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 306°F. 7
Exit temperature should not exceed 350-400°F; 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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