Page 213 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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176 Applied Process Design for Chemical and Petrochemical Plants

Now, the h b coefficient can be used with the overall U equa-
tion, including shell-side fouling, to calculate a final overall
coefficient for boiling.
The maximum flux equation of Zuber 128 is suggested as
another check for kettle reboilers:

D b L g 1 L v 2 0.25
q max 176c dp v c d (10-164)
2
A n v
where
q max tube bundle maximum flux, Btu/hr-ft 2
D b tube bundle diameter, ft
L length of tube bundle, straight tube, or average for
U-bundle, ft
A n net effective total bundle outside tube surface area,
ft 2
8
g gravitational constant, 4.17 10 ft/hr 2
The usual range of q max for organic fluids is 15,000—25,000
2
Btu/hr (ft ). For aqueous solutions, the range is 30,000—
40,000 Btu/hr (ft )
2
Figure 10-105. Nozzle connections for vertical thermosiphon reboilers.
Example 10-18. Kettle Type Evaporator—Steam in Tubes

where Evaporate 25,000 lb/hr of CCl 4 at 55 psia and saturation
T LM log mean temperature difference between liquid and
temperature on shell side of a kettle-type U-tube evaporator.
hot fluid, °F
Use steam as heating medium:
C s surface condition constant
1.0 for commercial tubes
1. Determine heat load, Q.
0.7 for highly polished tubes
B.P. of CCl 4 at 55 psia at 128°C 262°F
For the entire bundle, the film boiling coefficient
(assumes all tubes boiling, no liquid head effect so that some Heat of vaporization: l v of CCl 4 at 128°C 71.5 Btu>hr
tubes are not boiling): Heat duty: Q 25,000 171.52 1,790,000 Btu>hr
1
1 0.2431.75 ln 1 Nvc 24 2. Determine steam conditions (saturated), and t.
5
h b h on 30.7141p f D o 24 4.2 10 E c d (10-163)
Use t of approximately 60°F (refer to Table 10-26 for
N vc
guide).
where
p f tube pitch, ft Steam temperature 262 60 322°F
D o tube O.D., ft Steam pressure 92 psia
Latent heat (from steam tables)
l v steam 893.4 Btu/hr
A o U¿ T LM
E
1p f D o 2 1,790,000
Steam required 2,000 lb>hr
893.4
2
A o surface area per ft of tube; ft /ft
N vc Number of tubes in vertical tier at centerline of
3. Estimate unit size.
bundle
2
Use maximum heat flux Q/A 12,000 Btu/hr/ft .
D B Note that Table 10-26 indicates this value is quite safe.

You could use a higher allowable flux. Many designs are
2p f cos
2 operating based upon this conservative value.
D B bundle diameter, ft A Q>1Q>A2
tube layout angle, degrees
90° for rotated square 1,790,000 2
A 149 ft
60° for triangular 12,000