Compressors are the most prominent devices used in industries to increase the pressure of gases. The screw compressor is a type of positive displacement compressor which achieves this with a pair of rotors. The gases are taken inside the compressor using suction valves. It is made to flow through the rotor and finally discharged through the exit valve with increased pressure.
This article on screw compressors give a detailed analysis of its construction, working principle, classification, advantages, disadvantages as well as its applications.
Construction of a Screw Compressor.
The main parts of a screw compressor are as follows;
- A pair of Rotors
A screw compressor consists of a pair of rotors placed inside a cylindrical chamber. The compression process takes place inside this chamber. Based on their construction, rotors can be either male or female. The male rotor has lobes, whereas the female rotor contains flutes or gullies. The male rotor usually has four lobes, and the female has six.
The male rotor forms the driving head. On the other hand female rotor is the driven one.
The rotors are helically grooved along its length. It gives them a screw-like appearance when viewed from sideways. This peculiar appearance gives them the name – Screw Compressor.
2) Compression Cylinder
The compression process takes place inside the compression cylinder. The rotor and other components for compression are mounted inside the cylinder. Like an axial flow compressor, a screw compressor can also be made multi-staged by incorporating multiple cylinders. Each cylinder will have its own rotor assembly.
The compression cylinder also support the filters and the oil lubrication systems.
The compressor draws air from the atmosphere, and it may contain several impurities. The presence of these particulates will reduce the overall efficiency of the system. When these substances accumulate on the rotors or other moving components, the machine will choke and may even stop working. To avoid these extreme cases, we use air filters to purify the intake air. Air filters ensure easy and efficient working of the compressor machine.
In the case of oil-injected air compressors, rotor and other equipments for compression are oil bathed. It facilitates proper lubrication and cooling. Oil filters are used in these types of compressors to remove oil from the pressurized air.
Valves are used for the intake of raw air and to expel the pressurized air from the machine. The suction valves at the top of the compressor, suck in air from the tank or surroundings. In the case of a screw compressor, the process is continuous, and hence the device draws a definite amount of air throughout.
Apart from the suction valves, it also has discharge valves. Its function is to release the pressurized fluid from the compression chamber.
5) Storage Tank:
Tanks are present in compressors to temporary store the pressurized fluid until it is delivered to the supply.
Working of a Screw Air Compressor
During the suction process, the suction valve opens and takes in the air from the atmosphere.
The air then moves through the gap between the male and female rotors. The rotors push the air through narrow gaps formed during the controlled meshing and unmeshing process.
The volume of the compression chamber goes on decreasing as the air reaches the end of the rotors. This reduction in volume causes the pressure of the system to rise.
The moving rotors cause the axial flow of air, and the reduction in volume causes the gas to compress.
The airflow has radial and axial components. In effect, the air moves in an oblique path within the rotor.
The discharge of the compressed air takes place through the discharge valve
Classification of Screw Air compressors
The screw air compressors are of two types, on the basis of lubrication and cooling;
1) Oil Injected Screw compressors:
In these compressors, oil completely surrounds the rotor. Though it ensures proper lubrication and dissipates heat easily, it cannot be used in areas where oil-free air is required.
Oil Injected screw compressors use oil filters to separate the oil from the air.
2) Oil-Free Screw Compressors:
These compressors restrain from using oil for lubrication and cooling. They are well suited in low-pressure applications and in areas where high purity air is required.
But since there is no oil for cooling, intercoolers should be used to avoid excess heating of air. Also, the compression process can only be done in stages. For example, if we need to increase the pressure of a fluid from 2 bar to 7 bar, then the process is done in two stages. In the first stage, the pressure is increased to 5 bar, followed by a cooling process. After this cooling process, the compression continues, and finally reaches 7 bar through a gradual increase.
Based on the number of screw rotors, the screw compressors can also be classified as follows;
- Twin Screw Compressor
Twin-screw compressors or more commonly called as the screw compressors, contains two rotating screws. Based on the drive and geometry, these rotors screws can be either male or female. The male has lobes and a separate motor drives these rotors. The female rotor, on the other hand, has flutes and are driven by the male rotors.
- Single Screw Compressors
The single screw compressors make use of a single helical screw for compressing the air. Two gate rotors surround the helical screw. The sequential mesh and unmesh of the screw-gate rotor arrangement aids in the compression of air.
Advantages of Screw Compressor
- Convenient and continuous service; The compression process by screw compressors is continuous. The intake is as per the required supply. These compressors can ensure a regular supply of pressurized air.
- Quiet Devices; The compressors make no noise while functioning. This is in stark contrast to other compressors that makes a thud sound due to the action of the piston for the compression process. The quiet feature of these devices also helps workshop technicians to concentrate on other critical devices in their laboratory.
- Compatibility; These compressors work well in all type of weather conditions. It makes them an easy pick in harsh climatic regions.
- Screw compressors consume less oil compared to other oil-flooded air compressors. As a result, the air at the exit is free from excess oil and does not require elaborate filtering processes.
- For a reciprocating compressor of the same size, screw compressors can deliver more compressed air. Also, it can operate at a 100% duty cycle, ensuring optimum performance.
Disadvantages of Screw Compressors
- Cost; The initial cost of construction and installation of this device can be high. Often, the cost is far more than that of reciprocating compressors.
- Maintenance is a critical factor in the case of screw compressors. Oil and the filters need to be constantly cleaned and replaced for optimum performance.
- When it comes to low-capacity operations such as those below 500 liters, reciprocating (piston) compressors are advantageous over the other due to low initial cost and low maintenance cost.
Applications of Screw Compressor
- Textile and clothing industry
- Metallurgy and Mining field
- Fuel gas compression
- Vapour recovery compression systems