Page 427 - Handbook of Energy Engineering Calculations

calculate the rate at which the steam is being generated in the tube bank.

Calculation Procedure:

2
1. Find the temperature of steam at 1000 lf /in (6900 kPa)
f
From Table 2, Saturation: Pressures, of the Steam Tables mentioned under
Related Calculations of this procedure, the saturation temperature of steam at
2
1000 lb/in (6900 kPa), t = 544.6°F (558 K), a constant value as indicated in
s
Fig. 9.

FIGURE 9 Temperature vs surface area of boiler tubes.

2. Determine the logarithmic mean temperature difference in terms of the
flue-gas leaving temperature
The logarithmic mean temperature difference,Δt = (Δt –Δt )/{2.3 × log 10
2
1
m
[(t – t ) /(t – t )]}, whereΔt = flue-gas entering temperature = steam
2
1
s
s
1
temperature = (t – t ) = (2000 – 544.6);Δt = flue-gas leaving temperature –
2
s
1
steam temperature = (t – t ) = (t – 544.6); (Δt –Δt ) = [(2000 – 544.6) – (t 2
1
2
2
2
s
– 544.6)] = (2000 – t ); [(t – t )/(t – t )] = [(2000 – 544.6)/(t – 544.6)].
s
2
s
1
2
2
Hence,Δt = [(2000 – t )/{2.3 × log [(1455.4)/(t – 544.6)]}.
2
m
2
10
3. Compute the flue-gas leaving temperature
Heat transferred to the boiler water, Q = w × c × (t – t ) = UA Δt , where
m
2
g
p
1