Page 85 - Handbook of Energy Engineering Calculations
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mercury is great enough to return the liquid mercury to the boiler by gravity,
making the use of a mercury feed pump unnecessary.
To avoid the high cost entailed with using mercury, a number of man-made
solutions have been developed for binary vapor cycles. Their use, however,
has been limited because the conventional steam cycle is usually lower in
cost. And with the advent of the aero-derivative gas turbine, which is
relatively low cost and can be installed quickly in conjunction with heat-
recovery steam generators, binary cycles have lost popularity. But it is useful
for engineers to have a comprehension of such cycles. Why? Because they
may return to favor in the future.
COGENERATION SYSTEM ENERGY EFFICIENCY FOR A
TRADITIONAL STEAM POWER PLANT CYCLE
An industrial plant has 60,000 lb/h (27,240 kg/h) of superheated steam at
2
1000 lb/in (abs) (6890 kPa) and 900°F (482.2°C) available. Two options are
being considered for use of this steam: (1) expanding the steam in a steam
2
turbine having a 70 percent efficiency to 1 lb/in (abs) (6.89 kPa), and (2)
2
expand the steam in a turbine to 200 lb/in (abs) (1378 kPa) generating
electricity and utilizing the low-pressure exhaust steam for process heating.
Evaluate the two schemes for energy efficiency when the boiler has an 82
percent efficiency on a HHV basis.
Calculation Procedure:
1. Determine the enthalpies of the steam at the turbine inlet and after
isentropic expansion
Cogeneration systems generate power and process steam from the same fuel
source. Process plants generating electricity from steam produced in a boiler
and using the same steam after expansion in a steam turbine for process
heating of some kind are examples of cogeneration systems.
Conventional steam-turbine power plants have a maximum efficiency of
about 40 percent as most of the energy is wasted in the condensing-system
cooling water. In a typical cogeneration system the exhaust steam from the
turbine is used for process purposes after expansion through the steam
