Page 341 - Mechanical Engineers' Handbook (Volume 4)

330 Heat Exchangers, Vaporizers, Condensers

The major problem is that there is very little relationship between actual fouling and the
fouling factor specified. Typically, the fouling factor contains a safety factor that has evolved
from practice, lived a charmed life as it is passed from one handbook to another, and may
no longer be necessary if modern accurate design programs are used. An example is the
frequent use of a fouling factor of 0.001 hr ft F/Btu for clean overhead condenser vapors.
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This may have evolved as a safety or correction from the failure of early methods to account
for mass transfer effects and is completely unnecessary with modern calculation methods.
Presently, the practice is to use fouling factors from TEMA Standards. However, these often
result in heat exchangers that are oversized by as much as 50% on startup, causing operating
problems that actually tend to enhance fouling tendencies. Hopefully, with ongoing research
on fouling threshold conditions, it will be possible to design exchangers to essentially clean
conditions. In the meantime, the user of computer programs should use common sense in
assigning fouling factors only to actual fouling conditions. Startup conditions should also be
checked as an alternative case.
Industrial experience has shown for a long time that arbitrary fouling factors may ac-
tually contribute to fouling by greatly oversizing exchangers and lowering velocities. Gil-
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mour presented evidence of this years ago. In general, crude oils may need fouling factors,
as may polymerizing fluids, but light hydrocarbons may not. We now recommend designing
with no fouling factor, then adding about 20% surface, as length, and rechecking pressure
drop.

NOMENCLATURE
Note: Dimensional equations should use U.S. units only.

Description U.S. Units S.I. Units
Inside surface area ft 2 m 2
A i
Mean surface area ft 2 m 2
A m
Outside surface area ft 2 m 2
A o
Outside surface per unit length ft m
a o
Baffle cut % of shell diameter % %
B c
BR Boiling range (dew–bubble points) F (U.S. only)
C Two-phase pressure drop constant — —
Bundle bypass constant — —
C b
Heat capacity, hot fluid Btu/lb F J/kg K
C p1
Heat capacity, cold fluid Btu/lb F J/kg K
C p2
D Tube diameter, general ft m
Bundle diameter ft m
D b
Tube diameter, inside ft m
D i
Tube diameter, outside ft or in. m or U.S. only
D o
Shell diameter ft m
D s
Effective length: ft m
D ƒ
D i for tubeside
P t D o for shellside
Fan efficiency (0.6–0.7, typical) — —
E ƒ
F MTD correction factor — —
Bundle convection factor — —
F b
Mixture correction factor — —
F c
Gravity condensation factor — —
F g
g Acceleration of gravity ft/hr 2 m/sec 2
G Total mass velocity lb/hr ft 2 kg/sec m 2