Page 412 - Reciprocating Compressors Operation Maintenance

Reciprocating Compressor Calculations 397

king temperature and rod load, discussed earlier. In addition, horsepower
savings can be made by having the same ratio for each stage. This ratio
may be calculated by the following equation:

I/ri ri
Stage ratio = [Overall ratio] " (A-i I)
where n = number of stages

Generally speaking, in process work, the ratio per stage seldom exceeds
3.5 or 3.6. The basic equation for reciprocating compressor horsepower is:

k-l
ACFM x 144 x P,
HP, ad (A-12)
33000 k -1

This equation has been modified and drawn as a curve, using capacity
in million cubic feet per day at 14.6 psia and suction temperature. This
curve is given in Figure A-4.

This horsepower is theoretical and must be modified by compressibili-
ty, valve efficiency, and mechanical efficiency.
Valve efficiency allows for the pressure drop across the valves, which
results in higher discharge and lower suction pressure within the cylinder
than is supplied to the compressor. This may be found from Figures A-5
and A-6.
Mechanical efficiency is based on the power loss in the crankcase, and
efficiency is usually expected to be 95%. Then, actual horsepower may
be calculated as follows:

MMSCFD T, BHP Z,+Z 2 1
'
'
VX
LJ O — ~" „-„-, T.-,--,rr r1r-,„- -,-,.,-,-,m I r . ,,.,„-, V A > A, _ _ _ _ _ _ xx * *• VJ«
NX
niA n l
Act
Z std T std MMCFD 2 Valve Eff.
x l - (A-I3)
Mech. Eff.
where, MMSCFD is million standard cubic feet per day at 14,4 psia and
60°F.
The above equation should be used for finding horsepower. For exist-
ing machines, displacement is known and volumetric efficiency can be
calculated: