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90 Applied Process Design for Chemical and Petrochemical Plants

Table 10-13B Fouling factor outside tube, r o 0.001
Preliminary Design Resistances Fouling factor inside tube, r i 0.0025
Tube wall 1 in.—16 ga, Admiralty, 0.065-in. thick,
Basis: Pressures Used in Commercial Fractionations 2
k w 768 Btu/(ft ) (hr)/(°F)/(in.)
Inside area/ft A i 0.2278 ft /ft
2
Heating Side, r p ' Clean Service
2
Outside area/ft A o 0.2618 ft /ft
Condensing steam 0.0005 0.0010
Cooling hot water 0.0025 0.0045
1
Cooling hot oil 0.0080 0.0100 U o
Combustion gases * * 1 0.001 0.065 0.0025 1
Boiling Side, r h ' Clean Service 175 768 0.2278
600a b
0.2618
C 2 —C 4 hydrocarbons 0.0030 0.0040
Gasoline and naphthas 0.0050 0.0060 U o 1
Aromatics 0.0030 0.0040 0.00571 .001 0.0000846 0.0025 0.00192
2
C 2 —C 7 alcohols 0.0030 0.0040 U o 1>0.01121 89.1 Btu>hr 1ft 2 1°F2
Chlorinated hydrocarbons 0.0040 0.0070
Water (atm. pressure) 0.0015 0.0025 Note the relative effects of the tube wall resistance when
*For direct-fired reboilers, estimate area on basis of heat flux: compared to the fouling factors in this case.
Radiant zone q/A 10,000 Btu/(hr)(ft )(°F)
2
2
Convection zone q/A 3,500 Btu/(hr)(ft )(°F)
Used by permission: Fair, J. R., Petroleum Refiner. Feb. 1960, reference 45. Approximate Values for Overall Coefficients
©Gulf Publishing Company, Houston, Texas. All rights reserved.
Various fluid heat transfer operations can be characterized
in a general way by values of the overall coefficient, U. The val-
ues given in Perry cannot be all-inclusive for every situation.
94
Table 10-14 However, they are suitable for use in estimating exchanger
Thermal Conductivity—Special Materials performance and in checking (approximately) the calculated
values and similar nonexact comparisons. Table 10-15 lists a
Material k, Btu/(hr) (ft ) (°F)/(ft)
2
variety of applications and the corresponding overall U values
and fouling factors. In general, these units have performed
Carbon 3
Graphite 90 without difficulty, although the questions that cannot be
®
Karbate , carbon base 3 answered are whether they may have been too large or how
®
Karbate , graphite base 80 much too large they may have been.
Teflon ® 0.11 Tables 10-16, 10-17, 10-18, and 10-18A give general esti-
Glass (chemical resistant) 6.9 mating overall coefficients, and Table 10-19 gives the range
*To convert to Btu/(hr) (ft ) (°F)/(in.), multiply by 12. of a few common film coefficients. Table 10-20 illustrates the
2
To convert to gram calories/(sec) (cm ) (°C)/(cm), multiply by 0.004134. effect of tube-wall resistance for some special construction
2
materials. Table 10-20A lists estimating coefficients for glass-
lined vessels. Also see Reference 215. See Table 10-24 for
This value is then used to represent the film coefficient suggested water rates inside tubes.
equivalent to the converted inside coefficient, as h io . For steam jacketed, agitated closed reactor kettles, the over-
2
Figure 10-45 is convenient for solving for a clean U using all U usually will range from 40—60 Btu/hr (ft )(ºF). Of course,
known or estimated film coefficients only. the significant variables are the degree or type of internal wall
turbulence and the viscosity and thermal characteristics of the
Example 10-8. Calculation of Overall Heat Transfer internal fluid. For water or other liquid cooling in the reactor
Coefficient from Individual Components jacket, the U value usually ranges from 20—30.
®
For duPont’s Teflon tube ( / 4 -in. diameter) heat exchang-
1
An exchanger has been examined, and the following indi- ers (Figure 10-8) for condensing, heating, and cooling
vidual coefficients and resistances determined. What is the service, the U values range from 15—35. Little or no fouling
overall coefficient of heat transfer referenced to outside occurs on the Teflon surface.
®
coefficients? (Methods for determining these film coeffi- Figure 10-39 presents the effect of total fouling on the
cients are given later). overall coefficient. For example, if a clean nonfouled coeffi-
cient is corrected to the fouled condition by one overall
Film coefficient outside tube, h o 175 fouling factor, the effect of changing the expected amount
Film coefficient inside tube, h i 600 of fouling to another value can be readily determined.