Page 181 - Handbook of Energy Engineering Calculations

evaporator = 460 + 20 = 480°F (248.9°C), and the water temperature leaving
the economizer = 460 − 10 = 450°F (232.2°C).

2. Compute the steam generation rate
The energy transferred to the superheater and evaporator = Q + Q = (rate of
2
1
gas flow, lb/h)(gas specific heat, Btu/lb°F)(entering gas temperature,°F −
temperature of gas leaving evaporator,°F) (1.0 percent heat loss) = (150,000)
(0.267)(900 − 480)(0.99) = 16.65 MM Btu/h (4.879 MW).

The enthalpy absorbed by the steam in the evaporator and superheater =
2
(enthalpy of the super-heated steam at 450 lb/in (gage) and 650°F − enthalpy
of the water entering the evaporator at 450°F) + (blowdown percentage)
(enthalpy of the saturated liquid at the superheated condition − enthalpy of

the water entering the evaporator, all expressed in Btu/lb). Or, enthalpy
absorbed in the evaporator and superheater = (1330.8 − 431.2) + (0.02)(442.3
− 431.2) = 899.8 Btu/lb (2096.5 kJ/kg).
To compute the steam generation rate, set up the energy balance,

899.8(W ) = 16.65 MM Btu/h, where W = steam generation rate.
s
s
3. Calculate the energy absorbed by the superheater and the exit-gas
temperature

Q , the energy absorbed by the superheater = (steam generation rate, lb/h)
1
(enthalpy of superheated steam, Btu/lb – enthalpy of saturated steam at the

superheater pressure, Btu/lb) = (18,502)(1330.8 − 1204.4) = 2.338 MM Btu/h
(0.685 MW).
The superheater gas-temperature drop = (Q )/(rate of gas-turbine exhaust-
1
gas flow, lb/h)(1.0 − heat loss)(gas specific heat) = (2,338,000)/(150,000)
(0.99)(0.273) = 57.67°F, say 58°F (32.0°C). Hence, the superheater exit-gas
temperature = 900 − 58 = 842°F (450°C). In this calculation the exhaust-gas
specific heat is taken as 0.273 because the gas temperature in the superheater

is different from the inlet gas temperature.

4. Compute the energy absorbed by the evaporator
The total energy absorbed by the superheater and evaporator, from the above,
is 16.65 MM Btu/h (4.878 MW). Hence, the evaporator duty = Q = 16.65 −
2
2.34 = 14.31 MM Btu/h (4.19 MW).