CHEMICAL ENGINEERING

HEAT TRANSFER 205

From equation 9.1: 350DUAð 30. 83

and: UAD 11 .35 kW/K.

ii)Co-current flow

Heat load,QD 

2. 8 ð 2. 5

363 TD 7. 0

363 TkW for the organic

and for the water,QD 

3. 35 ð 4. 18 

T^0  293 D 14. 0 

T^0 kW

whereTandT^0 are the outlet temperatures of the organic and water respectively.

∴ 

363 TD 

14. 0 / 7. 0 

T^0  293

and: T^0 D 

474. 5  0. 5 TK.

T 1 D 

363  293 D70 deg K,T 2 D

TT^0

and from equation 9.9,TmD 70 

TT^0 /ln[70/

TT^0 ]degK.

In equation 9.1:

70

360 TD 11. 35

70 TCT^0 /ln[70/

TT^0 ]

∴ 7. 0

360 TD 11. 35

70 TC 474. 5  0. 5 T/ln[70/

T 474. 5 C 0. 5 T]

or: 0. 617

360 TD 

544. 5  1. 5 T/ln[70/

 1. 5 T 474. 5 ]

Solving by trial and error,TD 319 .8Kwhich is very close to the value suggested,

320.6 K.

∴The outlet temperature of the water is:

T^0 D 474. 5  

0. 5 ð 319. 8 D 314 .6K

which agrees almost exactly with the given value.

Thus for co-current flow:QD 7. 0

363  319. 8 D 302 .4kW

T 1 D70 deg K (as before),T 2 D 

319. 8  314. 6 D 5 .2degK

and from equation 9.9,TmD 

70  5. 2 /ln 

70 / 5. 2 D 24 .92 deg K

∴in equation 9.1: 302. 4 DUAð 24. 92

and: UAD 12 .10 kW/K

which is in relatively close agreement with the counter-current value.

PROBLEM 9.74

An organic liquid is cooled from 353 to 328 K in a single-pass heat exchanger. When the
cooling water of initial temperature 288 K flows countercurrently its outlet temperature
is 333 K. With the water flowing co-currently, its feed rate has to be increased in order
to give the same outlet temperature for the organic liquid, the new outlet temperature of
the water is 313 K. When the cooling water is flowing countercurrently, the film heat
transfer coefficient for the water is 600 W/m^2 K.