Page 209 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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172 Applied Process Design for Chemical and Petrochemical Plants

91
where Palen and Taborek modified the Gilmour equation to
C s surface factor, for clean copper and steel tubes 1.0 better accommodate the effect of surface types and the
and for clean chromium 0.7 effect of pressure, with the results agreeing with all the data
2
g acceleration of gravity, ft/hr 4.17 10 8
50% and 30%, which is better than other proposed cor-
latent heat, Btu/lb
relations.
h 1 nucleate boiling coefficient for single isolated tube,
2
Btu/(hr)(ft )(°F)
0.4 0.6 0.275
0.3
h 9.0 10 4 C s 1c L k L L > 2
c l liquid specific heat, Btu/(lb)(°F)
0.7
2
surface tension, lb/ft 31q> 2 L 4 1P > 0.425 2114,400>P2 m (10-155)
2
P reboiler operating pressure, lb/in. abs.
m 6.0 e 0.0035 T c
To account for tube and bundle geometry, Gilmour’s 90, 91
Maximum errors for data tested is 50% to 30%,
equation is modified based on the single tube cal-
restricted to hydrocarbons with T c 600°R.
culation, 91, 190
where
h b h st 1N rv 2 0.185 1v s 2 0.358 (10-153)
T c critical temperature, °R
q heat flux, Btu/(hr)(ft )
2
v s superficial vapor velocity, ft/sec
C s surface factor, noted with the McNelly equation cited
N rv number of tubes in center vertical row of bundle
earlier
h st h theoretical boiling film coefficient for a single h theoretical boiling coefficient for a single tube,
2
tube, Btu/(hr)(ft )(°F)
Btu/(hr)(ft )(°F)
2
h b heat transfer coefficient for a reboiler bundle, P pressure, lb/ft 2
2
Btu/(hr)(ft )(°F)
latent heat, Btu/lb
surface tension, lb/ft
B. Gilmour Equation 190 reportedly was used successfully in L liquid density lb/ft 3
many reboilers and vaporizers: v vapor density lb/ft 3
L liquid viscosity, lb/hr-ft
2
0.6
h 0.001 1c l G2 1P > L 2 0.425 1k l >c l l 2 1 l >D o G2 0.3 (10-154) c L liquid specific heat, Btu/lb-°F
g acceleration of gravity, ft/hr-hr
where h theoretical boiling coefficient for a single tube,
2
G (V/A) ( l / v ), mass velocity normal to tube surface, Btu/(hr)(ft )(°F)
2
lb/ (hr) (ft ) q heat flux, Btu/(hr)(ft )
2
2
For all other factors constant: h (V/A) 0.706 0.706 a 0.416 q max maximum heat flux, Btu/(hr)(ft )
h film coefficient of boiling heat transfer, m coefficient for equation for h 91
2
Btu/(hr)(ft )(°F)
V vapor produced, lb/hr The maximum heat flux recommended by Zuber 191 and
A surface area, ft 2 confirmed by Palen and Taborek: 91
a proportionality constant 1.0
k k l thermal conductivity of liquid, Btu/(hr)(ft)(°F) 2 0.25 0.5
q max 25.8 1 v 2 1 2 3 1 L v 2 g> v 4 31 v L 2> L 4
c l specific heat of liquid, Btu/lb -°F (10-156)
P pressure of boiling liquid, lb/ft 2
D D o O.D. of tube, ft
where symbols are as listed earlier. To avoid confusion with sub-
surface tension of liquid, lb/ft
scripts: L l (liquid) and V v (vapor).
l viscosity of liquid, lb/(hr)(ft)
L density of liquid, lb/ft 3
91
v density of vapor, lb/ft 3 Examination of plant data by the authors revealed that
tubes spaced closely together tend to create a vapor blan-
In tube bundles, if the disengaging space between the keting effect and the consequence of lower heat flux than
bundle and the kettle is small and insufficient to allow the for wider-spaced tube pitches. This author’s experience has
vapor bubbles to “break-free” of the liquid and thus tend to been to spread out the tube spacing (pitch) from normal
blanket the upper tubes with gas, heat transfer will be heat exchange design to ensure free boiling bubble move-
91
restricted. 190 For best design the superficial vapor velocity ment to avoid the very problem expressed in reference. To
should be in the range of 0.6—1.0 ft/sec to prevent the bub- account for Gilmour’s effect of the bundle on single tube
bles from blanketing the tube through the bundle and calculations, see Equation 10-153.
91
thereby preventing liquid contact with the tubes. When the Palen and Taborek proposed as the best choice for cir-
maximum heat flux is approached, this condition can occur, cular tube bundles (as compared to square) the following
so the 1.0 ft/sec vapor velocity is recommended. film boiling coefficient after analyzing available data (their