Types and working principle of Air Compressors: Rotary, Screw, Vane, Lobe

What is the main purpose of a compressor?

A reciprocating compressor is employed for compressing a small volume of air or gas to a relatively high-pressure ratio. But there is the presence of reciprocating parts lowers its mechanical efficiency, balancing problems and increases maintenance expenses. Therefore, whenever a large quantity of air is required to be delivered up to a pressure of 10 bar with continuous flow, the rotary compressor is used.

compressor



Classification of rotary compressor:

The rotary compressor may be classified as shown below

Rotary compressor :
(i)positive displacement rotary compressor
(a)Roots blower
(b)Vane type blower
(c)Screw compressor
(d)Scroll compressor

(ii)Roto dynamic rotary compressor
(a)Radial flow compressor(centrifugal compressor)
(b)Axial flow compressor
Classification of rotary compressor
Classification of rotary compressor

Positive displacement rotary compressor :


In this compressors, air/gas is compressed by trapping it in a reducing passage formed by a set of engaging surfaces. The gas pulls suction side and push to the delivery side by the help of engaging surfaces.

Rotodynamic rotary compressors :

In this type of compressors, the compression of gas/vapour is carried out by rotating elements imparting velocity to the flowing gas and developed desired pressure and compression is achieved by dynamic action or rotor. In rotodynamic compressors, the gas is not trapped in specific volume but it flows continuously through the compressor.

Roots blowers :

The root blower is as shown in fig consists of two lobes. For higher pressure ratio, three or four lobes may be used. Here the pressure of air delivered is slightly above the atmosphere. The machine consists of a fixed casing which contains two shafts fitted with two-lobe rotors. One of the rotors is driven by electric motor and other is driven through the gears from first rotor. The air is drawn through the inlet pipe due to the rotation of the rotors.

Root Blower
Root Blower


The volume of air is entrapped between one rotor and casing for a very short interval. Due to the rotation of lobes trapped air is carried to the discharge side. Continued rotation of rotors. opens the trapped space to the discharge port. The air is pushed to the receiver due to the continued rotation of the rotor. The detail operation is shown in fig 

Advantages of roots blower :

1. There are no reciprocating parts, only two rotating parts (normally identical in shape and in size). therefore it is very simple in construction.

2. Since rotors are symmetrical about their centre of rotation and hence it is dynamically balanced.

3. No clearance and it is positive displacement type machine.

4. It delivers gas/air continuously.

Vane type compressor or blower :

This type of compressor is shown in fig. It consists of rotor drum mounted eccentrically in the cylindrical casing. The rotor is provided with vanes in the slots. These vanes or blades are made from non-metallic materials usually fibre or carbon. The vanes remain in contact with the wall of casing due to centrifugal force or spring force or both. The vanes can slide in and out in the slots. The volume between two vanes keeps on changing due to the eccentric motion of the rotor.

          


The rotation of rotor causes space between the vanes, the rotor and casing. This space is connected to suction pipe so that air enters into the created space and filled it. A volume V1 of air is trapped between the vanes and casing. With the rotation of rotor, the air gets compressed due to reduction or space toward delivery side. The fluid volume reduces to V2 when it communicates with the delivery side. Since the receiver pressure is higher than the compressed air between the vanes, the backflow of air from the receiver will take place.


It causes further rise in the internally compressed air until the pressure reaches the delivered pressure. Due to rotation of vanes air is delivered to the receiver. In this type of compressor, the total pressure rise is partially due to internal compression between the vanes and partially due to backflow of air from the receiver.

This type of compressor can produce pressure up to 8.5 bar and delivered up to 150 m3/min. This compressor requires less work compared to roots blower. But it requires proper lubrication.

Screw compressor :

The rotary screw compressor is a positive displacement type compressor. It consists of two mating helically grooved rotors suitably housed in a cylinder equipped with appropriate suction and discharge ports as shown in Fig. The rotors are driven through a set of synchronized gears. One rotor (male) has three or four lobe, while another rotor (female) has four or six recesses.

Screw Compressor
Screw Compressor


Gas enters at the suction side and progressively compressed as it moves through the narrowing passageways formed by the lobes. Gas compresses due to reduction of clearances between the rotor which having helical groove. The suction and discharge of screw compressors are neither radials (in case roots blower nor axial but oblique).

The compression is obtained by direct volume reduction with pure rotary motion of rotor, the working cycle of screw compressor has four phases as follows:

(l) Suction: As the lobes of driving rotor begin to unmesh from recess of driven rotor, a void is created on both male side and female side, gas enters through suction (inlet) port. As the rotors continue to turn, interlobe space increase in size and gas flows continuously into space. Just to the point at which the interlobe space leaves suction port, the entire length of the interlobe space is completely filled with gas.

(2) Transportation: A further rotation of the rotor, the interlobe space which filled with gas is transported around the outer periphery without change of pressure until a male lobe begins to mesh with the considered space.

(3) Compression: Further rotation starts the meshing of another male lobe with another female interlobe space on suction end progressively compresses the gas in the direction of the discharge port. Thus the occupied volume of the trapped gas within the interlobe space is decreased and the gas pressure consequently increased.

(4) Discharge: As the space filled with gas is uncovered by passing over the delivery port, the gas is displaced with discharge line due to further meshing of the lobe into the recess. Screw compressor available with large capacities as 3m3/min to 1000m3/min with pressure up to 4 bar in single stage and 8 to 11 bar in two stage compressors.

The speed of this compressor is very high and large unit runs at 3000rpm and small unit run at speeds up to 32000 rpm.

Advantage of screw compressor :

1. It is a positive displacement machine, hence it produces high pressure of gas.

2. It is also high speed rotary compressor, the large volume of gas can be handled by it. Therefore it is extremely suitable for large capacity low temperature applications such as food refrigeration and industrial refrigeration.

(3) No need of lubrication oil singe there is no metal to metal contact in the cylinder. It is for this reason that is not only used in the food industry but also in the chemical industry and transportation of loose materials.

(4) The simplicity of screw compressor is its asset and it has the advantages of both turbomachine and reciprocating compressor without their shortcomings.

The main disadvantage of the screw compressor is the noise due to discontinuous suction and discharge of gas.

Scroll compressor :

The scroll compressor is a positive displacement rotary compressor. The scroll compressor consists of two matching scrolls. One of the two scrolls is fixed and other orbiting scroll is made to orbit but not rotate, around a fixed point on the stationary scroll as shown in Fig. These two scrolls are placed together form a series of space pockets between the two mating spirals.

Scroll Compressor
Scroll Compressor

These spaces are filled with gas when it is working. The suction of gas occurs at the outer edge of the scroll assembly and discharge occurs through the port at the centre of fixed scroll.

Working principle of scroll compressor :

The compression in scroll compressor is taken place by sealing suction gas in pockets of given volume at the outer periphery of the scrolls and progressively decreasing the of these pockets as relative motion of scroll move them inwards towards the discharge port.

The relative movement between the orbiting scroll and fixed scroll causes the gas pockets to move towards the discharge port at the centre of the assembly with continuously decreasing volume and increasing the pressure of the gas.

1. Suction: When the shaft rotates, the orbiting scroll orbits and opens suction port creates space between two scrolls. The gas enters the space between two scrolls through the suction port as shown in Fig. At the end of the first revolution of the shaft, the scroll surfaces again meet.
forming gas pockets as shown in Fig.

2. Compression: Further rotation of shaft (second revolution), the volume of gas pockets is progressively decreased and gas is compressed. At the end of second revolution of shaft produces the maximum compression of gas as shown in Fig.

3. Discharge: During the third revolution of-shaft, the compressed gas discharged through the discharge port as shown in Fig. At the end of the third revolution, the volume of pocket is reduced to zero as shown in fig. All gas is discharged from the scroll.

Advantages of scroll compressor :

(l) The scroll compressors are known for operating more smoothly, quietly and reliably than conventional compressor in some application, especially in refrigeration application.

(2) The compression process occurs over approximately 2 to 2 1/2  rotations of the crankshaft, compared to one rotation for rotary compressors, and one-half rotation for reciprocating compressors. In the scroll compressor, the suction and discharge processes occur for a full rotation.

Therefore the more steady flow and less pulsation, lower sound lower vibration and more efficient flow. Also, there is no reciprocating parts, so less vibration and balancing problem.

3. There is no valve and valve mechanism, hence no loss of throttling.

4.  The isentropic efficiency of the Scroll compressor is slightly higher than that of the reciprocating compressor when the compressor is running at design rating.

5. The scroll compressor has nearly 100 per cent volumetric efficiency because the suction process separates from compression and discharge processes.

6. Scroll compressors have fewer moving parts than reciprocating compressors which improve reliability. Scroll compressors tend to be very compact and smooth running so do not require spring suspension. This allows them to have very small shell enclosures which reduce cost but also result in smaller free volume.

Application of scroll compressor:

The scroll compressors are used for compressing air or refrigerant. Many residential central heat pump and air conditioning systems and a few automotive air conditioning systems employ a scroll compressor instead of the more traditional rotary and reciprocating compressors.

Types and working principle of Air Compressors: Rotary, Screw, Vane, Lobe  Types and working principle of Air Compressors: Rotary, Screw, Vane, Lobe Reviewed by Harish on April 24, 2020 Rating: 5

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