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Heat Transfer 83

r i (hr-ft 2 -°F/Btu)

Fouling Resistance—r o or

Figure 10-40B. Fouling resistance for various conditions of surface fouling on heat exchanger surfaces. Thermal resistance of typical uniform
deposits. Note that the abscissa reads for either the inside, r i , or outside, r o , fouling resistance of the bulidup of the resistance layer or film on/in
th
the tube surface. (Used by permission: Standards of Tubular Exchanger Manufacturers Association, 6 Ed, p. 138, © 1978. Tubular Exchanger
Manufacturers Association, Inc. All rights reserved.)

with considerable examination of the fouling range, both
density of the fouling material. These authors 137 suggest the
inside and outside of the tubes, and by determining the
need for specific time-dependent data to better define foul-
effects these have on the surface area requirements. Just a
ing, and they propose calculation techniques but no actual
large unit may not be the proper answer. Fouling of the tube physical data. If the time required to reach a certain level of
surfaces is usually expressed 107 asfollows:
fouling is measured or observed operationally, then clean-
ing maintenance schedules can be better coordinated by
r o fouling resistance on outside of tube,
considering production downtime, rather than having the
2
1hr21°F21ft outside surface2 need to improve the heat transfer become a surprise or
1Btu2 “crater” situation.
Epstein 145, 146 lists six types of fouling:
r I fouling resistance on inside of tube,
• Precipitation or scaling fouling: precipitation on hot
2
1hr21°F21ft inside surface2
surfaces or due to inverse solubility.
1Btu2
• Particulate fouling: suspended particles settle on heat
Fouling of the tube surfaces (inside and/or outside) can transfer surface.
be an important consideration in the economical and ther- • Chemical reaction fouling: deposits formed by chemical
mal design of a heat exchanger. Most fouling can be catego- reaction in the fluid system.
rized by the following characteristics. 107 Note that biological • Corrosion fouling: corrosion products produced by a
fouling is not included. reaction between fluid and heat transfer surface, and
tube surface becomes fouled.
• Linear
• Solidification fouling: liquid and/or its components in
• Falling-rate
liquid solution solidify on tube surface.
• Asymptotic
• Biological fouling: biological organisms attach to heat
Essentially all three of these types are time-dependent transfer surface and build a surface to prevent good
regarding the buildup or increase in the thickness and/or fluid contact with the tube surface.