Vaccum Pumps Design Rules of the Thumb:

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 stages down to 1 Torr, and five stages 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.

5. Theoretical adiabatic horsepower (THP)=[(SCFM) T1/8130a] [(P2/P1)^a –1] where T1 is inlet temperature in ºF +460 and a = (k − 1) / k , k = Cp / Cu .
6. Outlet temperature T 2 = T 1(P2 / P1)a .

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

8. Exit temperature should not exceed 350-400˚F; for diatomic gases ( CP / CV =1.4) this corresponds to a compression ratio of about 4.
9. Compression ratio should be about the same in each stage of a multistage unit, ratio = ( (Pn / P1)1/n , with n stages.

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

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

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