Page 426 - Handbook of Energy Engineering Calculations

FIGURE 8 Temperature vs surface area of economizer.

3. Compute the economizer heat-transfer coefficient

All the heat lost by the gas is considered to be transferred to the water, hence
the heat lost by the gas, Q = w(h – h ) = UA Δt , where the water rate of
2
1
m
flow, w = 150,000 lb/h (68,000 kg/h); U is the overall heat-transfer
2
2
coefficient; heating surface area, A = 4530 ft (421 m ); other values as
before. Then, 150,000 × (279.81 – 178.41) = U(4530)(238). Solving U =
2
[150,000 × (279.81 – 178.14)]/(4530 × 238) = 14.1 Btu/(h · ft · °F) [80
2
2
W/(m · °C)] [288 kJ/(h · m · °C)].
Related Calculations. The Steam Tables appear in Thermodynamic

Properties of Water Including Vapor, Liquid, and Solid Phases, 1969,
Keenan, et al., John Wiley & Sons, Inc. Use later versions of such tables
whenever available, as necessary.

STEAM GENERATING CAPACITY OF BOILER TUBES

2
2
A counterflow bank of boiler tubes has a total area of 900 ft (83.6 m ) and
2
2
its overall coefficient of heat transfer is 13 Btu/(h · ft · °F) [73.8 W/(m ·
2
K)]. The boiler tubes generate steam at a pressure of 1000 lb/in absolute
(6900 kPa). The tube bank is heated by flue gas which enters at a temperature
of 2000°F (1367 K) and at a rate of 450,000 lb/h (56.7 kg/s). Assume an

average specific heat of 0.25 Btu/(lb · °F) [1.05 kJ/(kg · K)] for the gas and
calculate the temperature of the gas that leaves the bank of boiler tubes. Also,