CHEMICAL ENGINEERING

HEAT TRANSFER 141

Heat load

QD 



360 / 3600 ð 4. 18 

Tw 280 D 0. 418 

Tw 280 kW for water

and: QD 



75 / 1000 ð 1. 9

370 To D 0. 143

370 To kW for the oil.

From these two equations,TwD 

406. 5  0. 342 To K

Area

For 19.0 mm o.d. tubes, surface areaD

$ð 0. 019 ð 1. 0 D 0 .0597 m^2 /m and for one

tube, surface areaD 

2. 0 ð 0. 0597 D 0 .1194 m^2

Temperature driving force

 1 D 

370 Tw,  2 D

To 280

and in equation 9.9:mD[ 

370 Tw 

To 280 ]/[ln 

370 Tw/

To 280 ]

D 

650 TwTo/ln 

370 Tw/

To 280

Substituting forTw:

mD 

243. 5  0. 658 To/ln 

0. 342 To 36. 5 /

To 280 K

Overall coefficient

hiD 2 .5kW/m^2 K

diD 19. 0  

2 ð 1. 3 D 16 .4mm

Therefore the inside coefficient, based on the outside diameter is:

hioD 

2. 5 ð 16. 4 / 19. 0 D 2 .16 kW/m^2 K

Neglecting the scale and wall resistances then:

1 /UD 

1 / 2. 16 C 1 / 1. 7 D 1 .052 m^2 K/kW

and: UD 0 .951 kW/m^2 K

Substituting in equation 9.1 gives:

0. 143
   370 To D


1. 951 ð 0. 1194


2. 5  0. 658 To/ln


3. 342 To 36. 5 /
   To 280

∴ ln 

0. 342 To 36. 5 /

To 280 D 0 .523 andToD324 K

PROBLEM 9.13

Waste gases flowing across the outside of a bank of pipes are being used to warm air

which flows through the pipes. The bank consists of 12 rows of pipes with 20 pipes, each