Page 425 - Handbook of Energy Engineering Calculations

humidifying, plastics heating, pipe heating, etc.
For pipe heating, a tubular heating element can be fastened to the bottom
of the pipe and run parallel with it. For large-wattage applications, the heater
can be spiraled around the pipe. For temperatures below 165°F (73.9°C),
heating cable can be used. Electric heating is often used in place of steam

tracing of outdoor pipes.
The procedure presented above is the work of General Electric Company.

HEAT-TRANSFER COEFFICIENT DETERMINATION FOR

BOILER ECONOMIZER

2
2
A 4530-ft (421-m ) heating surface counterflow economizer is used in
conjunction with a 150,000-lb/h (68,040-kg/h) boiler. The inlet and outlet
water temperatures are 210°F (99°C) and 310°F (154°C). The inlet and outlet
gas temperatures are 640°F (338°C) and 375°F (191°C). Find the overall
2
2
2
heat-transfer coefficient in Btu/(h · ft · °F) [W/(m · °C)] [kJ/(h · m · °C)].
Calculation Procedure:

1. Determine the enthalpy of water at the inlet and outlet temperatures
From Table 1, Saturation: Temperatures, of the steam Tables mentioned
under Related Calculations of this procedure, for water at inlet temperature, t 1

= 210°F (99°C), the enthalpy, h = 178.14 Btu/lb (414 kJ/kg), and at the
1
outlet temperature, t = 310°F (154°C), the enthalpy, h = 279.81 Btu/lb m
2
2
(651 kJ/kg).

2. Compute the logarithmic mean temperature difference between the gas

and water
As shown in Fig. 8, the temperature difference of the gas entering and the
water leaving,Δt = t – t = 640 – 310 = 330°F (166°C) and for the gas
3
2
a
leaving and the water entering, Δt = t – t = 375 – 210 = 165°F (74°C).
1
b
4
Then, the logarithmic mean temperature difference,Δt = (Δt –Δt )/[2.3 ×
m
a
b
log (Δt –Δt )] = (330 – 165)/[2.3 × log (330/165)] = 238°F (115°C).
10
b
10
a