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Introduction
This Article takes an In-depth look at Industrial Furnaces
You will learn more about topics such as:
What is An Industrial Furnace?
What Are The Uses For An Industrial Furnace?
Types of Industrial Furnaces
The Components Of An Industrial Furnace
What is An Industrial Furnace?
An industrial furnace is used to heat treat metals for annealing, tempering, carburizing, or pre-treatment of metals for forging. Though metal is a common material to be treated using an industrial furnace, other materials are also heated to prepare them for shaping, which include glass and certain types of ceramics. The function of an industrial furnace is to superheat materials to extreme temperatures using a variety of fuels and formats.
Though industrial ovens and industrial furnaces serve the purpose of heating materials for processing, there is a distinct difference between them. Many times the two terms are used interchangeably to describe a heating process. There are several differences between but the most distinct is temperature range. As a normal rule, ovens operate at 250° F to 900° F while furnaces can reach temperatures of 2000° F or higher.
Industrial furnaces come in different configurations and designs. Some are compact and portable with shelving and swing-open doors. They are used in laboratories for product or material testing. R and D labs use muffle furnaces to test new products while dental labs use them to do zirconia binder burnout.
In large metallurgy facilities, furnaces haveconveyor systems to handle high volumes of material. They are fully automated and have direct fired or induction heating. Their most common function is the extractive process of metallurgy or smelting.
Depending on the material's melting point or the desired product results, industrial furnaces have different processing temperatures, the temperature at which the furnace will most effectively and safely operate. They can have multiple chambers including loading/unloading, heat zone, and cooling chambers. Heat zones can reach temperatures upwards of 3000° F for extreme applications. The components of the heat zone are made of silicon carbide or a nickel chromium alloy, which are heat resistant, refractory materials able to withstand long term use. The cooling chamber uses either cold water or air to lower the material's temperature. In some cases, cooling chambers may have a cold water bath.
Uses for an Industrial Furnace
It may seem that the only industries that rely on industrial furnaces are metallurgical operations that do metal molding and shaping. In reality, wherever heated materials are required, it is very likely they use an industrialfurnace. Much of modern technology requires products and parts that need to be plasticized or deformed to be shaped for specific applications. In the majority of cases, industrial furnaces are the source for the heating process whether they are fossil or electrically fueled. Though the raw materials, fuel, and means of creating heat vary, industrial furnaces have proven to be a safe means for reshaping and pre-treating a wide variety of products.
For centuries, metallurgical companies have depended on industrial furnaces for smelting, heat treating, and tempering of various forms of metal at very high temperatures. Their flexibility makes them a central asset in the process since they can melt 550 lb. (250kg) of metal down to as few as 2 lb. (1kg). Their wide power range adds to their popularity.
Chemical processors use the heat from an industrial furnace to synthesize chemicals or perform heat treatments. Power companies use them to heat water to operate steam turbines for thermal power stations or to incinerate landfill waste. In the mining industry, they help in extracting raw metals from crushed ores. Ferrous and non-ferrous metal manufacturers that require high volume smelting and casting operations have used industrial furnaces for years.
There are different types of industrial furnaces for each industry. The fuel oil industries use ashing furnaces to determine the amount of ash formed by a sample. Annealing furnaces heat treat samples to soften them for cold or mechanical working. The types and designs of furnaces is endless and includes an assortment of industries from metalworking to food production.
The applications and uses of industrial furnaces grow as new technology and innovations appear on the market for mass production. Though there have been problems regarding their environmental friendliness, devices and mechanisms have been developed to overcome the negative effects to keep industrial furnaces operating.
Types of Industrial Furnaces
When industrial furnaces came into existence thousands of years ago, their sole purpose was to heat metals for molding and shaping. From their inception, and for many years after, that type of furnace remained the only variety. As industrialization has progressed, industrial furnaces have become a major part of multiple operations serving a wide range of applications.
Blast Furnaces
Blast furnaces are large towers or cylinders with a heat-resistant lining. They force hot air and gasses through the furnace load. As air flows into the furnace, ore, fuel, and limestone are pushed out the top creating a chemical reaction in the ore, fuel, and limestone as they move down the cylinder. The result is molten metal and slag at the bottom of the furnace while flue gasses move upward and out through the top of the furnace.
Blast furnaces are used for smelting industrial metals and extracting iron from iron ore in the form of a liquid. They are used for smelting and processing of lead, copper, and other metals. The main reason for using blast furnaces is the speed at which they can smelt metals, which is twice as fast as other furnaces.
Blast furnaces are made of several kinds of heat resistant elements. Their enclosed space holds the materials, gas, or air being heated until the desired temperature is reached. They come with varying temperature control options or can be set for a single temperature. The shape and design of the structure allows easy control of the temperature within the cylinder.
Temperatures in the blast furnace cylinder range from 1600°F to 2300°F. When needed, oil, tar, natural gas, coal, and oxygen can be added to increase productivity. Blast furnaces are relatively inexpensive. Modern blast furnaces can be 70’ to 120’ with a hearth diameter of 20’ to 45’.
Electric Furnaces
An electric furnace uses electrical resistance as a heat source to produce high temperatures. Several kinds of heat resistant materials are used in their construction. An enclosed space holds the materials, gas, or air being heated until the desired temperature, which is measured by an external sensor.
Electric furnaces can have varying temperatures or a single one. To increase heating, coal and oxygen can be added. They normally have atmospheres that can include air or oxidizing, inert, reducing, salt bath or vacuum and come in three or multi-zone varieties. Over-temperature protection, service or entry holes, view ports, computer interface and application software are available options. A common type of electric furnace is the electric arc furnace, which is used for making steel from scrap materials.
A benefit of electric furnaces is the precision control of their temperature and internal atmosphere. Important factors to consider when selecting an electric furnace are pressure range, process temperature, chamber length, and the height, width, or tube outer diameter of the furnace. Pressure range is the furnaces operating pressure, and temperature are the furnaces operating temperatures.
Gas Fired Furnaces
Rising electricity prices has made gas fired furnaces an economical choice for heating applications. Gas furnaces are grouped according to power combustion efficiency at 80-82% annual fuel utilization efficiency (AFUE) and have a condensing efficiency of at least 90%. While traditional furnaces lose heat during operation, a condensing furnace has a second heat exchanger that makes use of the potentially lost heat making a gas furnace more efficient.
Gas furnaces can have over-temperature protection, service or entry holes, be three or multi-zone, have view ports, a computer interface, and application software. They use natural gas as a source of heat, which can produce nitrogen oxide (NOx) as a waste product.
Heat Treating Furnaces
Heat treating furnaces heat a material and then bring it to a desired temperature. From there the material is cooled, which allows the furnace to instill increased strength, or aging within the material. This allows manipulation of the material to alter the physical, chemical and mechanical properties. The physical properties most often refer to how hard or soft the material is.
Heat treating furnaces add billions of dollars per year in value to metal products, especially steel and nonferrous products, by imparting specific properties to the parts. These properties are often critical to the proper function of the heat-treated parts.
Things to keep in mind when purchasing are uniformity, control, airflow and rate of rise. Uniformity is ensuring the temperature is the same throughout the system. Control regards how easily the system is monitored and adjusted. Airflow refers to the type of ventilation you want/need. Finally, the rate of rise regards how fast you want to be able to increase the temperature.
Double Chamber Furnace
Double chamber furnaces, referred to as dual chamber furnaces, have two areas for heating and cooling, which are separated to prevent temperatures and atmospheres from mixing. The design allows the heating chamber to retain and sustain its level of heat, a feature that makes double chamber furnaces more efficient and cost effective. Dual chamber furnaces are lightweight with low mass ceramic insulation. They are heated by coil wound heating elements that are mounted on ceramic rods for quick heating and cooling.
Each chamber of a double chamber furnace works independently using separate digital controls, thermocouples, and contactors, or the chambers can operate together. The lower chamber is normally equipped with a fan and stainless steel liner for recirculation of the heat and temperature uniformity. For safety purposes, the doors of dual chamber furnaces are vertical lift spring operated for the protection of equipment and workers. Their design makes it possible for the furnace to retain its heat and not need to be reheated with each heating cycle, a factor that makes dual chamber furnaces more efficient and cost effective.
The design of double chamber furnaces makes it possible to create two different atmospheres during a single cycle. This aspect of their structure makes it possible for a double chamber furnace to perform multiple thermal processes at the same time.
Muffle Furnace
Muffle furnaces have a separate combustion and heating chamber with a retort being a gas sealed chamber in which material is placed for heating. The separation of combustion chamber and heating chamber prevents the by-products of the heating process from contaminating the materials being heated. Modern muffle furnaces use electrical heating elements to further avoid possible contamination.
Most muffle furnaces are front loading boxes or tubes that are used for melting glass, creating enamel coatings, technical ceramics, and soldering and brazing. They are mainly found in research labs to test the combustibility and non-volatility of materials. Modern heating elements make it possible for a muffle furnace to reach temperatures of 1800°C (3727°F), which makes it possible to complete more sophisticated metallurgical applications.
Muffle furnaces are used to treat low carbon steel, stainless steel, and copper. Unlike blast furnaces and reverberatory furnaces, materials in muffle furnaces are isolated from the fuel, which is referred to as indirect contact. The process of a muffle furnace is referred to as a jacketed process where material is heated to a high temperature while it is contained and isolated from external contaminants, chemicals, and other substances. Muffle furnaces are lined with stainless steel to make them corrosion resistant and to avoid the effects of combustion heating.
Humpback Furnace
The humpback furnace is a conveyor type or belt furnace that is used for low dew point and bright firing applications. The heated section of the furnace is raised higher than the entrance and exit of the furnace, which gives the furnace the name of humpback. The raised heated section is designed to reduce the intrusion of atmospheric elements during the heating process. Oxygen molecules are isolated and prevented from entering the heating chamber, ensuring the presence of a pure hydrogen.
The mesh belt conveyor of a humpback furnace makes it possible to have larger door openings making it possible to process larger parts of up to two feet in height. Humpback furnaces are made of heavy gauge, highly durable steel that is welded, sealed, and reinforced. Computerized controls are located in the mainframe for easy access and provide overall control of the complete system using a microprocessor temperature control, phase angle power controller, and various forms of temperature instrumentation.
The sections of a humpback furnace include a continuous pusher, atmosphere doors, a horizontal heated section, and horizontal cooling section. Materials being heated are carried through the furnace in the form of a train on pusher plates or carrier trays. Applications that are perfect for humpback furnaces are annealing, bright firing, and brazing due to the pure hydrogen heating atmosphere.
Ashing Furnace
The ashing process, also known as dry etching, is used to remove organic material from a sample prior to the sample being analyzed. The success of the ashing process is dependent on complete combustion of the tested sample in order for the analysis to supply sufficient data.The ashing process requires controlled airflow and high temperature elements in order to burn samples, which assists in precisely calculating the volume of the incombustible compounds in the sample. The heating chamber has its use of air and exhaustion closely monitored to record the numerous changes per minute.
In the production of semiconductors, ashing is used to remove photoresist and other materials from the surface of silicon wafers. The photoresist material is a masking that is used during photolithography to define the pattern that will appear on the wafer. The final step in the process is the removal of the photoresist that serves as masking. Ashing burns off the photoresist and ensures that the surface of the wafers is clean and free of contaminants, factors that can affect the performance of the final product.
An ashing furnace heats samples until the only substances that remain are non-combustible ash, which is studied for its composition. The study of ash is used to make a nutritional evaluation of food to estimate the mineral content of food samples. In the petroleum industry, ashing furnaces are used to test the combustibility of petroleum products.
Regardless of the industry, the heating chamber of an ashing furnace is heated to approximately 600°C (1112°F) with a steady flow of air. During the ashing process, incombustible elements convert to oxides and sulfates, which makes it possible to quantify the content based on its day weight ash.
Calcination Furnace
Calcination furnaces are designed to change the physical and chemical properties of materials to form new crystal structures or phases. During calcination certain substances, such as water, carbon dioxide, and gases, are released, leaving behind salt and non-volatile components. The calcination process is a chemical process that requires the heating of materials to a very high temperature in the presence of oxygen. This thermal aspect of the process causes the decomposition or change of a material, including the removal of volatile components and impurities.
Applications that involve the use of a calcination furnace include removing water from samples and volatile substances. A very common use for calcination furnaces is in the manufacture of cement, where calcium carbonate is decomposed into calcium oxide and carbon dioxide. In the production of zeolites, a crystalline aluminosilicate material, calcination furnaces are used to remove ammonium ions.
For the calcination process to be successful, a calcination furnace includes a calciner, which is a long steel cylinder that rotates inside the furnace and reaches temperatures between 550°C and 1150°C (1000°F and 2100°F) that have to be held for an extended period of time. The process was originally designed to remove calcium carbonate (limestone), calcium oxide (lime), and carbon dioxide. The name calcination is from the Latin word calcinate meaning “to burn lime”
Tempering Furnaces
Tempering is a common practice that is used in the production of iron based alloys to toughen the metal and is completed after hardening to remove any excess hardness. The process is completed by heating a metal in a furnace to 900°C (1650°F) and soaking the metal in a liquid bath, after which it is cooled and placed in a tempering furnace.
A tempering furnace has ceramic heating elements that are lined with electrical wires to produce uniform heat in the sample chamber. The ranges of temperature vary in accordance with the desired material characteristics that are to be achieved. Iron based metals are heated in a tempering furnace to cause phase transitions in the metal’s martensite structure and reduce the metal's hardness to improve its ductility. The use of tempering furnaces for the process is due to their ability to maintain the same level of temperature and the uniformity of heating throughout the process. These factors ensure that the metal will not warp or fracture.
The tempering process is an ancient process that dates back a thousand years and has been used to give ferrous metals the greatest possible strength and elasticity. Most modern tempering furnaces are based on the ancient practices with some adjustments that have been added due to technological advancements.
Sintering Furnace
Sintering, like tempering, is a heat treatment process that changes loose, fragmented materials into solid masses. As with all forms of heat treatments, the amount of heat required depends on the consistency of the material being treated but is always below the melting point of the material. During the process, through the use of heat and pressure, the porous spaces between the molecules in the workpiece are squeezed, closed, and shaped without melting the material.
The process of sintering covers a wide range of methods and materials, all of which have advantages. The fundamental concept regarding the use of a sintering furnace is its ability to produce materials with exceptional purity and uniformity that exceed the characteristics of the original material. This aspect of the process is due to the simplicity of the fabrication process. As materials are sintered, they create uniform, controlled porosity, which adds to their purity.
The use of sintering is commonly found in processes that transform powders into metal parts. Sintering furnaces are capable of changing the properties of materials including their thermal and electrical conductivity, strength, and translucence. The process of sintering, like many other metalworking processes, has been used for centuries. Modern sintering furnaces are used to produce structural steel, porous metal parts, wiring, bearings, and magnets.
Rotary Tube Furnaces
Rotary tube furnaces are heat treatment furnaces that rotate during the heat treatment process. They are a circular furnace that transports samples through the furnace along a circular path. Rotary furnaces are burn pulverized, solid, gaseous, or liquid fueled furnaces that have metal drums lined with refractory metals that are highly resistant to heat and mounted at an angle within the metal drum.
Classified by their method of heating, rotary tube furnaces are used for continuous material processing using multiple thermal heating zones where heat is transferred from a heat source to the rotating tube holding the heated materials. The various applications of rotary tube furnaces include oxidation to increase oxygen in a sample and calcination for phase transformation. They are a critical part of the manufacturing of alumina, vermiculite, iron ore pellets, and cement clinker.
The ideal use for rotary tube furnaces are processes that necessitate continuously running batches that need to be heated in a short period of time. They are slightly inclined with material being fed at the top of the incline and exiting by falling out at the lower end of the tube. How long materials remain in the tube is dependent on the angle of the inclination, the speed of the rotation, the length of the tube, and the flow properties of the batched materials.
Certain models of rotary tube furnaces have closed feeding systems and can be used for defined atmospheres or vacuums. The various types of rotary tube furnaces are differentiated by the size of their batches, maximum temperatures, and materials being processed.
Induction Furnaces
Induction furnaces use a combination of electrical resistance and hysteresis losses to heat metal. They are cleaner and more energy efficient than other types of furnaces. With growing environmental concerns, induction furnaces are replacing other traditional heating methods.
Induction furnaces operate by a process called electromagnetic induction where an electric current is passed through a metal coil creating a magnetic field where electrical resistance raises the temperature of the metal to be processed. To be able to withstand long-term use, induction furnaces are made of heat-resistant materials. The heated area is controlled by the shape and size of the induction coils. Induction furnaces are widely used for melting iron and have become popular for working non-ferrous metals since they have excellent metallurgical control and are pollution-free. Features that have made induction furnaces popular include rapid heating, low levels of noise, fumes, and radiated heat.
Applications for induction furnaces include baking, aging, brazing, annealing, curing, firing, drying, burn-off, foundry, hot pressing, laboratory uses, heat treating, quenching and preheating of a wide assortment of metals. They have an operating frequency ranging from 50 Hz to 400 kHz or higher.
There are two types of induction furnaces: coreless or channel. A coreless furnace has a hollow tubing of heavy duty copper and is used for melting steels, irons, and non-ferrous alloys, while a channel furnace has a steel lined shell and is used for low melting point or higher melting point alloys. Important factors to consider when selecting an induction furnace are its configuration, operating frequency, and atmosphere control.
Vacuum Furnaces
A vacuum furnaces operate in a vacuum, airtight structure to alleviate surface reactions. The lack of air and gasses prevents contamination. Vacuum furnaces are able to heat various metals to high temperatures and perform processes such as brazing, sintering and heat treatment under precise and controlled conditions with consistency.
Vacuum furnaces are ideal for processing parts for bright annealing, stress relieving, normalizing, tempering, degassing, brazing and industrial purposes where high pressure cooling or quenching mediums are used. Typically, vacuum furnaces use low atmospheric pressure for heating.
Vacuum furnaces maintain the temperature throughout the enclosure making it ideal for metals that are sensitive to variations in temperature. They protect against contamination from carbon, oxygen and other gasses. With oxygen removed, oxidation is avoided. Vacuum furnaces use quenching for quick cooling. The processes of a vacuum furnace can be computer controlled to ensure metallurgical repeatability. Factors to consider when selecting a vacuum furnace are its pressure range, process temperature, control systems, chamber length, single or multiple chambered, height, width, and outer tube diameter.
Pit Furnace
Pit furnaces are installed in the form of a pit with an easily removable lid and are used to melt small quantities of ferrous and nonferrous metals for casting. They have a refractory lining and chimney with coke as their fuel or electricity. Pit furnaces have a vertical orientation and are batch furnaces that have atmospheric or direct fired heating. They can come with or without retorts or vessels depending on the requirements of a process.
The processes completed by a pit furnace include carburising, carbonitriding, gas nitriding, temperament, hardening, stress relief, and reclamation. Pit furnaces are electrically or gas fired and can be integrated into automatic lines. The wide use of pit furnaces is due to the precise temperatures they provide for metallurgical processes and their ability to harden steel alloys by evenly heating the alloys.
Pit furnaces are a very valuable asset as one of the best tools for changing the properties of metals to meet specified and exacting requirements. They have a maximum temperature of 1000°C (1832°F) and provide uniform heat with a ± 5° variance. Pit furnaces are highly efficient and have excellent atmospheric circulation and cooling systems.
A common use for pit furnaces is tempering steel and heat treating aluminum due to pit furnaces being easy to load and unload by hooking parts and lifting them out of the furnace. Long parts are heat treated in pit furnaces by being hung from the center post of the furnace.
Conveyor Furnaces
Conveyor type industrial furnaces are designed for rapid heat treatments. They have a tunnel shaped heating chamber that is separated into sections of heating modules that are combined to perform a single heating procedure. The various sections include preheating, heating or firing, and cooling. Since each section performs a different function, they are separated with heavy insulation to ensure efficient processing. Conveyor industrial furnaces are capable of treating products at temperatures up to 1000°C (1832°F).
The different types of conveyor furnaces include electronic mesh belt, brazing mesh belt, tabletop, and sintering mesh belt. Aside from conveyor belts for conveyor furnaces being made of metal mesh, roller designs are used for the heating treating of individual products. A common form of conveyor furnace is an overhead monorail type that is used for curing and drying applications. When the heating of heavy loads is necessary, slat and chain conveyor furnaces are used, which can easily be integrated into a production line.
The main reason for using a conveyor furnace is increased throughput and an increase in the rate at which products are processed. Efficient and streamlined heat treatments avoids delays in production. Automatic heat treatment processes removes the need for workers to handle components that are being heat treated and provides for the safety of workers. The consistency of a conveyor furnace ensures repeatability and standard treatment for all parts and components.
Tensile Testing Furnaces
The high temperature resilience of materials is a concern for applications that involve power generation, aerospace, and automobiles. Tensile testing, also known as tension testing, is where a sample is exposed to various stressful conditions until it fails. Tensile testing furnaces are used to expose metal products to conditions that involve high temperatures and tension. The testing process is performed regularly to assess the performance of steel and other metals that will be exposed to high temperatures during their use.
Tensile testing furnaces quantify the elasticity, yield quality, strain-solidifying, and other properties of a material. They expose samples to controlled pressure until the sample fails. The results of tensile testing furnaces makes it possible for manufacturers to determine the point at which a product will fail and allows manufacturers to make adjustments to products to increase a products longevity. In essence, tensile testing furnaces are an important quality control device.
Testing labs and R and D departments depend on tensile testing furnaces to assess the mechanical properties of metals, including a metal’s strength, ductility, impact resistance, fracture toughness, elongation, and stress. During elevated temperature testing, the sample is placed in a tensile testing furnace capable of heating to 982°C up to 1093°C (1800°F up to 2000°F). The control system of the tensile test furnace raises the temperature to the specified level and allows the sample to soak in the high temperature. Hot tensile data is compared to the performance of a metal or product at room temperature to determine how it can be used and under what conditions.
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Industrial Furnace Heat Treatments
Industrial furnace heat treatments vary according to the product to be treated and the type of furnace with certain types of furnaces specializing in one type of treatment. There is an endless list of treatments that industrial furnaces can perform that vary from an annealing process to melting a material for casting. The key to the success of an industrial furnace’s treatments is precision and accurate control of the temperature.
Annealing - During the annealing process, a metal is heated to improve its machinability and decrease its brittleness. Annealing creates a uniform internal structure to increase the strength of the final product. As with many industrial furnace heat treatments, timing is crucial in relation to reaching the correct temperature, holding that temperature, and lowering it at the proper rate.
Brazing - Brazing is a unique process that joins components using a dissimilar metal and is used with copper, carbon steel, stainless steel, and brass. Filler metals can be copper, silver, or certain nickel alloys.
Sintering - Sintering is part of the heat treatment of powdered metal production. It is where metal powders are mixed with a wax lubricant and compacted into a certain shape. The process of sintering is designed to improve the strength of a part while reducing porosity as metal particles join without melting.
Steam Treatment - Steam treatment is a process used for the oxidation of metals. During a steam treatment process, a thin layer of oxide forms on the surface of a metal part to provide increased corrosion and wear resistance, surface hardness, improved appearance, and seals porosity and increases density.
Melting - Industrial furnaces for melting metals cover different sectors. In some cases, industrial furnaces for melting metals are used to melt scrap steel and aluminum for remanufacturing. Blast furnaces are used to produce pig iron for steel manufacturing. Regardless of the type of metal, industrial furnaces used for melting metals for reprocessing are far more profitable and efficient than metals made from raw materials.
Components of an Industrial Furnace
While the individual designs of industrial furnaces differ, they do have certain commonalities. The most obvious feature found in all industrial furnaces is its fuel, which can be electricity, fossil fuel, non-fossil fuel, or a combination of two or three of them. The concern for the environment has increased efforts to control the CO2 and NOx from industrial furnaces, a major issue for climate control. Described below are the basic components of most industrial furnaces.
Power Source or Fuel
A power source or combustible fuel is necessary for an industrial furnace to generate heat. Electricity or a combustible fuel such as coal or natural gas are the most common. Wired electricity uses current to heat an electrical element while gas is pumped under pressure through a pipe. Coal is loaded by hand or dumped using a mechanized device.
Heat sources available for industrial furnaces include radiant, natural gas, induction, conduction, electrical, and dielectric. Each method has its benefits, limitations, and applications. As with all industrial furnaces, the design of the furnace and the type of available fuel determines what kind of power the furnace will have.
Radiant heat furnaces are similar to wood stoves and portable heaters. They use a flame to heat a material, which is commonly a form of ceramic plate. The heated material produces heat that is transferred to the area to be heated. The process is considered to be very efficient and economical.
Natural gas furnaces are very common. They are an economical method for creating a high temperature environment. They burn natural gas or propane and are chosen for their ability to produce extreme high temperatures. Most gas powered industrial furnaces have highly sophisticated control systems for environmental protection and safety.
Induction furnaces use a combination of electrical resistance and hysteresis losses to heat metal parts. Electrical current is sent to a hysteresis magnetic field around a coil that carries alternating current to produce an eddy of current, which becomes resistant and heats the material. Induction industrial furnaces are known for being clean, energy efficient, and a means for controlling melting metals. Over the past few years, they have gained popularity and are replacing other traditional methods.
A common type of electrical industrial furnace is an electrical arc furnace. The principle behind an arc furnace is the creation of heat from electrical resistance. The process begins with an electrical current being sent to a metal arc that heats as it resists the current. The created heat is transferred to the metal, which is heated to its melting point.
Heat Access
It is important that items being heat treated receive the heat produced from the heat source. Industrial furnaces require a way to direct the heat, under controlled conditions, at the targeted item. Whether a furnace melts metal, heat treats a chemical compound, or burns to power a mill assembly, the furnace must have a mechanism for the heat to interact with the material. Depending on the type of furnace, access may be direct where the material is placed over a flame or indirect where the material is placed on a form of heat exchanger. For all types, the first consideration is safety and the protection of operators.
The names for industrial furnaces serve as descriptors as to how the heat is directed at the items being heated and include terms such as direct heater and direct fired heater. Fire powered industrial furnaces have burners that heat up tubes with a fluid that is used to supply radiant heat to the materials being heat treated. When combustion occurs, heat is transferred to tubes that surround the combustion chamber and to the fluid in the tubes that heat the furnace. With electrical industrial furnaces, heat is created by electricity and transferred by radiation.
The key to each process is the radiation of heat from the tubes. In most cases, direct heat is only used for melting materials and not for annealing, sintering, and other treatment processes. Regardless of the method of heating, industrial furnaces make it easy to achieve the proper temperatures for the items being processed.
Emissions
An issue that has become a major concern for industrial furnace users and manufacturers is emissions, which, in the past, have done damage to the world’s air quality and contributed to the greenhouse effect. Recent designs have developed methods to redirect carbon dioxide emissions away from the combustion chamber, where incomplete combustion of some fuels generates toxic carbon monoxide.
Stringent environmental emissions regulations require proper disposal of carbon dioxide waste. To meet the requirements, producers are installing oscillating combustion technology (OCT) to efficiently control the problem. An OCT offers a low cost method for reducing Nitrogen Oxide emissions The technology uses a valve to oscillate the fuel flow rate to the burner creating fuel-rich and fuel-lean zones in the flame to retard the formation of NOx while increasing heat transfer to the load.
Several other methods of controlling emissions include dry and wet scrubbers that absorb or watch waste before it is released into the air. One of the first methods manufacturers used was a system of filters attached to smokestacks. With the centrifugal force process, pollutants are removed from the air by being spun. The force of the circular motion pushes the heavy particles, that are heavier than air, against the walls of the centrifuge and precipitate into a containment chamber.
A recent method for controlling Nitrogen Oxide emissions and controlling the flame temperatures of natural gas furnaces is a flame image sensing technique. Flame data is collected with a fiber-optic detector or spectrometer and video camera. The collected information provides data for control strategies for efficient methods to monitor industrial furnaces.
The mounting concerns for the environment have challenged scientists to continue to develop methods to control pollutants. What many experts are proposing is using sources of energy other than fossil fuels that can be generated by natural power. Electrical, induction, and radiant devices have become popular as a part of their efforts.
Insulation
Industrial furnaces operate at extremely high temperatures making insulation material a necessity for the furnace to run efficiently and safely. The outer layer of the furnace becomes extremely hot during operation causing operators to be at risk of serious burns. Insulation helps to reduce the amount of heat of the outer body of the furnace protecting operators.
The tremendous amount of heat generated by an industrial furnace needs to be properly directed and controlled. Regardless of any preventative measures, heat escapes leading to wasted energy. Insulation helps to control energy waste by containing heat in the chamber and preventing it from escaping.
Each of the described components of industrial furnaces are used and incorporated in the various models in ways that fit the designs and specifications of the manufacturer. When shopping for an industrial furnace, it is important to have a general knowledge of the basic requirements to make a wise buying decision. The main concern is the available energy source, which is the final determining factor.
Industrial Furnace Control Systems
All controllers work by holding or controlling variables or parameters at a set value. The two most important required variables are input signal and setpoint value with the input signal being the process value, which is sampled several times per second to the setpoint value. If the two do not match, the controller generates an output change based on the difference. The output signal initiates a response to correct the input signal to make it match the setpoint value.
The control system of an industrial furnace is designed to control a furnace’s temperature, heat delivery system, and other variables. Input data is received from thermocouples and sensors, which are matched against the setpoint for each aspect of the furnace.
As with all forms of industrial equipment, there are various forms of control system interfaces with the majority of systems having digital interfaces that allow for easy data entry. Digital controls can be human machine (HMI) software or have supervisory data acquisition and supervisory control and data acquisition (SCADA) functionality.
ON/OFF Control - ON/OFF control is the simplest type of control system and works using a hysteresis band. If the temperature in a system falls below the setpoint, a signal is sent to the heater to raise the temperature. Once the setpoint is achieved, the system turns the heater to off.
Analog Controllers - After ON/OFF controllers are analog controllers, which are a step up from ON/OFF controllers. They are a low cost control system that are reliable in harsh manufacturing conditions. Much like ON/OFF controllers, analog controllers provide direct or reverse on and off control for applications. They accept thermocouple or RTD input and have a power control mode without temperature sensors. The disadvantage of analog controllers is the absence of a readable display and sophistication.
Proportional Integral Derivative (PID) Controller - A PID control determines the necessary output value required to maintain the temperature in the system.
Time Proportioned Controller - Timed proportioned controllers use cycle times to proportion the output value.
Limit Control - Limit controllers provide safety limit control of process temperatures. They are unable to control temperatures but work independently as safety devices alongside a control system. Limit controllers accept thermocouple, RTD, or process limit inputs that are set for high or low temperatures. They shut a system down when over limit conditions are met.
Linear Control - Linear controls have variable control signals.
Proportional, Integral, Derivative (PID) Control - PID systems provide real time system feedback.
Feed Forward Control - Feed forward controllers are designed to anticipate the needs of the system. Instead of using feedback as a reference, the system collects data and anticipates the needs of the furnace.
Fuzzy Logic Control - Fuzzy logic controllers create heuristics that align with human thoughts and perceptions of problems to provide a method for dealing with the lack of precision and nonlinear control situations. The foundational concept of a fuzzy logic controller is very flexible and able to deal with if-then situations.
Nonlinear Control - Nonlinear controllers use various types of algorithms such as neural networking and adaptive gain.
Closed Loop Control - In a closed loop control system, the output temperature is measured and adjusted to maintain a constant output temperature. The temperature is constantly fed back to the controller to ensure maintenance of the temperature.
Multi-Loop Controllers - Multi-loop controllers can accept multiple input variables and are viewed as being multiple controllers enclosed in a single device.
Profile Controllers - Profile controllers, also known as ramp soak controllers, make it possible for an operator to program several setpoints and the time for each setpoint to be activated. Programming the setpoint is referred to as the ramp while the amount of time at each setpoint is referred to as the soak. Each individual ramp and soak is considered one segment. The setpoints and time settings of an operator are referred to as a recipe. Profile controllers allow for multiple recipes to be saved and accessed at a later time.
Conclusion
Industrial furnaces have become a staple for industrial operations from the production of plastics to the deforming of heavy metals.
An industrial furnace is a form of heat source that is used by manufacturing for producing heat by mixing fuel with oxygen or electrical energy for annealing, tempering, carburizing, or pre-treatment of metals for forging.
Industrial furnaces come in different configurations and designs with some being compact and portable with shelving and swing-open doors.
As industrialization has progressed, industrial furnaces have become a major part of multiple operations serving a wide range of applications.
Though metal is a common material to be treated using an industrial furnace, other materials are also heated to prepare them for shaping, which include glass and certain types of ceramics. The function of an industrial furnace is to superheat materials to extreme temperatures using a variety of fuels and formats.
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