Compressed air is a safe, clean, and convenient form of energy. But unregulated or improper pressure settings can result in increased compressed air demand, resulting in higher energy consumption. Excessive pressure can also increase equipment wear, resulting in higher maintenance costs and shorter tool life. A rule of thumb states that every 2-psig increase in operating pressure adds an additional 1% to compressed air energy cost.

Perhaps more important is having clean, dry air. Clean air prevents equipment malfunctions and premature component failures. Filters are an essential component of any compressed air system to remove all the dirt drawn in from the atmosphere by the compressor. Some filters can remove a small portion of moisture from the air, but they should not be used as a substitute for air dryers.

Many pneumatically powered machines — especially those with air motors — can also benefit from using lubricated air. Lubricated air contains an atomized mist of oil that extends life and improves operation of components by coating internal parts with a film of oil, thereby reducing friction. This atomized mist is created by an airline lubricator located downstream of the filter and regulator.

However, the trend in most new systems is to use unlubricated air and components that are lubricated for life — primarily valves and cylinders. Lubricators are beyond the scope of this article, but detailed information on lubricators can be found by clicking here.

Particulate filters
Contaminant particles in compressed air usually are measured in micrometers (µm), or 0.000039 of an inch. Filters are rated according to the minimum particle size that their elements will trap. Although filters rated 40 to 60 µm are adequate for protecting most industrial applications, many point-of-use filters are rated at 5 µm.

Finer ratings increase the pressure drop through the filter, which equates to higher energy cost to compress the air. In addition, finer filters clog more rapidly, also increasing pressure drop. In other words, although filters finer than necessary do no harm to downstream components, they will have a negative impact on air system operating cost.

Many filter manufacturers define the expected pressure loss and dirt holding capacity using curves related to pressure and flow. Therefore, particulate filters should be selected based on acceptable pressure drop and pipe connection size. A typical pressure drop through such filters would be between 1 and 5 psig. A filter with larger body size will produce less initial pressure loss and provide longer operating life than a smaller size filter with the same removal ratings. The charts in Figure 1 compare pressure drop through several particulate filters that all have identical 5-µm removal capacity.

Most filters can remove condensed water, typically via a form of cyclone separator at their inlet end, Figure 2. The water-removal efficiency of such filters depends on the incoming air velocity. Therefore, these filters must be matched to the intended air flow, rather than acceptable pressure drop.

If the filter is intended to remove moisture, an integral automatic float-type drain should be provided to periodically remove accumulated liquids from the filter bowl. Generally, such filters have transparent polycarbonate bowls to allow easy visual inspection of the liquid level.

Coalescing filters

Most oil entrained in a compressed air stream, as well as some of the condensed water, will be in the form of mists or aerosols that can pass through the openings in standard airline filters. Aerosol carry-over through such filters is commonly stated as parts per million (ppm) of oil vs. air by weight and will range from 1 to as little as 0.01 ppm. Coalescing-type filters, Figure 3, can remove these contaminants.

Coalescing filters are often rated to remove aerosols that are substantially smaller than the nominal size of the smallest solid particle that would be captured. Some models offer dual-stage filtration; the first removes solid particles to protect the coalescing element in the second stage.

Because all coalescing filters create a greater restriction to the air flow, pressure losses will be higher than those of simple particulate filters. Coalescing filters have an initial (or dry) pressure drop and a working (or saturated) pressure drop, both based on pressure and flow rate. The effective removal efficiency of such filters depends greatly on the air velocity passing through the filter assembly. A coalescing filter rated at 0.1 ppm will typically have a clean, wetted pressure drop of 2 to 5 psig, while a high-efficiency filter rated at 0.01 ppm can cause as much as 10-psig drop once it becomes wet or fully saturated during service, Figure 4.