A furnace, abbreviated as FURN, is a heating system or device that is used to generate heat by burning fuel or using electricity.  Furnaces are commonly used in residential, commercial, and industrial applications to provide warmth or to melt materials, among other uses.  In a typical furnace, fuel (such as natural gas, oil, or coal) or electricity is used to heat air or water, which is then circulated through a building or process to provide heat.  Furnaces can be classified based on the type of fuel they use or the way in which heat is transferred, such as radiant heating or forced air heating.

Furnace Index

Furnaces are also used in industrial applications such as steel production, where they are used to melt metals or other materials at high temperatures.  In these applications, furnaces can operate at extremely high temperatures and may be fueled by natural gas, coal, or electricity.  Furnaces require regular maintenance and safety checks to ensure they operate efficiently and safely.  Modern furnaces are often equipped with safety features such as automatic shut-off valves and sensors to detect gas leaks or other hazards.


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Furnace Design Classification

  • Electric Furnace  -  This furnace has a heating chamber with electricity as the heat source for achieving very high temperatures to melt and alloy metals and refractories.  The electricity has no electrochemical effect on the metal but simply heats it.
    • Arc Furnace  -  A type of electric furnace in which heat is generated by an arc between carbon electrodes above the surface of the material (commonly a metal) being heated.
      • AC Arc Furnace  -  A type of electric furnace in which heat is generated by an AC arc between carbon electrodes.
      • DC Arc Furnace  -  A type of electric furnace in which heat is generated by a DC arc between carbon electrodes.
      • Submerged Arc Furnace  -  The SAF uses the arc heat at the end of the electrode and the resistance heat of the charge or slag to convert electrical energy into heat energy so that use elements such as metals are reduced from ores or oxides.
    • Induction Furnace  -  Has a coil carrying alternating electric current surrounds the container or chamber of metal.  Eddy currents are induced in the metal (charge), the circulation of these currents producing extremely high temperatures for melting the metals and for making alloys of exact composition.
      • Coreless Furnace  -  A type of induction furnace that uses a magnetic field to heat metal objects.  The magnetic field is created by the use of an electric coil, which has a high frequency alternating current passed through it.  The electric coil is placed inside the induction heating chamber and the metal object to be heated is placed on top of it.  There are no coils or cores that generate heat by passing electricity through them.
      • Mains Frequency Furnace  -
        • Chennel Furnace  -  Basically consists of a tiltable furnace vessel with refractory lining onto which single or multiple inductors are mounted.  The melt in this furnace is guided in a closed or open channel.
    • Resistance Furnace  -  The heat is developed by the passage of current through a suitable internal resistance that may be the charge itself.  Electric energy is converted into heat when a current flows through the heating element.  The heat is transmitted to the material to be heated by conduction, convection, or radiation.
  • Fuel Fired Furnace  -  Fired process furnaces have the same function as electric process heaters, which is to heat a fluid to a desired working temperature.  The fluid flows through tubes that are heated by a combusting fuel.
    • Coal Fired Furnace  -
      • Cupola Furnace  -  This furnace is used mainly for the manufacture of different grades of bronze and cast iron in foundry processes.  Similar to the blast furnace, the cupola is a refractory-lined steel stack, resting on a cast-iron base plate with four steel legs. The bottom of the cupola furnace has two hinged doors supported in the closed position.
      • Pit Furnace  -  A type of furnace formed like a pit and is used for melting quantities of ferrous and non ferrous metals for production of castings.  They are preferable when work pieces are very long and need to hang vertically to minimize distortion during heating, or for very large loads which are best handled with an overhead crane.
      • Rotary Furnace  -  Rotary furnaces are often used for the heat treatment of bulk materials.  All  materials are uniformly exposed to the heating because their position within the bulk material changes constantly.  A continuous material movement through the heated space can be achieved by slightly and adjustably tilting the rotary furnace.
    • Gas Fired Furnace  -  Gas furnaces burn gas to produce heat for a variety of industrial processes.  An enclosed space contains the gas until it reaches the temperature for the application.  Gas furnaces can contain air, oxidized gas, inert gas, reducing, salt bath, or vacuum atmospheres.  Natural gas is the main type of gas used for gas furnaces.
    • Oil Fired Furnace  -  Liquid fuels produce combustible fumes.  The majority of liquid fuels are made from fossil fuels, with other variations being hydrogen, ethanol, and biodiesel. Oil is the most common type of liquid fuel used to heat and reheat materials for treatments.  The efficient operation of a liquid-fueled furnace means complete combustion of the fuel without any residue.


Furnace Standards

API Standards

  • API RP 573 - Inspection of Fired Boilers and Heaters


Furnace Glossary


  • Annealing Furnace  -  Annealing is a heat treatment that softens metals to allow for their cold working to improve their mechanical, electrical, and other physical properties.  Annealing furnaces relieve the internal stress of materials by heating them to their recrystallization temperature to make them ductile for further machining.
  • Ashing Furnace  -  Ashing is a process of quantifying the change in the weight of a sample as various components of the sample are burned away.  It is a process of expelling organic material before it is analyzed.  The method of ashing involves the complete combustion of the material being tested.


  • Bell Furnace  -  These are batch heat treatment furnaces that are capable of sintering or drying processes.  The load is heated inside an enclosed dome.  Bell furnaces are necessary when the workload is very high.  They are not used for small batches and are powered by electricity, gas, or a thermal circulation system.
  • Blast Furnace  -  A blast furnace is a cylindrical furnace that is used for smelting, which is the process of extracting metals from their ores.  The furnace is loaded from the top with ore, fuel, and limestone.  As the components move down the cylinder, a reaction takes place between them that produces molten metal and slag. At the bottom of the furnace are parallel pipes that push hot blast air up the cylinder to create the reaction between the materials.
  • Box Furnace  -  Used for heat treatment, calcining, curing, annealing, stress relieving, preheating, and tempering.  They have a very simple design, which makes them very versatile and problem-free.  Box furnaces can be designed with single or multiple zone heating.
  • Brinell Hardness Number  -  Is a value assigned to the hardness of metals and alloys.
  • Burner  -  A device which combines fuel and air in proper proportions for combustion and which enables the fuel-air mixture to burn stably to give a specified flame size and shape.
  • Burner Capacity  -  Amount of heat release a burner can deliver at a given set of operating conditions.


  • Calculation Furnace  -  Calcination is a heat treatment wherein samples are heated to a point just below their melting temperature to produce thermal decomposition or to remove volatile substances.  When ores are mined as carbonates or sulfates, the only way to extract the metal from the ore is to apply reduction, this is done in a calcination furnace.
  • Condenser  -  A component inside an air conditioner that removes heat from the system. A condenser can transfer heat to air or an intermediate fluid.
  • Convection Section  -  Above the radiant section, you will find the convection section, this area cooler is used to recover any additional heat.  Heat transfer also occurs in this section by convection.




  • Flame Detector  -  A device that was designed to stop gas from building up to a dangerous level.
  • Flame Failure  -  When the burner pilot or main flame goes out on its own.
  • Flue Gas Stack  -  Located at the top of all heat transfer chambers, the flue gas stack’s primary purpose is to protect the user of the furnace by collecting the flue gas and releasing it high into the atmosphere.



  • Heat Transfer  -  The exertion of power that is created by heat, or the increase in temperature.  It is the transfer of heat from one system to another.  There are four ways to transfer heat: conduction, convection, mixing, and radiation.
  • Heat Treatment  -  The heating and cooling of metals or alloys.


  • Igniter  -  A burner smaller than the main burner, which is ignited by a spark or other independent and stable ignition source and which provides proven ignition energy required to immediately light off the main burner.




  • Latent Heat  -  The energy absorbed or released by a substance during a constant temperature or phase change from a solid to liquid, liquid to gas or vise versa.





  • Pilot Light  -  On many natural gas furnaces a pilot light is used to facilitate ignition.  Pilot lights should be checked regularly to verify that they are working properly.
  • Process Furnace  -  An essential part of several industrial operations as a method for preparing fluids.  The two main types of process furnaces are electric and fired.  Of the two types, electric process furnaces are the more expensive to operate but have the advantage of not producing pollutants.


  • Quenching Furnace  -  A process of rapidly cooling a workpiece from a high temperature, and it is used to form martensite in steel.  The cooling material can be water or oil.  Quenching furnaces are normally paired with a batch furnace, roller hearths, or pusher furnaces. Different quenching furnaces are designed to meet the specific needs of a given application. A necessity for quenching furnaces is precision control of the temperature to avoid uneven heating and overheating.


  • Radiant Section  -  Where the heating element is found, which is often a flame.  The tubes or coils, often known as the radiant coil, in this section of the industrial furnace are heated by the radiation from the heating element.
  • Refinery Furnace  -  An essential part of oil refining process.  Crude oil is heated in a furnace to the desired inlet temperature for the distillation column.  They are used before the preflash and at the atmospheric and vacuum columns. Crude oil has to be heated before it enters the distillation tower.  The furnaces burn off waste gasses from the refining process and use energy efficient heat exchangers.
  • Refractory  -  Any material that has an unusually high melting point and that maintains its structural properties at very high temperatures.  Composed principally of ceramics, refractories are employed in great quantities in the metallurgical, glassmaking, and ceramics industries, where they are formed into a variety of shapes to line the interiors of furnaces, kilns, and other devices that process materials at high temperatures.
  • Rotary Tube Furnace  -  A heat treatment circular furnace that rotates during heat treating.  Materials travel a circular path through the furnace as they are treated.  Rotary tube furnaces use a continuous processing method to apply heat in thermal zones where the heat source supplies heat to a rotating tube.


  • Sintering Furnace  -  Sintering is a heat treatment process designed to transform loose, fragmented material into a solid mass.  The amount of heat provided during sintering varies in accordance with the type of material, but it is always slightly below the material’s melting point.  During sintering, the porous spaces in a workpiece are minimized as the material is squeezed and shaped at high temperatures and pressures.


  • Temperature  -  Normally described as the amount of heat or cold, but it is neither heat or cold.
  • Tempering Furnace  -  Designed to heat treat metal products to increase their durability and hardness.  Increasing the toughness of a metal product enhances the product's ability to withstand deformation and energy absorption before it cracks.  A tempering furnace brings out the beneficial properties of a metal and improves its mechanical characteristics.
  • Tensile Testing Furnace  -  A process for testing materials by subjecting them to tension until they break or fail. The properties tested are strength, elongation, and area reduction.  The process is a destructive method of testing products to determine their point of failure and durability.
  • Thermal Conductivity  -  The ability to transfer heat within a material without any motion of the material.
  • Thermostat  -  A device that is typically mounted in a centralized location to regulate the indoor air temperature. There are many different types of thermostats that will facilitate different functions, but all accomplish the same general purpose of sending a signal to the furnace when temperatures have dropped below the specified limit.



  • Vacuum Furnace  -  When a product is processed in a vacuum furnace, it is surrounded by a vacuum that prevents heat transfer through convection and removes contaminants.  Normally, heating products to high temperatures causes oxidation.  This is not present in a vacuum furnace since all oxygen has been removed.


  • Walking Beam Furnace  -  These furnaces are efficient methods for processing large, heavy parts.  The main uses of walking beam furnaces are annealing, forging, heating, stress relieving, quenching, and tempering.  The material to be processed is gradually fed through the furnace by water-cooled beams that lift and move materials in short steps.




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