What Contaminants Can Be Found in Compressed Air?

Contaminants in Compressed Air Compressed Air Wiki Filtering Air Treatment

It is vitally important to the user that the compressed air be of the right quality. If air that contains contamination comes into contact with the final product, rejection costs can quickly become unacceptably high. In this article we will take a look at the most common contaminants found in compressed air?

Water vapor in compressed air

Air in the atmosphere always contains moisture in the form of water vapor. Some water vapor is included in the compressed air and can potentially cause problems. Examples include: high maintenance costs, shortened service life and impaired tool performance, high rate of rejection with spray painting and plastic injection, increased leakage, disturbances in the control system and instruments, shorter service life for the pipe system due to corrosion and more expensive installation. The water can be separated using accessories: aftercoolers, condensation separators, refrigerant dryers and adsorption dryers.

A compressor that works with 7 bar(e) overpressure compresses air to 7/8 of its volume. This also reduces the air's ability to hold water vapor by 7/8. The quantity of water that is released is considerable. For example, a 100 kW compressor that draws in air at 20°C and 60% relative humidity will give off approximately 85 liters of water during an 8 hour shift. Consequently, the amount of water that will be separated depends on the compressed air's application area. This, in turn, determines which combination of coolers and dryers are suitable.

Oil in compressed air

The quantity of oil in compressed air depends on several factors, including the type of machine, design, age and condition. There are two main types of compressor design in this respect: those that function with lubricant in the compression chamber and those that function without lubricant. In lubricated compressors, oil is involved in the compression process and also is included in the (fully or partially) compressed air. However, in modern, lubricated piston and screw compressors the quantity of oil is very limited. For example, in an oil-injected screw compressor, the oil content in the air is less than 3 mg/m3 at 20°C. The oil content can be reduced further by using multi-stage filters. If this solution is chosen, it is important to consider the quality limitations, risks and energy costs involved.

Micro-organism in compressed air

More than 80% of the particles that contaminate compressed air are smaller than 2 µm in size and can therefore easily pass through the compressor's inlet filter. From that point, the particles spread throughout the pipe system and mix with the water and oil residue and pipe deposits. This can result in the growth of micro-organisms. A filter positioned directly after the compressor can eliminate these risks.

Nevertheless, to have pure compressed air, bacteria growth after the filter must be kept fully under control. The situation is complicated further as gases and aerosol can be concentrated into droplets (through concentration or electrical charging) even after passing several filters. Micro-organisms can germinate through the filter walls and therefore exist in the same concentrations on the inlet as well as the outlet sides of the filter.

Micro-organisms are extremely small and include bacteria, viruses and bacteriophages. Typically, bacteria can be as small as 0.2 µm to 4 µm and viruses from 0.3 µm to as small as 0.04 µm. Contamination smaller than 1 µm diameter and, consequently, micro-organisms can pass easily through the compressor inlet filter. Despite their size, these micro-organisms are a serious problem in many industries, because as 'living' organisms they are able to multiply freely under the right conditions. Investigations have established that micro-organisms thrive in compressed air systems with non-dried air at high humidity (100%).

Oil and other contamination act as nutrients and allow micro-organisms to flourish. The most effective treatment involves drying air to a relative humidity of <40% (this can be achieved by using any type of dryer) and fitting a sterile filter in the system. The sterile filter must be fitted in a filter housing that allows in situ steam sterilization or that can be easily opened. Sterilization must be performed frequently to maintain good air quality.

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