Page 291 - Pipeline Rules of Thumb Handbook

278 Pipeline Rules of Thumb Handbook

CENTRIFUGAL COMPRESSOR DATA

Centrifugal compressor performance calculations

Centrifugal compressors are versatile, compact, and gen- ature required to compress 5,000lbm/min of gas from 30psia
erally used in the range of 1,000 to 100,000 inlet cubic ft per at 60°F to 100psia. The gas mixture molar composition is as
minute (ICFM) for process and pipeline compression appli- follows:
cations.
Centrifugal compressors can use either a horizontal or a
5%
vertical split case. The type of case used will depend on the Ethane 80%
Propane
pressure rating with vertical split casings generally being n-Butane 15%
used for the higher pressure applications. Flow arrange-
ments include straight through, double flow, and side flow
configurations. This is the same gas mixture that was considered in Sample
Centrifugal compressors may be evaluated using either Calculation 1, p. 214, where the gas properties were calcu-
the adiabatic or polytropic process method. An adiabatic lated. The properties of this mixture were determined to be
process is one in which no heat transfer occurs. This doesn’t as follows:
imply a constant temperature, only that no heat is trans-
ferred into or out of the process system. Adiabatic is nor- MW = 45.5
mally intended to mean adiabatic isentropic. A polytropic = 611psia
process is a variable-entropy process in which heat transfer P c = 676°R
can take place. T c = 17.76
When the compressor is installed in the field, the power c p
required from the driver will be the same whether the pro- k 1 = 1.126
= 0.955
cess is called adiabatic or polytropic during design. There- Z 1
fore, the work input will be the same value for either process.
It will be necessary to use corresponding values when making Before proceeding with the compressor calculations, let’s
the calculations. When using adiabatic head, use adiabatic review the merits of using average values of Z and k in cal-
efficiency and when using polytropic head, use polytropic effi- culating the polytropic head.
ciency. Polytropic calculations are easier to make even though The inlet compressibility must be used to determine the
the adiabatic approach appears to be simpler and quicker. actual volume entering the compressor to approximate the
The polytropic approach offers two advantages over the size of the compressor and to communicate with the vendor
adiabatic approach. The polytropic approach is independent via the data sheets. The maximum value of q is of interest
of the thermodynamic state of the gas being compressed, and will be at its maximum at the inlet to the compressor
whereas the adiabatic efficiency is a function of the pressure where the inlet compressibility occurs (although using the
ratio and therefore is dependent upon the thermodynamic average compressibility will result in a conservative estimate
state of the gas. of q).
If the design considers all processes to be polytropic, an Compressibility will decrease as the gas is compressed.
impeller may be designed, its efficiency curve determined, This would imply that using the inlet compressibility
and it can be applied without correction regardless of pres- would be conservative since as the compressibility de-
sure, temperature, or molecular weight of the gas being com- creases, the head requirement also decreases. If the varia-
pressed. Another advantage of the polytropic approach is that tion in compressibility is drastic, the polytropic head re-
the sum of the polytropic heads for each stage of compres- quirement calculated by using the inlet compressibility would
sion equals the total polytropic head required to get from be practically useless. Compressor manufacturers calculate
state point 1 to state point 2. This is not true for adiabatic the performance for each stage and use the inlet compress-
heads. ibility for each stage. An accurate approximation may be sub-
stituted for the stage-by-stage calculation by calculating the
polytropic head for the overall section using the average com-
Sample performance calculations pressibility. This technique results in overestimating the first
half of the impellers and underestimating the last half of the
Determine the compressor frame size, number of stages, impellers, thereby calculating a polytropic head very near that
rotational speed, power requirement, and discharge temper- calculated by the stage-by-stage technique.