Air compressors

Air compressors are used to compress air from atmospheric pressure to higher pressure levels. The purpose is to store energy in the air. Compressed air can be used as power source, driving other utilities and machines. The largest benefit doing this is that the other machines and utilities do not need to have their own motor. Instead, the compressor producing compressed air can be used to drive several different tools in a factory for example.

How to compress air

Air can be compressed using several methods, and these are usually divided into positive displacement and dynamic displacement. Positive displacement involves piston-type compressors where work is applied to a piston compressing air trapped in a chamber thus increasing the pressure. Screw compressors are also positive displacement where two rotating screws compresses the air trapped in the gap between the two screws. Dynamic displacement can be a centrifugal compressor where the energy is added by the centrifugal force. The choice of compressor type depends on many factors but can, for example, be efficiency, noise levels, maintenance needs, capacity, cost, compactness etc.

Heat exchangers

In most compressors, some components generate heat. The heat is in some cases absorbed by oil. The oil is pumped to a heat exchanger where water cools it. The oil can then be pumped back to the compressor and cool the engine, gear-box etc.

Compressing air in itself generates heat and increases the air temperature which needs to be lowered. In screw compressors, this can be done by having oil injected in the gaps between the screws absorbing some of the excess heat. In those cases, the temperature of the air is increased from 20°C to around 80°C. However, in many applications and due to environmental issues, most compressor manufacturers also provide what they call “oil-free compressors”. It means that there is a limit on how much oil the pressurised air can contain after leaving the compressor. This limit is usually even lower than the oil content in the ambient air, so oil separation is crucial in those machines. In oil-free compressors, there is no oil cooling of the air being compressed. Due to this the air is heated much more compared to oil cooled compressors. Having no oil forces the compressor to be built with two separate compression stages. Otherwise, the air would become so hot that the compressor would become damaged. After the first compression stage, the air reaches temperatures of around 160°C - 180°C and is then cooled in an intercooler with water or air down to around 30°C. After the intercooler, the air will be compressed further in a second compression stage. When fully compressed, the aftercooler will cool the air from 140°C - 180°C to around 25°C.

In both the intercooler and the aftercooler moisture will condense in the heat exchangers. A compressed gas cannot contain the same amount of moisture at higher pressures as at ambient conditions.

Cooling air in an intercooler and aftercooler is something that traditionally has been done with big bulky Shell & Tube heat exchangers. The reason being that they do not generate a big pressure drop and are very robust. Since the purpose of the compressor is to compress air, the pressure drop in the heat exchanger should be minimised. With Airecs unique plate design it is possible to keep as low pressure drop as with the Shell & Tube, but with a much more compact and efficient unit. Reasons like this have interested several of the biggest compressor producers in Europe, and they are now discussing product developments utilising Airecs design.