Page 252 - Modelling in Transport Phenomena A Conceptual Approach

232 CHAPTER 7. UNSTEADY-STATE MACROSCOPIC BALANCES

area of the jacket is 4.5 m2 and the heat capacity of the liquid is 1850 J/ kg. K.
Calculate the average overall heat transfer coefficient if the variation of liquid
temperature as a function of time is recorded as follows:

t T
(min) ("C)
0 15
2 59
4 90
6 112
8 129
10 140
12 150
(Answer: 564 W/ m2. K)

7.20 An insulated rigid tank of volume 0.1m3 is connected to a large pipeline
carrying air at 10 bar and 120°C. The valve between the pipeline and the tank is
opened and air is admitted to the tank at a constant mass flow rate. The pressure
in the tank is recorded as a function of time as follows:

t P
(min) (bar)
5 1.6
10 2.1
15 2.7
20 3.3
25 3.9
30 4.4
If the tank initially contains air at 1 bar and 20 "C, determine the mass flow
rate of air entering the tank. Air may be assumed an ideal gas with a constant 6'p
of 29 J/ mol. K.
(Answer: 7.25 g/ min)

7.21 An insulated rigid tank of volume 0.2m3 is connected to a large pipeline
carrying nitrogen at lobar and 70°C. The valve between the pipeline and the
tank is opened and nitrogen is admitted to the tank at a constant mass flow rate
of 4g/s. Simultaneously, nitrogen is withdrawn from the tank, also at a constant
mass flow rate of 4g/s. Calculate the temperature and pressure within the tank
after 1 minute if the tank initially contains nitrogen at 2bar and 35°C. Nitrogen
may be assumed an ideal gas with a constant Cp of 30 J/ mol. K.
(Answer: 326.8 K, 2.12 bar)