Air Compressor

Air Compressor | Plant Technical Notes 

Air Compressor blog contains detailed study about Industrial Air Compressor related details and Interview questions

Today’s Blog – Industrial Air Compressor, Types, Uses

Air Compressor | Plant Technical Notes

Air compressor is equipment used in industry which provides compressed dry air which used to operate Valves which are operated pneumatically, Agitation purpose air, cleaning purpose. Today we will learn about Air compressor related definitions, types, dryer operation, working, applications, and drawbacks in easy terms.

We will also practice interview questions related to Air compressor which are asked in chemical industries.

Blog Content

• Air compressor types
• Positive displacement compressors

1. Single acting reciprocating air compressor
2. Double acting reciprocating air compressor
3. Diaphragm reciprocating air compressor 

• Rotary Compressors

1. Liquid ring compressor
2. Screw compressor
3. Scroll compressor
4. Lobe compressor

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Air compressor Interview Questions

How much types of air compressor used in industry?

Which are main parts of reciprocating air compressor?
What is dew point& Why it is important in instrument air?

1. Air compressor types

Air compressor is classified as below

1. Compression mechanism – Check detailed image below.

2. Number of stages – Single acting, Double acting, and multiple stages.

3. Type of cooling – Air cooling, Water cooling and oil cooling.

4. Types of drive used – Motor drive, Turbine drive, and Engine drive.

5. Lubricating type – Splash lubricated or forced lubricated, Oil free compressor.

6. Discharge pressure – High, Medium, and Low.

You can check general classification of air compressor by compression mechanism 

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Positive displacement compressors

There are three types of Positive displacement compressors available

1. Single acting reciprocating air compressor
2. Double acting reciprocating air compressor
3. Diaphragm reciprocating air compressor 

Single acting reciprocating air compressor

Single acting air compressor’s piston is working in one direction. Another part is free or open which is not perform any work

In cylinder air compressor takes place and another side is only crank is there. Crank has only work to push Piston

Parts of Single acting reciprocating air compressor

1. Suction NRV
2. Discharge NRV
3. Crank
4. Connecting rod
5. Piston
6. Cylinder

Working of Single acting reciprocating air compressor 

Step 1

Piston goes downward

Cylinder creates Vacuum

Suction NRV [Non return valve] open

Air suck through suction port into cylinder at that time

Discharge NRV is in closed condition

Cylinder Status –

Volume is High

Pressure is low

Step 2

Piston goes upward

Piston movement pressurize cylinder

Discharge NRV is opening due to pressurize

At that time Suction NRV will be closed

Cylinder Status –

Volume is Low

Pressure is High

Complete Cycle layout

Air Compressor - Working of single acting reciprocating compressor

Advantages

Small in size so installed in smaller area near to utility area

Air and water cooling system is available

Simple in design so simple maintenance procedure

Disadvantages

Requires ‘Air receiver’ due to non-continue air supply

High cost of compression due to less efficiency.
Liquid cool system requires frequent cooling jacket cleaning or strainer cleaning.
 NRV – Non return valves are stuck up or chocking problem

Operating efficiency: 22 to 24 kW / 100 cfm

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Double acting reciprocating air compressor

Double acting reciprocating air compressor

This type of compressor having two sets of compression system in single cylinder with single piston. In following image you can see pistons both sides are used. When first system is in compression another side is in suction air.

Double acting reciprocating air compressor gives continuous compressed air.

Advantages:

Compression efficiency is higher

Disadvantages:

This compressor has complex design so high initial purchase cost.  Special skilled persons required.

High space required as compared to single acting reciprocating air compressor

Require high capacity motor required for high flywheel mass to provide torque and current pulsations in motor

High vibrations makes frequent breakdown. High maintenance cost.

Operating efficiency – 15 to 16 kW / cfm.

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Diaphragm Compressor

Diaphragm compressor has diaphragm attached to connecting rod.

Air compressor - Diaphragm compressor

There is also diaphragm with oil chamber operation is available.

Advantages:

1. Service life is 10 times longer than other reciprocating compressor

2. Power consumption is 20% lesser as compared to others.

Disadvantages

1. Diaphragm changing job periodically.

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Rotary Compressors

There are four types of air compressors available

1. Liquid ring compressor
2. Screw compressor
3. Scroll compressor
4. Lobe compressor

Liquid ring compressor

In this type of compressor liquid ring is created for compression operation.

Construction of compressor

There are two types are available

1. Compressor with offset impeller position

2. Compressor with oval circulation ring

1. Compressor with offset impeller position

There is a casing and inside casing impeller is installed inside. Impeller is not in center it is slightly away from center. There are one inlet port and one outlet port.

Liquid ring compressor

Material of construction depends upon compatibility of liquid ring material and gas. When gas is compressed its temperature is increasing so, Liquid temperature is maintained by cooled / chilled exchanger by lines installed to casing or sending it partially through outlet and insert to inlet. Liquid used to create ring is changed periodically.

2. Compressor with oval circulation ring

There is same working method but only difference is – Impeller placed in center and gas compression area is managed by oval casing design.

Easy understand working of it

Liquid ring compressor working 

1. When compressor is started liquid ring is created due to centrifugal force by impeller

2. Gas suction is created because liquid ring is constant while suction area is increasing with impeller rotation.  When impeller is rotating liquid ring is closer to suction port and increasing due to impeller is not in center of liquid ring.

3. When this trapped gas is moved to Discharge port its area is reducing and makes gas pressurize.

In this type of compressor double suction and double discharge ports are available

Advantages:

1. Used for pressure and vacuum services

2.  This compressor is a continuous air pressure developing compressor. Constant & continuous air pressure delivery without pressure flatuations.

3. Simple construction & Less mechanical parts like piston cylinder with jacket for cooling NRV – Non return valves etc.

4. Less vibration & noise.

Disadvantages:

Higher power consumption

Erosion takes place due to continuous circulation of liquid ring

This have built capacity of 7 to 12000 cfm.

Read more on Wikipedia – Click here

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Vane type compressor

Construction

There is a circular rotating disk which has fitted with adjustable vanes.

Vane type compressor

These vanes are adjustable which can travel in the direction of disk center.

This disk is fit inside of casing in offset position [not in center]. Disk inside casing is connected with motor.

Casing has gas inlet port and gas outlet port for gas movement.

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Vane compressor working

When vane compressor is starting disk rotation started.

Compressor suction is created because initially vanes are pressed due to lower space between casing and vanes.

Vane compressor working

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During half of the rotation area of gas is increasing and gas sucked & trapped inside.

When gas is travelling towards outlet port its area is reducing. This area reduction compressing air and build pressure

Advantages

1. Less vibrations and noise

Disadvantages

1. Vane compressors best efficiency is 50% and generally it is 30 %.

2. Multiple rotating parts causes’ frequent breakdown as compared to liquid ring compressor.

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Screw compressor

Screw compressor is a positive displacement compressor.

Construction

• There are two screws are inside the casing which are fitted to work intermeshing with each other.
•  One screw is driving screw [Male] and another is driven screw [Female].
• Driving screw is connected to motor.
• Both screws are mount on timing gears which allows adjusting timing of rotation.
• For smooth operation of screw compressor oil is feed to inlet and it discharged to outlet port.
• Space between screws are called flute

Working

Screw compressors both screw inside the casing screws are rotating in opposite direction. When screw rotation starts near suction port gas is trapped inside spiral teeth of screw. Trapped gas is travelling to forward because screws are steady.

Screw compressor is efficient in low air requirement

Advantages

Gas compression is continuous so no need of air receiver.

There are no requirements of NRV – Non return valve.

Less maintenance cost & minimum breakdown due to only screws are rotational parts.

Lower vibration and noise

Disadvantages

Screw compressor is giving best efficiency on low pressure range.

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Scroll Compressor

Construction

Scroll compressor has different design. It is like two papers folded loosely with one another. One scroll stationary [Fixed] and another is rotating [free]. 

Working

When rotating scroll is moving air is trapped and passing through smaller chambers which increase pressure. When air pressure is increased it outlet from fixed scroll outlet.

Advantages

There is no NRV required in inlet and outlet.

Disadvantages

Dynamic compressor [ Non positive displacement compressor

Thanks for reading - Air Compressor
Naitik Patel
Industrial Guide