Page 193 - Chemical process engineering design and economics

Process Heat Transfer 175

a. Source: Reference 8 except where indicated. Multiply by 0.9869 to obtain atm.
Multiply by 0.9869 to obtain atmospheres,
b. Source: Reference 7.
c. The letters in the parenthesis is the normal installation
position, H for horizontal, V for vertical, and I for inclined,
d. Steam, organic, hot gases
e. Source: Reference 1.

Also, listed in Table 4.4 are pressure-drop ranges for heat exchangers for
making preliminary estimates. The pressure drop depends on whether the fluid is a
gas or a liquid, or if the fluid is condensing or vaporizing. For gases, the pressure
drop depends on the total pressure. Below atmospheric pressure, the pressure drop
is critical and should be small because of the cost of vacuum pumps.

SIZING HEAT EXCHANGERS

There are two general classes of problems encountered by a process engineer. One
class is the design problem, which requires calculating the size of a process unit.
The other class is the rating problem, which requires determining if an existing
process unit will satisfy process conditions. For a heat exchanger, the sizing prob-
lem is calculating surface area for transferring a specified amount of heat. Then, a
heat exchanger can be designed in detail to give the calculated area. For the rating
problem, the heat-transfer area is fixed. The heat exchanger may be available in a
plant, at a used equipment dealer, or supplied by a manufacturer, who usually pro-
duces standard heat exchangers in discrete sizes. Rating a process unit is a fre-
quently occurring problem. We will consider the design problem first.
We have now developed sufficient background material to outline a sizing
procedure for a preliminary estimate of the heat-exchanger surface area. Equations
for sizing heat exchangers are summarized in Table 4.5. Table 4.6 outlines the
calculation procedure. Because heat transfer coefficients and fouling factors are
contained in Table 4.3, we represent this mathematically by using functional nota-
tion as shown by Equations 4.6.10 to 4.6.13 inTable4.5.

Table 4.5 Summary of Equations for Sizing Shell-and-Tube
Heat Exchangers______________________________

First subscript: Process stream = 1,2,3 or 4
Second subscript: Component =1 or 2

Mass Balance

m 3' = m4 (4.5.1)

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