Furnaces (also called Fired Heaters)

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), and combustion 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 gas or oil) is inside tubes. Cooler process stream enters the top of the furnace (convection section) and exits near the bottom (radiant section).

Fired Heaters

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.

Fired Heater

Components of a Furnace What is a fuel train? Fuel Train 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 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 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 process stream (oil, etc.).

Convection Tubes Why “finned tubes”? Convection tubes (process fluid inside them) are in the roof of the furnace so NOT in contact with the direct flames in the fire box. Hot combustion gases transfer heat through 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.). Why “finned tubes”?

Damper and Stack Warm air and combustion gases leave the furnace 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 fired heater Typically 0.05 inches water (vacuum) by the stack damper

Draft Profile

Damper Often 10 ft up in the stack and allows adjustment 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 vacuum inside 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 causes coke (carbon) to form inside the tube where the process fluid flows 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 injecting superheated steam to remove the coke.

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.

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 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 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 Technology Handbook”, Uhai Publishing, Berne, NY, 1997.