Page 428 - Handbook of Energy Engineering Calculations

the flow rate of flue gas, w = 450,000 lb/h (56.7 kg/s); flue-gas average
g
specific heat, c = 0.25 Btu/(lb/°F) [1.05 kg/(kg · K)]; overall coefficient of
p
2
2
heat transfer of the boiler tubes, U = 13 Btu/(h · ft · °F) [73.8 W/(m · K)];
2
2
area of the boiler tubes exposed to heat, A = 900 ft (83.6 m ); other values as
before.
Then, Q = 450,000 × 0.25 × (2000 – t ) = 13 × 900 × (2000 – t )/{2.3 ×
2
2
log [(1455.4)/(t – 544.6)]}. or, log [(1455.4)/(t – 544.6)] = 13 ×
2
10
10
2
900/(2.3 × 450.000 × 0.25) = 0.0452. The antilog of 0.0452 = 1.11, hence,
[(1455.4/(t – 544.7)] = 1.11, and t = (1455.4/1.11) + 544.6 = 1850°F (1280
2
2
K).
4. Find the heat of vaporization of the water
From the Steam Tables, the heat of vaporization of the water at 1000 lb/in 2

(6900 kPa), h = 649.5 Btu/lb (1511 kJ/kg).
fg

5. Compute the steam-generating rate of the boiler tube bank

Heat absorbed by the water = heat transferred by the flue gas, or Q = w × h fg
s
= w × c × (t – t ), where the mass of steam generated is w in lb/h (kg/s);
1
2
s
p
g
other values as before. Then, w × 649.5 = 450,000 × 0.25 × (2000 – 1850) =
s
6
6
16.9 × 10 Btu/h (4950 kJ/s) (4953 kW). Thus, w = 16.9 × 10 /649.5 =
s
26,000 lb/h (200 kg/s).
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 required.

ENERGY DESIGN ANALYSIS OF SHELL-AND-TUBE HEAT
EXCHANGERS

Determine the heat transferred, shellside outlet temperature, surface area,

maximum number of tubes, and tubeside pressure drop for a liquid-to-liquid
shell-and-tube heat exchanger such as that in Fig. 10, when the conditions