Compressed air contamination
Ask most maintenance or production engineers what is the major contaminant found in their compressed air system and the answer would be oil.
In reality, oil is not the major problem everyone thinks it is. The most prolific contaminant is in fact water. Up to 99.9% of the total liquid contamination found in a compressed air system is water, with oil being only a very small part of the overall contamination problem.
To understand why, we must first understand where compressed air contamination comes from and the types of contaminants that can be found.
Contaminants in a compressed air system can generally be attributed to the following:
The quality of the compressed air being drawn into the compressor. Air compressors draw in large volumes of air from the surrounding atmosphere. This atmospheric air contains a large number of airborne contaminants, which will be covered shortly in more detail.
The type and operation of the air compressor. The air compressor itself 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 store and distribute the compressed air and will store the large amounts of contamination drawn into the system. Additionally, they cool the moist compressed air further promoting damage and corrosion.
What type of contamination is found in a compressed air system?
Atmospheric air in an industrial environment typically contains between 140 and 150 million dirt particles in every cubic metre of air. About 80% of these particles are less than two micron in size and are too small to be captured by the compressor intake filter, passing directly into the compressed air system.
Water vapour, condensed water and water aerosols
Atmospheric air contains water vapour (water in a gaseous form). The airs ability to hold water vapour is dependent on its temperature. The higher the temperature, the more water vapour can be held by the air.
During compression, the air temperature is increased significantly, which allows the air to easily retain the incoming moisture. After the compression stage, air is typically cooled to a usable temperature. This reduces the airs ability to retain water vapour, resulting in a proportion of the water vapour being condensed into liquid water, which is removed by a drain fitted to the compressor after-cooler. The air leaving the compressor is now 100% saturated with water vapour and any further cooling of the air will result in more water vapour condensing into liquid water. Further condensation does in fact occur 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, damages production equipment and end product, reduces production efficiency and increases maintenance costs. Both liquid water and water vapour must be removed for 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 that were operated wet prior to purification equipment being installed. Over time, this contamination constantly breaks away to cause damage or blockage in production equipment and also contaminate final product and processes.
Bacteria and viruses are also brought into the compressed air system through the compressor intake and warm, moist air provides an ideal environment for the growth of microorganisms. Ambient air can typically contain up to 3850 microorganisms per cubic metre. If only a few microorganisms were to enter a clean environment, a sterile process or a production system, 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
Almost all air compressors use oil in the compression stage for sealing, lubrication and cooling. During their operation, they allow lubricating oil to be carried into the compressed air system as liquid and as an aerosol. This oil mixes with water vapour in the air and is often very acidic, causing damage to the compressed air storage and distribution system, production equipment and final product.
Along with 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 vapourised oil from the compression stage of a lubricated compressor will carry over into a compressed air system where it can cool and condense, causing the same contamination issues as liquid oil. Typical oil vapour concentrations can vary between 0.05 and 0.5 mg per cubic metre of ambient air.
So why is oil seen as the major contaminant?
Oil is only perceived to cause the most problems because it 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 2.8 m3/min (100 cfm) compressor and refrigeration dryer combination, operating for 4000 hours in cooler climatic conditions can produce approximately 10,000 L or 2200 gallons of liquid condensate per year. Typical Australasian conditions can produce much more water.
If the compressor is oil lubricated with a typical 2 mg/m3 (2 ppm) oil carryover, 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 assumes 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 on the type of compressor used. As we have just seen, the contamination in a compressed air system originates from many sources and is not related solely to the compressor or its lubricants.
One common misconception is that by installing an oil free compressor into the system, there is no need for downstream filtration, however, the term oil free simply means that oil is not used in the compression chamber and therefore does not contact the air being compressed. An oil-free compressor does not supply contaminant-free air and no matter what compressor type is selected, adequate filtration and separation products will be required to remove the large volume of dirty contaminated water as well as the dirt, rust, pipescale and microbiological contamination entering the system.
Compressed air and its purification
Having identified the different types of contamination that can be found within a compressed air system, we can now examine the purification technologies available for its removal.
Coalescing filters are probably the most important purification equipment in a compressed air system. They are designed to remove aerosols (droplets) of oil and water using mechanical filtration techniques, but also have the additional benefit of removing solid particulate to very low levels (as small as 0.01 micron in size).
Installed in pairs, most users believe one to be an oil removal filter and the other to be a particulate filter, when in fact the pair of filters both perform the same function. The first filter, a general-purpose filter, is used to protect the high-efficiency filter from bulk contamination. This dual filter installation ensures a continuous supply of high-quality compressed air with low operational costs and minimal maintenance time.
Adsorption (desiccant) dryers
Water vapour is removed from compressed air using a dryer. A dryers efficiency 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 or -70C, which means for water vapour to condense into a liquid, the air temperature would have to drop below -40 or -70C respectively (the actual air temperature after an adsorption dryer is not the same as its dewpoint).
Beneficially, a pressure dewpoint of less than -26C will not only prevent corrosion, it will also inhibit the growth of microorganisms within the compressed air system.
Refrigeration dryers work by cooling the air, so are limited to positive pressure dewpoints to prevent freezing of the condensed liquid. Ideal for general-purpose applications, they provide pressure dewpoints of +3, +7 or +10C. Refrigeration dryers are not suitable for installations where piping is installed in ambient temperatures below the dryers dewpoint, ie, systems with outside piping.
Important note regarding compressed air dryers
Because 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 simply 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 must be employed as these provide a large bed of activated carbon adsorbent for the effective removal of oil vapours and provide ultimate protection against oil contamination.
Dust removal filters
Dust removal filters are used for the removal of particulate when no liquid is present. They usually provide identical particulate removal performance as 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 because they also provide sterilised compressed air. Housings are manufactured from stainless steel to allow steam or chemical sterilisation of the filter and element. It is important to note that the 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 todays 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 protecting air receivers and distribution piping.
Point-of-use purification should also be employed, with specific attention being paid to the application and the level of air quality required.
This approach to system design ensures the air is not over treated and provides the most cost-effective solution to high quality compressed air.
Are all compressed air filters and dryers the same?
Today, many manufacturers offer products for the filtration and purification of contaminated compressed air, with many being selected only on the basis of their initial purchase cost, with little or no regard for the air quality they provide or the cost of operation throughout their 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 is the reason for installing the purification equipment in the first place. All Atlas Copco / CPS purification equipment has been designed to provide compressed air quality in accordance with the recommendations shown in ISO 8573.1: 2001, the latest edition of the international air quality standard.
Pressure loss is the major contributor to operational costs of filtration products. The AC filter range have been designed using technology to ensure minimal pressure loss and thus 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% compared with a traditional filter are not uncommon. Equipment with a low purchase price may turn out to be a costly investment in the long run. Ask yourself what is the cost to your business of poor air quality and consider the initial purchase costs, plus the cost of operating and maintaining the purification equipment.
In reality, oil is not the major problem everyone thinks it is. The most prolific contaminant is in fact water.
Bacteria and viruses are also brought into the compressed air system through the compressor intake and warm, moist air provides an ideal environment for the growth of microorganisms.
Air quality is the reason for installing the purification equipment in the first place.
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