What I need to know
Compressed air can be quite dirty. The contamination is 7 times more dense at 100 PSIG than at atmospheric pressure. Filtration of compressed air is recommended for all applications.
There are three basic types of contamination in compressed air
The first is solid materials and is generally referred to as particulate matter. Particulate filters are generally surface filters that use one porous surface to intercept solid particles. They generally are the most open with rating of 50 micron to 20 micron. Most of the filters supplied by compressed air using equipment is of this type. They are effective at trapping solid particles generated by construction and deterioration of piping (rust). Most all air compressor filters are finer than this. Particulate filters do little to remove contamination coming into the air compressor. Since they have only one surface to filter, they plug easily in even moderately contaminated compressed air streams. They are not very effective in removing liquids entrained in the compressed air stream.
Liquid contamination is the second type of contamination. Coalescing filters remove this type of contamination. Small fine droplets of water can be carried for great distances and for a long time. A common example of this that is quite visible is fog and clouds. Coalescing filters generally are between 1 micro down to 0.01 micron size. They can offer removal down to 0.01 parts per million of liquid by weight. When these very clean conditions are needed, it is recommended that several stages with each stage being finer than the proceeding to avoid plugging. Large amounts of liquid can overload a very fine filter resulting in high pressure drop and poor performance with a lot of liquid escaping the filter. These filters cannot removing oil and water vapors, only liquids and solids.
Vapor contamination generally is water vapor or oil vapor. Water vapor is best removed by compressed air dryers. Oil vapor can be removed by passing the air though a bed of materials such as activated charcoal. These material are surface active and attract the oil vapor to the surface on the material wetting it. These materials have very large surface areas but will become inoperative as the surfaces are wetted with the oil. The elements need periodic changes. To provide the maximum service life, these units need to protected by as efficient as possible coalescing filter upstream from the vapor removal device. Any liquid will also be trapped and use surface area. These filter also remove most odors from compressed air. Oil vapor content can be reduced to 0.003 parts per million by weight which can be lower than the air we breathe.
Filters cannot remove all the water from an air compressor. The air leaving and air compressor is saturated with water and water vapor. Since the compression of air heats it, the air continues to cool as it flows away from the air compressor. As this cooling takes place water is constantly being condensed into the air stream. We recommend the use of dryer which will reduce the dew point of the compressed air below the coldest temperature the air is to be used avoiding the formation of water. The combination of this water and other contaminates (primarily oil and oil vapor) form acidic compounds the destroy or plug air cylinders, valves, and other devices using or transporting the compressed air.
Maintenance
Saving money by extending filter change intervals is false economy. Every 2 PSIG pressure drop increases the energy to produce it by 1% for systems operating at 90-125 PSIG at the compressor. At 8 cent per KWH the electrical power to run an air compressor at its rated capacity for 1 year at 16 hours a day 5 days a week is about the cost of a new air compressor. Filter elements should be changed once a year or more frequently. Pressure drops waste energy. Additionally the materials used to make the filter elements deteriorate or become coated with contamination and become less effective at removing contamination. Many oil vapor removing filter elements need to be changed more often than once a year. |
