Furnacesalso called Fired Heaters Julie King Originally prepared by Todd King. I edited for use in CM4120.
Fired Heaters • Often in a large chemical plant or refinery, there will be 50 furnaces. • Often you will preheat a feed to get it up to the temperature needed for a reaction in a reactor (i.e., for an endothermic reaction). • Furnaces are often used to preheat the feed before it goes into the reactor.
Direct Fired Furnaces • We will focus on direct fired furnaces. • Have air/fuel (fuel oil or natural gas) /combustion gases in the “firebox” and this heats the process stream such as heavy hydrocarbons (oils in a refinery) etc. • The process stream being heated (like a process gas oil) is inside tubes. • The cooler process stream enters the top of the furnace (convection section) and exits near the bottom (radiant section).
Burner Sketch -Burners located “under’ the furnace. -Air/fuel mixes -Get combustion -Atomizing steam used to get better air/fuel mixing
Combustion • Rapid chemical reaction that occurs when the proper amounts of a fuel and oxygen are combined with an ignition source to release heat and light. CO2 and H2O are the combustion products for a complete combustion reaction. • Different fuels release different amounts of heat (energy) as they are burned.
Components of a Furnace • Fire box • Radiant Tubes • Convection Tubes • Damper and Stack • Refractory Lining • Burners and Air Registers (lets air in by burners)
Fire Box and Refractory Layer • This section contains the burners (under it) the open flames, and combustion gases. • Fire box is lined with refractory brick (usually white/tan in color, lightweight, chalk-like, ceramic material) lining that can handle high temperatures and reflects heat back into the furnace.
Fire Box and Refractory Layer • Refractory layer include a brick layer and stainless steel rods (sometimes a wire mesh) for the brick to attach to • “Peep” holes so the operator/engineer can view the firebox ‘flame’, get a temperature reading, etc. • Fire box temperature typically 1,800 oF.
Radiant Tubes • Radiant tubes (process stream inside them, i.e. oil, etc) are along the walls in the fire box. They receive direct heat from the flames (burners). This section of tubes sees higher temperatures and has a faster accumulation of coke (carbon – like when you BBQ on your grill) deposits on inside of the tubes. • Radiant heat transfer typically accounts for 65% of the total heat absorbed by the process stream (oil, etc.).
Convection Tubes • Convection tubes (process fluid inside them, i.e. oil, etc.) are in the roof of the furnace so NOT in contact with the direct flames in the fire box. • The hot combustion gases transfer heat thru the metal tubes (often finned tubes to increase efficiency) and into the process fluid. • Convective heat transfer typically accounts for 35% of the total heat absorbed by the process stream (oil, etc.).
Damper and Stack • Warm air and combustion gases leave the furnace thru the stack and enter the atmosphere. • This natural draft (like your chimney in your house that carries the combustion gases up) creates a lower pressure inside the furnace. • Draft = atm pressure – pressure inside fired heater • Typically 0.05 inches water (vaccum) by the stack damper
Damper • Damper is often 10 ft up in the stack and allows the adjustment of the stack draft. • Damper also allows how much air gets into the furnace. Open the damper, and more air comes in. Use the damper to control the excess O2 in the furnace. • Typically want about 2 mol% excess O2 or else you waste energy (just send too much hot air out the stack that you did not need to heat).
Furnace Types of Furnace Drafts • Natural Draft: draft is induced by buoyancy forces as the hot air rises thru the stack and creates a vacuum inside the fire box. Pressure in fire box < atm pressure • Forced Draft : fans are used to force air into the burners (below the fire box)
Furnace Types of Furnace Drafts • Induced Draft: a fan is put in the stack that enhances the low pressure in the fire box. • Balanced Draft : uses 2 fans • 1 fan pulls air out the stack • 1 fan forces air into the burners
Common Furnace Problems • Flame Impingement: flames from the burner touching a tube . • This weakens the metal tube and causes coke (carbon) to form inside the tube where the process fluid is • Solve by reducing the fuel supply to the affected burner.
Common Furnace Problems • Coke Formation • Coking always occurs inside the process fluid tubes (typically the radiant tube section where it is hottest) in a furnace. • Remove coke by shutting down the furnace (typically once/3 yrs) and injection superheated steam to remove the coke.
Common Furnace Problems • Replace Refractory • Refractory in the fire box becomes brittle and start to fall off over time at high temperatures. • Solve by shutting down the furnace (typically once/3 yrs) and removing old refractory and installing new refractory.
Common Furnace Problems • Fuel Composition Changes • The fuel composition of the fuel oil or natural gas can change. • The more heat a fuel produces during combustion, the more air is needed. • Your process control programs can help you here. Control the % excess O2 (open/close damper), allow more or less fuel into burner, etc.).
Common Furnace Problems • Process Fluid Feed Pump Failure • The furnace will get too hot, causing coking and damaging the equipment (too hot for the furnace materials). • Try to restart the feed pump or start the back up process feed pump FAST! • Then isolate (block off) the primary feed pump and get it fixed ASAP.
Common Furnace Problems • Flameout • Occurs when the burner flame goes out with the fuel still being pumped into it. • Now we have unburned fuel inside the furnace. • Often happens when there is not enough air in the burner. • Solution: shutdown the furnace. Stop fuel into the burner/furnace. This is a dangerous situation.
References • W. L. Luyben, B. D. Tyrus, M. L. Luyben, “Plantwide Process Control”, McGraw Hill, NY, 1999. • C. E. Thomas, “The Process Technololgy Handbook”, Uhai Publishing, Berne, NY, 1997.