Excess air in a rotary drier

[op9]

I have been involved with burner application on aggregate driers for the asphalt industry for some time. Modern burners are total air burners where all combustion air enters through the burner. Older burners supplied only about 40% combustion through the burner, the remaining being induced around the burner by the exhaust (ID) system. In the past I have heard conjecture that for drying purposes additional air MUST be induced through the drier. In my aggregate drying experience as long as the exhaust system is capable of dealing with the efficiently combusted products of combustion AND the water vapour/steam. Additional extraneous air only suceeded in carrying away useful heat and lowering the efficiency.

I recently attended a drier for clay. Its burner fired into a stationary brick lined chamber, the gases then entering the rotary drier (counter flow). The burner mounting plate had a number of 3" dia ports arranged concentrically around the burner. These were fitted with shutters but were wide open. The burner is capable of supplying all its combustion air so does not need more induced air. In fact the burner is presently supplying more than is required. I have yet to go back to do more extensive tests but I tried restricting the concentric ports. No appreciable difference in emissions or operation was observed. As I said burner is probably supplying more air than is required anyway. However I now again wonder what is the purpose of these ports? The only reason I can think of is to induce some air flow across the brick hot face to control the brick face temperature. However I would appreciate anyone's opinion who may be experienced with this type of application/arrangement.

Thanks in advance,
Rod


Rod Nissen.
Combustion & Engineering Diagnostics

 

[MintJulep]

For some products the difference between temperature and heat is important, and the excess air allows control of temperature and heat independently.

Some products, like clay and rubber, are subject to "case hardening" if hit by too high a temperature. The outer surface dries quickly and forms a crust that makes drying the center of the material almost impossible.

 

[dvd]

Make-up air can be used to control the mass fraction of moisture in the drying air. Not until the product (and moisture) reaches the wet-bulb temperature of the air in the dryer can additional product warming begin. If the wet-bulb temperature is very high, then water must be heated to a higher temperature before it vaporizes. With higher wet-bulb temperatures there comes a higher vapor pressure in the dryer; higher vapor pressure makes it more difficult for moisture in the product to be removed if the vapor pressure of the product moisture is lower than the vapor pressure of the dryer atmosphere.

 

[op9]

Thanks to both of you for the responses.

Rod Nissen.
Combustion & Engineering Diagnostics