Page 288 - Pipeline Rules of Thumb Handbook

Gas—Compression 275

Figure 2. Ratio of specific heat (n-value).

Altitude—Multiplier Altitude—Multiplier
1,500ft 1.000 4,000ft 1.12
2,000ft 1.03 4,500ft 1.14
2,500ft 1.05 5,000ft 1.17
3,000ft 1.07 5,500ft 1.20
3,500ft 1.10 6,000ft 1.22

8. For a portable unit with a fan cooler and pump driven
from the compressor unit, increase the horsepower
1
figure by 7 /2 %.
The resulting figure is sufficiently accurate for all purposes.
The nearest commercially available size of compressor is then
selected.
The method does not take into consideration the super-
compressibility of gas and is applicable for pressures up to
1,000psi. In the region of high pressures, neglecting the devi-
ation of behavior of gas from that of the perfect gas may lead
to substantial errors in calculating the compression horse-
power requirements. The enthalpy-entropy charts may be
used conveniently in such cases. The procedures are given in
references 1 and 2.

Example. What is the nominal size of a portable com-
pressor unit required for compressing 1,600,000 standard
cubic ft of gas per 24 hours at a temperature of 85°F from
40psig pressure to 600psig pressure? The altitude above sea
Figure 3. Brake horsepower required for compressing natural level is 2,500ft. The N value of gas is 1.28. The suction tem-
gas. perature of stages, other than the first stage, is 130°F.