CHEMICAL ENGINEERING

150 CHEMICAL ENGINEERING VOLUME 1 SOLUTIONS

T 2 

T 2  340 hb QT 1 Ts

(K) (K) kW/m^2 K kW/m^2 (K) (K)

362.2 22.2 4.43 98.4 369.2 378.1

367.8 27.8 5.91 164.3 379.5 394.4

373.3 33.3 7.38 245.8 390.9 413.3

376.1 36.1 7.30 263.5 394.9 418.9

378.9 38.9 6.81 264.9 397.8 421.9

381.7 41.7 6.36 265.2 400.7 424.8

384.4 44.4 5.73 254.4 402.6 425.7

390.0 50.0 4.54 227.0 406.2 426.8

It is fairly obvious that the rate of evaporation will be highest when the heat flux is a
maximum. On inspection this occurs whenTsD425 K.

PROBLEM 9.18

It is desired to warm an oil of specific heat 2.0 kJ/kg K from 300 to 325 K by passing it
through a tubular heat exchanger containing metal tubes of inner diameter 10 mm. Along
the outside of the tubes flows water, inlet temperature 372 K, and outlet temperature
361 K.
The overall heat transfer coefficient from water to oil, based on the inside area of the
tubes, may be assumed constant at 230 W/m^2 K, and 0.075 kg/s of oil is to be passed
through each tube.
The oil is to make two passes through the heater and the water makes one pass along
the outside of the tubes. Calculate the length of the tubes required.

Solution

Heat load

If the total number of tubes isn,therearen/2 tubes in one pass on the oil side, that is
the oil passes through 2 tubes in traversing the exchanger.
The mass flow of oil is thereforeD
0. 075 ðn/ 2 D 0. 0375 nkg/s and the heat load:

QD 0. 0375 nð 2. 0

325  300 D 1. 875 nkW

Temperature driving force

 1 D 

361  300 D61 deg K, 2 D 

372  325 D47 deg K

and, in equation 9.9: mD
61  47 /ln
61 / 47 D 53 .7deg K

In equation 9.213:

XD

 2  1 /

T 1  1 andYD

T 1 T 2 /

 2  1

whereT 1 andT 2 are the inlet and outlet temperatures on the shell side and 1 and 2 are
the inlet and outlet temperatures on the tube side.