Exercise-2

A brick chimney 3.5m inside diameter (round) and 45m high is to handle flue gases (average molecular weight 30) at 603K. The atmospheric pressure outside the chimney is 734mm Hg and outside air is at 300K. It may be assumed that the gases do not cool as they rise in the chimney. Make the necessary calculations and prepare the following graphs:

1. Draft at the bottom of the chimney vs. flow rate of waste gases.
2. Horsepower equivalent of the flow energy available for draft at the bottom of the stack vs. flow rate of flue gases, Ignore the losses due to contraction and expanses of gases.

For both the plots an a and b, the graph should cover the entire range of chimney flow rates from 0 to the flow rate at which the available draft at the bottom of the chimney is nil
Use   and Viscosity of  where  is in .

Solution   
a)  Mechanical energy balance for flow of gases

.

and  

Substituting values and after simplification

 

 
Putting  and  and other values of variables we get.

 

We note at  .

And at

b)  Flow energy in

This equation shows that flow energy will be maximum
. and zero at