What are the dangers of moisture in compressed air?
Ambient air always contains some moisture in the form of water vapor. What leads to the condensation of this water vapor is temperature variation. If the water vapor content in compressed air is too high, it will condense in the pipework, fittings, or the machines using it. Such condensation can lead to:
- high maintenance and repair costs
- increased production downtime
- reduced performance of tools and machinery
- higher reject rate for painting and spraying systems
- faults in control valves
- corrosion in compressed air lines leading to more leaks and, as a result, higher energy consumption
Various air treatment ancillaries can help to remove the water contained in compressed air. Typically, multiple ancillaries form a system to achieve an acceptable water level. Such ancillaries are:
- aftercoolers
- oil-water separators
- refrigerant dryers or desiccant adsorption dryers
Background information on compressed air preparation is available here.
The compressed air quality required by the operator - or the machine using compressed air - determines how dry the compressed air must be. The maximum pressure dew point (PDP) is the measurement of compressed air dryness, specified in °C. Above the PDP, air compressed at the given pressure will not condense water vapor into moisture. e.g., compressed air with a pressure of 8 bar(g) and a PDP of +3°C does not condense unless exposed to a temperature below +3°C
These values determine which coolers and dryers you must include in the planning of a compressed air installation. You can learn more about humidity in compressed air in the Atlas Copco compressed air wiki.
How does oil get into compressed air, why does it cause problems and how can it be eliminated from compressed air?
The oil content in compressed air depends mainly on the type of compressor and ambient conditions. If the compressor draws in air containing oil because the air intake is near a busy road, the compressor cannot provide "pure" compressed air.
Oil contamination in compressed air can disrupt production processes and make products unusable in food and pharmaceutical industries, and also in painting lines. For instance, the atomized powder for painting could clump when it comes into contact with oil.
A fundamental distinction is made between compressors with or without oil injection into the compression chamber. Oil-lubricated compressors inject oil in the compression chamber for lubrication between the screw elements and sealing; therefore, a part of this lubricant initially remains in the compressed air. Most of the residual oil gets removed in the compressor's integrated air-oil separator.
Although the residual oil content of the compressed air produced by a modern piston or screw compressors is low, it is still present. Depending on the quality requirements of the application, the compressed air needs treatment downstream of the compressor. For example, a typical oil-injected screw compressor such as our GA VSD+ range has a residual oil content of less than 3 mg/m3 of compressed air at a temperature of 20 °C. Filters can further reduce this oil content. Our article on compressed air treatment explains what filters perform which tasks. The removal of liquid oil and oil vapor through filtration is enough for many applications.
In contrast, oil-free compressors, such as our Z series, never add oil to the compression chamber. As such, this avoids the risks of contamination, damaged or unsafe products, as well as production losses and damage to the operator's reputation. Atlas Copco was the first-ever manufacturer whose compressors were certified by TÜV according to ISO 8573-1, class 0 (2010).
Drying the compressed air also reduces its oil content, as the oil precipitates together with the water content. An oil-water separator can then absorb the oil content from the water. The resulting condensate then contains so little residual oil that it is suitable for discharge into a foul drain, without harming the environment or violating strict environmental regulations.
Particles and microorganisms: the importance of drying compressed air
Although every compressor has an intake air filter, micro-particles can pass through this filter. Most of the particles contained in the compressed air are smaller than 2 mm; mixed with water and oil residues, they get distributed in the compressed air supply system and can serve as a breeding ground for microorganisms. A way to reduce the risk of bacteria is to install a line filter directly downstream of the compressor to capture the micro-particles.
Even with a filter, though, there are still other ways for bacteria to grow in your compressed air system. When water vapor condenses in the pipe system, the moisture attracts microorganisms and acts as a breeding ground. So the bacteria migrate through the filters, settle in the pipe system, and thus continue to contaminate the compressed air.
The most effective measure against microorganisms is, therefore, to dry the air to a relative humidity of less than 40% with refrigeration dryers or adsorption dryers. You can find out which equipment, in addition to compressors, you need to consider when planning a compressed air installation in our article on compressed air treatment.
