Page 149 - Handbook of Energy Engineering Calculations

FIGURE 11 Gas/steam profile and data. (Chemical Engineering.)

To determine the critical gas inlet-temperature, T , get from the steam
1
tables the properties of the steam generated by this boiler: t = 390°F
s
(198.9°C); h , heat of saturated liquid = 364 Btu/lb (846.7 kJ/kg); h , total
s
1
heat of saturated vapor = 1199.6 Btu/lb (2790.3 kJ/kg); h , heat of saturated
w
liquid of feedwater leaving the economizer at 370°F (187.8°C) = 342 Btu/lb
(795.5 kJ/kg); and h , heat of saturated liquid of the feedwater at 227°F
f
(108.3°C) = 196.3 Btu/lb (456.6 kJ/kg).
Writing an energy balance across the evaporator neglecting heat and blow-
down losses, we get: (T − T )/(T − T ) = (h − h )/(h − h ) = X, where T =
1
2
w
3
s
s
1
1
f
gas temperature in boiler, °F (°C); T = pinch-point gas temperature, °F (°C);
2
T = outlet gas temperature for TEG, °F (°C); enthalpy, h, values as listed
3
above; X = ratio of temperature or enthalpy differences. Substituting, X =
(1199.6 − 342)/(1199.9 − 196.3) = 0.855, using enthalpy values as given
above.
The critical gas inlet-temperature, T = (t − Xt )/(1 − X), where t =
s
s
f
lc
temperature of saturated steam, °F (°C); t = temperature of feedwater, °F
f