Compressed air moves the machines that make the things we work (and play) with every day. To maximize the efficiency and service life of pneumatic tools and equipment that push, pull, lift, position, or convey, compressed air must be clean, dry, and delivered at the appropriate pressure. A basic understanding of air preparation – filtration, regulation and lubrication – will help plant engineers and maintenance staff keep things running smoothly.
Compressing air concentrates contaminants and moisture, which cause premature wear and tool damage. The right filter removes particles and water. Air leaves the compressor at higher pressure than most end uses require, so regulating the flow to the correct pressure prevents damage and improves efficiency. Finally, compressed air is an efficient way to deliver lubrication to tools continuously, so adding lubrication to clean, regulated air prolongs tool life and protects performance.
There are three main types of filters: general-purpose filters for removing moisture and particles, coalescing filters for removing oil aerosols, and activated carbon filters for removing oil vapors.
General-purpose filters are used for most applications. Uses include main headers, branch lines, tools, cylinders, valves, and valve circuits. They are available in different pipe sizes and filtration levels, typically ranging from 40 microns down to 5 microns. They work by centrifugally removing entrained liquids and dirt, which then fall to the bottom of the bowl. A baffle quiets the air turbulence and prevents re-entrainment. The air, now free of water droplets and large particles, passes through the filter element, which removes small particles. The condensate at the bottom of the bowl exits through a drain operated manually or automatically. (Figure 1)
Oil-removal filters are used where very clean, oil-free air is required, such as for supply to fluidic devices, air gauging equipment, and air bearings. Oil mist coalesces as air passes through the filter’s fine fibrous filtration media. Oil droplets are collected in the outer sock (saturation will be clearly visible and is normal) and then drop to the bottom of the bowl for removal through a drain.
Activated-carbon filters are used where oil vapor in the air is not acceptable, such as with instrumentation or paint spraying. The porous structure of activated carbon results in a large surface area to which oil vapors are attracted and adhere.
Filter elements that remove fine particles, oil droplets, or oil vapor will last longer and perform better if a general-purpose prefilter is placed upstream to remove water and larger particle contaminants first.
The service life of a filter element depends on the quality of the inlet air. In general, air service equipment should be maintained annually, but use, air quality and condition at inspection may dictate adjusting the maintenance interval. Elements in activated carbon filters should be replaced every 1,000 hours or when odor is detected.
Pressure regulators are used to reduce pressure to the level required for downstream equipment, limit the force of cylinders, or minimize pressure variation at point of use.
General-purpose regulators are designed to deliver maximum flow capacity (based on size) while maintaining outlet pressure to the set level accurately. (Figure 2) General-purpose regulators are available in relieving and nonrelieving types.
Relieving regulators can be adjusted from a high pressure to a low pressure. Even in a dead-end configuration, relieving regulators will allow excess downstream pressure to exhaust.
Nonrelieving regulators will not allow downstream pressure to escape when they are adjusted. The trapped air needs to be released some other way, such as by operating a downstream valve.
Pilot-operated regulators are used in high-flow applications and where access to the main regulator is limited. This type of regulator does not use a control spring. Instead, an air pilot signal controls the outlet pressure of the main regulator. This air pilot signal is controlled by a small pilot operator regulator. Pilot-operated regulators are typically more precise than spring-operated general-purpose regulators.
For even greater control of outlet pressure, a “feedback” pilot regulator can be used. Here, the piloted regulator outlet pressure from the pressure-critical point in the system can be fed back to the controlling pilot regulator, which adjusts pressure accordingly. Feedback systems are very responsive to downstream pressure requirements. Note that very few applications require feedback, so in many cases a general-purpose regulator is an acceptable choice for a pilot regulator.