AIR COMPRESSORS

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AIR COMPRESSORS

Function

Air compressor increases the pressure and reduces the volume of air. It consumes electrical energy. It increases the pressure and temperature.

 Types of compressors

There are many types of air compressors.

(a) reciprocating, rotary

(b) low-medium and high pressure air compressors

(c) single and multi-cylinder air compressors

(d) air and water cooled compressors

(e) direct drive

(f) belt driven chain driven and gear driven compressors

(g) Rock drill

(h) Pneumatic hand tool

(i) Trench digging

(j) Mine prospecting

(k) Spray painting

(l) Sand blasting compressors

Comparison Criterians

(a) noise level

(b) volumetric displacements

(c) power required

(d) maintenance required

(e) efficiency at full and partial loads.

  1. Reciprocating compressors are single stage and multistage.
  2. Rotary compressors are positive and non positive displacement compressors.
  3. Positive displacement type includes Root blower, Vane type and Screw type compressors.
  4. Non-positive displacement type includes centrifugal axial flow compressor.
  5.  Compression ratio is the ratio of pressure after compression to pressure before compression.
  6. Compressor capacity is volume supplied (m3/min) per minute after compression
  7. Free air delivered is volume sucked/minute for compression
  8. Work done under adiabatic conditions is n(p2v2—p1v1)/(n-1)
  9. Work done under iso-thermal conditions W = p2v2lnV1/V2
  10. . Work input is least under iso-thermal compression. Therefore cooling is must during compression process.
  11. In a two stage compressor, efficiency will be maximum under perfect cooling. The intermediate pressure is geometric mean of inlet and final pressures i.e. p2 = (p1p3)0.5
  12. In a two stage compressor with imperfect cooling,                                    p2/p1 =  [(Td/T1)(n-1)/n(p3/p1)] 0.5
  13. Heat rejected per stage per kg of air in a reciprocating compressor with perfect inter-cooling is                                                                                    Heat rejected  =        [Cp +Cv [(γ-n) /(n-1)] (t2-t1)
  14. Various efficiencies of a compressor are as follows:

(a)Iso-thermal efficiency

ηiso-th = Work done under iso-thermal/Actual work done

(b) Adiabatic efficiency

ηadb = Work done during adiabatic process/Actual work done

(c)  Mechanical efficiency

ηmech = Theoretical power required/Actual power required

(d) Overall efficiency

overall = Air power available/power supplied by the prime mover

(e) The clearance volumetric efficiency

ηcl.vol = Volume actual sucked for compression/Stroke volume

15. The volumetric efficiency is greatly reduced by the increase of clearance volume.

16. Multistage compression reduces work input as compared to single stage compression

17. Multistage compression improves clearance volumetric efficiency.

18. Multi-stage compression provide better mechanical balance and improved cooling during compression.

19. Multistage compression reduces leakage of compressed air.

20. There is less difficulty in selecting lubricants for the number of compressors in the multi-stage compression.

21. Selection of compressor depends on pressure required and compressed air flow per minute required.

(a) Reciprocating compressors are suitable for pressure difference greater than 3.5 atm and low volumetric flow requirements.

(b)Rotary compressors are suitable for pressure difference greater than 1.5 and less 3.5 atm and medium volumetric flow requirements.

(c) Centrifugal compressors are suitable for pressure difference less than 1.5 atm per stage  and very high volumetric requirements..