Compressors and Vacuum Equipment

A.  The following chart is used to determine what type of compressor is to be used:

B.  Fans should be used to raise pressure about 3% (12 in water), blowers to raise to less than 2.75 barg (40 psig),
and compressors to higher pressures.
C.  The theoretical reversible adiabatic power is estimated by:
Power = m z1 R T1 [({P2 / P2}a-1)]/a
	where:
	T1 is the inlet temperature, R is the gas constant, z1 is the compressibility, m is the molar flow rate,
	a = (k-1)/k , and k = Cp/Cv
D.  The outlet for the adiabatic reversible flow, T2 = T1(P2/P1)a
E.  Exit temperatures should not exceed 204 0C (400 0F).
F.  For diatomic gases (Cp/Cv = 1.4) this corresponds to a compression ratio of about 4
G.  Compression ratios should be about the same in each stage for a multistage unit,
the ratio = (Pn / P1) 1/n, with n stages.
H.  Efficiencies for reciprocating compressors are as follows:
	65% at compression ratios of 1.5
	75% at compression ratios of 2.0
	80-85% at compression ratios between 3 and 6
I.  Efficiencies of large centrifugal compressors handling 2.8 to 47 m3/s (6000-100,000 acfm) at suction is about 76-78%
J. Reciprocating piston vacuum pumps are generally capable of vacuum to 1 torr absolute, rotary piston types can achieve vacuums of 0.001 torr.
K. Single stage jet ejectors are capable of vacuums to 100 torr absolute, two stage to 10 torr, three stage to 1 torr, and five stage to 0.05 torr.
L. A three stage ejector requires about 100 lb steam/lb air to maintain a pressure of 1 torr.
M.  Air leakage into vacuum equipment can be approximated as follows: Leakage = k V(2/3) where k =0.20 for P >90 torr, 0.08 for 3 < P < 20 torr, and 0.025 for P < 1 torr            V = equipment volume in cubic feet            Leakage = air leakage into equipment in lb/h