12 Types Of Industrial Compressors And Their Working Principle
1. Introduction
A compressor is a device that compresses a gas or air to a pressure higher than atmospheric pressure. This raises the temperature of the gas, its specific mass (absolute Density), its volumetric flow QV decreases. In contrast, its mass flow QM stays constant (equality of mass per unit of time through the compressor.
If the pressure is lower than the atmospheric pressure, the instrument is called a vacuum pump.
If the compression ratio is 1.25 or lower, then it is called a fan.
If the compression ratio is 1.25-2, then the equipment is known as a blower
Above 2.0 compression ratio equipment is called a compressor.
In this article we will study in detail about types of compressors and their working principle.
2. Types Of Compressors
There are two main types of compressors used in industry.
- Positive Displacement compressors
2. Dynamic compressors
3. Positive Displacement Compressors
positive displacement compressors are such compressors in which a specific amount of fluid ( Air or gas ) enters in chamber. And work is done done to reduce the volume of gas . As a result gas is compressed. following are the types of Positive displacement compressors.
- Reciprocating compressor
2. Roots Compressor
3. Vane Compressor
4. Screw Compressor
5. Mono-screw Compressor
6. Scroll Compressor
7. Liquid Ring Compressor
3.1. Reciprocating Compressor
A reciprocating compressor is shown in Fig below like in reciprocating pump crank connecting rod mechanism is used. The valves are freely floating. This means that they open by a pressure difference over the valve (not by cam like fuel motors). Springs press the valves on their seat and help to close them.
Working Principle:
A horizontal piston compressor is shown, together with its indicator diagram. Vertically the pressure in the cylinder is set out horizontally, the volume in the cylinder, that is taken by the gas that has to be compressed. The task is to compress gas from inlet pressure p1 to an outlet pressure p2 (mostly P2 is the pressure of a press reservoir).
A piston is in the “dead” point on the left side. Both valves, suction and discharge, are closed. The “dead” volume V0 is filled with gas on end compression pressure p2. The piston moves to the right so that the rest gas of the precious cycle is expanded in the cylinder. The pressure in the cylinder decreases. See a → b. When the pressure in the cylinder in state b becomes lower than the suction pressure p1, the suction valve opens. The gas is sucked isobar. The state of the gas does not change. Only the mass in the cylinder increases. The piston ends in the dead point on the right side. At state c, the movement of the piston reverses. The piston exercises and overpressure on the gas. The curve c d gives the compression.
Reciprocating compressors are consist of following parts.
- Cylinder
2. Piston
3. Connecting Rod
4. Crankshaft
5. Seals
6. Valves
7. Crossheads
3.2. Roots Compressor
The roots compressor is shown in the figure below is composed of two rotors in the form of a figure of eight that rotate in opposite directions in a housing in the form of a flattened cylinder.
Working Principle
While rotating, at first, a gas is aspired out of the suction line, then locked up pin in the space between the rotor and housing and then transported to the discharge side where it is displaced. During the transporting in the compressor, the gas is not compressed. There is no internal compression like this, as is the case with a piston compressor and many other displacement compressors.
When the rotor head rotates along the edge of the discharge port, compressed gas flows back from the discharge pipe. Then the transported gas is displaced against the discharge pressure to the discharge side. The operation principle of the roots compressor is essentially different from that of the piston compressor because there is no internal compression, and there are no valves.
3.3. Vane Compressor
The vane compressor (Fig) consists of a rotor of radial slots where vanes can move in and out. The centrifugal force pushes the vanes against the housing. And the housing can push the vanes back in their slots. Because of the eccentric mounting of the rotor in the pump housing, chambers are created during rotation that can become bigger and smaller. Like a roots compressor, this compressor has no valves but ports. But the vanes compressor has an internal compression.
Working Principle
When the inlet side chamber I (Figure) with volume VI presents itself, it is filled with gas on suction pressure p1. The gas is then locked up in the rotor in the space limited by the rotor, pump housing, vanes, and cover. This space decreases so that the gas is compressed.
Finally, the gas is confined in the smallest chamber II with volume V11. The pressure that the gas has at that moment is called the design pressure p2.
3.4. Screw Compressor
The screw compressor consists of two screw-shaped rotors with different profiles that can grip one another (Figure). They rotate in opposite directions. They can have different numbers of lobes. The main rotor (male rotor) has four lobes, while the female rotor or side rotor has six. When the rotors do not have the same number of lobes, they must be driven differently. The flow through the compressor is axial.
Working Principle
The operation mode is shown in the Figures. First, the grooves or recesses in both rotors are filled with gas from the suction line because these chambers present themselves before the suction port. By rotating, the chambers become bigger. The gas cannot leak to the outlet port situated at the other side of the cylinder block because the screw blocks the outlet port at the time of compression.
When the rotors rotate, the female rotor is first locked from the inlet port, and the male rotor gets free and is filled with gas. Then these couple of chambers will be completely locked off from the inlet port. In the chambers, the gas is now at inlet pressure. The male lobe grips then into the female recess, leading to the chamber becoming smaller, and an internal compression of the gas occurs.
When the male rotor has travelled a whole rotation, then the holes of the profiled ends of the rotors present themselves before the discharge port and the compressed gas is displaced. After that, the same thing happens with the other chambers.
3.5. Mono-Screw Compressor
A mono-screw compressor shown in Figure. Uses one screw gripped by two gears. A motor drives the screw. The gears are of metal or composite material. The gas is locked up at the inlet side in the groove of the screw and then axially transported to the outlet side. During the compression, there is a volume reduction like with the classic two-rotor screw compressor.
Working Principle
Inlet
Atmospheric gas flows in the compression element and fills the grooves of the main rotor. The rotors grip in the grooves and form two compression chambers at the top and bottom of the main rotor
Compression
The rotors follow the main rotor automatically. The volume in the rotor decreases, and the gas is compressed. Purified water is injected into the compression element and takes care of the sealing and lubrication.
Outlet
The mix of water and gas is evacuated by the compression element and flows to a water separator. Because of the low-end temperature of the gas, and is not needed.
3.6. Scroll Compressor
Another displacement compressor that is used mainly for small capacities is the scroll compressor. It consists of a fixed spiral. Another spiral, the rotor, makes an eccentric movement in the stationary spiral.
The figure shows the operation for compressed air applications. Scroll compressors are often used for refrigeration applications and heat pumps. The scroll compressor is oil-free because there is no Metal-on-metal contact between the compression spirals. Furthermore, it is driven directly instead of a gearbox drive, and so the compressed gas is absolutely oil-free.
The delivered pressure attains 10 [bar(g)]. The flow in FAD (free air delivery) is as high as 70 [m3/h].
3.7. Liquid Ring Compressor
A rotor with vanes is eccentrically positioned in a cylindrical housing. When the housing is partially filled with liquid, and the wheel rotates quickly, a liquid ring caused by centrifugal force will be formed. Between the inner surface of the liquid ring and the outer surface of the wheel’s hub, chambers appear that change size cyclically: on the suction side, they become more significant. On the discharge side, they become smaller so that an internal compression takes place. The inlet and outlet lines are axial; there are no valves.
Because the compressed gas is in very tight contact with the liquid ring, which has a great heat storage capacity, the temperature rise in the gas is very small. The gas is compressed nearly isothermally. These compressors are used in processes where the temperature rise must be limited.
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4. Dynamic Compressors
There are generally two different types of dynamic compressors.
Centrifugal compressors
Axial Flow compressors
4.1. Centrifugal Compressors
The operational principle of a centrifugal compressor (aka a radial turbofan) accords with that of a centrifugal pump, but the fluid is different. Now it’s a gas. It is built up out of an impeller with vanes that rotates fast in an abutting volute (Figures). The rotating movement of the impeller produces a pressure rise on the locked-in gas; the gas is propelled out of the impeller and caught in the volute. An under-pressure arises in the inlet, and this causes the gas to be sucked into the suction line.
4.2. Multi Stage Compressors
When higher pressures are wanted, multiple impellers are placed on one shaft. Such a multistage compressor (Figures) has 6–10 stages and a pressure rise of 6–10 bar, but some implications go as high as 30 bar.
The operating speeds by turbomachines are very high compared with other types of compressors. For airplane applications, where weight is of paramount importance, turbo compressors have speeds of 50–100 000 [rpm]. Commercial machines work at 20 000 [rpm] or lower. Small airplanes or helicopter motors limit the number of stages mostly to two.
Centrifugal compressors are used for very large refrigeration installations cooling Power from 350 [W] to 3.5 [kW] per compressor.
4.3. Axial Flow Compressors
The operating principle of axial compressor accords to that of an axial pump, apart from fluid use (Figure). The fan is applied for high volumetric flows and low pressures. The achievable pressure ratio is 1.05 to 1.8. The regulation of the pressure occurs mainly by variable voltage, often in multiple steps.
If higher pressures are desired, multiple stages are to be placed on one shaft. After each rotor comes to a stator (guide wheel). The job of the rotor is to decrease the relative velocity of the gas to increase the pressure. The rotor is to reduce the relative velocity of the gas to increase the pressure.
