Page 64 - Handbook of Energy Engineering Calculations

(c) The heat supplied per pound (kg) of throttle steam = (H − H ) + (H −
3
1
6
H ) = (1362.3 − 265.27) + (1380.1 − 1178) = 1299.13 Btu/lb (3026.97 kJ/kg).
2
5. Compute the ideal thermal efficiency
(d) Use the relation, ideal thermal efficiency = (heat converted to work)/(heat
supplied) = 467.55/1299.13 = 0.3598, or 35.98 percent.

6. Show other ways to heat feedwater while increasing the turbogenerator

output
For years, central stations and large industrial steam-turbine power plants
shut off feedwater heaters to get additional kilowatts out of a turbogenerator
during periods of overloaded electricity demand. When more steam flows
through the turbine, the electrical power output increases. While there was a

concurrent loss in efficiency, this was ignored because the greater output was
desperately needed.
Today steam turbines are built with more heavily loaded exhaust ends so

that the additional capacity is not available. Further, turbine manufacturers
place restrictions on the removal of feedwater heaters from service. However,
if the steam output of the boiler is less than the design capacity of the steam
turbine, because of a conversion to coal firing, additional turbogenerator
capacity is available and can be regained at a far lower cost than by adding

new generator capacity.
Compensation for the colder feedwater can be made, and the lost efficiency
regained, by using a supplementary fuel source to heat feedwater. This can be

done in one of two ways: (1) increase heat input to the existing boiler
economizer, or (2) add a separately fired external economizer.
Additional heat input to a boiler’s existing economizer can be supplied by
induct burners, Fig. 3, from slagging coal combustors, Fig. 4, or from the
furnace itself. Since the economizer in a coal-fired boiler is of sturdier

construction than a heat-recovery steam generator (HRSG) with finned
tubing, in-duct burners can be placed closer to the economizer, Fig. 3. Burner
firing may be by coal or oil.

Slagging coal combustors are under intense development. A low-NO ,
x
low-ash combustor, Fig. 4, supplying combustion gases at 3000°F (1648.9°C)
may soon be commercially available.