Oil-free compressed air means…well what exactly does it mean? The output of an oil-free compressor perhaps, or it could be ISO 8753 specified with the possibility to specify something better than Class 1 for hydrocarbons. Another possibility is that compressed air is taken from a lubricated machine and treated to become ‘technically oil free’. Steve Riley of Beko Technologies outlines the pros and cons of both systems and suggests a third option – the Bekokat catalytic converter. Oil Free Compressor: There are many types and manufacturers offering customers a choice of machines. They are reliable, and up to 10 year service/extended warranty packages are widely available. The initial capital cost is high compared to lubricated types. Maintenance costs are usually spread over a 10 year period to mask the high cost of changing LP and HP air ends at around 40,000h Having sold many units, I am convinced there is no problem of screw or centrifugal types adding oil to the inspired air. However, there could be a question mark over the quality of the air being drawn into the compressor. Normal urban/ industrial ambient air contains about 150 million particles. More than 75% of these are below the pore size of the compressor inlet filter. Hydrocarbon content will vary but could be as high as 0.5mg/m3. Is the air leaving an oil-free compressor in Ellesmere Port or Grangemouth really 100% oil free? Only if it is subsequently treated to remove contaminants. It is very common to see an oil-free compressor installation with no further filtration downstream except to protect a dryer. There is, of course, an option to use a standard lubricated compressor and a series of coalescing filters and carbon filters to reach the specified quality. There will be a significant cost benefit in doing this for reasons already mentioned. The questions that need to be asked are firstly which filtration system or components are needed, then how much cost will this involve? There will be pressure drops at each component stage of the filtration system and an annual or more frequent filter element change will be needed. A system with filtration to ISO 8573 Class 1 could easily exceed 1 bar in pressure drop from new, increasing as the filter clogs. This equates to an increase in power which is easy to calculate and a number of filter and carbon elements that are known costs. It should be borne in mind that the performance of filters is specified at 21°C and that to achieve this temperature the air would be taken in at 11°C. So the performance of the filters is as specified during the winter months and some nights in the spring and autumn periods but how many compressor houses will be adequately ventilated and the air taken from outside? The performance graph of oil carry-over versus temperature is logarithmic so it is important to be realistic about the temperature that the air will go on to the filters. Activated carbon filter life will be considerably lower than the other components and unless installed as a ‘carbon tower’ rather than a filter can be expensive because of short operating life. BEKO Technologies offers an alternative method of producing oil-free compressed air. The Bekomat system is based on the catalytic converter method. The first unit to be installed is in a food factory in North Yorkshire and has been running for more than a year with zero maintenance costs. The only incident was a site power outage requiring a resetting of thermal protection. The operating principle of the catalytic converter is simple, like the one in your car which, together with heat, will break down long hydrocarbon chains into water and carbon dioxide. There is no need to worry about green credentials as we are talking about treating compressed with, say, 2 or 3ppm of oil content. The advantages are that the quality of the compressed air output is not dependent on the quality of air at the inlet. Routine maintenance costs are low. Some will have heard that there is a high energy input and possibly that there have been fires in the catalyst vessels. There is more than one brand on the market; the BEKO design ensures that the lifetime cost analysis is competitive against other systems and could be considered as the only way to be 100% sure that there are no hydrocarbons getting through the compressor or the treatment system. The design has a lower temperature and no direct contact between heaters and catalyst, minimising risk of fire. The system is suitable for point-of-use applications where the quality of the air required is significantly higher than normal factory air and the usual solution would be to buy a dedicated oil-free compressor or a filtration system. The disadvantages of catalytic converters are that the range is not suitable for very large flows and Bekokat is not a low cost product. Maintenance is simple in that the catalyst will need to be changed at 20,000-30,000h. A solution would be a maintenance contract with the cost of the catalyst amortised. In conclusion, there is another option to arrive at high quality – 0.003mg/m3 using a simple catalyst and heater system.
