3Fired HeatersOften 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.
4Direct 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).
6Burner Sketch -Burners located “under’ the furnace. -Air/fuel mixes -Get combustion-Atomizing steamused to get betterair/fuel mixing
7CombustionRapid 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.
9Components of a Furnace Fire boxRadiant TubesConvection TubesDamper and StackRefractory LiningBurners and Air Registers (lets air in by burners)
10Fire 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.
11Fire 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.
12Radiant TubesRadiant 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.).
13Convection TubesConvection 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.).
14Damper and StackWarm 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 heaterTypically 0.05 inches water (vaccum) by the stack damper
16DamperDamper 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).
18Furnace 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 pressureForced Draft : fans are used to force air into the burners (below the fire box)
19Furnace 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 fans1 fan pulls air out the stack1 fan forces air into the burners
22Common 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 isSolve by reducing the fuel supply to the affected burner.
23Common Furnace Problems Coke FormationCoking 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.
24Common Furnace Problems Replace RefractoryRefractory 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.
25Common Furnace Problems Fuel Composition ChangesThe 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.).
26Common Furnace Problems Process Fluid Feed Pump FailureThe 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.
27Common Furnace Problems FlameoutOccurs 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.
28ReferencesW. 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.