Drying compressed air

Last month I covered the correct piping for compressed air systems. This month I’ll be talking about other components in the compressed air system that remove water from the air stream.

First, a quick note about air and temperature: the hotter the air, the more water vapor it can hold. As the air cools, this water vapor condenses into liquid water. Liquid water in compressed air systems is a contaminant stripping away helpful lubricants, degrading rubber and plastic parts, and corroding and rusting metal components. It has been my experience that liquid water is the leading cause of failure for air-operated machinery. Several devices are available to reduce the amount of water that gets to the compressed air components of machinery. Compressors are often located in boiler rooms that characteristically have high heat and humidity levels. Locate a compressor’s air intake as close to an outside vent as practicable. Usually, providing the compressor air intake area with a vent line to the outside of the boiler room is fairly simple, providing cooler drier air at the compressor air intake.

1. Drains and Auto-Drains– Somewhere, usually at the bottom of the tank on which the compressor pump is mounted, there is a drain valve of some sort. Water that has condensed out of the hot compressed air and fugitive oil from the compressor pump collects in the tank. When this drain valve is opened, this water-oil mixture is blown out of the tank removing it from the compressed air system. THIS WATER AND OIL MAY BE CONSIDERED A HAZARDOUS WASTE. Opening the drain valve and draining the compressor tank should be done regularly, at least once daily.

Very effective, inexpensive and easily installed automatic drain valves are widely available. These auto-drains contain an adjustable timer that regularly opens an electric solenoid valve allowing the tank to drain. Besides the obvious advantage of knowing your compressor is being drained regularly without you having to squat down in a hot boiler room to open, wait and then close a hot greasy valve, you’re also spared the maintenance on this inaccessible area.

Even a small leak in the compressor drain valve will significantly affect your compressor, causing cycling more often, thus incurring greater wear and higher electric bills.

2. After Coolers and Refrigerated Dryers- Compressed air exiting the compressor pump is extremely hot and can hold lots of water vapor. As this air cools, some of the water vapor condenses out as liquid water in the compressor tank requiring the draining procedure discussed in paragraph 1. However, the air is still hot, and lots of water vapor remains in the air stream, which will later condense in the cooler air lines and equipment.

In the dry cleaning plant, the most effective way to reduce the remaining water vapor before it condenses and travels into the machinery is to cool the airflow further. The simplest method of achieving this is using a device called an After Cooler. An After Cooler is usually a finned coil with a fan mounted on it. Compressed air leaving the compressor pump enters the tubing with the fins attached while the fan blows air across the coil cooling the compressed air, condensing out much of the remaining water vapor.

A variant of this method uses a water-cooled heat exchanger instead of a finned air-cooled coil. Refrigerated Dryers are the most effective device to remove troublesome liquid water from the compressed air stream. They apply the same principle as the aftercooler, but a refrigeration system is used instead of using air or water to cool the compressed air. Refrigeration provides a much lower condensing temperature, hence a lower level of water vapor in the compressed air stream. Keep in mind that you will still need some type of drain on any system used. After Coolers and Refrigerated Dryers must be appropriately sized to the air compressor. Well, that’s about it for this month; I’ll be continuing next month with more easily accomplished tasks that save big bucks.