Air compression effects

When compressed air is delivered into a pipeline for use, the air will almost certainly be very hot, and it may contain a lot of water and oil. The power required to provide the compressed air, and hence the operating cost, may also be very high. 

Whether or not the air can be used without being cooled will depend to a large extent on the properties of the material to be conveyed. The most efficient form of compression is to carry out the process isothermally, and so cylinders of reciprocating compressors, for example, would be water-cooled, and if staging was employed for achieving high pressures, inter-cooUng would be employed here as well. 

2 Oil-free air Oil-free air is generally recommended for most pneumatic conveying systems and not just those where the material must not be contaminated, such as food products, pharmaceuticals and chemicals. Lubricating oil, if used in an air compressor, can be carried over with the air and can be trapped at bends in the pipeline or obstructions. Most lubricating oils eventually break down into more carbonaceous matter which is prone to 

Although conventional coalescing after-filters can be fitted, which are highly efficient at removing aerosol oil drops, oil in the super-heated phase will pass straight through them. Super-heated oil vapour will turn back to liquid further down the pipeUne if the air cools. Ultimately precipitation may occur, followed by oil breakdown, and eventually a compressed air fire. The only safe solution, where oil-injected compressors are used, is to use chemical after-filters such as the carbon absorber type which are capable of removing oil in both liquid droplet and super-heated phases. 

3 Water removal As the pressure of air is increased, its capability for holding moisture in suspension decreases. As the temperature of air increases, however, it is able to absorb more moisture. If the air is not initially saturated it may well reach the saturation point during the compression process, or in a following after-cooler. Where air is compressed isothermally quite large quantities of water vapour can be condensed, and in many cases the air leaving the compressor will be saturated. In adiabatic compression the temperature of the air will rise, and because of the marked ability of warmer air to support moisture, it is unlikely that any condensation will take place during the compression process. 

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For projectile velocities less than about a third of the speed of sound in air, compressibility effects may be neglected and the drag coefficient is a fimction of Reynolds number alone, defined as... 

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