Page 184 - Chemical process engineering design and economics

166 Chapter 4

1
U 0= ———————————— (4.15)
Do 1 1
R fi + —— +— +Rf 0

Individual heat-transfer coefficients and the fouling resistance or fouling
factor, are listed
in Table 4.3. The heat transfer coefficients in Table 4.3 are divided according to
whether the fluid is inorganic or organic, a gas or a liquid, and whether it is heated
or cooled, with or without a phase change. The inorganic fluids are water and
ammonia, whereas the organic fluids are divided into three categories, light, me-
dium, or heavy, depending on their viscosity. If the fluid is a gas, pressure will
also affect the transfer properties. The footnotes in Table 4.3 define a light, me-
dium, and heavy organic fluid.
For boiling liquids, the heat flux cannot be too large or a vapor blanket will
form on the heat transfer surface, effectively insulating the surface and thus reduc-
ing the heat-transfer coefficient. Gases or vapors have a much lower heat-transfer
coefficient than a boiling liquid. If heat is supplied by a condensing vapor or hot
liquid, a considerable reduction in heat transfer will occur if a vapor blanket
forms. If the fluid is being heated electrically, however, heat transfer will remain
essentially the same, and the heater surface temperature will rise until the heater
"burns out". To avoid this problem, Walas [3] recommends designing a heat ex-
changer for a boiling heat flux of less than 1.3xl05 W/m2 (4.12xl04 Btu/h-ft2).

Terminal Temperatures of the Fluid Streams

Before calculating the logarithmic-mean temperature difference, determine the
terminal temperature of each fluid stream. Three of the four terminal temperatures
are usually specified, and the fourth can be found by optimizing the fixed and op-
erating costs for the heat exchanger. If we consider cooling a process stream, then
the stream temperature at the inlet and outlet of the heat exchanger will usually be
known. The stream leaves one process unit and enters the heat exchanger. Then,
the stream is cooled to a specified temperature, depending on the requirements of
the next process unit. Also, if the coolant is water, which is generally the case, its
temperature varies throughout the year. Take the worst case, which is approxi-
mately 30 °C (86 °F) in the New York area. The next step is to calculate the exit
water temperature, which is discussed in Example 4.1.

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