Exercise-3

A brick flue must be designed to discharge  (300K and 1 atm) of flue gas from furnace to stack. The flue is horizontal with a total length of 100 m and the four sharp 90 degree bends (L/D for one sharp bens is 20). The flue is rectangular in cross section with a 2:1 ratio of height to width. The average temperature of the flue gas is 350 degree .

Calculate the following:

1. Pressure drop in mm water to be expected if the internal cross section of the flue were
2. Energy consumed by friction in the flue (watts)
3. What would be the minimum cross- sectional dimensional of the flue if the pressure drop is limited to 2.5 mm of waste gases.

 Use the following values:

Molecular weight of flue gas 29,    and  

Universal gas constant

Viscosity , where T is in  of water

SOLUTION:   

a) Mechanical energy balance gives

 

 

 

and  

 

Substituting the values we get

 

b)

Where

c) Let the height of the rectangular cross section is h

 
 

Mechanical energy balance

 
Substituting the values
 

Solving, we get,