Furnace

There are two main types of furnaces: those used to heat indoor spaces during winter, like wall furnaces, which are self-contained room heaters, and industrial furnaces, which are enclosed structures that contain high heat chambers for material fabricating, melting or heat treating in manufacturing environments. Furnaces obtain their heat through many different sources. Electric furnaces, gas furnaces and induction furnaces all use a variety of heating methods, but are able to obtain high heat. High temperature Furnaces are used only in industrial applications, and usually on materials with high melting points, such as metals like copper, titanium, steel, aluminum and bronze, as well as glass and some plastics. Other applications for powerful furnaces include heat treating furnaces like annealing furnaces, which alter or improve material properties,  sintering furnaces, which form parts and products from powder materials, and finally, metal melting furnaces such as blast furnaces, which employ extremely high heat to turn scrap or raw metal molten for casting purposes. Industrial furnaces are used in chemical manufacturing, ceramics, forgeries, foundries, steel service centers, metal fabricators, glass fabricators and many other metallurgy applications and industries. All furnaces for industrial use are well insulated chambers with controlled atmospheres. Some replace oxygen with an endothermic gas, while others like vacuum furnaces create a vacuum environment.

Industrial furnaces come in many different specifications and designs according to their specific function. Some are compact and portable, with shelving and a swing-open door. These are common in laboratories during product or material testing. In large metallurgy facilities, furnaces are often composed of large, continuous conveyor systems that handle extremely high volumes of parts or products. These furnaces are fully automated and usually CAD/CAM operated. For facilities with smaller runs, manually loaded/unloaded furnace chambers are employed. Small furnaces usually require the use of handling tools and crucibles, which are containers that can withstand direct contact with molten metal. Depending on the material's melting point or the desired product results, all furnaces have differing process temperatures, which are the maximum temperatures at which the furnace will effectively and safely operate. Many industrial furnaces that contain multiple chambers include a loading/unloading chamber, heat zone chamber and cooling chamber. The heat zone can reach temperatures upwards of 3000º F for the most extreme applications. Therefore, all components are made of silicon carbide or a nickel chromium alloy, both of which are extremely heat resistant, refractory materials that are able to withstand long term use. The cooling chamber uses either cold water or air to lower the material's temperature. Some contain cold water baths, which are used for the quenching process.

There are many different furnace heat sources available today. They include radiant, natural gas, induction, conduction, electrical and dielectric, and each method has its specialized benefits, limitations and applications. Radiant heat furnaces are similar to wood stoves and portable heaters. They use a flame to heat an object, commonly a ceramic plate. This object gives off heat that transfers throughout the area. Natural gas furnaces are very common. Compared to electric furnaces, they are an economical method of creating a high heat environment. They burn natural gas or propane in order to generate heat, and are used for their high temperature abilities. Induction furnaces use a combination of electrical resistance and hysteresis losses to heat metal parts. They are exposed to a magnetic field around a coil-carrying alternating current. Induction furnaces are the first choice in metal melting applications and are often used by iron foundries. Electric furnaces are also popular for melting metal. The most common type is an electrical arc furnace, which uses high amounts of electrical current, which travels through a metal arc and is conducted onto large amounts of scrap metal. The current heats the scrap to a high enough degree to melt it completely. These furnaces are mostly used for recycling metal parts to be formed into new products.
 
Furnaces often come with varying temperature control options or are set for a single temperature, depending on the process for which the furnace will be used. Overheating protection, service or entry holes, sight glass, temperature control, computer interface and application software are some of the features available on these furnaces. Furnace designers produce machinery and equipment that are used for various heat treatments of materials. Heat treating adds billions of dollars per year in value to metal products, especially steel and nonferrous products, by imparting high heat to the parts that alter their specific properties. These properties are critical to the proper function of the parts. The heat treating process involves three basic steps. The glass or metal parts, which often include rods, tubes and sheets are first heated to a specified temperature up to 2400º F. They are then held at that temperature for the required amount of time, which may be as little as a few seconds or as much as 60 hours. Finally, the parts are transported to a cooling chamber and rapidly chilled far below room temperature.
 
The steel industry, among others, uses high-temperature, natural gas-fired furnaces, which produce large amounts of nitrous oxide (NOx) per unit of processed material. To meet increasingly stringent emissions regulations, oscillating combustion technology has been developed and is continuing to be improved. This technology provides efficiency, productivity and reduced NOx emission from all types of furnaces. A valve is used to oscillate the fuel flow rate to the burner. Oscillation creates fuel-rich and fuel-lean zones within the flame that retard the formation of NOx, increasing heat transfer to the load. Another project is underway to reduce NOx emissions and carefully control flame temperatures of natural gas furnaces with flame image sensing techniques. Flame data will be collected with either fiber-optic detectors or spectrometers and video cameras. The data will be used to develop control strategies for the most efficient control of these furnaces.

Furnace Types

• Annealing furnaces use hydrogen to protect materials from oxidation, relieving the residual stresses produced during the processing of steel strips, tubes and rods. The hydrogen also has high heat transfer properties that aid in the reduction of production cycles.
• Batch furnaces heat treat one load at a time. Batch furnaces are used in applications in which large parts and complex alloy grades must be treated.
• Belt furnaces are continuous-type furnaces that move parts using a mesh-type or cast-link belt.
• Blast furnaces are large towers or cylinders that have a heat resistant lining and force gusts of hot air and gases up through the furnace load.
• Car furnaces are batch-type furnaces that are used for lower stress-relieving ranges. Car furnaces utilize a car on rails for entering and exiting the furnace area.
  
• Continuous type furnaces are heat-treating furnaces through which materials continuously enter one door and are discharged through another.
  
• Direct-fired tunnel-type furnaces are continuous-type furnaces in which the parts are conveyed through a tunnel-like heating area on hooks or fixtures, in order to minimize distortion.
  
• Electric furnaces are machines that use electricity to heat.
• End port regenerative furnaces, used to melt glass, have burner ports and the checkers on the same end. While end port regenerative furnaces are initially energy efficient, the checker volume eventually decreases through plugging, resulting in the gradual decrease of melting efficiency over the life of the furnace.
• Gas-fired furnaces burn gas to create heat.
  
• Heat treating furnaces heat the material and cool it to bring it to a desired point for increased strength, aging or manipulation.
• Induction furnaces use combined electrical resistance and hysteresis losses to heat metal by exposing it to the varying magnetic field around a coil-carrying alternating current.
  
• Pusher furnaces are continuous-type furnaces in which parts are charged into the furnace in containers on a periodic basis. These containers are pushed against a line of containers that have already been heated, advancing the containers towards the discharge end of the furnace, where they are put into use.
• Rotary retort furnaces are a type of continuous furnace that utilize an internal screw to advance the parts, giving good control of the retention time within the heated chamber.
  
• Shaker-hearth furnaces are a type of continuous furnace that advances the parts along the hearth using a reciprocating shaker motion.
• Side port regenerative furnaces, used to melt glass, have burner ports on opposite ends and incorporate a greater checker volume than the end port regenerative furnace, resulting in higher pull rates.
• Sintering furnaces heat materials below their melting point to make them more dense or compact and solid, which can lead to shrinkage.
• Slot furnaces are furnaces in which stock is charged and removed in batches through a slot or opening.
• Vacuum furnaces are airtight and use low atmospheric pressure as the atmosphere for heating.

Furnace Terms

Annealing - The processes of heating, holding temperature and cooling in order to accomplish a specific objective with the material being heated.
 
Banded Structure - A structure that can be developed from the hot rolling of steel, resulting in a layering effect.
 
Bright Annealing - Annealing done in an extremely hot atmosphere that prevents the occurrence of discoloration and may reduce oxides.
 
Carburizing - Heating the surface of steel in contact with carbon-rich solids, liquids or gases in order to add carbon to the material.
 
Case Hardening - Making the surface layer of a steel material substantially harder through heat treating.
 
Control Zone - The separate sensor or instrument of a piece of thermal processing equipment which controls its own temperature individually.
 
Controlled Cooling - A process that is used to deter cracking or hardening or to achieve a desired microstructural result. The controlled cooling process involves cooling the material from an elevated temperature in a selected method.
 
Cooling Stresses - Stresses resulting from irregular temperature distribution during cooling.
 
Industrial Ovens - Enclosures that are used for a wide variety of process heating applications.
 
Preheating - Heating material or products prior to a subsequent thermal or mechanical treatment.
 
Quenching - Quickly cooling from a high temperature.
 
Racking - Placing parts on a rack or tray prior to heat treatment in order to prevent heat-related distortions and to keep the parts separated.
 
Refractory - A term describing materials that have a very high melting point, making them ideal for linings of furnaces and kilns.
 
Spalling - Surface flaking or chipping resulting from improper heat treatment or material dissociation.
 
Spray Quenching - A quick cooling process in which spray nozzles are used to spray water or other liquids on a part.
 
Stabilizing Treatment - The process of heating a part to slightly above its normal operating temperature and then cooling it to room temperature. Stabilizing treatments are done prior to finishing the part to final dimensions and for the purpose of ensuring dimensional stability.
 
Stress Equalizing - A heat treatment done at low temperatures in order to balance stresses in a cold-worked part without decreasing the mechanical strength imparted to the product through the cold-working process.
 
Stress Relieving - Heating and holding the desired temperature long enough to reduce residual stresses, then slowly cooling.
 
Superheating - Heating at a temperature above the point where equilibrium phase transformation should occur but without actually making a transformation.
 
Thermocouple - A device that measures temperature through thermal electromotive force.
 
Transformation Temperature - The temperature equilibrium at which a change in the phase occurs.
 
Water Quenching - A quick cooling process using water as the quenching medium. Water quenching is inefficient at the beginning, or hot, stage of the quenching process.