Page 207 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10F 5/30/2001 4:35 PM Page 170

170 Applied Process Design for Chemical and Petrochemical Plants

separate shell, as it is inserted into the circular shell of the p sin 1>2
sidewall of the distillation column or tank, for example, see 0.359 a bc d (10-147)
D o N
Figure 10-111. It is inserted into the body of the liquid to be
g 1 l v 2 0.25
heated/boiled. The level of the liquid is also controlled as v c d (10-148)
2
described earlier. v
The mechanism of boiling is essentially nucleate pool q max K (10-149)
boiling. In both styles of reboiler the liquid velocity is rela-
tively low compared to thermosiphon units. 90, 188 Jacobs 188 Gilmour’s bundle correction is
provides an extensive comparison of advantages and disad- h b h (N rv ) 0.185 (v s ) 0.358 , improved to
0.185
0.385
vantages of essentially all the reboiler types used in industrial h b h (N rv ) (v s /v c )
91
plants. Palen and Taborek conducted extensive studies of N rv number of holes in vertical center row of bundle
v s superficial vapor velocity
available data and proposed nucleate boiling equations to
v c maximum flux
correlate various data from the available 14 equations down
v c q max / v , where q max comes from the Zuber
to a selected 6 for detailed study. The study was limited to equation discussed separately
various hydrocarbons and hydrocarbon mixtures. Their
conclusions after computer correlations of the results from The results for small bundles do not agree as well as the
several equations were as follows. Palen and Taborek equation.
91
90
Palen recommendation corrects single tube boiling data Determine q max from Figure 10-103A. Use a safety factor
(outside) to the bundle effect in a horizontal reboiler by: of 0.7 with Equation 10-149 per the recommendation of ref-
erence 91 for conservative results.
Revised boiling coefficient, h b h 1t 1BCF2
where
This is limited by the maximum heat flux of approxi- D b bundle diameter, ft
2
mately 12,000—25,000 Btu/(hr)(ft ). L average bundle length, ft
The bundle correction factor for vapor blanketing: A bundle heat transfer surface, ft (outside)
2
tube layout angle, degrees
4.2110 52 G 0.2431.75 ln 11>N N number of tube holes/tubesheet. Note: U-tubes have
1BCF2 h ow 30.714 1p D o 31>N vc 4 vc 24
(10-144) 2 holes per tube, so N 2 number of tubes
where v vapor density, lb/ft 3
l liquid density, lb/ft 3
a o 1U 1 21 T2
G mass velocity of vapor (10-145) g acceleration of gravity, ft/(hr)(hr)
1 2p D o
latent heat, Btu/lb
U 1 is found by Equation 10-161 overall coefficient for surface tension, lb (force)/ft
2
isolated single tube, Btu/(hr)(ft )(°F). K empirically determined constant used as 176 in the
range of
for bundles
2
G mass velocity of vapor from a bottom tube based on q max maximum heat flux, Btu/(hr)(ft )
2
the (p D o ) spacing, lb/(hr) (ft ) maximum flux physical property factor,
3
a o tube outside heat transfer surface, ft /ft Btu/(ft )(hr)
2
p tube pitch, ft p tube pitch, ft
D o tube O.D., ft D o tube O.D., ft
N vc number of tubes in the center vertical row of bundle
latent heat, Btu/lb The original Zuber 191 equation for maximum heat flux as
T mean temperature difference between the bulk of modified by Palen 90
the boiling liquid and the bulk of the heating
medium, °F
2 0.25 0.5
h b corrected boiling coefficient, Btu/(hr)(ft )(°F), for q max 25.8 1 v 2 1 2 3 1 l v 2 g > v 4 31 v l 2 > l 4 ,
2
bundle
3
Btu/(hr)(ft ) (10-150)
h 1t nucleate boiling coefficient for an isolated single
2
tube, Btu/(hr)(ft )(°F)
Symbols are as defined previously.
Maximum Bundle Heat Flux 91 The tube density coefficient,
, is given in Table 10-27.
The tube wall resistance cannot be ignored for reboilers.
91
Recommended limiting maximum heat flux for the tube Based on the outside tube diameter, 91
density coefficient:
D b 1L2 T wall a o ln1a o >a i 2 a o ln 1D o >D i 2

(10-146) r w c d (10-151)
A k w a o a i 2 k w