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COMPRESSORS and VACUUM PUMPS














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COMPRESSORS AND VACUUM PUMPS

1                 Fans are used to raise the pressure about 3% (12 in. water), blowers raise to less than 40 psig, and compressors to higher pressures, although the blower range commonly is included in the compressor range.

2                 Vacuum pumps: reciprocating piston type decrease the pressure to 1 Torr; rotary piston down to 0.001 Torr, two-lobe rotary down to 0.0001 Torr; steam jet ejectors, one stage down to 100 Torr, three stage down to 1 Torr, five stage down to 0.05 Torr.

3                 A three-stage ejector needs 100 lb steam/lb air to maintain a pressure of 1 Torr.

                    4. In-leakage of air to evacuated equipment depends on the absolute pressure, Torr, and the volume of the equipment, V cuft, according to w = kV2/3 lb/hr, with k = 0.2 when P is more than 90 Torr, 0.08 between 3 and 20 Torr, and

                    0.025 at less than 1 Torr.

4                 Theoretical adiabatic horsepower (THP) = [(SCFM)T1/8130a] [(P2/P1)a-1], where Tl is inlet temperature in F+460 and a = (k -1)/k, k = Cp/Cv.

5                 Outlet temperature T2= T1(P2/P1)a.

6                 To compress air from 100 F, k = 1.4, compression ratio = 3, theoretical power required = 62 HP/million cuft/day, outlet temperature 306F.

7                 Exit temperature should not exceed 350-400F; for diatomic gases (Cp/Cv = 1.4) this corresponds to a compression ratio of about 4.

8                 Compression ratio should be about the same in each stage of a multistage unit, ratio = (Pn/P1)1/n, with n stages.

9                 Efficiencies of reciprocating compressors: 65% at compression ratio of 1.5, 75% at 2.0, and 80-85% at 3-6.

10             Efficiencies of large centrifugal compressors, 6000-100,000 ACFM at suction, are 76-78%.

11             Rotary compressors have efficiencies of 70%, except liquid liner type which have 50%.
















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