Homework Answers

A packed counter-current water cooling tower is used to cool

1.356 kg/s.m2

of water from 43.3 °C to 26.7 ºC. The air enters from the bottom

of the tower at 23.9 ºC with a wet bulb temperature of 21.1 ºC. The whole of the

resistance to heat and mass transfer can be considered as being within the gas

phase and the product of the mass transfer coefficient and the transfer surface

per unit volume of column (KG.a) may be taken as 1.207×10-7

kmol/s.m3

.Pa.

The tower operates at atmospheric pressure. Calculate the minimum air flowrate

and the necessary height of the tower when 1.5 times of minimum is used as

actual air flowrate.

Data: Latent heat of water at 0 ºC = 2501 kJ/ kg

Heat capacity of dry air = 1.005 kJ/ kg. ºC

Heat capacity of water vapour = 1.88 kJ/ kg. ºC

Density of air = 1.198 kg/ m3

Enthalpy of saturated air-water vapour data are as follows:

Hyi, kJ/kg dry air 84 97.2 112.1 128.9 148.2 172.1 197.2

Temperature, ºC 26.7 29.4 32.2 35 37.8 40.6 43.3

A 30-plate bubble cap column is to be used to remove n-pentane

from oil by means of pure steam stripping. The inlet oil contains 6% n-pentane

by volume and it is desired to reduce the n-pentane to 0.1% by volume. Assume

isothermal operation and an overall plate efficiency of 30%. Find out the ratio

of actual to minimum steam consumption. The equilibrium relation for the

system is y=3x.