April 2019
When purchasing new compressors, an increasing number of factory managers are stipulating ‘oil-free’ and ‘class zero’ requirements as part of the specification process. However, research shows that these products can be less energy efficient and cost more to maintain than oil-lubricated equivalents. Andy Jones, managing director at Mattei, considers how oil-free air can best be achieved. Are ‘oil-free’ compressors a myth? A recent report into the global air compressor market has found that the oil-free segment now holds the largest share of the market and will grow at a faster rate than its lubricated counterpart. These trends are driven by the fact that some compressed air users believe that oil-free options offer improved air quality, as well as reduced costs for purification equipment and maintenance. Historically, it was only the most sensitive of environments, such as pharmaceutical and food and drink manufacturing, that required compressed air to have such a high standard of purity. This was for compliance with regulations allowing only trace amounts of residual oil in their air. Today, however, ‘oil-free’ has almost become a buzz word and machines that don’t use any oil at all are being hailed as the optimum solution in many more industry sectors. The problem is that there are many myths surrounding oil-free compressors and the quality of air they deliver, including the assumption that the air is totally oil-free. So, what is the case for oil-free compressors, and should factory Process, Controls & Plant Focus on: Compressed Air managers be looking for alternative solutions to achieve their objectives? Assessing the standards One of the main myths surrounding oil-free compressors is that they do not use oil. Let’s take a look at this in more detail. The international standard for compressed air quality (ISO8573-1:2010) specifies the amount of contamination allowed in each cubic metre of compressed air. Contaminants are separated into three areas – solid particulate, water and oil – each having a range of classes (one to nine). When specifying the purity of air required, a purity class is selected for each of these three contaminants respectively. Purity class 1 states that in each cubic metre of compressed air not more than 0.01mg of oil is allowed. Meanwhile, class 0 (zero), despite its name, does not mean zero contamination. In reality, class zero simply has to have less contamination in a cubic metre of air than class 1. By design, oil-free compressors do not use oil in the compression chamber and therefore oil does not contact the air being compressed. However, oil is often used for the lubrication of moving parts and cooling of the machine. Given that ‘oil-free’ compressors are not completely oil- free, factory managers should consider whether class 1 or class zero for oil is truly needed. Cost savings It is often stated that using an oil-free compressor reduces the user’s overall cost of ownership as costly filters can be eliminated, as is the need to change filter elements. System pressure losses are also slightly reduced, again lowering costs. However, purification equipment is still necessary to stop contaminants entering through the compressor intake. For example, contaminants within the atmospheric air or added by the compressor itself, along with rust and pipescale in the compressed air system, mean that no matter what type of compressor is installed, the same level of purification equipment is required. The cost could be high too, if the elaborate and complicated sealing mechanism which is used to prevent the lubricating oil from entering the compressed air, fails. Therefore, air from oil-free 24 | Plant & Works Engineering www.pwemag.co.uk April 2019 Andy Jones, managing director at Mattei
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