A compressor is a mechanical device that increases the pressure of a gas by decreasing its volume. There are different types of compressors classified by the pressure they can achieve and method of compression. Positive displacement compressors, like reciprocating compressors, work by trapping gas in a space and reducing the volume to increase pressure before releasing it. Rotary compressors use impellers or rotors to continuously compress gas. Compressors have various applications but also limitations related to temperature increases during compression.

1. COMPRESSORS

2. WHAT IS A COMPRESSOR?
• It is a mechanical device that increase the pressure of a gas by decreasing it’s
volume.
• They are similar to pumps but since pumps only transport fluid through pressure
increase; compressors reduce the gas volume.
• It basically converts mechanical energy into gas energy.
• PURPOSE : to move gas from place to place.
• TYPES OF COMPRESSION DEVICES :
• COMPRESSORS – Moves air/ gas from 35 psi to 65000 psi
• BLOWERS – moves air/gas up to 50 psi
• FANS - maximum 1psi

3. METHODS OF COMPRESSION
• 4 ways to compress a gas :-
• Trap the gas; reduce it’s volume and then push the compressed gas out of the jail !
• Do the above things but don’t reduce it’s volume. Compress it by backflow i.e. flow
which comes through the discharge system and then push it out of the jail !
• Compress the gas by IMPELLERS !!(also known as pressure risers) through circular
motion
• Increase the speed of the gas and finally convert this high velocity into pressure.
• FIRST TWO METHODS GIVE YOU POSITIVE DISPLACEMENT COMPRESSORS WHILE
THIRD GIVE YOU ROTARY COMPRESSORS
• FOURTH METHOD GIVES YOU – ejectors!

4. SOME BORING TERMS IN COMPRESSOR
• Vanes : Vanes were originally used by mankind to make arrows. They are used so
that the arrow doesn’t miss it’s target ! Thus, it was used as a ‘stabilizer’ for arrows.
Same is the purpose of vanes in a compressor. Think of the air as an arrow. The
vanes help in guiding the air properly so that it doesn’t miss it’s target.

5. CLASSIFICATION
Compressors
Positive displacement
Reciprocating
Diaphragm Double acting
Single Acting
Rotary
Lobe Screw
Liquid Ring Scroll
Vane
Dynamic
Centrifugal Axial

6. POSITIVE DISPLACEMENT
• It is a system which compresses the air by means of displacement of a mechanical
linkage thereby reducing it’s volume.
• Displacement of a compressor is the volume swept through the first-stage cylinder;
usually expressed in cubic feet per minute.

7. REAL BORING TERMS IN + DISP COMPRESSOR!
• Actual capacity : capacity during intake. Think of it as the capacity your lungs have to take in
oxygen.
• Volumetric efficiency = actual capacity / displacement
• Compression efficiency = theoretical horsepower / actual indicated horsepower
• Mechanical efficiency = indicated power / brake power
• Overall efficiency = mechanical + compression efficiency
• Frame load = the force which the compressor frame and the running gear( they are more
easily known as the connecting rods, bolts, crossheads and all that stuff)
• Compression ratio = discharge pressure / inlet pressure ( inlet pressure = 14.696 psi)
• Piston displacement = net volume displaced by the piston at the rated speed

8. SOME GENERAL KNOWLEDGE
• Until now, many people are confused with the term vacuum. Most of them think
that there is nothing in a vacuum and thus vacuum means nothing.
• However, you might be surprised to know that vacuum is basically a form of
pressure. Although it is expected from engineering students to know this; if this is
new to you don’t worry! At least you know it know!
• Vacuum = pressure way below atmospheric pressure!
• I am sure if you view vacuum as a form of pressure; you will view this as something
that solves all your confusion when you see ‘vacuum’ term during further study !!

9. RECIPROCATING COMPRESSORS
• Use pistons driven by a crankshaft.
• Gases which are compressed :
• 1. AIR
• 2. HYDROGEN, OXYGEN
• 3. LIGHT HC FUELS
• 4. VARIOUS STORAGE GASES
• They are also known as intermittent flow compressors
• The manipulation of clearance volume is a major compressor performance parameter.
• They are more generally used as ‘constant volume variable pressure’ machines today.

10. RECIPROCATING COMPRESSOR
COOLING
• Water cooled – By using water jackets to cool them.
• Air cooled – natural cooled

11. REVIEW OF + DISP. COMPRESSORS
• Compressors – machines designed for compressing air from an initial intake to a higher
discharge pressure
• Vacuum pumps – compressing gas from a below atmospheric pressure to a final pressure
near atmospheric
• Reciprocating compressors – each compressing element consists of a piston moving back
and forth in a cylinder.
• Single-acting – compression takes place on one side of a piston; away from the crankshaft.
• Double-acting – compression on both sides of the piston
• Single stage – compression from initial to final pressure takes place in a single step or stage
• Multi-stage – two or more distinct stages

12. COMPRESSOR WORKING

13. COMPRESSOR LIMITATIONS
• Being in accordance with Boyle’s law, compressing a gas by means of reducing it’s
volume also results in increasing it’s temperature according to Gay Lussac’s law.
• This unfortunately; brings a halt to the compressor’s wonderful applications.
• Some of these major limitations are :-
• Discharge pressure
• Pressure rise (also known as differential)
• Compression ratio
• Effect of clearance
• Desire to save power..!

14. MULTISTAGING
• Multi-staging is useful for volumetric efficiency.
• For reciprocating compressors; multi-staging is used :
• To save power!
• To limit gas discharge temperature
• To limit pressure differential

15. ROTARY COMPRESSORS
• Gas compressor that uses positive displacement rotary compressors.
• Used where LARGE volumes of high pressure is needed; generally for industrial
purposes.
• It is a continuous flow type compressor
• It consists of two meshes, the male mesh with five teeth and female with six teeth.

16. ROOTS BLOWER
• Positive displacement lobe pump operates by pumping a fluid with a pair of
meshing lobes not unlike a set of stretched gears.
• Consists of three processes – Intake; pumping and
forced air