Page 315 - Mechanical Engineers' Handbook (Volume 4)

304 Heat Exchangers, Vaporizers, Condensers

In a properly designed heat exchanger, it is unusual for F to be less than 0.7, and if
there is no temperature cross (T t ), F will be 0.8 or greater. As a first approximation for
2
2
preliminary sizing and cost estimation, F may be taken as 0.85 for multitubepass exchangers
with temperature change of both streams and 1.0 for other cases.

2.3 Overall Heat-Transfer Coefficient
The factor (U ) in Eq. (1) is the overall heat-transfer coefficient. It may be calculated by
o
procedures described in Section 3, and is the reciprocal of the sum of all heat-transfer
resistances, as shown in the equation
U 1/(R R R R R ) (7)
w
o
ƒ o
h o
ƒ i
h i
where
R 1/h (8)
h o o
R (A /Ah ) (9)
h i o i i
Ax
R ow (10)
w
Ak
mw
Calculation of the heat-transfer coefficients h and h can be time consuming, since they
i
o
depend on the fluid velocities, which, in turn, depend on the exchanger geometry. This is
usually done now by computer programs that guess correct exchanger size, calculate heat-
transfer coefficients, check size, adjust, and reiterate until satisfactory agreement between
guessed and calculated size is obtained. For first estimates by hand before size is known,
values of h and h , as well as values of the fouling resistances, R ƒ o and R ƒ i , are recommended
o
i
by Bell for shell and tube heat exchangers. 10
Very rough, first approximation values for the overall heat-transfer coefficient are given
in Table 1.
2.4 Pressure Drop
In addition to calculation of the heat-transfer surface required, it is usually necessary to
consider the pressure drop consumed by the heat exchanger, since this enters into the overall

Table 1 Approximate Values for Overall Heat Transfer
Coefficient of Shell and Tube Heat Exchangers (Including
Allowance for Fouling)

U o
2
2
Fluids Btu/hr ft F W/m K
Water–water 250 1400
Oil–water 75 425
Oil–oil 45 250
Gas–oil 15 85
Gas–water 20 115
Gas–gas 10 60