trap particulates and oil residues, together with separate drying systems to remove water droplets and vapour. There should be at least one filter on

the intake side of the compressor, to prevent large particulates from entering, and potentially damaging, the precision components within. Downstream, coalescing oil filters or even activated carbon adsorber towers can be used to remove small particles and oil vapours. Using an oil-free compressor removes the requirement for high specification oil filters in many applications, but as the air entering the compressors may contain hydrocarbon residues from the environment, some applications still require extra filter elements. The removal of water from the

by Mark Whitmore, general manager, BOGE M

any industrial and commercial operations rely on compressed air,

and facilities management teams work hard to ensure their compressor installations can deliver an appropriate supply. The design and operation of an efficient, reliable compressed air system depends on a host of factors, including the pressure and volume requirements of on-site equipment and the variability of demand over time. There’s one aspect of compressed air

system design that often receives insufficient attention, however: the quality of the supplied air. The output of a compressor won’t be pure air, it will also contain a variety of other materials including water, oil residues and particulates. Some of those impurities are present in the intake air that is drawn into the compressors, others may be added as a by-product of the air compression process. Impurities in the compressed air supply

can have a number of undesirable effects on downstream equipment and processes. Water vapour can cause corrosion in machinery, pipes and fittings. Particulates can block nozzles, damage seals and harm the operation of valves. Oil residues can damage sensitive materials or contaminate products. Impurities can also present real health and safety risks to exposed personnel. Those risks are why all, but the very

simplest compressed air installations require some form of air treatment. In


fact, air treatment equipment typically accounts for more than a third of the total capital cost of an installation. Air treatment equipment also has its own operating costs: treatment systems require energy to run, effort to maintain, and they may include consumable elements such as filters that must be monitored and periodically replaced. Those extra costs mean that the choice

of air treatment technologies can play a decisive role in the overall cost of ownership of a compressed air installation. That’s why it is essential to select solutions that are appropriate to the needs of your installation. Air quality specifications are defined in

the ISO 8573-1 standard, which establishes a range of different numbered classes for particulate, oil and water contamination. Specific maximum contamination levels are provided in the standard, from Class 1 at the cleaner end of scale up to Class 4 for oil contamination, 7 for particulates and 9 for water. At the top of each scale is Class 0, which is defined only as air with a lower level of contamination than that required for Class 1. There is no single air treatment

technology capable of removing oil, water and particulates from the air supply, so most compressor installations require a combination of devices to meet the required quality specifications. A complete air treatment system may include a number of filter elements to

Modern systems offer high levels of performance and operational flexibility, for example using variable speed drives and smart control technology to adjust the speed of refrigerant drying systems

compressed air supply is usually a multi- stage process. Cyclone separators mechanically remove liquid condensates from compressed air using centrifugal force. Refrigerant dryers cool compressed air so water vapour condenses into liquid form, allowing it to be separated and removed. The effectiveness of refrigerant systems is limited by physics: they cannot operate below the freezing point of water. This means that applications in cold environments, or those which require extremely low levels of residual moisture, may need to use an adsorption drying technology. In these systems, air is passed over a hygroscopic material, such as silica gel. As the desiccant material in the dryer will eventually become saturated, adsorption dryers must periodically be regenerated. This is done using expanded air from the compressor system, or ambient air which is electrically heated and blown over the material. In addition to these primary air

treatment elements, larger compressor installations may include oil/water separators and other systems to allow the safe handling and disposal of the oil and water collected during treatment. The overall efficiency and effectiveness

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of an air treatment system depends on how well each of its elements work with the others, and how the whole system performs across the expected range of operating conditions. Modern systems offer high levels of performance and operational flexibility, for example using variable speed drives and smart control technology to adjust the speed of refrigerant drying systems. If your air quality is letting you down, it pays to seek specialist advice.


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