A rotary air compressor is a type of positive displacement compressor that operates by trapping air between rotating lobes or screws and reducing the volume of the trapped air to increase its pressure. In this article, I will discuss the types and the working of Rotary Air compressors in vehicles.

Rotary Air Compressor

Rotary air compressors are used in a wide range of industries, including manufacturing, automotive, construction, and pharmaceuticals. They are employed for tasks such as powering pneumatic tools, providing compressed air for industrial processes, and supporting various applications in production lines.

These compressors consist of two intermeshing helical rotors (screws) within a casing. As the rotors turn, air is drawn in and compressed by the decreasing volume between the rotors. They are known for their continuous, quiet operation and are widely used in various industries.

In reciprocating compressors, the pressure of the air is increased in its cylinder with the help of a moving piston. But in a rotary air compressor, the air is entrapped between two sets. of engaging surface and the pressure of air is increased by squeezing action or backflow of the air.

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Differences Between Reciprocating and Rotary Air Compressor

There are certain differences available in these types of compressors. These differences are-

Sl.No	Reciprocating Air Compressor	Rotary Air Compressor
01.	The speed of air pressure is low.	The speed of air pressure is high.
02.	The air supply is intermittent.	The air supply is continuous.
03.	The lubricating system is complicated.	The lubrication system is simple.
04.	The air delivered is less clean, as it comes in contact with the lubricating oil.	The air delivered is more clean, as it does not come in constant with the lubricating oil.
05.	Isothermal efficiency is used for all sorts of calculations.	Isentropic efficiency is used for all sorts of calculations.
06.	The maximum delivery pressure may be as high as 1000 bar.	The maximum delivery pressure is 10 bar only.
07.	The maximum free air discharge is about 300m/min.	The maximum free air discharge is about 3000m/min
08.	They are suitable for low discharge of air at very high pressure.	They are suitable for large discharges of air at low pressure.
09.	The size of the air compressor is large for the given discharge.	The size of the air compressor i small for the same discharge.
10.	Balancing is a major problem.	There is no balancing problem.

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Types of Rotary Air Compressors

Rotary air compressors are of the following types :

1. Roots blower compressor,
2. Vane blower compressor,
3. Centrifugal blower compressor; and
4. Axial flow compressor.

The first two compressors are popularly known as positive displacement compressors, whereas the last two as non-positive displacement compressors.

#1. Roots Blower Compressor

A roots blower compressor consists of two rotors with lobes rotating in an air-tight casing that has inlet and outlet ports. Its action resembles that of a gear pump. There are many designs of wheels, but they generally have two or three lobes (and sometimes even more). In all cases, their action remains the same.

The lobes are so designed that they provide an air-tight joint at the point of their contact. The mechanical energy is provided to one of the rotors from some external source, while the other is gear-driven from the first. As the rotors rotate, the air at atmospheric pressure, is trapped in the pockets formed between the lobes and casing.

The rotary motion of the lobes delivers the entrapped air into the receiver. Thus, more and more flow of air into the receiver increases its pressure. Finally, the air at a higher pressure is delivered from the receiver.

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#2. Vane Blower Compressor

A vane blower compressor consists of a disc rotating eccentrically in an air-tight casing with an inlet and outlet ports. The disc has several slots (generally 4 to 8) containing vanes. When the rotor rotates the disc, the vanes are pressed against the casing, due to centrifugal force, and form air-tight pockets.

The mechanical energy is provided to the disc from some external source. As the disc rotates, the air is trapped in the pockets formed between the vanes and casing. First of all, the rotary motion of the vanes compresses the air. When the rotating vane uncovers the exit port, some air (under high pressure) flows back into the pocket in the same way as discussed in the case of the roots blower compressor.

Thus, the pressure of air, entrapped in the pocket, is increased first by decreasing the volume and then by the backflow of air. Now, the air is delivered to the receiver by the rotation of the vanes. Finally, the air at a high pressure is delivered from the receiver.

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#3. Centrifugal Blower Compressor

A centrifugal blower compressor consists of a rotor(or impeller) to which several curved vanes are fitted symmetrically. The rotor rotates in an air-tight volute casing with inlet and outlet points. The casing for the compressor is so designed that the kinetic energy of the air is converted into pressure energy before it leaves the casing.

The mechanical energy is provided to the rotor from some external source. As the rotor rotates, it sucks air through its eye, increases its pressure due to centrifugal force, and forces the air to flow over the diffuser. The pressure of air is further increased during its flow over the diffuser.

Finally, air at a high pressure is delivered to the receiver. It will be interesting to know that the air enters the impeller radially and leaves the vanes axially.

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#4. Axial Flow Compressor

An axial flow compressor consists of several rotating blade rows fixed to a rotating drum. The drum rotates inside an air-tight casing to which are fixed stater blade rows. The blades are made of aerofoil sections to reduce the loss caused by turbulence and boundary separation.

The mechanical energy is provided to the rotating shaft, which rotates the drum. The air enters from the left side of the compressor. As the drum rotates, the air flows through the alternately arranged stator and rotor. As the air flows from one set of stators and rotors to another, it gets compressed. Thus, successive compression of the air, in all the sets of stator and rotor, the air is delivered at a high pressure at the outlet point.

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Comparison of Centrifugal and Axial Flow Air Compressors

The differences between these types of Air compressors are-

SL.No.	Centrifugal Compressor	Axial Flow Compressor
01.	The flow of air is perpendicular to the axis of the compressor.	The flow of air is parallel to the axis of the compressor.
02.	It has low manufacturing and running costs.	It has high manufacturing and running costs.
03.	It requires low starting torque.	It requires high starting torque.
04.	It is not suitable for multi-staging.	It is suitable for multi-staging.
05.	It requires a large frontal area for a given rate of flow.	It requires less frontal area for a given rate of flow. It makes the compressor suitable for aircraft.

Conclusion

In summary, rotary air compressors play a vital role in providing a reliable source of compressed air for a diverse range of industrial applications, contributing to the efficiency and productivity of various processes.

Frequently Asked Questions(FAQ’S)

What are the advantages of rotary air compressors?

Rotary air compressors are known for their compact design, continuous operation, and lower maintenance requirements compared to some other types of compressors. They are also suitable for applications where a constant supply of compressed air is needed.

Are rotary air compressors energy-efficient?

Rotary air compressors can be energy-efficient, especially when properly sized for the intended application. Regular maintenance and choosing the right type based on the workload contribute to their overall energy efficiency.

Can rotary air compressors be used for continuous operation?

Yes, rotary air compressors, especially rotary screw compressors, are designed for continuous operation. They provide a reliable and steady source of compressed air, making them suitable for applications requiring uninterrupted performance.