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CONTAMINANTS

Contaminants include:

Solids
Water
Oil in various forms

Oil comes from the air compressor, moisture comes from the cooling compressed air and dirt is present in the intake air. More contaminants are made as these contaminants damage pipe and compressed air components. In addition to hard particles, bacteria will grow and a sludge of oil, water, bacteria and hard particles forms.

Solids — dust, dirt and pollen — are contained in the ambient air at the compressor intake. Measurements indicate that in a typical metropolitan area, the atmosphere may contain more than four million dust particles in a cubic foot of air. Consider the amount of dust in free air, and then consider that its concentration after compression (at 100 psig) will be eight times greater.

In addition, very small particles in a compressed air stream can be serious contaminants.

In terms of particle size, contaminants in a typical compressed air system range from 10 microns to 0.01 micron. A micron (or micrometer) is one-millionth of a meter or approximately 0.00004 inch. particulate size comparison

If a 5-micron particle were represented by a basketball, then on the same scale, a 0.5-micron particle would be the size of a golf ball. This size difference (golf ball vs. basketball) explains why filters designed to remove particles 5 microns in diameter cannot take out oil mist at 0.5 micron and smaller.

While the basketball and golf ball comparison is easy to visualize, it is much more difficult to visualize and deal with the actual contaminant particle size. If a 0.5-micron particle were represented by a golf ball, then an actual golf ball would be approximately 5 miles around!

Moisture is ever present in the ambient air. In the compression process, air is heated. As it flows through the distribution system it is cooled, causing water vapor to condense. Condensed moisture combines with oil and solid contaminants forming a sludge, which is detrimental to air-operated devices and to processes. Sludge becomes a real concern when the compressed air has contact with the actual product. For effective protection against moisture, a compressed air dryer should be installed in the air distribution system.

Moisture also causes rusting in pipelines. Rust particles picked up by the air-stream add to the overall contaminant load on the system.

Oil is the most troublesome contaminant in compressed air systems and the most difficult to remove. It enters the air-stream in the form of fine aerosols and vapor (i.e., oil in the gaseous state).

Lubricating oil enters the air-stream in liquid, aerosol and vapor forms. In reciprocating compressors lubricating oil applied to cylinders is fragmented into fine particles by the shearing action of the piston the higher the speed of the compressor, the higher the concentration of oil aerosol entering the air-stream.

Rotary screw compressors operate by the action of a pair of helical rotors whose mating parts do not touch. For lubricated machines to maintain a seal during compression, a large volume of oil is injected directly into the compressor chamber at all times. Although air/oil separators remove most of the oil, fine mists pass though to contaminate the system downstream.

LIQUID OIL

Oil mist particles have diameters in the range of 0.01 micron to 10.0 microns. Conventional air line filters remove only the larger particles. The smaller particles that pass through the filter eventually collide with one another and with confining surfaces, forming droplets large enough to settle out in the air line network as liquid oil. The formation of larger droplets from small particles is known as coalescence, and occurs more readily in regions of high turbulence where small oil particles are forcefully thrown against enclosing surfaces and against each other. This is why liquid oil is commonly found in orifices, pressure reducers and nozzles as well as in other areas of turbulence downstream of these devices.
compressed air filter coalescer example

OIL IN THE VAPOR STATE

While compressor operating speed, temperature and shearing rates determine the amount of oil in fine mist form, temperature determines the amount of oil in vapor form. Vapor pressure is an indication of the amount of oil vapor that exists at a given temperature. Most common compressor lubricants have a very low vapor pressure at ambient temperatures. However, at elevated temperatures the vapor pressure, along with the amount of oil in the vapor state, increases substantially. This phenomenon is illustrated in the tabulation below.

TYPICAL COMPRESSOR LUBRICANT
Variation of oil vapor concentration with temperature at 100 psig pressure
Vapor Temperature (°F)	Concentration (ppm by weight)
35	0.004
50	0.019
100	0.62
150	5.8
200	26.8
300	206
400	22,700

Note that at I00°F the oil concentration in the vapor state is 0.62 ppm; at 200°F the amount of oil vapor is increased by a factor of 43 to over 26 ppm.

When cooling occurs in a 100 psig air system, some of the oil condenses into the liquid state. Reducing the air temperature from 200°F to 100°F reduces the oil vapor content from 26.8 ppm to 0.62 ppm. At this temperature, the air-stream could contain as much as 26.8 ppm of condensed oil plus any liquid oil carried over from the compressor. Oil removal filters are capable of removing the liquid oil, but are not effective in capturing oil vapor. Therefore, the temperature of the inlet air to the filter should always be as low as feasible to limit the concentration of oil vapor in the outlet air-stream. When selecting compressed air filters, consider not only the maximum but also the recommended operating temperature specified by the manufacturer.

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