Most compressed air systems incorporate one or more deliberately installed “engineered air leaks” to drain water and lubricant from the system. Unfortunately, these devices often waste significant volumes of expensive air and, ultimately, energy.
When air is compressed and cooled to atmospheric conditions (or dried, in the case of refrigerated air dryers), water vapor contained in it is squeezed and condensed. Similarly, air compressed by lubricated screw compressors contains a small amount of lubricant. The problem: If moisture and lubricants aren’t eliminated from compressed air, they will contaminate downstream air-powered devices—or worse, cause product-quality or other issues.
Whenever compressed air encounters a reduction in temperature or goes through a quality improvement (i.e., is filtered), something is left behind. If this “something” is left in the compressed air system, it will grow to unmanageable levels. It needs to be removed.
There are a number of ways to remove liquid contaminants from compressed air—ranging from very simple to extremely complex and sophisticated. Your choice of method can affect your energy costs:
Air valves located at a low point of a system or at the bottom of separators are often left cracked open and blowing to remove condensate from a system. This is the simplest, but least desirable approach in terms of energy efficiency, in that it wastes a large, cumulative volume of compressed air to expel a small volume of condensate.
For example, on a 100 hp system, a drain flowing 5 cfm wastes 37 gallons of air per minute, even though the compressor might produce two-tenths of a gallon of water per minute on the hottest, most humid summer day, and a quarter of that during average conditions. This drainage would cost about $900 per year in electricity costs at $0.10 per kWh.
These devices can be adjusted to run for a few seconds at a time, every few minutes. Simple in design, they require less maintenance and typically waste less compressed air than manual drainage. They still must be set to expel enough air to drain the condensate that collects in worst-case conditions (hottest and most humid summer day), even though those conditions may exist only a few times per year. A timer drain might consume a quarter to half the compressed air volume of a manual drain.
Typically found in the bottom of filters and separators, float drains will operate when the level of condensate rises to the trip-point using small volumes of compressed air. They save more air than manual and timer drains, but often can’t be tested, leaving drain failures undetected.
With regard to energy efficiency, an airless drain that expels only condensate—not air—is the preferred device. Properly installed, it can remove contaminates without wasting expensive compressed air and will adust for all conditions.
More information can be found in the Library on the CAC Website, or in our Best Practices for Compressed Air Systems Manual. MT