Illustration

Regenerator receives hot flue gases at  and cold air at , the flue gases leave at  and the air is preheated to . As estimated of the heat given up by the flue gases is heat lost to the regenerator surroundings, and the rest  is recovered in the preheated air. It may be assumed for estimating purposes that  for flue gases and  for air, independent of temperature. Estimate over all thermal efficiency, efficiency limit, and relative efficiency for this heat exchange operation.

Suppose now that the depth of the regenerator is increased to 2.5 times in such a way to double the heat exchange area   while keeping constant the over-all heat transfer coefficient .The quantities and entering temperatures of the flue gases and air will be kept the same. Heat losses are same as that in a). Estimate for the enlarged regenerator (a) air preheat temperature, (b) over-all thermal efficiency and relative thermal efficiency    

Solution:

(a) Heat balance: reference temperature  

 

 

 
 
.

(b) Air preheat temperature and exit temperature of flue gas are not known. Since quantities and entering temperatures of flue gas and air are same. We can write 

 

                                                       (1)

Heat balance for the enlarged regenerator:

                           (2)

In equation 2,   and  are air and flue gas temperature at the exit of the regenerator.

Or                                               (3)      
                  
By solving 1 and 3

We get  and

 

 

References:

S.C.Koria: Video lectures on fuel,furnace and refractory, NPTEL phase 2

S.C.Koria: Video lectures on material and heat balance in metallurgical processes, NPTEL phase 2

Rosenquist : Principles of extractive metallurgy

Butts, Allison: Metallurgical Problems, McGraw Hill Book Company, 1943

R. Schuhmann, JR. Metallurgical engineering, volume 1