Page 330 - Mechanical Engineers' Handbook (Volume 4)

4 Common Operational Problems 319

the heat-exchanger designer, the following subsections relating the more obvious problems
are included to help make the learning curve less eventful.

4.1 Fouling
The deposit of solid insulating material from process streams on the heat-transfer surface is
known as fouling, and has been called ‘‘the major unresolved problem in heat transfer.’’ 44
Although this problem is recognized to be important (see Ref. 45) and is even being seriously
researched, 45,46 the nature of the fouling process makes it almost impossible to generalize.
3
As discussed by Mueller, fouling can be caused by (1) precipitation of dissolved substances,
(2) deposit of particulate matter, (3) solidification of material through chemical reaction, (4)
corrosion of the surface, (5) attachment and growth of biological organisms, and (6) solid-
ification by freezing. The most important variables affecting fouling (besides concentration
of the fouling material) are velocity, which affects types 1, 2, and 5, and surface temperature,
which affects types 3–6. For boiling fluids, fouling is also affected by the fraction vaporized.
As stated in Ref. 25, it is usually impossible to know ahead of time what fouling mechanism
will be most important in a particular case. Fouling is sometimes catalyzed by trace elements
unknown to the designer. However, most types of fouling are retarded if the flow velocity
is as high as possible, the surface temperature is as low as possible (exception is biological
48
fouling ), the amount of vaporization is as low as possible, and the flow distribution is as
uniform as possible.
The expected occurrence of fouling is usually accounted for in practice by assignment
of fouling factors, which are additional heat-transfer resistances, Eq. (7). The fouling factors
are assigned for the purpose of oversizing the heat exchanger sufficiently to permit adequate
on-stream time before cleaning is necessary. Often in the past the fouling factor has also
served as a general purpose ‘‘safety factor’’ expected to make up for other uncertainties in
the design. However, assignment of overly large fouling factors can produce poor operation
caused by excessive overdesign. 49,50
For shell and tube heat exchangers it has been common practice to rely on the fouling
factors suggested by TEMA. Fouling in plate heat exchangers is usually less, and is dis-
1
cussed in Ref. 38. The TEMA fouling factors have been used for over 30 years and, as
Mueller states, must represent some practical validity or else complaints would have forced
their revision. A joint committee of TEMA and HTRI members has reviewed the TEMA
fouling recommendations and slightly updated for the latest edition. In addition to TEMA,
fouling resistances are presented by Bell 10 and values recommended for reboiler design are
given in Ref. 25. For preliminary estimation, the minimum value commonly used for design
is 0.0005 F hr ft /Btu for condensing steam or light hydrocarbons. Typical conservative
2
2
estimates for process streams or treated cooling water are around 0.001–0.002 F hr ft /Btu,
2
and for heavily fouling streams values in the range of 0.003–0.01 F hr ft /Btu are used. For
reboilers (which have been properly designed) a design value of 0.001 F hr ft /Btu is usually
2
adequate, although for wide boiling mixtures other effects in addition to fouling tend to limit
performance. These commonly used estimates can contain large built-in safety factors, and
should not necessarily be accepted for modern computerized designs. A more realistic ap-
proach for most fluids is proposed in Section 5.6 under Fouling.
On the other hand, heavily fouling fluids such as crude oils may require even larger
fouling factors for reasonable on-stream times than those given in TEMA. In this case,
detailed physical characteristics of the fluid must be determined by experiment before real-
istic design fouling allowances can be assigned.
Heat Transfer Research, Inc., in cooperation with a task force of industry experts, has
an ongoing research program to measure fouling rates over a range of process conditions
and compare with fluid characteristics.