Page 150 - Handbook of Energy Engineering Calculations

(°C); other symbols as before. Using the values determined above, T = [390
lc
− (0.855)(227)]/(l − 0.855) = 135l°F (732.8°C).

2. Determine the system pinch/point and gas/steam profile
Up to a gas inlet-temperature of approximately 1351°F (732.8°C), the pinch

point can be arbitrarily selected. Beyond this, the feedwater inlet temperature
limits the temperature profile. Let’s then select a pinch point of 25°F
(13.9°C), Fig. 11. Then, T the gas-turbine gas temperature at the pinch point,
2
°F (°C) = t + pinch-point temperature difference, or 390°F + 25°F = 415°F
f
(212.8°C).

Setting up an energy balance across the evaporator, assuming a heat loss of
2 percent and a blow-down of 3 percent, leads to: Q evap = W (1 − heat loss)
e
(TEG heat capacity, Btu/°F) (T − T ), where W = TEG flow, lb/h; heat
e
1
2
capacity of TEG = 0.27 Btu/°F; T = TEG inlet temperature,°F (°C).
1
6
Substituting, Q evap = 150,000(0.98)(0.27)(950 − 415) = 21.23 × 10 Btu/h
(6.22 MW).
The rate of steam generation, W = Q evap /[(h − h ) + blowdown percent ×
w
s
s
(h − h )], where the symbols are as given earlier. Substituting, W = 21.23 ×
t
s
w
6
10 /[(1199.6 − 342) + 0.03 × (364 − 342)] = 24,736 lb/h (11,230 kg/h).
Determine the boiler economizer duty from Q econ = (1 + blowdown)(W )
s
(h − h ), where symbols are as before. Substituting, Q econ = 1.03(24.736)
w
f
6
(342 − 196.3) = 3.71 × 10 Btu/h (1.09 MW).
The gas exit-temperature, T = T - Q econ /TEG gas flow, lb/h)(l − heat loss)
3
2
(heat capacity, Btu/lb °F). Since all values are known, T = 415 - 3.71 ×
3
6
10 /(150,000 × 0.98 × 0.27) = 317°F (158°C). Figure 11 shows the
temperature profile for this installation.

Related Calculations. Use this procedure for heat-recovery boilers fired by
gas-turbine exhaust in any industry or utility application. Such boilers may be

unfired, supplementary fired, or exhaust fired, depending on steam
requirements.
Typically, the gas-pressure drop across the boiler system ranges from 6 to
12 in (15.2 to 30.5 cm) or water. There is an important tradeoff: a lower