Page 180 - Fluid mechanics, heat transfer, and mass transfer

158 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

➢ Compression ratios should be about the same in . What is the reason that specific speeds of centrifugal
each stage for a multistage unit. compressors are the lowest compared to fans and
blowers?
1=n
Compression ratio ¼ðP n =P 1 Þ ; withnstages:
& Because they have narrow impellers.
ð5:29Þ
. What is percent clearance for a reciprocating
compressor?
& Polytropic power:
& Clearancevolume is usually expressed as a percent of
a=h piston displacement and referred to as percent clear-
mz 1 RT 1 ½P 2 =P 1 Þ 1Š=ða=h Þ; ð5:30Þ
p p
ance, or cylinder clearance, C.
where h p is the polytropic efficiency (use 75% for & Percent clearance ¼ 100(clearance volume)/(volume
preliminary work). of piston displacement).
. What is the outlet temperature for reversible adiabatic & For double-acting cylinders, the percent clearance
process? is based on the total clearance volume for both the
head end and the crank end of a cylinder. These two
a
T 2 ¼ T 1 ðP 2 =P 1 Þ ð5:31Þ clearance volumes are not the same due to the
presence of the piston rod in the crank end of the
. How are the outlet temperatures estimated for multi-
cylinder.
stage compressors?
& Sometimes, additional clearancevolume (external) is
n
Outlet temperature T 2 ¼ T 1 ½P 2 =P 1 Š : ð5:32Þ intentionally added to reduce cylinder capacity.
. Define volumetric efficiency for a reciprocating
. What is the maximum exit temperature for compressors compressor.
handling diatomic gases? & The term volumetric efficiency refers to the actual
& 167–204 C (350–400 F).

pumping capacity of a cylinder compared to the
. What is the normally recommended maximum outlet piston displacement.
temperatures for multistage compressors? & Without a clearance volume for the gas to expand and
& 50–60 C. delay the opening of the suctionvalve(s), the cylinder

& Intercooling can be used to hold desired temperatures could deliver its entire piston displacement as gas
for high overall compression ratio applications. This capacity.
can be done between stages in a single compressor & When a nonlubricated compressor is used, the vol-
frame or between series frames. umetric efficiency should be corrected by subtracting
➢ Sometimes for high compression ratio applica- an additional 5% for slippage of gas.
tions, the job cannot be done in a single compres- . “Efficiencies for pumps are generally more than for
sor frame. Usually, a frame will not contain more compressors.” True/False?
than about eight stages (wheels). & False. Efficiencies for pumps are in the range of
& Sometimes, economics rather than a temperature
40–60% and for compressors the efficiencies are in
limit dictate intercooling. the range of 60–80%.
. Define specific speed for a compressor. . What are the usual efficiencies for large (170–3000 atm.
3
& Speed in rpm at which an impeller would rotate if m /min) at suction condition) centrifugal compressors?
reduced proportionately in size so as to deliver 1 cft Illustrate.
of gas per minute against a total head of 30.48 cm & 76–78%.
(1 ft) of fluid. . What is the range of efficiencies of rotary compressors?
. What is the range of specific speeds for a centrifugal & 50–70%.
compressor?
. What are the normally used compression ratios for
& 1500–3000 rpm at the high-efficiency point.
different compressors? What are the considerations
. “Axial flow blowers and fans have high specific speeds involved in limiting compression ratios?
compared to centrifugal compressors.” True/False? & Typical compression ratios of reciprocating
& True. compressors:
. “Mixed flow blowers and fans have higher specific ➢ Large pipeline compressors: 1.2–2.0.
speeds than axial flow types.” True/False? ➢ Process compressors: 1.5–4.0.
& False. ➢ Small compressors: up to 6.0.