Bits and Pieces: Fluid Power Fact #4/2
Understanding Air Compressors – Part 2
[Note: Part 1 of this article presents basics of air compressors. Part 2 presents the classification and the construction details of air compressors. Part 3 presents a brief about the sizing of air compressors.]
Classification of compressors
In a positive displacement compressor, air is confined within an enclosed space where it is compressed by decreasing its volume. In a dynamic displacement compressor, air is accelerated by the rapidly rotating elements of the compressor, causing a relatively small increase in pressure and a large increase in velocity.
Compressors can also be classified according to the specific design of the element used to create the flow of air. That is, reciprocating type or rotary type.
The term single-stage compressor indicates that an increase in pressure takes place only in a single cylinder. In a multi-stage compressor, the exhaust of one cylinder feeds the in-stroke of another in order to obtain higher outlet pressures. The multi-stage compressor is usually provided with an inter-cooler to remove the heat of compression.
Reciprocating piston compressor
The figure shows the basic single-cylinder reciprocating compressor. As the piston moves down during the inlet stroke, the inlet valve opens and draws air into the cylinder. During the upward motion of the piston, air is compressed and discharged through the opened outlet valve. Piston compressors have relatively complex design with many moving parts.
In piston compressors, there is a likelihood of introducing small amounts of lubricating oil from the piston walls into the compressed air. This very small contamination is unwanted in food, pharmaceutical, and chemical industries. For such applications, diaphragm compressors may be used as a power source. Here a flexible diaphragm partitions the compressor chamber and the actuating piston. This feature allows the lubricating oil to be excluded from the compressed air supply. Diaphragm compressors are limited in output and pressure and they are used most often for light-duty applications.
A screw compressor consists of two helically grooved screws meshing with each other. The design of the screws makes them possible to move air from the inlet to the outlet of the compressor. Compression is achieved by pushing the trapped air into a progressively smaller volume as the screws move ahead. Since there are no surfaces that actually make contact with one another, this type of compressor does not necessitate cooling and is characterised by the low noise level and small loss of efficiency. They have the benefit of simplicity with fewer moving parts rotating at a constant speed and of a steady delivery of compressed air without pressure fluctuations.
Depending upon the working requirements, compressors are driven either by electrical motors or by internal combustion engines. In factories, large compressors are mostly driven by three-phase induction motors. Compressors are driven either by constant-speed drives or by variable-speed drives. Typically, power is transmitted through V-Belt, gear or direct drive configuration. Belts and coupling must be properly shielded for safety.
Pressure Regulation in Compressors
In order to match the delivery volume of the compressor to the fluctuating air consumption and as a safety measure, it is necessary to regulate the pressure developed by the compressor. The pressure is usually regulated by the on-off regulation method. In this method, the drive motor has two operating positions (i.e., ON or OFF). The switching limits can be set on the regulator. The drive motor is switched off upon reaching the preset maximum pressure, Pmax. The motor is switched on again, when the pressure has dropped to a preset minimum pressure, Pmin. A large gap between Pmax and Pmin and a large compressed air receiver are necessary to reduce the switching frequency of the motor.
Air Compressor Packaged Units
An air compressor packaged unit is fully assembled air compressor system, complete with an air compressor, electric motor, air receiver, belt/gear/direct drive, and automatic controls. Optional equipment includes air receiver, dryer, after-coolers, automatic moisture drain, low oil safety control, magnetic starter, water separator and particulate filters.
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