Page 211 - Compression Machinery for Oil and Gas

200 SECTION II Types of Equipment

As compression ratio increases the capacity decreases as less gas is drawn
into the compression chamber and compressed. As V 3 increases there is more
gas that must expand thus reducing capacity. V 3 is referred to as the fixed clear-
ance volume.
Fixed Clearance
The fixed clearance volume (V 3 in Fig. 5.2) is the volume of gas remaining in the
compression chamber at the discharge end of the stroke that must expand from dis-
charge pressure (P D ) to suction pressure (P S ) before the suction compressor valve
can open and the suction event begin. Fig. 5.22 is a cross-section of a typical com-
pressor cylinder assembly with some of the fixed clearance volume highlighted.
The cavity between the compressor valve and cylinder bore might represent
70% of the total fixed clearance volume. There is also some volume between the
head and the bore, the piston and the bore, and the piston and head. All of this
volume adds together to form the total fixed clearance volume.
Fixed clearance (CL) is expressed as a percentage of the piston displacement
(referring to Fig. 5.2):
Volumetric Efficiency
VE is the percentage of stroke that fills with gas at suction pressure and suction
temperature. In equation form (referring to Fig. 5.2):
V 1 V 4
VE ¼ 100%
V 1 V 3
Some notes about VE:
l VE represents the capacity.
l VE is not suction valve open time. The suction valves do not have to be open
for the full VE.

FIG. 5.22 Compressor cylinder cross-section highlighting fixed clearance volume.