Compressors - Part 1
- 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
- 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