Page 251 - Modelling in Transport Phenomena A Conceptual Approach

PROBLEMS 231

0 = exp (g
where
)

in which T is the temperature of the tank contents at any instant and C is the
heat capacity of hot water.
b) Write down the unsteady-state energy balance and show that the time required
to increase the temperature of the tank contents from TI to T2 is given by

c) Bondy and Lippa (1983) argued that when the difference between the outlet
and inlet jacket temperatures is less than 10% of the ATLM between the average
temperature of the jacket and the temperature of the tank contents, Eq. (1) in
Problem 7.16 can be used instead of Eq. (3) by replacing T, by the average jacket
temperature. Do you agree? For more information on this problem see Tosun and
Akgahin (1993).

7.18 600kg of a liquid is to be heated from 15°C to 150°C in a well stirred,
jacketed tank by steam condensing at 170 "C in the jacket. The heat transfer surface
area of the jacket is 4.5 m2 and the heat capacity of the liquid is 1850 J/ kg. K. The
overall heat transfer coefficient, U, varies with temperature as follows:

T U
("(3 ( W/ mz. K)
15 390
30 465
60 568
90 625
120 664
150 680

a) Calculate the required heating time.
b) Correlate the data in terms of the expression

where T is in degrees Kelvin, and calculate the required heating time.
(Answer: a) 11.7min; b) 13.7min)

7.19 500kg of a liquid is to be heated from 15°C to 150°C in a well stirred,
jacketed tank by steam condensing at 170 "C in the jacket. The heat transfer surface