons per day. If you are not getting that volume out of the compressor and dryer, then the separator, dryer, and drains are not doing their jobs.
For a quick and easy test, put a graduated bucket under the drains and see the water volume, compare the predicted volume with the CAGI table.
Of course, when it is -40 outside, the water doesn’t even condense in the compressor, so having timer drains blasting as if it is the worst-case day is a waste of power. Hence the need for drains that work on demand.