CHEMICAL ENGINEERING

HEAT TRANSFER 151

∴ XD

325  300 /
372  300 D 0. 347

and: YD
372  361 /
325  300 D 0. 44

For one shell side pass, two tube side passes, Fig. 9.71 applies andFD 0 .98.

Area

In equation 9.212,ADQ/UFmD 1. 875 n/
0. 230 ð 0. 98 ð 53. 7 D 0. 155 nm^2.

The area per unit length based on 10 mm i.d.D
$ð 0. 010 ð 1. 0 D 0 .0314 m^2 /mand
total length of tubingD 0. 155 n/ 0. 0314 D 4. 94 nm.

Thus the length of tubes requiredD 

4. 94 n/nD 4 .94 m.

PROBLEM 9.19

A condenser consists of a number of metal pipes of outer diameter 25 mm and thickness
2.5 mm. Water, flowing at 0.6 m/s, enters the pipes at 290 K, and it should be discharged
at a temperature not exceeding 310 K.
If 1.25 kg/s of a hydrocarbon vapour is to be condensed at 345 K on the outside of the
pipes, how long should each pipe be and how many pipes would be needed?
Take the coefficient of heat transfer on the water side as 2.5, and on the vapour side as
0 .8kW/m^2 K and assume that the overall coefficient of heat transfer from vapour to water,
based upon these figures, is reduced 20% by the effects of the pipe walls, dirt and scale.
The latent heat of the hydrocarbon vapour at 345 K is 315 kJ/kg.

Solution

Heat load

For condensing the organic at 345 K,QD

1. 25 ð 315 D 393 .8kW
   If the water outlet temperature is limited to 310 K, then the mass flow of water is given by:

393. 8 DGð 4. 18

310  290 orGD 4 .71 kg/s

Temperature driving force

 1 D 

345  290 D55 deg K, 2 D 

345  310 D35 deg K

Therefore in equation 9.9,mD
55  35 /ln
55 / 35 D 44 .3deg K.

No correction factor is necessary with isothermal conditions in the shell.

Overall coefficient

Inside:hiD 2 .5kW/m^2 K.
The outside diameterD 0 .025 m anddiD
25  2 ð 2. 5 / 103 D 0 .020 m.
Basing the inside coefficient on the outer diameter:

hioD 

2. 5 ð 0. 020 / 0. 025 D 2 .0kW/m^3 K