Air Compressor

air compressor screw type

air compressor screw type

Air compressors supply compressed air to pneumatic control machines or tools. For critical environments, the air has to be clean, dry and oil free. For these types of environment, the compressors are oil-free types.

Air compressors works on several principles: screw, centrifugal, reciprocating. They may also have single stage or multistage compressing, be oil-flooded type and oil-free type.

 

Principles of Operation

air compressor screw type

air compressor screw type

For all types of compressors, the principle of operation is almost the same. Atmospheric air is sucked into a compressing element through a suction filter. The compressing element, which can be a single screw, a double screw, an impeller or a piston, then compresses the incoming air and discharge the air to another chamber.

The air that is compressed becomes hot. Heat exchangers are installed to cool the hot air. The cooling of the hot compressed air takes away some of the moisture contained in the air as well as prevents seizure of the moving machine parts.

air compressor screw type

air compressor screw type

For higher pressures, a second stage compressor element is used to further compress the air. The second stage also incorporates a heat exchanger at the discharge side.

The first stage heat exchanger is called the intercooler, and the second stage heat exchanger the aftercooler.

air compressor capacity valve

air compressor capacity valve

Pressure switches are installed on the discharge piping or reservoir to start or stop the compressor as needed. Some compressors use air pressure sensors to control a suction valve at the compressor so that it will load and unload according to the demand for compressed air.

air compressor capacity valve dismantled

air compressor capacity valve dismantled

air compressor parts

air compressor parts

air compressor capacity valve

air compressor capacity valve

A cooling water system is necessary to bring the temperatures of the air and compressor housing down. This is external to the compressor unit. They consist of water pumps, and cooling towers to expel the heat from the compressor to the atmosphere.

Air Reservoir

reciprocating air compressor

reciprocating air compressor

The compressed air is led into an air reservoir. The air reservoir stores the compressed air that is then fed to the various end users through piping.

Screw Compressor

reciprocating air compressor

Screw compressor elements consists of two screw shafts held together by bearings and arranged with timing gears at the end of the shafts. The screw helixes are very close together but not touching. This assembly is housed together as a unit and is usually dismantled as one unit.

air reservoir

air reservoir

Through a system of gears, the driver and driven screw shafts are rotated by a prime mover, usually a motor.

The lubricating system consisting of an oil pump, filter, and oil cooler built into the whole compressor unit for lubrication of bearings and gearing.

The cooling water system is usually external to the unit, and has to be incorporated into the system if the compressor is to be operated. This consists of water pump, cooling tower and the necessary piping.

The condensed water collected in the intercooler and aftercooler are discharged to outside through a condensate trap.

The pressure switch installed at the air discharge piping is used to control the opening of a suction damper. This limits the incoming air and constant pressures at the discharge can be achieved.

Centrifugal Compressor

screw air compressor

screw air compressor

Centrifugal compressors are like fans or centrifugal pumps. The impeller is mounted on a shaft, supported by bearings. Through a system of gearing, the speed of rotation is brought up to a very high speed. A motor drives the gearing system through shaft coupling. Because of the very high speed involved, the balancing of the rotating impeller and shaft assembly is very important in order to avoid vibration.

The assembly is also very sensitive to the user demand, and control systems for these compressors are more sophisticated than the screw compressor. As with other rotating machines, oil lubrication system consisting of pump, oil filter and oil cooler is installed to lubricate the rubbing parts like bearings, and gearing. A water cooling system as described in the screw compressor also applies.

Centrifugal compressors are susceptible to surging. Centrifugal fans become unstable when operated at certain combinations of speed and load. Surging can occur when the load demand or air temperature changes suddenly, or due to restrictions to its flow.

Reciprocating Compressor

Reciprocating compressors can produce high air pressures. Because of the rubbing effect of the piston rings on the liner, it is difficult to make this oil free. Some designs using Teflon rings do not need oil and are classified as oil free.