Page 175 - Handbook of Energy Engineering Calculations

entering temperature,°F − gas temperature leaving evaporator,°F). Or, Q =
1
(140,000)(0.99)(0.27)(980 − 408) = 21.4 MM Btu/h (6.26 MW). The
enthalpy absorbed by the steam in the evaporator, Btu/lb (kJ/kg) = (enthalpy

of the saturated steam in the HRSG outlet – enthalpy of the feedwater
entering the evaporator at 373°F) + (blowdown percentage)(enthalpy of the
saturated liquid of the outlet steam − enthalpy of the water entering the
evaporator, all in Btu/lb). Or, enthalpy absorbed in the evaporator = (1199.3

− 345) + (0.05)(362.2 − 345) = 855.2 Btu/lb (1992.6 kJ/kg). The quantity of
steam generated = (Q , energy absorbed by the evaporator, Btu/h)/(enthalpy
1
6
absorbed by the steam in the evaporator, Btu/lb) = (21.4 × 10 )/855.2 =
25,023 lb/h (11,360 kg/h).

2. Determine the economizer duty and exit-gas temperature
The economizer duty = (steam generated, lb/h)(enthalpy of water entering the

economizer, Btu/lb – enthalpy − enthalpy of the feedwater at 230°F, Btu/lb)(l
+ blowdown percentage) = (25,023)(345 − 198.5)(1.05) = 3.849 MM Btu/h
(1.12 MW).
The gas temperature drop through the economizer = (economizer
duty)/(gas flow rate, lb/h) (1 − heat loss percentage)(specific heat of gas,

6
Btu/lb°F) = (3.849 × 10 )/(140,000)(0.99)(0.253) = 109.8°F (60.9°C). Hence,
the exit-gas temperature from the economizer = (steam saturation

temperature,°F − exit-gas temperature from the economizer,°F) = (408 − 109)
= 299°F (148.3°C).

3. Calculate the constant K for evaporator performance
In simulating evaporator performance the constant K is used to compute
1
revised performance under differing flow conditions. In equation form, K , =
1
ln[(temperature of gas-turbine exhaust gas entering the HRSG,°F − HRSG

saturated steam temperature,°F)/(gas temperature leaving the evaporator,°F −
HRSG saturate steam temperature,°F)]/(gas flow, lb/h). Substituting, K , =
1
ln[(980 − 388)/(408 − 388)]/140,000 = 387.6, where the temperatures used

reflect design condition.

4. Compute the revised evaporator performance
Under the revised performance conditions, using the given data and the above