Abstract: A coiled arrangement of pipes convey a heated medium, such as a slurry, into, through, and out of a heater or fire box provided with an arrangement of high velocity burners. The heater is fired to a preselected elevated temperature. A plurality of radiation shields are positioned in close proximity to the surfaces of the pipes conveying the medium for heating. The position of the radiation shields relative to the pipes forms inner and outer heating zones within the heater to provide different flame temperatures. The radiation shields function as a physical barrier to prevent direct contact of the fuel flame and combustion products with the pipes. Thus the radiation shields are maintained at a higher temperature than the surfaces of the pipes and at a lower temperature than the combustion products.