Back Pressure Type. Back pressure turbines usually operate with high
pressure, high temperature throttle steam supply, and exhaust at steam pressures in the
range of 5 to 300 psig (34 to 2068 kPa gage). Un-controlled steam extraction openings
can be provided depending on throttle pressure and exhaust pressures. Two methods of
control are possible. One of the methods modulates the turbine steam flow to be such as
to maintain the turbine exhaust pressure constant and, in the process, generate as much
electricity as possible from the steam passing through the turbine. The amount of
electricity generated, therefore, changes upward or downward with like changes in steam
demand from the turbine exhaust. A typical back pressure cycle is shown in Figure 13.
The other method of control allows the turbine steam flow to be such as to provide
whatever power is required from the turbine by driven equipment. The turbine exhaust
steam must then be used, at the rate flowing through the turbine, by other steam
consuming equipment or excess steam, if any, must be vented to the atmosphere.
Atmospheric Exhaust. Atmospheric exhaust is the term applied to mechanical
drive turbines which exhaust steam at pressures near atmospheric. These turbines are
used in power plants to drive equipment such as pumps and fans.
Turbine Generator Sizes. See Table 9 for nominal size and other
characteristic data for turbine generator units.
Noncondensing and Automatic Extraction Turbines. The sizes of turbine
generators and types of generator cooling as shown in Table 9 generally apply also to
these types of turbines.
Geared Turbine Generator Units. Geared turbine generator units utilizing
multistage mechanical drive turbines are available in sizes ranging generally from 500
to 10,000 kW. Single stage geared units are available in sizes from 100 kW to 3,000 kW.
Multistage units are also available as single valve or multi-valve, which allows further
division of size range. Because of overlapping size range, the alternative turbine
valve and stage arrangements should be considered and economically evaluated within the
limits of their capabilities.
Turbine Throttle Pressure and Temperature. Small, single stage turbines
utilize throttle steam at pressures from less than 100 psig (689 kPa gage) and saturated
temperatures up to 300 psig and 150 (66 degrees C) to 200 degrees F (93 degrees C) of
superheat. Steam pressures and temperatures applicable to larger multistage turbines
are shown in Table 10.
Selection of Throttle Pressure and Temperature. The selection of turbine
throttle pressure and temperature is a matter of economic evaluation involving
performance of the turbine generator and cost of the unit including boiler, piping,
valves, and fittings.