Page 420 - Handbook of Energy Engineering Calculations

subcooling of the condensate.

Calculation Procedure:

1. Compute the LMTD for the heater
2
Steam at 65 + 14.7 = 79.7 lb/in (abs) (549.5 kPa) has a temperature of
approximately 312°F (155.6°C), as given by the steam tables. Condensate at

this pressure has the same approximate temperature. Hence, the entering and
leaving temperatures of the heating fluid are approximately the same.
Oil enters the heater at 60°F (15.6°C) and leaves at 125°F (51.7°C).
Therefore, the greater temperature G across the heater is G = 312 – 60 =

252°F (140.0°C), and the lesser temperature difference L is L = 312 – 125 =
187°F (103.9°C). Hence, the LMTD = (G – L)/[ln(G/L)], or (252 – 187)/[ln
(252/187)] = 222°F (123.3°C). In this relation, ln = logarithm to the base e =
2.7183. (Figure 4 could also be used to determine the LMTD.)

2. Compute the heat required to raise the oil temperature
Water weighs 8.33 lb/gal (1.0 kg/L). Since this oil has a specific gravity of
0.85, it weighs (8.33) (0.85) = 7.08 lb/gal (0.85 kg/L). With 1000 gal/h (1.1
L/s) of oil to be heated, the weight of oil heated is (1000 gal/h)(7.08 lb/gal) =

7080 lb/h (0.89 kg/s). Since the oil has a specific heat of 0.5 Btu/(lb · °F) [2.1
kJ/(kg · °C)] and this oil is heated through a temperature range of 125 – 60 =
65°F (36.1°C), the quantity of heat Q required to raise the temperature of the
oil is Q = (7080 lb/h) [0.5 Btu/(lb · °F) (65°F)] = 230,000 Btu/h (67.4 kW).

3. Compute the heat-transfer area required

Use the relation A = Q/(U × LMTD), where Q = heat-transfer rate, Btu/h; U =
2
overall coefficient of heat transfer, Btu/(h · ft · °F). For heating oil to 125°F
2
(51.7°C), the U value given in Table 1 is 20 to 60 Btu/(h · ft · °F) [0.11 to
2
2
2
0.34 kW/(m · °C)]. Using a value of U = 30 Btu/(h · ft · °F) [0.17 kW/(m ·
°C)] to produce a conservatively sized heater, we find A = 230,000/[(30)
2
2
(222)] = 33.4 ft (3.1 m ) of heating surface.
4. Choose the coil material for the heater
Spiral-type tank heating coils are usually made of steel because this material
has a good corrosion resistance in oil. Hence, this coil will be assumed to be