Chemical Engineering June 2011 - 35

Feature Report
CompaCt, HigH-effiCienCy Heat exCHangers:
Understanding Fouling
Engineers should plan to avoid fouling of
heat exchangers instead of reacting to it
Jeff Kerner
Jennings Alberts Inc.
T
he topic of fouling in heat exchangers
has been of increasing
interest in recent years, since it
is so closely linked to the cost
of building and running a chemical
plant. It has been reported that fouling
costs are about 0.25% of the gross
national product (GNP) of highly industrialized
countries, which, based
on current estimates of GNP for the
U.S. of $15 trillion, makes the staggering
cost of fouling $37 billion a year.
One can easily envision that the cost
for the chemical process industries
(CPI), both U.S. and worldwide, are
unthinkably high.
When considering fouling and its
associated costs due to downtime,
labor and cleaning, the first issue that
comes to mind is maintenance expense.
However, fouling also directly
impacts capital expense (Capex) and
operating expense (Opex). Capex
is affected because larger heat exchangers
are required to compensate
for fouling. In addition, greater
floor space and beefier foundations
are needed for these larger heat exchangers.
Increased diameter piping
is an additional cost, since a higher
flowrate of coolant will generally be
required to achieve the desired heat
transfer in a fouled heat exchanger.
There are also the costs of both online
and offline cleaning equipment.
While maintenance costs are usually
factored into Opex, operating
expenses are also affected by the reduced
production rate of the fouled
heat exchanger as well as increased
fuel or electrical costs in those applications
where additional steam or
electricity is needed to overcome the
effects of a fouled heat exchanger.
This article summarizes
the fouling mechanisms in
high-efficiency compact heat
exchangers (CHEs) and also
describes fouling mitigation
and monitoring methods
that can be used when designing
or using CHEs.
Table I. Recommended FoulIng
Process Fluid
Soft water
ResIsTances FoR PHes vs. Tema values
Rf -PHe
Rf -Tema
m2/K-kW m2/K-kW
0.018
Cooling tower water 0.044
Sea water
River water
Lube oil
Organic solvents
0.026
0.044
0.053
Steam (oil bearing) 0.009
PHE = plate heat exchanger
Compact heat exchangers
Compact heat exchangers, in general,
are characterized by a large heattransfer
surface area per unit volume
of the exchanger, (characterized by
the term b) where b > 700 m2/m3 for
heat exchangers handling gases and
b > 400 m2/m3 for those handling liquids.
Compact heat exchangers that
are commercially available are the
plate-and-frame, fluted-plate-block
(also known as the welded-plate-fin),
spiral, enhanced-tube and printedcircuit
heat exchangers. For the scope
of this article, only the three compact
heat exchangers commonly used for
liquid-to-liquid services in the CPI
will be discussed (plate-and-frame
heat exchangers, fluted-plate-block
heat exchangers and spiral heat exchangers;
see box on p. 36). These
three types are chosen for this article
since, in the author's experience,
liquid-to-liquid services represent
the bulk of heat exchanger applications
that the chemical engineer will
encounter in CPI operating plants as
well as those applications in which
fouling is most often seen.
General overview of fouling
Except in the cleanest of applications
(deionized [DI] or purified water, refrigerants
and well-treated closedloop
systems), deposits that tend to
0.18-0.35
0.18-0.35
0.18-0.35
0.35-0.53
0.36
0.018-0.053 0.36
0.18
form on heat transfer surfaces reduce
the heat transfer efficiency of the exchanger.
The five recognized mechanisms
of fouling were first described
by Epstein in 1983 and are listed in
[1] as the following:
1. Particulate matter (sedimentation
of fine, suspended particles or flow
blockage by large particles)
2. Crystallization (precipitation followed
by deposition of dissolved salts)
3. Chemical reaction (deposit formation
on the heat transfer surface
by a chemical reaction in which the
surface itself is not a reactant)
4. Corrosion (fouling of the heat transfer
surface)
5. Bio-fouling (microbial fouling due to
growth and deposition of biological
films or slimes)
In addition to the fouling tendency of the
fluid, the physical parameters that affect
fouling have been well-documented
[2, 3]. These include the following:
* Metal wall temperature
* Fluid temperature
* Wall shear stress
* Fluid velocity
* Surface material or finish
* Chemical treatment
Fouling factors
Most heat-exchanger designs include
a resistance term known as the fouling
factor, Rf. The selection of too high
ChemiCal engineering www.Che.Com june 2011 35
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Chemical Engineering June 2011

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