You may have noticed water coming out of your compressor. Some people ask where does it come from and why is it there? An additional important question to ask is: how well is it being removed?
Atmospheric air contains water in vapor form, the amount depending on the temperature and relative humidity of the inlet flow. For example, air at 70° F and 70% relative humidity contains about one tenth of a gallon of water per 1,000 cubic feet. When this air is compressed, the moisture carrying capability of the compressed air reduces, and some of the water drops out. The same air at 100 psi pressure and 70° F can hold only about one fifth of the amount of water as it did at ambient pressure. This extra water needs to be removed from your system or it will travel downstream contaminating your compressed air consuming tools and equipment.
A typical compressor has some method of removing this water, usually located after the air cooler, on the bottom of a centrifugal water separator. You should be aware that not all compressors have water separators and drains at this point; therefore, to avoid moisture problems, some method of removing the water must be added. A good check to always incorporate to your maintenance rounds is to see if water is being removed at this point—a surprising number of compressors have failures of the drains, allowing water to pass downstream. If this point is dry, you may have problems.
How much water is formed? At the previously mentioned conditions, every 10 hp of fully loaded compressor capacity produces about 2,500 cubic feet of compressed air per hour. This would condense about 0.2 gallon per hour, or about 5 gallons of water per day for every 10 hp of compressor power. At different inlet conditions, the amount of water in the compressed air would go up or down, every 20° F increase in temperature roughly doubles the amount of water in the air.
Want to avoid problems? Always make sure your compressor drains are working and you have a backup location where water can be captured and removed. A good location is a well-sized wet receiver, through which the air passes before going downstream.
And, if you want to avoid water problems altogether, make sure your system has an air dryer that would reduce the dew point of the compressed air at least 10° lower than the coldest point at which the system piping is exposed. Refrigerated dryers, which produce dew points of about 35° F also have drains that must be working to expel water, so this is an additional maintenance item. Desiccant dryer would be used if the air is subject to freezing temperatures—these would have drains on the inlet filters.
In case of dryer failure, make sure your piping system is designed with a slight slope away from the compressors, with automatic drains located at any low point. Drains should be selected and set so they waste a minimal amount of compressed air, expelling only water. There are many varieties of “airless” drains that can do this. Lastly, make sure that you tap the air drops to equipment from the top of the pipe rather than the bottom, lest free water flow into your expensive machines.
Faisul says
Ron, can you share a decision tree in selecting compressed air dryer