Oil is not what it seems…
August 03rd 2007

Mark White, product manager – Industrial Compressed Air,Parker domnick hunter dispels the myth about compressed air contamination

Ask most maintenance or production engineers, what they considered to be the major contaminant found in their compressed air system and their answer would almost certainly be oil. In reality however, oil is not the major problem perceived by so many. In fact, the most prolific contaminant is water and up to 99.9% of the total liquid contamination found in a compressed air system is water. This means that oil only attributes a very small percentage of the overall liquid contamination.

Understanding the sources of compressed air contamination and the types of contaminants which must be eliminated is fundamental to understanding the principles of effective purification system design.

Sources of contamination in a compressed air system

The atmospheric air

Air compressors draw in vast volumes of air from the surrounding atmosphere. This atmospheric air contains a large number of airborne contaminants (highlighted later in article).

The type and operation of the air compressor

The air compressor can also add contamination, from wear and tear particles to coolants and lubricants. Compressed air storage devices and distribution systems The air receiver and system piping distributes the compressed air and will also store the large amounts of contamination drawn into the system. Additionally, they cool the moist compressed air which causes condensation on a large scale and promotes corrosion within the compressed air system.

Types of contamination found in a compressed air system

Atmospheric dirt

Atmospheric air in an industrial environment typically contains 140 million dirt particles for every cubic metre of air. 80% of these particles are less than 2 microns in size and are too small to be captured by the compressor intake filter and therefore pass directly into the compressed air system.

Water vapour, condensed water and water aerosols

Atmospheric air contains water vapour (water in a gaseous form). The ability of compressed air to hold water vapour is dependent upon its temperature. The higher the temperature, the more water vapour that can be held by the air. During compression, the temperature increases significantly, which allows the heated air to easily retain the incoming moisture.

Prior to exiting the compressor, compressed air is normally cooled to a usable temperature. This reduces the air’s ability to retain water vapour, resulting in a proportion of the water vapour being condensed into liquid water which can now be removed by a condensate drain fitted to the compressor after-cooler. The air leaving the after-cooler is now 100% saturated with water vapour and any further cooling of the air will result in more water vapour condensing into liquid water.

Condensation occurs at various stages throughout the system as the air is cooled further by the air receiver, piping and the expansion of air in valves, cylinders, tools and machinery. The condensed water and water aerosols cause corrosion to the storage and distribution system, damage production equipment and can also spoil the end product. Overall, water contamination reduces production efficiency and increases maintenance costs and must be removed to enable the system to run correctly and efficiently.

Rust and pipescale

Rust and pipescale can be found in air receivers and the piping of ‘wet systems’ (systems without adequate purification equipment) or systems which were operated ‘wet’ prior to purification equipment being installed. Over time, this contamination breaks away to cause damage or blockage in production equipment which can also contaminate final product and processes.

Micro-organisms

Bacteria and viruses will also be drawn into the compressed air system through the compressor intake and warm, moist air provides an ideal environment for the growth of micro-organisms. Ambient air can typically contain around 3800 micro-organisms per cubic metre. If only a few micro-organisms were to enter a clean, sterile environment, or production process, enormous damage could be caused that not only diminishes product quality, but may even render a product entirely unfit for use and subject to recall.

Liquid oil and oil aerosols

Most air compressors use oil in the compression stagefor sealing, lubrication and cooling. During operation, lubricating oil is carried over into the compressed air system as liquid oil and aerosols. This oil mixes with water in the air and is often very acidic, causing damage to the compressed air storage and distribution system, production equipment and ultimately, the final product.

Oil vapour

In addition to dirt and water vapour, atmospheric air also contains oil in the form of unburned hydrocarbons. The unburned hydrocarbons drawn into the compressor intake as well as vaporised oil from the compression stage of a lubricated compressor will carry over into a compressed air system where it will cool and condense, causing the same contamination issues as liquid oil.

Typical oil vapour concentrations can vary between 0.05

and 0.5mg per cubic metre of ambient air.

So why is oil seen as the major contaminant?

Oil is only perceived to cause the most problems as it is can be seen emanating from open drain points and exhausting valves. In the majority of instances, it is actually oily condensate (oil mixed with water) that is being observed.

How much water is actually in a compressed air system?

If we measured the amount of water in a small compressed air system, the volume is staggering. A small 2.8m3/min (100 cfm) compressor and refrigeration dryer combination, operating for 4000 hours in typical Northern European climatic conditions can produce approximately 10,000 litres or 2,200 gallons of liquid condensate per year. If the compressor was an oil lubricated machine, then although the resulting condensate would visually resemble oil, oil would in fact account for less than 0.1% of the overall volume and it is this resemblance to oil to which a false association is made.

The example above illustrates the use of a small compressor to highlight the large volume of condensate produced. If a compressed air system was operated in warmer, more humid climates, or with larger compressors installed, running for longer periods, the volume of condensate would increase significantly. Contamination and types of compressor It is often believed that the level of compressed air purification equipment required in a system is dependent upon the type of compressor used, however this is incorrect as contamination in a compressed air system originates from many sources and is not related solely to the compressor or its lubricants.

One common practice when installing an ‘oil free’ compressor is to omit some or all of the downstream filtration, because ‘oil free’ compressors do not use oil in the compression chamber and therefore oil does not contact the air being compressed.

Although lubricating oil is not added to the air during compression, even an oil free compressor does not supply contaminant free air and regardless of which compressor type is selected, adequate filtration and separation equipment is required to remove the large volume of water aerosols, particulate, rust, pipescale and microbiological contamination from entering the system.

Compressed air and its purification

Having identified the different types ofontamination found within a compressed air system, the purification technologies available for its removal can now be examined.

Coalescing filters

Coalescing filters are probably the most important pieces of purification equipment in a compressed air system. They are designed not only to remove aerosols (droplets) of oil and water using mechanical filtration techniques, but also to remove solid particulate to very low levels (as small as 0.01micron in size). Installed in pairs, most users believe one to be an oil removal filter and the other to be a particulate filter, however, the pair of filters both perform the same functions. The first filter is a ‘general purpose filter’ which protects the ‘high efficiency filter’ from bulk contamination. This dual filter installation ensures a continuous supply of high quality compressed air with the additional benefits of low operational costs and minimal maintenance.

Adsorption (desiccant) dryers

Water vapour is removed from compressed air using a dryer, the efficiency of which is measured as pressure dewpoint. Adsorption or desiccant dryers remove moisture by passing air over a regenerative adsorbent material which strips the moisture from the air. This type of dryer is extremely efficient and typical pressure dewpoints for adsorption dryers are -40°C or -70°C, which means for water vapour to condense into a liquid, the air temperature would need to drop below -40°C or -70°C respectively (the actual air temperature after an adsorption dryer is not the same as its dewpoint). Typically, a pressure dewpoint of -40°C is used in most applications as compressed air with a dewpoint below - 26°C will not only prevent corrosion, it will also inhibit the growth of micro-organisms within the compressed air system.

Refrigeration dryers

Refrigeration dryers work by cooling the air and are therefore limited to positive pressure dewpoints to prevent freezing of the condensed liquid. Ideal for general purpose applications, they provide pressure dewpoints of +3°C, +7°C or +10°C.

Refrigeration dryers are not suitable for installations where piping is installed in ambient temperatures below the dryer dewpoint i.e. systems with external air receivers and piping.

Important note regarding compressed air dryers As adsorption and refrigeration dryers are designed to remove only water vapour and not water in a liquid form, they require the use of coalescing filters to work efficiently.

Adsorption (activated carbon) filters

Oil vapour is oil in a gaseous form and as with water vapour, will pass through a coalescing filter just as easily as the compressed air itself. Therefore, oil vapour removal filters utilise a large bed of activated carbon adsorbent for the effective removal of oil vapours and provide the ultimate protection against oil contamination.

Dust removal filters

Dust removal filters are used for the removal of particulate where no liquid contamination is present. They usually provide identical particulate removal performance to the equivalent coalescing filter and use the same mechanical filtration techniques to provide up to 99.9999% particle removal efficiency. For absolute particulate retention (100% at a given size), a sieve retention membrane filter must be used.

Microbiological (sterile) filters

Absolute removal of solid particulate and microorganisms is performed by a sieve retention or membrane filter. They are often referred to as ‘sterile’ air filters as they also provide sterilised compressed air. Housings are manufactured from stainless steel to allow in-situ steam or chemical sterilisation of the filter and element. It is important to note that the piping between the sterile filter and the application must also be cleaned and sterilised on a regular basis.

Cost effective system design

To achieve the stringent air quality levels required for today’s modern production facilities, a careful approach to system design, commissioning and operation must be employed. Treatment at one point alone is not enough and it is highly recommended that the compressed air is treated prior to entry into the distribution system to a quality level suitable for protecting air receivers and distribution piping. Point of use purification should also be employed, with specific attention being given to the application and to the level of air quality required. This approach to system design ensures that the air is not ‘over treated’ and provides the most cost effective solution to high quality compressed air.

Not all compressed air filters and dryers are the same Today, many manufacturers offer products for the filtration and purification of contaminated compressed air, with many being selected based only on their initial purchase cost, with little or no regard to the air quality they provide or to the running costs throughout their operational life.

Compressed air purification equipment is vital for the removal of system contamination, therefore when purchasing this type of equipment, air quality, energy efficiency and lifetime costs must always be considered.

Air quality

Air quality should always be the main reason for installing purification equipment. All domnick hunter purification equipment has been researched and designed to provide compressed air quality in accordance with the quality classes described in ISO8573.1 : 2001, the latest edition of the international air quality standard. Additionally, domnick hunter product performance has been independently verified by Lloyds Register and is supported by a 12 month performance guarantee, which can be extended simply by conducting annual maintenance in accordance with the domnick hunter recommendations.

Energy efficiency

During the design of domnick hunter filtration and drying products, the company’s engineers strive to provide the lowest operating costs whilst achieving the air quality required by the international standards. Pressure loss is the major contributor to operational costs of filtration products. domnick hunter OIL-X EVOLUTION filters have been designed using aerospace technology to ensure both pressure loss and energy consumption is kept to an absolute minimum. By considering pressure losses after 12 months of operation and not just at start-up, energy savings in excess of 60% can be made when compared to a conventional filter. domnick hunter adsorption dryers are also optimised to ensure that regeneration costs are minimised and energy management systems are available to further reduce operational costs during periods where the water vapour entering the dryer is reduced (possibly due to weather conditions, shift patterns or a variable air demand).

Low lifetime costs

Equipment with a low purchase price may turn out to be a more costly investment in the longer term. One must consider the initial purchase costs in addition to the operational and maintenance costs of the purification equipment. Moreover, the user might also wish to consider what the cost of poor air quality is ultimately to the business, i.e. the cost of making an ill advised decision regarding air quality requirements. By guaranteeing air quality and ensuring that energy consumption is kept to an absolute minimum, domnick hunter purification equipment can reduce the total cost of ownership and improve the bottom line through improved manufacturing efficiencies.