Page 409 - Reciprocating Compressors Operation Maintenance

394 Reciprocating Compressors: Operation and Maintenance

pD= A a!L xSxRPM
1728

where:
A HE = head end area of piston, square inches.
S = stroke, inches.
PD = piston displacement in CFM.

For double-acting cylinders:

AHB*SXRPM , A XSXRPM
PD = CE
DA
1728 1728

or:
SxRPMx2 ( . A
pn A HF -- 2- (A-7)
DA = - x
1728 V 2

where A R = rod area, square inches.

Compression Ratio

P 2/Pj is the ratio of the discharge pressure to the suction pressure with
both pressures expressed as absolute pressures. In the pV diagram (Fig-
ure A-2, Position 3), the line 2-3 is at discharge pressure, and line 4-1 is
at suction pressure.
It should be stressed that the compression ratio is not controlled by the
compressor, but by the system conditions at compressor suction and dis-
charge. During compression, the cylinder pressure will rise until it is high
enough to discharge the gas through the valves and into the system. Dur-
ing expansion, the cylinder pressure will fall until the system can force
gas into the cylinder and maintain the pressure. Naturally, the compres-
sion ratios cannot be infinite. As the compression ratio is increased at
constant suction pressure, the piston rod loading will increase and the
piston rod will fail. Further, the cylinder cooling provided may not be
adequate for the discharge temperature associated with higher compres-
sion ratios. Nevertheless, the machine will always attempt to meet the
system compression ratios.