Page 265 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:38 PM Page 227

Heat Transfer 227

or even approached too closely. This is to avoid a film 1. Heat duty:
boiling condition, rather than nucleate boiling.
14. Determine the number of tubes: Q (187)(8.33)(60)(82 40) 3,935,000 Btu/hr
Shell-side boiling temperature 30°F
No. tubes L T >l (10-247)
2. Arithmetic average tube-side temperature,
where l assumed length of tube, ft.
82 40
t a 61°F
Remember to keep a standard length if possible and 2
maintain a tube-side pass condition to realize the film
conditions established in Step 4. U-tubes are a good 3. Overall shell temperature drop:
selection for this type of service, and a kettle-type shell
is usually used. t o t a t s 61 30 31°F
15. Determine the pressure drops in the usual manner.
4. Tube-side film coefficient. Assume minimum water
In general, at low boiling temperature film drops, the velocity of 5 ft/sec, using 1-in. 14 BWG tubes.
finned tubes give considerably higher coefficients than plain
2
tubes, but in the general region of a 10–12°F boiling film tem- From Figures 10-50A and 10-50B, h i 1,215 Btu/hr (ft )(°F)
perature difference, the two tubes become about the same. Correction 0.925
where l assume length of one tube, ft In terms of outside surface:
L t total tube length, ft 1
2
L f total finned tube length, ft h io h t 1,21510.9252 310 Btu>hr 1ft 21°F2
3.66
L p total plain tube length, ft
2
R t total resistance to heat transfer, (hr)(°F) (ft )/Btu
2
r b r o outside (tube) fouling factor, (hr)(°F) (ft )/Btu 5. Assumed fouling resistances:
2
r i inside (tube) fouling factor, (hr)(°F) (ft )/Btu
2
A o outside tube surface area, ft /ft Tube side 0.002
2
A i inside tube surface area, ft /ft Shell side 0.001
t o overall t between average tube-side bulk tempera-
ture, °F, and evaporating (boiling) side fluid 1 1
6. B R oa 13.66210.0022 0.001 0.01144
t b temperature drop across boiling film, °F h s 310
2
U o overall coefficient of heat transfer, Btu/(hr) (ft )(°F)
U L overall coefficient of heat transfer per ft of tube 7. Assume temperature drop across film, t b 5°F.
length, Btu/(hr) (ft of tubing)(°F) 8. From Figure 10-150, h s 620 Btu/hr (ft )(°F)
2
2
h b boiling film coefficient, Btu/(hr) (ft )(°F) 9. R oa 0.01144 1/620 0.01144 0.00162 0.01306
2
h t h w inside water film coefficient, Btu/(hr) (ft )(°F) 10. Calculate,
2
A o outside tube surface area, ft /ft
Q total heat duty, Btu/hr
1>620
t b 31a b 3.95°F
0.01306
Example 10-23. Boiling with Finned Tubes
Not a check.
See Figure 10-151.
7a. Reassume: t b 4.5°F.
A direct evaporation water chiller is to use Freon 12 on
8a. h s 550
the shell side, cooling 187 gpm of water for a closed system 1
9a. R oa 0.01144 / 550 0.01326
from 82°F to 40°F. Because the Freon is to come from an
10a. Calculated,
already existing system, operating at 30°F evaporator tem-
perature, this same condition will be used to avoid com-
1>550
pressor suction pressure problems. Note: Care must be t b 31a b 4.25°F
given to avoid water freezing on tubes. Keep the evaporating 0.01326
temperature slightly above the freezing point of fluid.
This is close enough.
Tubes are copper 1-in. nominal O.D. 14 BWG (0.083-
in. thick at finned section) 19 fins/in. Wolverine Trufin ® 11. Overall coefficient:
(standard tube (unfinned) wall thickness 0.095 in.). U o 1 1 75.5 Btu>hr 1ft 21°F21outside2
2
2
Finned surface area/ft length 0.678 ft /ft. Plain tubes are R oa 0.01326
2
0.5463 ft /ft.