Cm 4120 Fired Heater Lecture

7/30/2019 Cm 4120 Fired Heater Lecture

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Furnaces

(also called Fired Heaters)

Julie King (2006)

Rev. John Sandell (2007)

CM4120

QuickTime™ an d aTIFF (Uncompressed) decompressor

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Fired Heaters

-What they look like….



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), 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).



Fired Heaters



Burner Sketch

-Burners located

“under‟ the

furnace.-Air/fuel mixes

-Get combustion

-Atomizing steamused 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.



Fired Heater



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.



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.



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.



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.).



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”?



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



Draft Profile



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!).



Furnace Controls



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)



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



Natural and Forced Draft



Induced and Balanced Draft



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




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.




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.




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.).




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.




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 Technology

Handbook”, Uhai Publishing, Berne, NY,

1997.

Cm 4120 Fired Heater Lecture

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