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How to Remove Contaminants from Compressed Air?

Air Treatment Compressed Air Wiki Filtering How-To Contaminants in Compressed Air

If compressed air is contaminated and comes into contact with the final product, the quality of these pruducts might be compromised. In this article we will explain how to easily remove these contaminants from your compressor system.

Using filters to remove oil from compressed air

removing oil from compressed air is done with a filter
Modern fiber filters are very efficient at removing oil. However, it is difficult to precisely control the quantity of oil remaining in the air after filtration as temperature, among other factors, has an significant impact on the separation process. Filter efficiency is also impacted by the oil concentration in the compressed air as well as the amount of free water. Data stated in the filter specification always applies to a specific air temperature, usually 21°C. This corresponds to the approximate air temperature after an air-cooled compressor working in an ambient temperature of 10°C. However, climate and seasonal changes may cause temperature variations, which will, in turn, affect the filter's separation capacity.

The air should be as dry as possible to achieve the best results. Oil, activated carbon and sterile filters all provide poor results if free water is present in the air (the filter specifications do not apply under such conditions). Fiber filters can only remove oil in the form of droplets or as aerosols. Oil vapor must be removed using a filter with activated carbon. A correctly installed fiber filter, together with a suitable pre-filter, can reduce the quantity of oil in the compressed air to approximately 0.01 mg/ m3. A filter with activated carbon can reduce the quantity of oil to 0.003 mg/m3.

Activated carbon is produced specifically to cover an extensive internal surface. Activated carbon is able to absorb 10-20% of its own weight in oil. A filter coated with activated carbon powder therefore contains only a small amount of carbon powder. This limits its lifetime, and its use is restricted to 20°C. The bulk activated carbon bead filter contains a large amount of activated carbon. This makes it more suitable for many applications (even at higher temperatures) and increases the lifetime of the filter.

This lifetime is influenced by the temperature of the air. As the temperature increases, the amount of oil vapor increases exponentially. Activated carbon filters should contain the appropriate quantity of carbon and should be dimensioned to create the lowest possible pressure drop. Filters with activated carbon only remove air contamination in the form of vapor and should be preceded by other, appropriate filters. For optimal effect, the filters should also be placed as close as possible to the application in question. Additionally, they must be checked regularly and replaced frequently.

An oil-free compressor eliminates the need for an oil filter. This means the compressor can work at a lower discharge pressure, thereby reducing energy consumption. It has been shown in many cases that oil-free compressors are the best solution, both from an economical standpoint and for the quality of air.

Reducing the water content with an after-cooler

The compressed air from the compressor is hot after compression, often at a temperature between 70–200°C. An aftercooler is used to lower this temperature, which in turn also reduces the water content. Today, this equipment is frequently included as standard equipment for a compressor installation. The aftercooler should always be placed directly after the compressor. The heat exchanger cools the hot air and then routes most of the condensation water, which would otherwise flow into the system, as quickly as possible. The aftercooler can be either water- or air-cooled and is generally fitted with a water separator featuring automatic drainage.

Using a water separator for the separation of condensation and compressed air

Most compressor installations are fitted with an aftercooler as well as a water separator, in order to separate as much condensation water as possible from the compressed air. With the right choice and sizing of the water separator, an efficiency of 80-90% can be achieved. The remaining water flows with the compressed air as water mist into the air receiver.

Separating oil and water

Oil in the form of droplets is separated partly in an aftercooler, condensation separator or a condensation tap and flows through the system with the condensation water. This oil/water emulsion is classed from an environmental point of view as waste oil and must not be drained off into the sewage system or directly into nature.

New and more stringent laws are continuously being introduced with regard to the handling of environmentally hazardous waste. The condensation drainage, as well as its collection, is complex and expensive. An easy and cost-effective solution to this problem involves installing an oil/water separator, for example, with a diaphragm filter to produce clean drainage water and to drain the oil off into a special receiver.

Purification of medical air

In addition to regular air purity requirements, there are special applications that require an even higher degree of air treatment purification. High quality air is of vital importance to many industries, but nowhere is this so literally true as in the medical sector. The purity of medical air for hospital patients must be 100% guaranteed. However, the air drawn from our environment to produce medical air, especially in cities or industrial areas, is rarely of a sufficient quality to begin with.

Medical air filtration consists of several purifying stages to treat compressed air so that the result is extremely clean. By using a water separator and two coalescing filters, contaminants like water, particles and oil droplets are eliminated from the air before it goes into the cold regenerative adsorption dryer. This dryer lowers the dew point to –40°C, which is the temperature required to qualify for medical use.

After going through the adsorption dryer, the air passes through an extra filtration stage, whose function is twofold. Activated carbon reduces hydrocarbons such as oil vapor and smells to harmless levels, and a catalyst converts excessive concentrations of carbon oxide into carbon dioxide. In this filtration stage, sulfur oxide and nitrogen oxide contaminants are also reduced to an absolute minimum. In the final stage, a particle filter eliminates dust particles that may have been introduced into the air by the dryer or the extra filtration unit. The requirements for the medical market differ for each country, and are governed by local legislation.

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