Page 232 - Chemical process engineering design and economics

214 Chapters

Again, for a real gas use the compressibility factor.

n |)/n
rfp v - i
2
Wp = ————— || —— | _i |
(n-l)/n l(fj J

To obtain the actual work of compression, divide the polytropic work of
compression by the polytropic efficiency.

W P
W A = —— (5.19)

The polytropic efficiency is defined by Equation 5.20.

TIP =————— (5.20)

Figure 5.15 contains plots of the polytropic efficiency for centrifugal and ax-
ial compressors as a function of the volumetric flow rate at the compressor inlet.

In addition to the isentropic and polytropic efficiencies, there are other effi-
ciencies that affect the actual power that is delivered to the gas. The isentropic and
polytropic efficiencies are hydraulic efficencies because some of the work done on
the gas is consumed by fluid friction. Other factional energy losses are caused by
the compressor seal, the bearings supporting the shaft, and any gears needed to
reduce or increase the rotational speed. Table 5.6 lists these efficiencies. The engi-
neering literature either reports these efficiencies separately or combined. The
seal, bearing, and gear efficiencies may be combined into a mechanical efficiency.
Then, the hydraulic and mechanical efficiencies are combined into an overall effi-
ciency for the machine, which is designated as the compressor efficiency, r| c.

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