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.
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.
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
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
