Page 218 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 207

Stroke (in.) Rotating Speed (rev/min) Piston Speed (ft./min)
21.0 257 900
19.5 277 900
18.0 300 900
16.5 327 900
15.0 360 900
13.5 400 900
12.0 450 900
10.5 514 900
9.0 600 900
7.5 720 900
6.0 900 900
4.5 1200 900
3.0 1800 900
Everything else being equal (admittedly very difficult) all of these combi-
nations would have the same relative compression efficiency.
l VLP varies directly with MW. For example, a hydrogen compressor
(MW¼2) would have 89% less VLP as compared to the same compressor
compressing natural gas (MW¼18) simply because of the very low MW.
3
l The very basic relationship between the cylinder bore diameter (A BORE )
2
and the number and size of the compressor valves (N A VLV PKT ) deter-
mines the basic efficiency of a given cylinder. In simple terms—the larger
the valves for a given cylinder bore diameter the better the efficiency.
l As the cylinder bore increases in diameter the relative efficiency decreases.
This results from simple geometry:
The bore area grows by the bore diameter to the second power, but the circum-
ference, which is the space available to locate compressor valves (represented by
the rectangles in Fig. 5.27), only grows by the bore diameter to the first power.

D
πD 2
Bore area =
4

Circumference = πD
FIG. 5.27 Drawing showing the space available for compressor valves in a typical compressor
cylinder design. (Courtesy of Ariel Corporation.)