fouling of heat exchangers, elsevier (1995

Fouling of Heat Exchangers
by T. R. Bott



• ISBN: 0444821864
• Pub. Date: April 1995
• Publisher: Elsevier Science & Technology Books
PREFACE
There are many textbooks devoted to heat transfer and the design of heat
exchangers ranging from the extreme theoretical to the very practical. The
purpose of these publications is to provide improved understanding of the science
and to give guidance on the design and operation of process heat exchangers. In
many of these texts the problem of the accumulation of deposits on heat transfer
surfaces is ignored or at best, dealt with through the traditional fouling resistance.
It is common knowledge that this approach is severely limited and inaccurate and
may lead to gross errors in design. Furthermore the very arbitrary choice of
fouling resistance more than offsets the accuracy of correlations and sophisticated
methods, for the application of fundamental heat transfer knowledge.
Little attention was paid to the heat exchanger fouling and the associated
inefficiencies of heat exchanger operation till the so-called "oil crisis" of the 1970s,
when it became vital to make efficient use of available energy. Heat exchanger
fouling of course reduces the opportunity for heat recovery with its attendant
effect on primary energy demands. Since the oil crisis there has been a modest
interest in obtaining knowledge regarding all aspects of heat exchanger fouling, but
the investment is nowhere near as large as in the field of heat transfer as a whole.
Although books have appeared from time to time since the 1970s, addressing
the question of heat exchanger fouling, they are largely based on conferences and
meetings so that there is a general lack of continuity. The purpose of this book
therefore, is to present a comprehensive appraisal of current knowledge in all
aspects of heat exchanger fouling including fundamental science, mathematical

models such as they are, and aspects of the practical approach to deal with the
problem of fouling through design and operation of heat exchangers. The
techniques of on and off-line cleaning of heat exchangers to restore efficiency are
also described in some detail.
The philosophy of the book is to provide a wide range of data in support of the
basic concepts associated with heat exchanger fouling, but written in such a way
that the non-mathematical novice as well as the expert, may find the text of
interest.
T.R. Bott
December 1994
oo
vii
ACKNOWLEDGEMENTS
The author wishes to record his sincere gratitude for the skill, dedication
and persistence of Jayne Olden, without which this book would never have been
completed.
All the diagrams and figures in this book were drawn by Pauline Hill and
her considerable effort is acknowledged.
ix
NOMENCLATURE
Note: In the use of equations it will be necessary to use consistent units unless
otherwise stated
Area or area for heat transfer
A~ Constant
A n
Hamaker constant
a+, ao aD, ar
Vector switches associated with the dimensionless deposition
parameters N I,
N o N o

and N r respectively
B
Correction term Equation 12.28
C Circulation rate
C
Cunningham coefficient or a constant
c Concentration
cb
C m
Concentration in bulk blowdown water
Concentration in make up water
cp Specific heat
%
c~
Specific heat of solid
Concentration of cells in suspension
D
Diffusion coefficient or dimensionless grouping as described by
Equation 10.50
D c Collector diameter
d
Diffusion coefficient for particles
Diameter
E
Activation energy or dimensionless grouping as described by
Equation 10.49
Fouling of Heat Exchangers
Eo
F
Eddy diffusivity

Shear force
F~
Adhesion parameter
F~
Repulsion force
FS
Slagging index
F.
Van der Waals force
f
Friction factor
f~
Ball frequency (Balls/h)
Lifshitz - van der Waals constant
K
Transfer coefficient or Constant
x~
Mass transfer coefficient of species A
x~
Deposition coefficient
x.
Mass transfer coefficient allowing for sticking probability
Mass transfer coefficient
Mass transfer coefficient of macro-molecules
xo
Constant in Equation 10.31
X,o
Solubility product
x,
Transport coefficient

Kt
*
Dimensionless transport coefficient
k~
Rate constants Equation 12.10
Rate constant
Length
t.
Characteristic length
M
Mass flow rate
M*
Asymptotic deposit mass
Nomenclature
xi
m~
m
Mass of fouling deposit
Mass
N
Dimensionless deposition parameter (Equation 7.40)
N~
Mass flux of cells
Dimensionless interception deposition parameter
Dimensionless diffusion deposition parameter or mass flux away
from reaction zone
N,
Dimensionless impaction deposition parameter
N~
N~

N~
Mass flux of macro-molecules
Mass flux of reactants or precursors
Dimensionless thermophoresis deposition parameter
Particle number density
Integer on concentration factor
P
Sticking probability
e,
Po
P~
Probability of scale formation
Sticking probability for impacting mechanisms
Sticking probability for non-impacting mechanisms
Overall sticking probability
P
Pressure
Ap
O
Pressure drop
Rate of heat transfer
Heat flux
R
Universal gas constant or parameter defined in Equation 9.14
Fouling resistance or Fouling potential (see Chapter 16)
Overall fouling resistance
Fouling resistance at time t
o~
Xll
Fouling of Heat Exchangers

a,
R T
R|
Slagging propensity
Total resistance to heat transfer
Asymptotic fouling resistance
t"
Radius
Rate of oxygen supply
r~
Rate of corrosion
Rate of oxygen supply
Stopping distance or parameter defined by Equation 9.13
SR
Silica ratio
Temperature
L
Cloud point temperature
Tcv
Temperature of critical viscosity
rl
r,
t
Freezing temperature
Pour point temperature
Time
Induction time
U
Average ball circulation time
Electrophoretic mobility of charged particles

Overall heat transfer coefficient for clean conditions
Overall heat transfer coefficient for fouled conditions
Velocity
U o
Initial velocity or velocity in the absence of thermophoresis
//r
U T
Radial velocity
Stokes terminal velocity
/At
Velocity due to thermophoresis
Nomenclature
xiii
U*
f,
Friction velocity
Mean particle volume
V
Volumetric flow
v,
Energy associated with double layers
Total energy of adsorption
Energy associated with van der Waals forces
Electrophoretic mobility
X
Number of cells per unit area
Number of cells to cover completely unit area
Thickness or distance
Subscripts
Av

Average
Bulk
B~o
Biomass
C
Cold or clean
Critical
Crystal face
f
Foulant, or freezing
g
Gas or growth
H Hot
Impact
Induction, or initiation, or interference or inside
/n
Inhibitory
irr
Irreversible
xiv
Fouling of Heat Exchangers
m Mean, or metal
max Maximum
P
Particle or sticking probability
p Pressure
rev Reversible
Scale, surface or solid, saturation
t Time
w Wall or surface

x~ Adsorbed cells
Asymptotic or infinite
Dimensionless numbers
Re = dvp Reynolds number
r/
Pr = cp ~ Prandtl number
St =~ Stanton number
17vc p
ad
Nu = Nusselt number
Sc = r/ Schmidt number
pO
Sh = KI. Sherwood number
D
Bi = a/~ Biot number
2
Nomenclature
xv
Greek
a
P
r/o
~tot
P
2"*
.(2
Heat transfer coefficient
Time constant
Distance over which diffusion takes place
Induced EMF

Viscosity
Particle collection efficiency
Combined collection efficiency for non-impacting mechanisms
Overall particle collection efficiency
Fraction of surface
Thermal conductivity
Thermal conductivity of foulant deposit
Thermal conductivity of scale
Kinematic viscosity
Dimensionless group described by Equation 10.48
Density
Foulant density
Shear stress or particle relaxation time
Dimensionless particle relaxation time
Rate of deposition
Particle flux
Particle volume
Particle volume function (see Equation 7.45)
Rate of removal
Scale strength factor
Water quality factor
Table of Contents

Preface

Acknowledgements

Nomenclature

1 Introduction 1

2 Basic Principles 7

3 The Cost of Fouling 15

4 General Models of Fouling 23

5 Fluid Flow and Mass Transfer 33

6 Adhesion 45

7 Particulate Deposition 55

8 Crystallisation and Scale Formation 97

9 Freezing Fouling or Liquid Solidification 137

10 Fouling Due to Corrosion 149

11 Chemical Reaction Fouling 185

12 Biological Growth on Heat Exchanger Surfaces 223

13
The Design, Installation, Commissioning and Operation o
f
Heat Exchangers to Minimise Fouling
269

14 The Use of Additives to Mitigate Fouling 287

15 Heat Exchanger Cleaning 357

16
Fouling Assessment and Mitigation in Some Common
Industrial Processes
409

17 Obtaining Data 479

Index 517


CHAPTER 1
Introduction
The accumulation of unwanted deposits on the surfaces of heat exchangers is
usually referred to as fouling. The presence of these deposits represents a
resistance to the transfer of heat and therefore reduces the efficiency of the
particular heat exchanger. The foulant may be crystalline, biological material, the
products of chemical reactions including corrosion, or particulate matter. The
character of the deposit depends on the fluid (liquid or gas) passing through the
heat exchanger. It may be the bulk fluid itself that causes the problem of deposit
formation, e.g. the decomposition of an organic liquid under the temperature
conditions within the heat exchanger. Far more often than not, the fouling problem
is produced by some form of contaminant within the fluid, often at very low
concentration, e.g. solid particles or micro-organisms.
Fouling can occur as a result of the fluids being handled and their constituents in
combination with the operating conditions such as temperature and velocity.
Almost any solid or semi solid material can become a heat exchanger foulant, but
some materials that are commonly encountered in industrial operations as foulants

include:
Inorganic materials
Airborne dusts and grit
Waterborne mud and silts
Calcium and magnesium salts
Iron oxide
Organic materials
Biological substances, e.g. bacteria, fungi and algae
Oils, waxes and greases
Heavy organic deposits, e.g. polymers, tars
Carbon
Fig. 1.1 is a photograph of the tube plate of a shell and tube heat exchanger
fouled with particulate matter deposited from high temperature flue gases passing
through the tubes.
The problems associated with heat exchanger fouling have been known since
the first heat exchanger was invented. The momentum of the industrial revolution
depended on the raising of steam, usually from coal combustion. In the early days
serious problems arose in steam raising equipment on account of the accumulation
of deposits on the water side of boilers. The presence of these deposits, usually
crystalline in character originating from the dissolved salts in the feed water,
Fouling of Heat Exchangers
caused the skin temperature of the boiler tube to reach dangerous levels allowing
failure to occur. Development of suitable feed water treatment programmes
however has largely eliminated the problem in modem boiler operating technology.
FIGURE 1.1. A fouled tube plate of a shell and tube boiler
In general the ability to transfer heat efficiently remains a central feature of
many industrial processes. As a consequence much attention has been paid to
improving the understanding of heat transfer mechanisms and the development of
suitable correlations and techniques that may be applied to the design of heat
exchangers. On the other hand relatively little consideration has been given to the

problem of surface fouling in heat exchangers. A review [Somerscales 1988] that
traces the history of heat exchanger fouling suggests four epochs in the
development of an understanding of the problem of fouling. The chronology
follows in general, the development of science and measurement techniques over
the same timescale. In the first period up to about 1920, concern was directed
towards observing the phenomenon and devising methods of reducing the problem
with less emphasis on the scientific understanding of the mechanisms involved.
The second period from 1920 - 1935 covered development in the measurement of
fouling and representation. The following ten years from 1935 - 1945 saw the
extended use of the so-called "fouling factor". The fouling factor may be defined
as the adverse thermal effects of the presence of the deposit, expressed in
numerical terms. Fouling factors or fouling resistance is discussed in more detail in
Chapter 2. From 1945 to the present time a more scientific approach to the
problem of fouling has been introduced with detailed investigations into the
mechanisms that underline the problem of heat exchanger fouling. Chapters 7 - 12
detail the physical and chemical conditions and interactions that lead to heat
exchanger surface fouling.
The review by Somerscales [1988] demonstrates how little has been done
towards a better understanding of the problem since the early 1800s except in
steam raising technology. It is indeed an anomaly that the accuracy of many
sophisticated design techniques is restricted by a lack of understanding of the
Introduction 3
fouling process likely to be associated with the particular process under
consideration.
Energy conservation is often a factor in the economics of a particular process.
At the same time in relation to the remainder of the process equipment, the
proportion of capital that is required to install the exchangers is relatively low. It is
probably for this reason that heat exchanger fouling has been neglected as most
fouling problems are unique to a particular process and heat exchanger design.
The problem of heat exchanger fouling therefore represents a challenge ['Boa

1992] to designers, technologists and scientists, not only in terms of heat transfer
technology but also in the wider aspects of economics and environmental
acceptability and the human dimension.
The principal purpose of this book is to provide some insight into the problem
of fouling from a scientific and technological standpoint. Improved understanding
of the mechanisms that lead to the accumulation of deposits on surfaces will
provide opportunities to reduce or even eliminate, the problem in certain situations.
Three basic stages may be visualised in relation to deposition on surfaces from a
moving fluid. They are:
1. The diffusional transport of the foulant or its precursors across the boundary
layers adjacent to the solid surface within the flowing fluid.
2. The adhesion of the deposit to the surface and to itself.
3. The transport of material away from the surface.
The sum of these basic components represents the growth of the deposit on the
surface.
In mathematical terms the rate of' deposit growth may be regarded as the
difference between
~D -~R (1.1)
where #~ and #R are the rates of deposition and removal respectively.
The extent of the adhesion will influence #R-
Fig. 1.2 shows an idealised asymptotic graph of the rate of growth of a deposit
on a surface. In region A the process of adhesion is initiated. In some fouling
situations the conditioning (or induction) period can take a long time, perhaps of
the order of several weeks. In other examples of fouling the initiation period may
be only of the order of minutes or even seconds.
Region B represents the steady growth of the deposit on the surface. Under
these circumstances there is competition between deposition and removal. The
rate of deposition gradually falls while the rate of removal of deposit gradually
Fouling of Heat Exchangers
G,I

w
.4,,
.4
o,
Vl
0
s
W
r~
Time
FIGURE 1.2. The change in deposit thickness with time
increases. Finally the rate of removal and the rate of deposition may become equal
so that a plateau steady state or asymptote is reached (Region C) when the deposit
thickness remains virtually constant.
Many system variables will affect the extent of the various stages and these will
be discussed in subsequent chapters.
The world consumption of energy is large, taking into account all sources and
methods of utilisation. Fossil fuels are of course, extensively used for the
generation of heat used to raise steam for the production of electrical power.
Under these circumstances a large fraction of the heat released from the fuel is
transferred across various heat exchangers to the cold utility (cooling water).
Under the prevailing conditions of operation there is ample opportunity for heat
transfer surfaces to become fouled with attendant reductions in the efficiency of
energy utilisation. In other processes the primary fuels, coal, oil or natural gas are
used for process stream heating. For instance the crude oil vaporisers in petroleum
refining are usually heated by means of fuel oil combustion.
Reduced efficiency of the heat exchangers due to fouling, represents an increase
in fuel consumption with repercussions not only in cost but also in the conservation
of the world's energy resources. The necessary use of additional fossil fuels to
make good the shortfall in energy recovered due to the fouling problem, will also

have an impact on the environment. The increased carbon dioxide produced
during combustion will add to the "global warming" effect.
Although reduced heat transfer efficiency is of prime importance there may also
be pressure drop problems. The presence of the foulant will restrict flow that
results in increased pressure drop. In severe examples of fouling the exchanger
may become inoperable because of the back pressure. Indeed the pressure drop
problems may have a more pronounced effect than the loss of thermal efficiency.
Introduction 5
In order to help reduce or overcome the problem of fouling, additives may be
used. For instance a whole industry has built up around the treatment of water
used for cooling purposes. The various chemicals added to the water fall into three
categories, i.e. control of biological growth, prevention of scale formation and
corrosion inhibition. Careful choice of treatment programmes will do much to
reduce the accumulation of deposits on heat exchange surfaces. The addition of
chemicals however, brings with it problems for the environment.
It is oRen the case that the water used for cooling is returned to its source, e.g.
a fiver or lake. Even water taken from some other source such as a bore hole will
eventually be returned to the natural environment. The presence of the additives
can represent a hazard for the environment since many of the chemicals may be
regarded as toxic.
Despite the best efforts of engineers and technologists to reduce or eliminate
heat exchanger fouling the growth of deposits will still occur in some instances.
Periodic cleaning of the heat exchangers will be necessary to restore the heat
exchanger to efficient operation. If the deposits are difficult to remove by
mechanical means chemical cleaning may be required. The chemicals used for this
purpose will of'ten be aggressive in character and represent an effluent problem
after the cleaning operation. Unless this effluent is properly treated it could also
represent an environmental problem. Even water used for cleaning can become
contaminated and may require suitable treatment before discharge.
Of direct concern to the operator of process equipment are the economic

aspects of heat exchanger fouling since this will affect the operating costs that in
turn affects the profitability of the operation as a whole. In the first instance the
heat exchanger is generally overdesigned to allow for the incidence of fouling.
Increasing the size of the exchanger will of course, increase the initial capital cost
and hence the annual capital charge.
The restriction to flow imposed by the presence of the deposit means that for a
given throughput the velocity will have to increase. The increased velocity
represents an increase in pumping energy and hence an increase in costs. Many
pumps are electrical and so the increased energy requirement is in terms of the
more expensive secondary energy. Other operating costs can accrue from the
presence of the deposits such as increased maintenance requirements or reduced
output. Emergency shutdown as a direct result of heat exchanger fouling can be
particularly expensive. In many examples of severe fouling the frequency of
cleaning may not coincide with ~ the planned periodic plant shutdown for
maintenance (annual basis) and it might be necessary to install standby heat
exchangers for use when the cleaning of heat exchangers becomes necessary.
Additional heat transfer capacity provided by standby equipment represents an
additional capital charge.
Finally, but by no means least, there is the human dimension to the problem of
fouling. Severe fouling can lead to the loss of employee morale [Bott 1992]. The
repeated and persistent need to shut down the plant to clean heat exchangers, or
difficulties in maintaining the desired output due to the accumulation of deposits
Fouling of Heat Exchangers
will inevitably lead to frustration on the part of those employees whose duty it is to
maintain production and product quality. The problem is compounded if financial
incentives are linked to quality and volume of production. Repeated subjection to
these difficult working conditions can lead to an indifferent attitude that can
compound the unsatisfactory character of production brought about by the
particular fouling problem.
The fouling of heat exchangers is a wide ranging topic coveting many aspects of

technology. It represents a challenge not simply in terms of reducing product cost,
and hence competitiveness in the market place, but also with the concerns of
modem society in respect of conservation of limited resources, for the environment
and the natural world, and for the improvement of industrial working conditions.
It is the purpose of this book to provide a background and basis that will enable the
reader to face up to the challenges presented by the problem of heat exchanger
fouling.
REFERENCES
Bott, T.R., 1992, Heat exchanger fouling. The challenge, in: Bohnet, M., Bott,
T.R., Karabelas, A.J., Pilavachi, P.A., S6m6ria, R. and Vidil, R. eds.
Fouling Mechanisms - Theoretical and Practical Aspects. Editions
Europ6ennes Thermique et Industrie, Pads, 3 - 10.
Somerscales, E.F.C., 1988, Fouling of heat transfer surfaces: an historical review.
25th Nat. Heat. Trans. Conf. ASME. Houston.
CHAPTER 2
Basic Principles
The accumulation of deposits on the surfaces of a heat exchanger increases the
overall resistance to heat flow. Fig. 2.1 illustrates how the temperature distribution
is affected by the presence of the individual fouling layers.
FIGURE 2.1. Temperature distribution across fouled heat exchanger surfaces
T 1 and T 6 represent the temperatures of the bulk hot and cold fluids respectively.
Under turbulent flow conditions these temperatures extend almost to the boundary
layer in the respective fluids since there is good mixing and the heat is carded
physically rather than by conduction as in solids or slow moving fluids. The
boundary layers (the regions between the deposit and the fluid), because of their
near stagnant conditions offer a resistance to heat flow. In general the thermal
conductivity of foulants is low unlike that of metals which are relatively high. For
these reasons, in order to drive the heat through the deposits relatively large
temperature differences are required, whereas the temperature difference across the
metal wall is comparatively low.

Fouling of Heat Exchangers
Thermal conductivities of some common foulant-like materials are given in
Table 2.1 that also includes data for common construction materials. The effects
of even thin layers of foulant may be readily appreciated.
TABLE 2.1
Some thermal conductivities of foulants and metals
Material
Thermal conductivity
W/mK
Alumina 0.42
Biofilm (effectively water) 0.6
Carbon 1.6
Calcium sulphate 0.74
Calcium carbonate 2.19
Magnesium carbonate 0.43
Titanium oxide 8
Wax 0.24
Copper 400
Brass 114
Monel 23
Titanium 21
Mild steel 27.6
The resistance to heat flow across a solid surface is given as
x
-~- (2.1)
where x is the solid thickness
and
2 is the thermal conductivity of the particular solid.
Referring to the diagram (Fig. 2.1) the resistances of the solids to heat flow are:
For Deposit 1 x L where 2~ is the thermal conductivity of Deposit 1

21
For Deposit 2 xz where 22 is the thermal conductivity of Deposit 2
Basic Principles
and for the metal wall xm where ,;1, m is the thermal conductivity of the metal
For steady state conditions the heat flux q
q=
T2-T3= T3-T4=
T4-T5
(2.2)
Also q= a,(~- g) = a,(~- ~)
(2.3)
where a~ and tz 2 are the heat transfer coefficients for the hot and cold fluids
respectively.
Equation (2.3) can be rewritten as
~-~_~-~
(2.4)
1 1
and
al a 2
respectively.
represent the resistance to heat flow of the hot and cold fluids
The total resistance to heat flow will be the sum of the individual resistances, i.e.
R r the total thermal resistance will be given by
Rr (x~)(x~22)(x~.) 1
1
= + + +~4 -
a I a 2
(2.5)
The overall temperature driving force to accomplish the heat transfer between the
hot and cold fluids is the sum of the individual temperature differences

i.e.
(~- r~) + (~ - ~)+(~ - ~) + (r, - ~)+(~ - ~)
or
TI-T 6
.'.q- T~-T6 (2.6)
ev
10
Fouling of Heat Exchangers
If the heat exchange area required for the required heat transfer is A, then the rate
of heat transfer Q
ev
(2.7)
In general the design of heat exchangers involves the determination of the
required area A. The necessary heat transfer, the temperatures and the fluids are
generally known from the process specification, the individual heat transfer
coefficients of the fluids may be calculated, and values of the fouling resistances on
either side of the heat exchanger would have to be estimated. It is the latter that
can be difficult and if the resistances are incorrectly estimated difficulties in
subsequent operation may be manifest.
At first sight it may be thought possible to calculate the fouling resistance, i.e.
x:12:
where
x:is
the deposit thickness
and
2 r is the foulant thermal conductivity
The difficulty is however, that this involves a knowledge of the likely thickness
of the deposit laid down on the heat exchanger surfaces and the corresponding
thermal conductivity. In general these data are not available. It is therefore
necessary to assign values for the fouling resistance in order that the heat

exchanger may be designed.
An alternative way of writing Equation 2.6 for clean conditions when the heat
transfer surfaces are clean, is:
q= Uc(T~ - T6)
(2.8)
where
U c
represents the overall heat transfer coefficient for clean conditions, i.e.
gc +~-t -
al a2
(2.9)
and allowing for the fouling resistances on either side of the heat transfer surface
1 x~ x 2 x,. +~+~ (2.10)
uo +
Rewriting Equation 2.8 for fouled conditions, to give the heat flux
Basic Principles
11
q = Uo ( T~ - T6 )
(2.11)
Because the temperature driving force across the heat exchanger usually varies
along the length of the heat exchanger, it is necessary to employ some mean value
of the temperature difference in using Equations 2.8 and 2.11.
If ATt and AT 2 are the temperature difference between the hot and cold fluids at
either end of the heat exchanger then the temperature difference may be taken as
the arithmetic mean, i.e.
AT,,, = ATe- AT 2 (2.12)
2
but more usually the log mean temperature difference is used, i.e.
AT,,, = AT~ - AT 2 (2.13)
ln(A~

For more background to the use of mean temperature differences in the design
of heat exchangers the reader is referred to such texts as Hewitt, Shires and Bott
[1994].
The mean temperature difference may be substituted in Equation 2.11 to give
the heat flux
q = UoAT ~
(2.14)
and if the total available heat transfer area A is taken into account
Q=UoAAT ~
(2.15)
In the design of heat exchangers A is usually unknown, so rearranging Equation
2.15 provides a means of estimating the required heat transfer area, i.e.
A= Q (2.16)
The choice of the individual fouling resistances for the calculation of
Uo
can
have a marked influence on the size of the heat exchanger and hence the capital
cost.
For a heat exchanger transferring heat from one liquid to another with the
individual liquid heat transfer coefficients of 2150 and 2940
W/mZK
and fouling
12
Fouling of Heat Exchangers
resistances of 0.00015 and 0.0002 m2/WK on the surfaces of the heat exchanger,
the total resistance to heat flow is
1 1
2150 2940
0.00015 + 0.0002 m2/WK
= 0.00047 + 0.00034 + 0.00015 + 0.0002 m2/WK

= 0.00116 m2K/W
For the given design conditions, i.e. thermal load and temperature difference
this fouling resistance represents an increase in the required heat exchanger area
over and above the clean area requirements of
0.00035
0.00081
x 100% = 43.2%
i.e. the cost of the heat exchanger will be increased considerably due to the
presence of the fouling on the heat exchanger.
If the same fouling resistances are applied to a heat exchanger transferring heat
between two gases where the individual heat transfer coefficients are much lower
due to the low thermal conductivity of gases, say 32.1 and 79.2
W/m2K,
the
situation is quite different.
Under these conditions the total thermal resistance is
1 1
+, + 0.00035 m2K/W
32.1 79.2
= 0.0312 + 0.0126 + 0.00035 m2K/W
= 0.0442 m2K/W
In these circumstances the increase in required area in comparison to the clean
conditions is
0.00035
0.0438
x 100% = 0.8%
For the liquid/liquid exchanger the choice of fouling resistances represents a
considerable increase in the required surface in comparison within the clean
Basic Principles 13
conditions. Using the same fouling resistances for a gas/gas heat exchanger

represents negligible additional capital cost.
The traditional method of designing heat exchangers is to consider the potential
fouling problem and assign a suitable fouling resistance to correspond. In order to
assist with this selection, organisations such as the Tubular Exchanger
Manufacturers Association (TEMA) issue tables of fouling resistances for special
applications. The first edition appeared in 1941. From time to time these data are
reviewed and revised. A review was carried out in 1988 and made available
[Chenoweth 1990].
The principal difficulty in this approach to design is the problem of choice. At
best the tables of fouling resistances give a range of mean fouling resistances, but
in general there is no information on the conditions at which these values apply.
For instance there is generally no information of fluid velocity, temperature or
nature and concentration of the foulant. As will be seen later these factors
amongst others, can have a pronounced effect on the development of fouling
resistance. Probably the largest amount of information contained in the tables is
concerned with water. Table 2.2 presents the relevant data published by TEMA
based on a careful review and the application of sound engineering acumen by a
group of knowledgeable engineers, involved in the design and operation of shell
and tube heat exchangers.
TABLE 2.2
Fouling resistances in water systems
Water type Fouling resistance
104 m2K/W
Sea water (43~ maximum outlet)
Brackish water (43~ maximum outlet)
Treated cooling tower water
(49~ maximum outlet)
Artificial spray pond (49~ maximum outlet)
Closed loop treated water
River water

Engine jacket water
Distilled water or closed cycle condensate
Treated boiler feedwater
Boiler blowdown water
1.75-3.5
3.5 - 5.3
1.75 - 3.5
1.75 - 3.5
1.75
3.5- 5.3
1.75
0.9- 1.75
0.9
3.5 - 5.3
Chenoweth [ 1990] gives the assumptions underlying the data contained in Table
2.2. For tubeside, the velocity of the stream is at least 1.22
m/s
(4fl/s) for tubes of
non-ferrous alloy and 1.83
m/s
(6
ft/s)
for tubes fabricated from carbon steel and
other ferrous alloys. For shell-side flow the velocity is at least 0.61
m/s
(2ft/s). In