CHEMICAL ENGINEERING

HEAT TRANSFER 159

The tube i.d.D 19. 0  

2 ð 1. 67 D 15 .7mm

the cross-sectional area for flowD
$/ 4
0. 01572 D 0 .000194 m^2

and hence the water velocity:uD 0. 217 /
1000 ð 0. 000194 D 1 .12 m/s.

From equation 9.221 :hiD4280[ 

0. 00488 ð 308 1]1. 120.^8 / 0. 01570.^2

D 

4280 ð 0. 503 ð 1. 095 / 0. 436 D5407 W/m^2 K

or, based on outside diameter:hioD
5407 ð 0. 0157 / 0. 019

D4468 W/m^2 Kor4.47 kW/m^2 K

Outside

The temperature drop across the condensate film,Tfis given by:

(thermal resistance of water filmCscale)/ total thermal resistance D
mTf/m

or:
1 / 4. 47 C 0. 5 /
1 / 0. 500 D
23. 0 Tf/ 23. 0

and: TfD 14 .7deg K

The condensate film is thus at 

331. 7  14. 7 D317 K.

The outside film coefficient is given by:

hoD 0 .72[

k^3 ^2 g/

jdoTf ]^0.^25 (equation 9.177)

At 317 K,kD 0 .13 W/m K,D508 kg/m^3 ,D 0 .000136 N s/m^2 andjD

p
678 D
26 .0.

∴ hoD 0 .72[
0. 133 ð 5082 ð 9. 81 ð 286 ð 103 /

26 ð 19. 0 ð 10 ^3 ð 0. 000136 ð 14. 5 ]^0.^25

D814 W/m^2 Kor0.814 kW/m^2 K

Overall

1 /UD 

1 / 4. 47 C 

1 / 0. 814 C 0. 50 D 1. 952

and: UD 0 .512 kW/m^2 K or 512 W/m^2 K

which is sufficiently near the assumed value. For the proposed unit, the heat load:

QD 

0. 512 ð 678 ð 4. 88 ð 0. 0597 ð 23. 0 D2328 kW

or an overload of:
2328  2145100 / 2145 D 8 .5%

PROBLEM 9.27

37.5 kg/s of crude oil is to be heated from 295 to 330 K by heat transferred from the
bottom product from a distillation column. The bottom product, flowing at 29.6 kg/s is to