Heaters process outlet temperature, increase

The heater process outlet temperature begins to increase as the excess air is reduced. This is because more heat is given to the process fluid and less heat goes up the stack. 

When overheated, hydrocarbons tend to breakdown, leaving carbon residues (coke). This coke builds up on the inside of the heater tubes, slowing the transfer of heat from the tube walls to the product by restricting the flow of product and acting as an insulator. As the control system attempts to maintain the process outlet temperature at the setpoint, the fuel valves will open and the tubes subjected to an increased heat load. With the diminished ability of this heat to be transferred to the process fluid, the temperature of the tubing will increase. 

Automatic operation linked to process outlet temperature while on the bad side of absolute combustion is potentially hazardous because the heater outlet temperature will drop as a result of the reduced heating efficiency of the fuel. The automatic control will then call for more fuel, which, in turn, produces more oxygen starvation. The only way out of this situation is to put the furnace back to manual operation and manually increase the combustion air or reduce the fuel to get back to the good side of the point of absolute combustion. 

The feed heater is used to increase the feed temperature and control the reactor inlet temperarnre. Although the heater efficiency depends on how it is designed and operated, the heater duty is determined by feed preheating requirement. The feed heater outlet temperature can also be a function of the reaction temperature profile—that is, an ascending temperature profile will lower the heater outlet temperature. For a given feed preheat, the heater duty is mainly a function of the heat of reaction and heat recovery. A process engineer can determine the ways to maintain process heat recovery, heater efficiency, and heat flux. 

Newer heaters typically have thin reflective tiles, rather than massive refractory brick walls. Such newer heaters will heat up more rapidly. Also, the process fluid outlet temperature responds more rapidly to changes in the firing rate. This improves the heater outlet temperature control. Perhaps for this reason, it seems that heaters with reflective refractory walls are less subject to process tube coking and shortened heater run lengths. There is also a process, called "alonizing," that increases the reflectivity of older brick refractory heater walls. 

Column was reboiled by a fired heater. Heater fuel was controlled by a tray temperature, and there was a hi -temperature trip at the process side heater outlet, l en the circulation pump briefly failed the column cooled and the controller increased heating rate. The trip failed to function. When circulation was reestablished an extremely hi v xnization rate resulted and produced a pressure surge that dislodged trays. 

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