Page 210 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10F 5/30/2001 4:35 PM Page 173

Heat Transfer 173

Table 10-28 The following is a calculation procedure suggested by
90
Error Comparison of Test Case from Reference 91* Palen and Small for the required boiling coefficient for a
horizontal tube bundle:
Existing Methods Proposed Methods
1. Assume a value for h l .
Statistical Tube-by-
Gilmour Model Tube-Model 2. Calculate U l from Equation 10-161.
Kern (Eq. 8) (Eq. 10) (Eq. 12)
2
r w wall resistance, 3Btu>1hr2 1ft 2 1°F24 1
Error with no safety 2 1
factor r i inside fluid fouling resistance, 3Btu>1hr2 1ft 2 1°F 24
1. Ave. % overdesign 61 15 17 15
2. Ave. % underdesign 0 40 19 10 3. Calculate h l from McNelly or Gilmour equations.
Errors with safety
4. Compare calculated h l with assumed value, if difference
factor included
is significant, use the calculated value and repeat from
3. Area safety factor
step 2 until convergence is acceptable
required 1.0 1.80* 1.25 1.25
4. Ave. % overdesign 61 48* 26 30 5. Calculate T b from:
5. Max. % overdesign 140 110* 75 60
*Excluding case 15, which showed very high error. T b 1U l >h l 2 1 T2 (10-158)
Used by permission: Palen, J. W., and Taborek, J.J. Chemical Engineering Pro-
gress, V. 58, No. 7, p. 43, ©1962. American Institute of Chemical Engineers, If T b is less than 8°F, free convection must be taken
Inc. All rights reserved. into account by a corrected h l h l :

3 2 0.25
D o L g L T b c L
h l ¿ h l 0.531k L >D o 2c d (10-159)
L k L
where
statistical analysis results). Also see Table 10-28 where the T b mean temperature difference between the bulk of
equation references are to their article reference, not this the boiling liquid and the tube wall, °F
91
current text. T mean temperature difference between the bulk of
the boiling liquid and the bulk of the heating
h b = 0.714 h (p d) 4.2 10 5 G (1/N rv ) 0.24 (1.75 + ln(1/N ) (10-157) medium, °F
rv
h l nucleate boiling coefficient for an isolated single
2
tube, Btu/(hr)(ft )(°F)
where
h l nucleate boiling coefficient for an isolated
G is the single tube mass velocity through the (p-d) tube space,
single tube corrected for free convection,
defined as:
2
Btu/(hr)(ft )(°F)
coefficient of thermal expansion of liquid
a t 1 t2U
G
1p d2 Other symbols as cited previously.
where most symbols are as defined earlier, plus 6. Calculate the correction to the nucleate boiling film
2
a t surface area, ft /ft of tube outside surface area coefficient for the tube bundle number of tubes in ver-
p tube pitch, ft
tical row, h b . See previous discussion.
d tube O.D., ft
7. Using Equation 10-161 to determine the overall U for
h b heat transfer coefficient for reboiler bundle,
2
Btu/hr-ft -°F bundle.
8. Determine the area required using U of step 7.
U overall heat transfer coefficient based on theoretical
single tube, h 9. Determine the physical properties at temperature: T b
T b /2.
The fit of the data to the proposed equation is on average
30% overdesign, which is good in terms of boiling data
and when compared to the Gilmour’s bundle coefficient of Nucleate or Alternate Designs Procedure
48% and Kern’s 61%. 91
The following nucleate or alternate designs procedure,
70
2
h theoretical boiling coefficient, Btu/(hr)(ft )(°F) suggested by Kern, is for vaporization (nucleate or pool
h b heat transfer coefficient for reboiler bundle, boiling) only. No sensible heat transfer is added to the boil-
2
Btu/(hr)(ft )(°F) ing fluid.