Page 250 - Chemical process engineering design and economics

Compressors, Pumps, and Turbines 231

rotor full, or

Figure 5.18 A radial-inflow turbine. (Source Ref. 27 with permission).

If the steam is expanded to atmospheric pressure or above, the turbine is
called noncondensing. Noncondensing turbines are used when the exhaust steam
is needed for process heating. On the other hand, if the steam is expanded to below
atmospheric pressure, the turbine is called condensing. Usually, the exhaust pres-
sure is between 0.0040 to 0.0053 bar (3 to 4 mm Hg, 0.058 to 0.0769 psia), but
can be anywhere from 0.0013 to 0.020 bar (1 to 15 mm Hg, 0.0189 to 0.29 psia).
In condensing turbines, the exhaust steam may contain as much as 15 % moisture
by mass, but 10 % is common practice [32],
Because centrifugal and axial compressors are high-speed machines, they
could be driven by steam turbines, which are designed for the same high speeds
and thus may be directly coupled. To improve efficiency, however, recent devel-
opments in steam-turbine technology are in the direction of achieving higher
speeds, which will require gears to match the speed of the driven machine [33].
About 2 to 3% of the shaft power is lost by gear friction [26].
To size a steam turbine requires calculating the steam flow rate, which will
eventually be needed to size a steam boiler. A summary of equations for sizing a
steam turbine are given in Table 5.11 and the calculation procedure in Table 5.12.
In this case, the mass balance is simple in that the steam flow rate into the turbine
is equal to the steam and the condensate flow rate out of the turbine.

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