Hi AndreChE,
I am not sure why you are wanting to separate the radiant and convection for efficiency calculation.  Doing it the way you are proposing gives you values for efficiency, but they don't have much meaning.  Normally, the heater efficiency would be based on the (radiant+convection)/heat input.  Sometimes the radiant only efficiency might have some importance, but the convection is different.  If you only consider the convection recovery only, you would consider the heat available as being the heat in the flue gas from the inlet temperature(bridgewall) and the flue gas at the temperature of the process inlet temperature(this would be the lowest temperature that could be achieved if all heat removed to that point).  this value would be divided into the heat absorbed(convection) for the recovery efficiency of convection.  
Thanks,
Jack Hardie

Hi jehar

I am calculating this for pyrolysis furnaces so radiant section is very important and since you have hundreds of chemical reactions I must use the convection in order to calculate the radiation.

AndreChE

Gulf has published a Handbook of Heat and Mass Transfer ISBN 0-87201-411-8 edited by Cheremisinoff. Its Volume 1: Heat Transfer Operations, Chapter 42: "Design Practices in Combustion and Incineration", would, no doubt, be good reading for the subject in hand.

I have design books and papers for furnaces. This is an issue of optimization and rating.

Thanks.
AndreChE