Rules Of Thumb : Compressors And Vacuum Pumps

Although experienced engineers know where to find information and how to make accurate computations, they also keep a mini mum body of information readily available, made largely of shortcuts and rules of thumb. This compilation is such a body of information from the material in this book and is, in a sense, a digest of the book.

Rules of thumb, also known as heuristics, are statements of known facts. The word heuristics is derived from Greek, to discover or to invent, so these rules are known or discovered through use and practice but may not be able to be theoretically proven. In practice, they work and are most safely applied by engineers who are familiar with the topics. Such rules are of value for approximate design and preliminary cost estimation, and should provide even the inexperienced engineer with perspective and whereby the reasonableness of detailed and computer-aided design can be appraised quickly, especially on short notice, such as a conference.

Everyday activities are frequently governed by rules of thumb. They serve us when we wish to take a course of action but we may not be in a position to find the best course of action. Much more can be stated in adequate fashion about some topics than others, which accounts, in part, for the spottiness of the present coverage. Also, the spottiness is due to the ignorance and oversights on the part of the authors. Therefore, every engineer undoubtedly will supplement or modify this material (Walas, 1988).

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 1⁄4 kV 2=3 lb/hr, with k 1⁄4 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) 1⁄4 [(SCFM)T1 /8130a] [(P2 =P1 Þa À 1], where T1 is inlet temperature in 8F þ 460 and a 1⁄4 (k À 1)=k,k 1⁄4 Cp =Cv .
6. Outlet temperature T2 1⁄4 T1 (P2 =P1 )a
7. To compress air from 1008F, k 1⁄4 1:4, compression ratio 1⁄4 3 theoretical power required 1⁄4 62 HP/million cuft/day, outlet temperature 3068F.
8. Exit temperature should not exceed 350–4008F; for diatomic gases (Cp =Cv 1⁄4 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 1⁄4 (Pn =P1 )1=n , with n stages.
10. Efficiencies of fans vary from 60–80% and efficiencies of blowers are in the range of 70–85%.
11. Efficiencies of reciprocating compressors: 65–70% at compression ratio of 1.5, 75–80% at 2.0, and 80–85% at 3–6.
12. Efficiencies of large centrifugal compressors, 6000–100,000 ACFM at suction, are 76–78%.
13. Rotary compressors have efficiencies of 70–78%, except liquid liner type which have 50%.
14. Axial flow compressor efficiencies are in the range of 81–83%.