Reducing heat transfer surface area requirements of direct fired heaters without decreasing run length

Abstract

This invention relates to the design of direct fired heaters which consist of vertically oriented refractory lined enclosures containing tubular heat transfer elements, the elements partially surrounding a cluster of burners. The burners fire gaseous fuel and generate high temperature combustion products which allow for the transfer of heat, by radiation and convection, from the combustion products to the heat transfer elements and the continuous flow of process fluid contained therein. The transferred heat raises the temperature of the fluid from the design temperature at the inlet to the design temperature at the outlet, at a heat transfer rate commensurate with the temperature differential existing at any given location. The surface area requirements of the heat transfer elements and that of the enclosure surrounding the heat transfer elements is significantly reduced by limiting firebox recirculation of burner generated combustion products, thereby increasing overall temperature differentials and heat transfer rates between combustion products and process fluid. Gains in heating surface reduction are not accompanied by losses in heater run length because low process fluid temperatures and high inside heat transfer coefficients are provided, which minimize process fluid film temperature in areas where high heat transfer rates prevail.