Using compressed air for tools and equipment is common. There are a variety of compressor styles to choose from. The challenge for users of compressed air is air cleanliness that is free of particulates, moisture and oil carry-over. In this article we’ll discuss three different compressed air treatment methods. Air receiver/ storage tanks, air after cooling and filters.
Storage Tank Cooling Method
All air compressor applications could benefit greatly by the installation of air receiver tanks. It is generally accepted that no receiver is too large for an application and no number of receivers are too many, and the installation of a vertical or horizontal receiver tank usually depends on physical space available.
The receiver serves many important functions. It dampens pulsations from the discharge line of a reciprocating compressor, resulting in essentially steady pressure in the system. It serves as a reservoir to take care of sudden or unusually heavy demands in excess of the compressor capacity. It prevents too frequent cycling of the compressor. In addition, it serves to precipitate some of the moisture that may be present in the air as it comes from the compressor or that may be carried over from the after-cooler.
The minimum receiver capacity for certain applications may be calculated, but experience and judgment are just as important. Receivers are also used to meet heavy, short time demands of certain equipment, and the manufacturers of this equipment can supply the information on the air requirements in such cases.
The storage of quantities of moisture in receiver tanks leads to the formation of rust and scale on the inside of the tank, which can become loose and get carried down-stream in the outgoing air. This rust and scale can cause problems of blockage in air using components and premature blockage of filters.
The reason for large volumes of liquids in the air receiver is that the tank is made of steel, and the wall temperature of the vessel is the same as the ambient temperature.
Even if the compressor is supplied with an air-cooled after-cooler, there is always a difference between the discharge air temperature of the after-cooler and the ambient air temperature (the approach temperature). As the cooling is not 100 percent efficient, the air discharging from the after-cooler will always be higher than the ambient air being used to cool the hot compressed air.
As soon as air leaving the after-cooler enters the receiver tank it comes in contact with the chiller steel wall of the tank, which is usually at ambient temperature. At this point, moisture starts to condense out of the compressed air as the air chills. If the air is stored in the tank for enough time, the temperature of the air in the tank will be the same as the ambient temperature, and no more moisture will condense out. At this point the air in the receiver tank is 100% saturated at a dewpoint equal to atmospheric temperature.
This is a typical mechanical float-type drain trap
Compressed Air After-cooling
Air-cooled versions
An after-cooler is a heat exchanger used to cool compressed air. Reduction of the compressed air temperature will cause moisture and oil droplets to precipitate out of the air. These contaminants are collected and drained off with a moisture separation device and drain trap.
The after-cooler should be located as close as possible to the compressor outlet.
A heat transfer will occur between two bodies of different temperature until temperature equilibrium is reached. This transfer of heat can take place in three different ways. Generally these take place simultaneously.
- Conduction
- Convection
- Radiation
The air-cooled after-cooler looks very much like a car radiator, and acts like one as well. However, rather than water filling the interior, the hot compressed air enters the bottom of the air-cooled after-cooler and travels within a tube system, discharging through the upper discharge port into a moisture separator. The tubes have fins, or metal plates between them to increase their surface area, and dissipate the heat more effectively.
As heat from the compressed air transfers to the cooler atmospheric air, some of the heat of compression is removed from the compressed air and carried away.
Water Cooled Versions
Water-cooled after-coolers effectively do the exact same thing only with much more control of the discharge air temperatures.
Compressed Air Filtering Basics
Sources of contamination
Contamination of compressed air results from several sources. The air contamination in industrial areas is very significant, with the average metropolitan environment containing in the order of 4 million dirt particles per cubic foot, as well as water vapor, which condenses out after compression and cooling.
Next, the compressor contributes its share of the pollution consisting of wear particles and, if oil-lubricated, carbonized oil.
Most contamination in a compressed air system can be removed simply by filtration. It is important however to select the correct type of filtration to obtain reliable results.
Mechanical separation of particles relies on three mechanisms of filtration:
- Direct interception
- Inertial impact
- Diffusion or Brownian movement
The first method, direct interception, affects the larger particles in a gas stream, which are literally sieved out. These present no real problem other than clogging filter material.
The second method, Inertial impact occurs when a particle traveling in a gas stream is deflected around the first, second or even third fiber in the filter material, but is eventually unable to negotiate the torturous path between the fibers and cannot change direction as quickly as the gas stream. Therefore, it collides with a fiber and remains attached to it.
The third method, diffusion or Brownian movement, affects the very fine particles which are subject to inter-molecular and electrostatic forces which cause them to spiral in the gas stream, thus increasing their effective diameter.
There is a critical particle size as a result of particles falling between the inertial impaction and diffusion mechanisms. This critical size is 0.3 micron and it is very relevant when considering removing fine particles, either liquid or solid. This particle size is typically used in filter efficiency and integrity tests.
If a filter tested on this particle size proves to be 100 % efficient, then it is quite safe to state that this filter is capable of removing any particle above this size.
Research has shown that every cubic meter of air in industrial areas contains about 140 million dirt particles. In heavy industrial areas the problem is even worse. Furthermore, up to 80 % of this contamination is smaller than 2 micron, allowing it to pass straight through the compressor intake filters, which range between 5 -10 microns.
Compress this air to 100 psig, and you have a staggering 980 million dirt particles per cubic metre, not to mention oil, operating dirt, scale, rust and condensed moisture.
Dirt and other contamination can have a serious effect on the wear and efficiency of pneumatic machinery. It will increase maintenance and downtime, and can be a determining factor in the early replacement of the pneumatic equipment.
The coalescing type filter, as depicted in these photos, is typical of a low capacity coalescing type filter. The flow pattern is the same as that of larger filters, however the filter media sometimes has an anti-re-entrainment barrier on the outside of the filter cartridge to prevent liquid contaminants from getting picked up by the outgoing, clean airflow.
The air enters through the inlet port and travels down through the inner core of the filter, passes through the filter media and leaves through the discharge port. Moisture and oil collect in the bottom of the housing and are expelled by a condensate drain valve, and other solid contaminants collect in the filter element.
This type of a filter frequently has a built-in, float-type auto drain trap and some manufacturers have a moisture level sight glass mounted on the side. This is to provide visual assurance that the moisture drain is functioning. This unit is not shown with a differential pressure gauge which is usually attached to the top of the filter.
The purpose of this type of gauge is to show the condition of the filter media by registering the pressure difference between the inlet side of the filter and the outlet side.
The manufacturers’ recommendations should be followed in respect to their suggested maximum allowable pressure drop before change out of the filter cartridge. A high pressure drop across filters must be compensated for with higher initial system pressure. Running with a dirty filter could be costly, from a fuel efficiency standpoint.
It is important to note that, as the airflow of the coalescing type filter is from the inside out. The particulate type filter, used for solid dirt and dust removal, is most often from the outside-in, as this configuration takes advantage of the larger surface area of pleated filter media, giving the filter element a longer life as it has more area to catch the dirt and dust particles before a high differential pressure is detected. If the media is not a pleated type media, the particulate filter may have an inside out flow as well.
The compressor inlet filter is a dry, pleated particulate filter, with a 5-10 micron rating.
Typical grades and specifications of filtration would be:
| Types of Filters | Particulate | Oil Removal Coalescing | Extra Fine Oil Removal Coalescing |
|---|---|---|---|
| General Shop Air | Instrument Air | Food Industry | |
| Liquid Removal | 100% of Water | 99.99=% of Oil | 99.99=% of Oil |
| Max. Liquid Loading | 2000 ppm w/w | 1000 ppm w/w | 100 ppm w/w |
| Solid Particulate Removal | 1 micron | 0.01 microns | 0.01 microns |
| Oil Carry-over | 1 ppm w/w | 0.01 ppm w/w | 0.001 ppm w/w |
| Pressure Drop | Dry: 1psi Wet:2 psi | Dry: 1psi Wet:3 psi | Dry: 2psi Wet:6 psi |
Regardless of your tool or equipment, treating your compressed air before it reaches your application will benefit your job and the longevity of your valuable assets.
If you have any questions about this article or anything mobile compressor related, please contact us.
