Page 297 - Compression Machinery for Oil and Gas

282 SECTION II Types of Equipment

Actual Suction Volume Flow and Power Consumption Versus
Molecular Weight for Dry Screw Compressors

The main thermodynamic property for the compressed gases is the molecular
weight. Gases compressed in dry screw compressors range from hydrogen, nat-
ural gas, air, hydrocarbons, CO 2 , and butadiene up to very heavy gases with a
molecular weight of 100kg/kmol.
The molecular weight and pressure ratio determine the optimum tip speed. For
heavy gases or low-pressure ratio the optimum tip speed is low, for light gases or
high-pressure ratio the optimum tip speedis high. Dry screw compressors operating
with different gases are compared using the circumferential Mach number at the
inlet conditions of the gas. Typical circumferential Mach numbers range between
0.2 and 0.4 although higher or lower values are possible for some applications.
Figs. 6.30 and 6.32 show the effect of different molecular weights on the
performance of two dry screw compressors. The data have been calculated
for a male-rotor tip speed of 120m/s with a pressure ratio of 3 with water injec-
tion. The gases in this example range from a mixture of 50% hydrogen and 50%
methane with a molecular weight of 9kg/kmol up to CO 2 with a molecular
weight of 44kg/kmol. Figs. 6.30 and 6.32 show that the suction volume flow
and the power consumption do not change very much within a large range of
molecular weights. This means that dry screw compressors are suited well
for variable gas applications like flare gas compression. For these applications
screw compressors are better suited than centrifugal compressors because they
do not have surge line. For very low molecular weights the drop in volume flow
is more pronounced than the drop in power consumption thus leading to an
increase in discharge temperature. If the discharge temperature can be limited
by liquid injection even larger variations of molecular weight are possible. The
final limitations must always be determined on a case-by-case basis.

60,000
Suction volume flow (m 3 /h) 40,000 100% rotor diameter
50,000

30,000
50% rotor diameter
20,000
10,000

0
0 10 20 30 40 50
Molecular weight (kg/kmol)
FIG. 6.30 Volume versus mole weight for dry screw compressors.