Page 407 - Handbook of Energy Engineering Calculations

2
2
2
2
1,350,000/[(25)(290)] = 186.4 ft (17.3 m ), say 200 ft (18.6 m ).
4. Compute the required quantity of heating steam
The heat added to the oil = Q = 1,350,000 Btu/h, from step 1. The enthalpy of
2
vaporization of 200-lb/in (abs) (1379-kPa) saturated steam is, from the
steam tables, 843.0 Btu/lb (1960.8 kJ/kg). Use the relation W = Q/h , where
fg
W = flow rate of heating steam, lb/h; h = enthalpy of vaporization of the
fg
heating steam, Btu/lb. Hence, W = 1,350,000/843.0 = 1600 lb/h (720 kg/h).

Related Calculations. Use this general procedure to find the heat-transfer
area, fluid outlet temperature, and required heating-fluid flow rate when true

parallel flow or counterflow of the fluids occurs in the heat exchanger. When
such a true flow does not exist, use a suitable correction factor, as shown in
the next calculation procedure.
The procedure described here can be used for heat exchangers in power

plants, heating systems, marine propulsion, air-conditioning systems, etc.
Any heating or cooling fluid—steam, gas, chilled water, etc.—can be used.
To select a heat exchanger by using the results of this calculation
procedure, enter the engineering data tables available from manufacturers at

the computed heat-transfer area. Read the heater dimensions directly from the
table. Be sure to use the next larger heat-transfer area when the exact
required area is not available.
When there is little movement of the fluid on either side of the heat-

transfer area, such as occurs during heat transmission through a building wall,
the arithmetic mean (average) temperature difference can be used instead of
the LMTD. Use the LMTD when there is rapid movement of the fluids on
either side of the heat-transfer area and a rapid change in temperature in one,

or both, fluids. When one of the two fluids is partially, but not totally,
evaporated or condensed, the true mean temperature difference is different
from the arithmetic mean and the LMTD. Special methods, such as those
presented in Perry—Chemical Engineers’ Handbook, McGraw-Hill, 2007,

must be used to compute the actual temperature difference under these
conditions.
When two liquids or gases with constant specific heats are exchanging heat
in a heat exchanger, the area between their temperature curves, Fig. 2, is a