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7/30/2019 Cm 4120 Fired Heater Lecture
http://slidepdf.com/reader/full/cm-4120-fired-heater-lecture 1/28
Furnaces
(also called Fired Heaters)
Julie King (2006)
Rev. John Sandell (2007)
CM4120
QuickTime™ an d aTIFF (Uncompressed) decompressor
are needed to see this p icture.
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Fired Heaters
-What they look like….
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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.
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Direct Fired Furnaces
We will focus on direct fired furnaces.
– Have air, fuel (fuel oil or natural gas), andcombustion gases in the “ firebox” and this heats
the process stream such as heavy hydrocarbons(oils in a refinery) etc.
– Process stream being heated (like a process gasor oil) is inside tubes.
– Cooler process stream enters the top of thefurnace (convection section) and exits near the
bottom (radiant section).
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Fired Heaters
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Burner Sketch
-Burners located
“under‟ the
furnace.-Air/fuel mixes
-Get combustion
-Atomizing steamused to get better
air/fuel mixing
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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.
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Fired Heater
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Components of a Furnace
Fuel Train
Fire Box
Radiant Tubes
Convection Tubes
Damper and Stack
Refractory Lining
Burners and Air Registers (lets air in by burners)
What is a fuel train?
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to s ee this picture.
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Fire Box and Refractory Layer
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 andreflects heat back into the furnace.
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Fire Box and Refractory Layer
Refractory layer includes a brick layer and
stainless steel rods (sometimes a wire mesh)
that attach to the brick.
“Peep” holes so the operator/engineer can
view the firebox „flame‟, get a temperature
reading, etc.
Fire box temperature typically 1,800 oF.
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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). Section of tubes sees higher
temperatures and has a faster accumulation of coke deposits on inside of the tubes (carbon – like when you BBQ on your grill).
Radiant heat transfer typically accounts for 65% of the total heat absorbed by the processstream (oil, etc.).
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Convection Tubes
Convection tubes (process fluid inside them)are in the roof of the furnace so NOT incontact with the direct flames in the fire box.
Hot combustion gases transfer heat through themetal tubes (often finned tubes to increaseefficiency) and into the process fluid.
Convective heat transfer typically accounts for
35% of the total heat absorbed by the processstream (oil, etc.).
Why “finned tubes”?
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Damper and Stack
Warm air and combustion gases leave thefurnace though the stack and enter 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 firedheater
Typically 0.05 inches water (vacuum) by thestack damper
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Draft Profile
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Damper
Often 10 ft up in the stack and allowsadjustment of the stack draft.
Controls amount of air into the furnace. Open
the damper, and more air comes in. Controls the excess O2 into the furnace.
Typically want about 2 mol% excess O2 or you waste energy (just send too much hot air out the stack that you did not need to heat!).
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Furnace Controls
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Types of Furnace Drafts
Natural Draft: draft is induced by
buoyancy forces as the hot air rises
through the stack and creates a vacuuminside fire box. Pressure in fire box <
atm pressure
Forced Draft : fans are used to force air into the burners (below the fire box)
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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
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Natural and Forced Draft
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Induced and Balanced Draft
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Common Furnace Problems
Flame Impingement: flames from the burner touching a tube
– Weakens the metal tube and causescoke (carbon) to form inside the tubewhere the process fluid flows
– Solve by reducing the fuel supply tothe affected burner
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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
injecting superheated steam to remove thecoke.
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Common Furnace Problems
Replace Refractory – Refractory in the fire box becomes brittle
and starts 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.
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Common Furnace Problems
Fuel Composition Changes – Fuel composition of the fuel oil or natural gas
can change.
– More heat a fuel produces during combustion,the more air is needed.
– Your process control programs can help youhere. Control the % excess O2 (open/closedamper), allow more or less fuel into burner,etc.).
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Common Furnace Problems
Process Fluid Feed Pump Failure
– Furnace will get too hot, causing cokingand 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.
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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!
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References
W. L. Luyben, B. D. Tyrus, M. L. Luyben,
“Plantwide Process Control”, McGraw Hill,
NY, 1999. C. E. Thomas, “The Process Technology
Handbook”, Uhai Publishing, Berne, NY,
1997.