BCAS 2018

Technology Guide 30 www.bcas.org.uk Filtration selection – making the right choice BCAS looks at filtration and why it is an important part of contamination removal. I n a typical compressed air system, there are 10 contaminants that require treatment for the system to operate safely, efficiently and cost effectively. This rises to 15 if using compressed air for breathing air applications. These contaminants come from 4 different sources and are not solely related to compressor design. Compressed air filters are required to treat 9 of the 10 main contaminants. Selection of filtration is important and there are many factors to consider when selecting filters: ● Sizing: for example, can the filter handle the compressed air flow rate at the minimum system pressure and maximum system temperature. ● Grade of filtration: a single filter is unable to treat all contaminants; therefore, a combination of filtration grades and filtration methods are used. ● Cost: filters are often seen as a commodity and purchased on price. Although filters may look similar and claim similar performance, the cost to operate the filter and maintain it can vary significantly. When considering cost, always use total cost of ownership rather than initial purchase cost. Filtration Types Water separators Although called water separators, they reduce the content of all liquids at the point of installation. Liquid in a compressed air system is usually a mixture of oil and water (even when using an oil free compressor). Water separators are usually the first piece of purification equipment installed downstream of an aftercooler or air receiver and should be used to protect coalescing filters from liquid contamination. It is worth noting that they are not always necessary, for example if coalescing filters are installed immediately after the air receiver and the air temperature is unchanged (no condensation occurs between the air receiver and the filters which means no liquid requires removal). If variable speed compressors are used, ensure the water separator selected can work at the variable flow conditions (many are designed for optimum efficiency only at their maximum flow conditions and reducing air flow reduces the velocity of the air in the separator and therefore separation efficiency). Coalescing filters Coalescing filters treat multiple contaminants (atmospheric particulate, rust, pipescale, micro-organisms and aerosols of oil and water). But more importantly, protect refrigeration and adsorption (desiccant) air dryers from contamination. They are usually installed close to where the compressor is located (either in the compressor room on larger installation or on the compressor itself for smaller fixed or portable compressors). These will typically be installed in pairs, with one being a General Purpose filter, the other being a High Efficiency filter. This is the most cost effective method of installation as the general-purpose filter protects the high efficiency filter from heavy contamination, extending the life of both elements up to 12 months. Dry particulate filters Dry particulate filters are very similar in construction and operation to that of a coalescing filter, however as their name suggests, they are not suitable for the treatment of oil or water aerosols and are therefore found installed downstream of the coalescing filters and dryer. Adsorption dryers use granular desiccant materials and dry particulate filters are usually installed on the outlet of such dryers to prevent excessive particulates entering the distribution system. They will also be found installed to protect point of use applications from particulate already in the distribution system such as rust and pipescale and micro-organisms A general purpose dry particulate filter will typically provide particle reduction down to 1 μm with an efficiency rating around 99.9%. Oil Vapour Removal Filters (Activated Carbon Filters) The previously mentioned stages of filtration will treat liquid water, water aerosols, liquid oil, oil aerosols, and particulate. However, oil vapour will not be removed by these stages, which is often evident by the smell of oil from compressed air. Oil vapour can cool and condense in the downstream piping and once again form liquid oil and oil aerosols. Additionally, if the compressed air is to be used for breathing, mixing with food, beverage or pharmaceutical products or other such duties”, oil vapour removal must be included. A combination of general purpose and high efficiency coalescing filters plus an oil vapour removal filter would typically be classified as ISO 8573-1:2010 Class 1.-.1. Some manufacturers are also available to meet ISO 8573-1:2010 Class 0 (0.003mg/m3) for total oil. Adsorption filters are available in a variety of designs, and can be split into the in-line type, carbon tower and modular system. In-line oil vapour filters are typically Filtration