The majority of air compressors work on positive displacement, this means a fixed volume of air is delivered from each rotation of the compressor.
The above image shows a combined two stage compressor which is fine as it is, however when high pressures are required from a compressor set up like this then the compressed air temperature can rise to over 200°C and motor power needed to drive the compressor rises with this temperature.
So for these higher desired pressures it is far more economical to include a cooling system between each stage, this would then be known as a multistage compressor. These cooling systems are commonly known as intercoolers which work by drawing the hot compressed air over a very large surface area where the heat can dissipate quickly, this cooled air will then enter the second stage compression to be compressed even further by the last piston.
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Screw compressors
Screw compressors are best suited when only medium pressures are required (<10 bar).
Screw compressors offer a steady flow of air without pulses and fluctuating pressure that can be associated with piston compressors, they also offer a simple system with fewer moving parts which equates to a more reliable system with less maintenance involved.
Screw compressors work on the basis of two intermeshing rotating screws with minimal clearance between each screw fin, they draw in air and squeeze it up along the screw with air getting compressed tighter the further along the screw its drawn until it reaches the discharge port, it is here that it is delivered out of the compressor.
The two intermeshed screws can be synchronised by external timing gears meaning that the screws should remain close but without touching each other. The other version is known as wet rotary screw compression, this is where the first screw is driven by a motor while the second screw isn’t, the second screw is rotated by being in physical contact with the first. This requires oil to be sprayed in with the air to relieve friction and wear; some of this lubricant will make its way into the air system so it is removed by later oil separation units.
Dynamic Compressors
Dynamic compressors are capable of delivering large volumes of air but little pressure (0.5-3bar). These compressors are usually known as blowers and work by drawing air in and throwing it out with the use of rotary fins, these fins rotate at very high speed.
There are 2 main types of dynamic compressor, they are centrifugal and axial.
Centrifugal systems use centrifugal force to hurl air out from the fins, centrifugal systems can generally obtain greater pressures than the axial type compressor.
Axial type compressor use a set of fan blades in line to generate large air flow, pressures from this method aren’t expected to reach much over 0.5bar. The axial compressors are largely used for ventilation and as part of air processing.
Axial Compressor
Air Saturation
The air we breathe from our atmosphere contains moisture in the form of water vapour; people often refer the amount of moisture in the air as levels of ‘humidity’. Humidity levels are a percentage of the saturation of air with water.
The maximum amount of water saturation in air varies greatly with temperature; hotter air can hold much more moisture than cold air. Due to the change in air pressure when compressing air its ability to hold moisture is seriously reduced, so some of the moisture content that was previously in the air will simply condense out of it when the air is compressed. This is why drainage systems and dryers are an important part of compressor systems.
If the water was allowed to travel out of the compressor into the pneumatic components that it was powering then this could cause big problems, for example ceasing valves and actuators.
Relative Humidity = Mass of water vapour present X 100
Mass of water needed to saturate it
Stages of air treatment
Filters
Compressed air requires filtration and drying before it leaves for pneumatic components, atmospheric air carries many particles of debris and moisture that unless filtered out can block valves or cause increased wear. Compressors usually filter air before its compressed, this is mainly to remove relatively large particles which could damage the compressor. A fine grade filter used after it is compressed helps filter the small particles which could clog or damage the pneumatic components that are used in pneumatic system set ups.
Air dryers
Before air can be used the excess moisture has to be removed, where well dried air isn’t a requirement all that may be needed is a basic intercooler followed by a separator unit; this is where the condensed water collects and is drained off.
Refrigeration units
The dew point can be lowered even further by cooling the air with a refrigeration unit which cools the air even further to around 0°C helping more moisture collect in the separator tank. This cold air is then routed through a separate section of the initial heat exchanger to help cool the warm air which is flowing through from the compressor.
This system of drying is suitable for most systems as air is dried out well.
Deliquescent dryer
Where completely dry air is a necessity then a chemical dryer should be used, the chemicals can remove moisture in two ways.
1) A chemical agent called a desiccant is used, this chemical agent collects water vapour and holds it while it eventually turns itself into a liquid once so much vapour has been collected and runs to the bottom of the unit where it is drained off. This chemical agent needs to be replaced quite regularly.
Absorption dryer
2) Within an absorption dryer a material called silicone dioxide or copper sulphate is used, this process works by attracting moisture to the sharp edges of these granular materials. Once these materials become saturated they are dried by passing heat through them, this process cant be carried out while it is still drying incoming air so they are usually set up with 2 of these moisture collectors which can be swapped over instantly, this allows one collector to collect the moisture while the remaining collector is left to dry itself out.
Oil is often added to the air to lubricate moving parts, the oil is released as a fine mist into well dried air, if the air is poorly dried then the oil will mix with the moisture and become tacky which will cause problems sticking valves ect.
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The oil is drawn up from the oil reservoir due to the way the air flows through the lubricator; a pressure difference between the lower and upper chambers is created as the air is channelled down past the oil mist opening. The screw valve is used to adjust this pressure difference.
Pressure Regulation
Pressure regulation in pneumatics is vital for the correct operation of circuits and for damage prevention to circuit components. As you would imagine all pneumatic components have a maximum operating pressure.
Relief valves
These are basic pressure regulators and are usually used in pneumatic circuits as a backup device should the main pressure controller fail. They work with the use of a ball valve which is under an adjustable amount of pressure to keep the valve shut, once this pressure is exceeded the ball forces the spring back allowing the excess pressure to escape. Care should be taken when employing a pressure relief valve, this is because the valve must have sufficient ability to vent the pressure and flow quickly enough should a fault occur, and should the worst happen the relief valve may have to dump the entire compressor output.
Once a fault has triggered the release mechanism and the pressure has dropped sufficiently then the valve automatically closes and re-seals itself. This type of valve is always ready to release pressure again if needed, however a safety valve must be manually reset once it has cracked to release pressure.
Non-relieving pressure regulators
Non-relieving pressure regulators work by restricting flow rather then venting it should over pressure occur. The regulator restricts flow when the pressure gets too high because the pressure acts on the diaphragm forcing it up against the spring pressure, the diaphragm has what is called a ‘poppet’ attached on the end of it which is drawn up with the diaphragm and restricts the passing air flow.
Relieving pressure regulators
Relieving pressure regulators use a similar diaphragm system to that of the non-relieving regulator. The diaphragm drops if the outlet pressure is too high closing the inlet valve and opening the outlet vent, releasing pressure until it falls back to the preset pressure.
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