Rating a Furnace

Normal heaters will handle about 70% of the total duty in the radiant section. On this basis, estimate the radiant heat absorption Qr (Btu per hour). From enthalpy or specific heat data for the process fluid, estimate the temperature at the crossover between the convection and radiant sections and the average fluid temperature in the 

Using the specified allowable radiant transfer rate, calculate the total radiant tube surface. 

The convection tube surface is usually about equal to the surface in the radiant section. On this basis, select a tube size and pass arrangement that will give the required total surface and meet the specified pressure drop limitation. 

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