Air Filters

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OMI AIR FILTER
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REMOVING CONTAMINANTS

Liquid Water
In compressed air systems water vapor exists as a contaminant originating at the compressor outlet in vapor form, but as the air cools, it will exist as both liquid and vapor. The amount of water vapor that can exist in any given volume of compressed air is directly proportional to the air temperature and inversely proportional to the pressure. Most liquid water will be present when the temperature is lowest and the pressure is highest and removal at this point will achieve the highest efficiency. In order to achieve this an essential element of any system following the compressor is an efficient after cooler of sufficient capacity to reduce the temperature of the outgoing air to within 8°C of the temperature of the water entering the after cooler. The outgoing air should then be piped to a receiver of adequate capacity located in the coolest location available, definitely not within the compressor house itself. This will permit further cooling of the air to occur and therefore more condensation. Generally the capacity of the receiver is about 30 times greater than the rated free air delivery of the compressor when operating in the 7 bar g region, typical of most industrial air supplies.

Water Vapor
A properly designed air line filter of the correct size, in the correct location will effectively and efficiently remove liquid water, but will not reduce the water vapor content of the air. Further air cooling may result in more water condensing out. If complete freedom from water contamination is essential then the water vapor content of the air must be lowered such that the ‘Dew Point’ of the air is lower than any temperature that the air can be exposed to in the system. Once all liquid water is removed from compressed air, then normally the air will be completely saturated with water vapor. The particular temperature and pressure at which the compressed air exists at that moment is known as the ‘Pressure Dew Point’.

Solid Particles
Like water, solid particles exist in any compressed air system regardless of the type of compressor. These can arise from four principle sources:-
a) Atmospheric dirt inhaled at the compressor inlet port.
b) Corrosion products due to the action of water and weak acids, formed by the interaction of water and gases such as sulphur dioxide inhaled by the compressor.
c) Carbon products formed by the action of the heat of compression on the lubricating oil or the normal wear of the carbon piston rings used in some types of oil free compressors.
d) Particles originating from the mechanical fixing of the metal pipe work and components into the air  distribution system. The size of dirt particles covers a very wide range from several hundred to below one micron and the level of filtration depends upon the degree of cleanliness needed for the particular process involved. Generally it is inadvisable to provide finer filtration than is absolutely necessary because the finer the filtration, the greater the quantity of dirt trapped by the filter element and the more rapidly it will become blocked.

Oil
The principle source of oil contamination within a compressed air system is from the compressor. An oil lubricated compressor of 105 scfm capacity may introduce as much as 0.17 quarts of oil per week into the system. Oil is used for lubrication of the compressor but when it emerges with the compressed air prior to distribution the oil is now in a totally unusable state. Having been subjected to high temperatures during air compression it becomes oxidized and acidic and can be considered as an aggressive contaminant rather than a lubricant and so must be removed. Normal air line filters will remove sufficient liquid oil (along with water) to leave the air in a suitable condition to supply most pneumatic tools and cylinders, but certain processes demand completely oil-free air. One solution is to use oil-free compressors. These will still produce air contaminated with dirt and water and it is often more economical to use lubricated compressors in conjunction with after coolers and standard air line filters, only fitting high efficiency oil removal filters at the points in the system which demand oil-free air. This ensures that the amount of air needing special treatment is kept to a minimum by allowing a smaller specialized filter in the affected area and not a large specialized filter for the whole plant. Oil in a compressed air system can exist in three forms, oil/water emulsions, aerosols (small particles suspended in the air) and oil vapors. Emulsions can be removed by standard air line filters but the aerosols are our next concern.

Oil Aerosols
These particulate oil droplets exist in the air stream and the most troublesome are in the size range 0.01 to 1 micron (approx 90%), the rest may be slightly larger. Most standard air line filters achieve water removal by centrifugal action but due to their small particle size these aerosols are unaffected and require special coalescing filters. In addition to removing the oil droplets these filters will also remove minute water droplets, but they must be protected against gross dirt or water contamination by means of standard air line filters mounted immediately upstream. It is normally advisable that these filters are capable of removing particles down to 5 microns or less otherwise the coalescing filter may quickly become choked and blocked with dirt, requiring a filter element replacement. Coalescing filters are normally rated by the amount of air which they can ‘process’ to achieve a given oil removal performance, normally a maximum remaining oil content in the exit air of 0.01 ppm. To try to overflow these units will not only result in a greater pressure drop across the unit and therefore extra energy cost but more importantly the remaining oil content will increase. This may be acceptable for some applications where oil removal down to the order of 0.5 ppm is quite adequate to give a degree of protection to a system particularly prone to gross oil contamination.

Oil Vapor
For most processes the removal of oil vapor is unnecessary since unlike water vapor, oil vapor exists only in minute quantities and is not objectionable except in circumstances where its odor is unacceptable eg. in food processing, pharmaceutical and beverage industries and breathing air applications. The most common method of removal is to pass the air through an adsorbing bed, usually of activated carbon, although other materials can be used. Such vapor removal filters will normally reduce the total remaining oil content when used in conjunction with a pre-filter (general purpose filter) and a coalescing filter to 0.003 ppm. A common misconception of these filters is that they will remove carbon monoxide or carbon dioxide - they will not. As with oil removal (coalescing) filters the vapor removal filters should only be employed where their function is needed, the maximum flow rating is not exceeded and they are preceded by a general purpose and a coalescing filter. This will minimize the size of the filters required and therefore the cost of the installation. The location of the compressor intake may also have an effect on the level of filtration required, if for example the intake is situated by a source of hydrocarbon vapors etc. Clean air intake will reduce the cost of producing clean compressed air.

Once all of the contaminants have been considered the degree of cleanliness of air for each part of an industrial plant or process can be determined. By only employing the correct filters in the right location energy and maintenance costs can be kept to a minimum. The volume of air involved in each stage must always be considered as undersized, inappropriate filters are a prime cause of high energy costs. A very general guide to the typical levels of cleanliness required for common processes is shown below. Each application should however be considered on its own merits. Recommendations on air drying are particularly difficult since this is dependant upon the temperature of the compressed air main adjacent to the application/machine the level of pressure reduction and air flow rate. For well laid out generation and distribution systems drying is seldom required in countries of typically low to moderate relative humidity and ambient temperatures. When choosing a filter to clean compressed air ensure:-
● The correct type of filter and element rating is selected for particle removal.
● The liquid removal efficiency is high and that re-entrainment is not possible.
● Ease of maintenance and liquid condensate collection is possible.
● Easy visibility of condensate and/or element ensures that function is achieved or shows if maintenance is required. This may be a pressure drop device, liquid level indicator or transparent bowl. In order to aid determining the type of water and particle removal.
 

RECOMMENDED FILTRATION LEVELS.

Please contact us today for further discussion email:
jontye1@streamyx.com

Sophotech (M) Sdn Bhd (408422T)
25 Jalan PM1, Taman Perindustrian Merdeka, 75350 Melaka, West Malaysia
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