Excess air in a rotary drier 1

[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 was 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,
Rod

Rod Nissen.
Combustion & Engineering Diagnostics