Removing Contaminants from Compressed Air

To clean compressed air, the air should be as dry as possible. Free water in the air prevents some filters from operating at their best. An effective way to dry air is with an aftercooler. 

After compression, the compressed air from the compressor is hot, usually at a temperature between 70–200°C. An aftercooler is used to lower this temperature, which in turn also reduces the water content. 

An aftercooler is a standard water filter accessory for a compressor installation. It should always be placed directly after the compressor. Aftercoolers can be water- or air-cooled and are generally fitted with a water separator that drains the collected water out of the system.

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

Filtration is a vital step in air treatment. However,  several factors can impact the separation process, making it difficult to control the quantity of oil remaining in the air.

Oil filter efficiency is affected by:

• air temperature, 

• oil concentration in the compressed air 

• and the amount of water present. 

The product specifications usually state the specific air temperature at which filters function. Therefore, check the ambient air temperature and choose a filter that works at that temperature. Climate and seasonal changes can cause temperatures to fluctuate, which will affect the filter's separation capacity. As temperature increases, the amount of oil vapor increases exponentially. 

It is worthwhile to check what type of contaminants you need to filter out and then choose a filter that is suitable. 

Modern fiber filters are very efficient oil removal filters for air compressors. However, fiber filters can only remove oil in the form of droplets or as aerosols. While oil vapor must be removed using an activated carbon filter. 

Consider that a correctly installed fiber filter and a suitable pre-filter can reduce the quantity of oil to approximately 0.01 mg/m3. A filter with activated carbon can significantly reduce the quantity of oil to 0.003 mg/m3.

Carbon can absorb 10-20% of its weight in oil. A filter coated with activated carbon powder has a limited lifetime and can only operate in fairly low temperatures. Whereas, bulk activated carbon bead filters contain much more activated carbon; this increases the filter’s lifetime and allows the filter to work at higher temperatures.

Activated carbon filters only remove vapor and should be preceded by other, appropriate filters. The filters should also be placed as close to the application in question for optimal effect. These filters should also be regularly checked and replaced frequently; a good standard is once a year. This also depends on to what extent the compressor is being used and how much water and oil is being removed.

To filter oil out of the water, oil in the form of droplets is separated partly in an aftercooler, condensation separator or condensation tap and flows through the system with the condensation water. This oil/water emulsion is classed as waste oil and must not be drained off into the sewage system or directly into nature. The condensation drainage and its collection is complex and expensive. An easy and cost-effective solution to this problem is installing an oil/water separator with a diaphragm filter to produce clean drainage water and drain the oil into a special receiver.

Often the oil that contaminates air comes from the lubricants used to run the air compressor. Choosing an oil-free compressor will eliminate the need for an oil removal filter. This is the best solution for the quality of air and from an economic standpoint as it reduces energy consumption.