b) Net Plant Heat Rate. This heat rate is determined by dividing the
total fuel energy (Btu/hour) added to the boiler by the difference between power
(kilowatts/hour) generated and plant auxiliary electrical power consumed.
Cycle Performance. Both turbine and plant heat rates, as above, are usually
based on calculations of cycle performance at specified steady state loads and well
defined, optimum operating conditions. Such heat rates are seldom achieved in practice
except under controlled or test conditions.
Long Term Averages. Plant operating heat rates are actual long term average
heat rates and include other such losses and energy uses as non-cycle auxiliaries, plant
lighting, air conditioning and heating, general water supply, startup and shutdown
losses, fuel deterioration losses, and related items. The gradual and inevitable
deterioration of equipment, and failure to operate at optimum conditions, are reflected
in plant operating heat rate data.
Plant Economy Calculations. Calculations, estimates, and predictions of steam
plant performance shall allow for all normal and expected losses and loads and should,
therefore, reflect predictions of monthly or annual net operating heat rates and costs.
Electric and district heating distribution losses are not usually charged to the power
plant but should be recognized and allowed for in capacity and cost analyses. The
designer is required to develop and optimize a cycle heat balance during the conceptual
or preliminary design phase of the project. The heat balance depicts, on a simplified
flow diagram of the cycle, all significant fluid mass flow rates, fluid pressures and
temperatures, fluid enthalpies, electric power output, and calculated cycle heat rates
based on these factors. A heat balance is usually developed for various increments of
plant load such as 25, 50, 75, 100 percent and VWO (valves, wide open). Computer
programs have been developed which can quickly optimize a particular cycle heat rate
using iterative heat balance calculations. Use of such a program should be considered.
Theoretical Steam Rate. When the turbine throttle pressure and temperature
and the turbine exhaust pressure (or condensing pressure) are known, the theoretical
steam rate can be calculated based on a constant entropy expansion or can be determined
from published tables. See Theoretical Steam Rate Tables, The American Society of
Mechanical Engineers, 1969. See Table 8 for typical theoretical steam rates.