Page 176 - Handbook of Energy Engineering Calculations

value of K and solving for T , the evaporator exit-gas temperature, ln[(880
g2
1
− 366)/(T − 366)] = 387.6(165,000) –0.4 ; T = 388 °F (197.8°C). Then, the
g2
g2
evaporator duty, using the same equation as in step 1 above = (165,000)(0.99)
(0.27)(880 − 388) = 21.7 MM Btu/h (6.36 MW).
In this calculation, we assumed that the exhaust-gas analysis had not

changed. If there are changes in the exhaust-gas analysis, then the gas
properties must be evaluated and corrections made for variations in the
exhaust-gas temperature. See Waste Heat Boiler Deskbook by V. Ganapathy
for ways to do this.

5. Find the assumed duty, Q , for the economizer
a
Let the economizer leaving-water temperature = 360°F (182.2°C). The
enthalpy of the feedwater = 332 Btu/lb (773.6 kJ/kg); saturated-steam

enthalpy = 1195.7 Btu/lb (2785.9 kJ/kg); saturated-liquid enthalpy = 338.5
6
Btu/lb (788.7 kJ/kg). Then, the steam flow, as before, = (21.5 × 10 )/[(1195.7
− 332) + 0.05 (338.5 – 332)] = 25,115.7 lb/h (11,043 kg/h). Then, the

assumed duty for the economizer, Q = (25,115.7)(1.05)(332 − 198.5) = 3.52
a
MM Btu/h (1.03 MW).

6. Determine the UA value for the economizer in both design and off-

design conditions
For the design conditions, UA = Q/(ΔT), where Q = economizer duty from
step 2, above;Δ T = design temperature conditions from the earlier data in this
6
procedure. Solving, UA = (3.84 × 10 )/{[(299 − 230) − (408 −
373)]/ln(69/35)} = 76,800 Btu/h°F (40.5 kW). For off-design conditions, UA
= (UA at design conditions) (gas flow at off-design/gas flow at design

conditions) 0.65 = (76,800) (165,000/140,000) 0.65 = 85,456 Btu/h°F (45.1 kW).

7. Calculate the economizer duty
The energy transferred = Q = (UA)(Δ T). Based on 360°F (182.2°C) water
t
leaving the economizer, Q = 3.52 MM Btu/h (1.03 MW). Solving for t as
a
g2
6
before = 382 − [(3.52 × 10 )/(165,000)(0.9)(0.253)] = 388 − 85 = 303°F
(150.6°C). Then,ΔT = [(303 − 230) − (388 − 360)]/ln(73/28) = 47°F
(26.1°C). The energy transferred = Q = (UA)(Δ T) = (85,456)(47) = 4.01
t