(207f) Analysis and Optimization of High-Emissivity Coil Inserts to Improve Overall Coil Heat Transfer at Lower Pressure Drop and Shorter Residence Time in Ethylene Furnaces

This paper presents analysis of high-emissivity cylindrical inserts to improve overall coil heat transfer in ethylene furnaces or other furnaces handling gaseous fluid in the coil. Due to enhanced internal heat transfer, we can achieve the same heat transfer duty at lower pressure drop, shorter coil length, and reduced total external surface area of the coil that receives radiant and convective heat from combustion flames.

Overall heat transfer in ethylene furnaces is generally limited by convective heat transfer from coil internal surface to gaseous fluid flowing inside the coil, which constitutes the highest resistance to overall heat transfer. Thus, the best way to improve overall heat transfer is to focus on minimizing this heat transfer resistance. There have been attempts to accomplish this by increasing flow turbulence so as to increase the convective heat transfer coefficient. Turbulence inside the coil can be increased by modifying the roughness and/or shape of coil internal surfaces, or twisting the coil tube into helical shape. However, the major drawback of this approach is higher pressure drop, increasing overall cracking reaction pressure and thus resulting in poorer product yields.

This paper analyzes another approach in the literature that uses a small-diameter high-emissivity cylindrical insert enclosed in a large-diameter outer coil to create effective internal radiant heat transfer. As a result, heat can simultaneously be transferred by both internal convection and radiation, thereby leading to a new optimal coil design that can achieve lower pressure drop and shorter residence time. These two features are known to improve cracking product yields from ethylene furnaces. This paper will focus on the analysis of heat transfer, pressure drop, and residence time to come up with an appropriate combination of the insert dimension and coil size.