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Show to the tubes has a relatively small effect on the correlation between cumulative absorbed duty and process temperature. 2.4 Model Inputs In addition to the necessary geometrical information, simulation of conditions within the radiant section of a process heater requires the specification of several model inputs. A m o n g these are the following: • Tube model • Tube thermal resistance • Refractory wall thermal resistance • Tube and refractory wall emissivity • Fuel composition, temperature, and mass flow rate • Oxidizer composition, temperature, and mass flow rate The manner in which conditions on the process side are coupled with those on the fireside is what is here termed the "tube model". The model used in the present simulations involves using a tabulated correlation between cumulative absorbed energy, process temperature, and the process side heat transfer coefficient. The internal heat transfer coefficient as well as the tube wall thickness and thermal conductivity are used to determine the overall tube thermal resistance, Rt, between the fireside tube wall and the process side fluid. This wall resistance represents the total resistance to heat transfer by series conduction through the tube wall, the coke layer formed on the inside wall, and by convection from the inside tube wall to the process fluid. Generally, the fireside tube metal temperature ( T M T ) is not known, a priori, and must be determined from an energy balance at the surface. The balance may be written as Ts~Tb Q rad + Q conv " Q cond = - ^ I[ > where q"rad' Q "conv anc* q"conv represent net radiative, convective, and conductive fluxes, respectively, at the surface. Assuming that the flameside convection coefficient (computed by the model), incident radiative flux, surface emissivity, backside temperature (Tb, process temperature), and the conductive thermal resistance (Rt) are known, the surface temperature, Ts, can be found. In a similar manner, thermal boundary conditions for the furnace refractory walls must be specified. As a percentage of the furnace fired duty, heat loss through the furnace walls typically ranges from 1-3%. Although small, this heat loss is not negligible and the spatial dependency of these losses within the furnace should be modeled. The thermal resistance, Rt, between the fireside refractory wall and the outside ambient air is specified taking into account the conduction through the refractory material and convection from the outside wall to ambient conditions. A s Rt 5 |
