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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 a Furnace?
How Furnaces Work
Types of Furnaces
Methods for Heating Furnaces
And much more…
Chapter One – What is a Furnace?
A furnace is a piece of equipment that provides direct electric or fired heat for industrial processes that require temperatures that exceed 752 °F (400 °C). Many industrial processes require heating for the preparation of materials for production or the completion of an application. In all cases, the dependability and durability of electric and fired industrial furnaces provide the necessary temperature control and reliability to complete a manufacturing process or operation.
The two general types of industrial furnaces are electrical and fired. Electrical industrial furnaces are either arc or high frequency induction. The arc type is used for refining, while high frequency induction is used for melting metals. Fired furnaces rely on the combustion of a fuel source to directly or indirectly heat raw materials or to sinter finished parts.
The construction of furnaces varies depending on the fuel source and type of furnace, with wide variation between electric and fueled furnaces. All furnaces are built of materials capable of withstanding extraordinary levels of heat without failing or breakage. Some factors considered during furnace design are process temperature, height of the furnace, outer diameter (OD), length, and desired pressure range.
Chapter Two – How Furnaces Work
Different furnaces function differently and burn different types of fuel. For many years, furnaces were powered by wood or coal; this required constant refueling for continuous heat. Modern furnaces have moved on to fuels that are supplied automatically.
How a Furnace is Fueled
The critical element in the operation of an efficient and economical furnace is its fuel. Though coal and wood were used for many years, they polluted the environment, required constant feeding, and made it difficult to keep a steady temperature. Modern furnaces have fuel fed directly into the furnace at a controlled rate, using electricity to maintain even temperature.
Fuel-fired furnaces are the most widely used. The nature of the fuel determines the design of the furnace but is not relevant to modern furnaces. As with any type of heat-operated device, the supply of oxygen is important to the furnace’s efficient operation.
Electric furnaces are efficient and environmentally sound because they do not release flue gasses. Unfortunately, they are expensive to operate. Electric furnaces can use either induction or resistance heating.
First, resistance heating is the most expensive type of electric furnace. These furnaces use a circulating fan to maintain temperature uniformity. Resistors can be made of various materials. The load to be heated may also serve as a resistor.
Induction heating is used for heating a localized area of a workpiece. With induction heating, electricity passes through a coil that surrounds the load. The type of load determines the frequency of this current. The coils are water-cooled to prevent them from overheating.
Furnace Burner Types
The fuel is supplied to the burners where, predictably, it is burnt. Most furnaces have more than one burner that can be mounted in different sections of the furnace depending on its design. The burner has an oxidizer to change the chemical energy into thermal energy. The type of fuel used in a furnace is determined by the burners, which mix the fuel and air and ignite them. Burners must be stable, cost effective, reliable, and energy efficient, and they must have proper flame dimensions.
The components of the burner include the nozzle, mixing tube, downstream connection, and air fuel ratio control. The fuel and air are mixed to produce the best quality flame; forced air is required for the mixing process.
Burners produce six types of flames: A, C, E, F, G, and H.
Type A – Type A is a conventional flame that burns forward and is shaped like a feather. It is used in all-purpose furnaces.
Type C – Type C is ball-shaped with swirl and has a hot reverse flow. It is used in cubicle-shaped furnaces.
Type E – Type E has a very high swirl with some recirculation. Convex types are used to avoid flame impingement, while concave types focus on hot spots. Both types increase direct radiation.
Type F – Type F has no swirl or recirculation and is long and luminous. Due to its luminous radiation, it is used in long furnaces.
Type G – Type G is also long and luminous without swirl. It supplies uniform coverage for long furnaces.
Type H – Type H has high velocity and low swirl with high circulation. It is fast-mixing and used to force flow around the backs of furnaces.
Heat Transfer in Furnaces
Heat transfer in a furnace takes place in three ways: radiation, convection, and conduction.
Radiation in Furnaces
In a furnace, the initial heat source, the burners, are located in a chamber with tubes on four sides. Radiation occurs when the burners are ignited and radiate heat to the fluid inside the tubes.
Convection in Furnaces
Convection requires the flow of a gas or liquid to carry heat. In a furnace, there are tubes located above the furnace that catch heat as it leaves the heating chamber before it exits through the stack. This process helps maintain the efficiency of the furnace by preventing wasted heat.
Conduction in Furnaces
Conduction is the transferring of heat through a solid surface. Heat conduction happens in a furnace when heat is transferred to the tubes, which act as the surface that transfer heat.
The diagram below depicts radiation and convection.
How Furnace Stacks Work
The stack is a chimney or vertical pipe that disperses the hot air or flue gasses from the furnace heating process. The emissions from the stack are strictly controlled and monitored to avoid the release of harmful gasses into the atmosphere. Flue gasses contain a variety of materials, including carbon dioxide (CO2), water vapors, nitrogen, and oxygen. With the rise of environmental concerns, most stacks have contaminating materials filtered through air scrubbing or some other method before release. The high pressure outside the stack is the force that drives the flue gasses out.
Inside some stack configurations are damper blades, which are thin metal plates that regulate the flow of air. In large furnaces, dampers have multiple blades designed to protect the stack and prevent materials from entering the furnace. They come in sizes made to fit the diameter of the stack.
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Chapter Three – Types of Furnaces
Furnaces serve the dual purposes of providing heat and assisting in production. Industrial furnaces tend to center around the annealing, melting, tempering, and carburizing of metals. Though these are critical functions of furnaces, they serve far more purposes and come in designs to fit those differing functions.
Residential furnaces are simple devices designed to provide a sufficient amount of heat. Those designed for industrial use are more complex and provide far greater amounts of heat. The basic designs of the two types are similar, but industrial use furnaces are more complex.
Selecting a residential furnace is rather easy since its purpose is to provide heat. In the case of industrial furnaces, there are a variety of parameters to consider, beginning with the purpose of the furnace and its importance in production. Industrial furnaces can be divided between direct contact and indirect contact furnaces.
Ashing Furnaces
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. Due to the complexity of the ashing process, ashing furnaces are designed specifically for the product(s) to be tested.
The typical ashing furnace has a heating element in an enclosure with a scale attached to weigh the sample before, during, and after it is burnt. An ashing furnace is relatively small. It holds a chamber, heating element, heat plate, and chamber threshold, all of which can be easily replaced.
Ashing furnaces are used in the food industry to estimate the mineral content of food. Samples are heated to 1112 °F (600 °C). The dry weight of the ash provides data on the mineral concentration of the sample. In addition, the petroleum industry uses ashing furnaces to test the combustibility of their products and crude.
Calcination Furnaces
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.
In the process of calcination, the ore is heated to a high temperature in the absence of air or oxygen; this removes moisture from the ore. In some instances, calcination is referred to as purification since volatile and oxidizing portions are removed from the ore.
Tempering Furnaces
A tempering furnace is 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.
Tempering furnaces have ceramic and quartz heating elements that are lined with electrical coils to provide uniform heating of the chamber. There are different heating ranges depending on the material to be processed. Tempering takes place at temperatures between 542 °F and 1382 °F (300 °C and 750 °C).
Annealing Furnaces
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. After a workpiece undergoes annealing, it is rolled, drawn, forged, extruded, headed, or welded—these are processes that cause internal stress.
Sintering Furnaces
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. The purpose is to heighten the material’s properties, such as thermal and electrical conductivity, strength, and translucency.
Tensile Testing Furnaces
Tensile testing is 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. Tensile testing is one of several tests applied to products. The majority of tensile testing furnaces are small and fit in a laboratory.
Rotary Tube Furnaces
A rotary tube furnace is 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.
Bell Furnaces
Bell furnaces 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.
Box Furnaces
Box furnaces are 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, with temperatures ranging between 1800 °F and 3100 °F (1000 °C and 1700 °C). They come in multiple configurations and sizes, from tabletop models to large heavy-duty multi-level models.
Pit Furnaces
Pit furnaces are located at floor level and are top loading. Workpieces to be treated are held in fixtures or baskets or can be placed at the base of the furnace. Pit furnaces are best suited for heating long shafts, tubes, and rods. The main purpose of a pit furnace is to melt small amounts of metals for casting. Pit furnaces are fueled by coke.
Quenching Furnaces
Quenching is 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.
Vacuum Furnaces
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.
Vacuum furnaces are an ideal method for quenching materials. They use an inert gas to quickly cool a treated piece. A vacuum furnace includes a vacuum unit, hydraulic system, and cooling system.
Walking Beam Furnaces
Walking beam 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 at a maximum temperature of 2012 °F (1100 °C). The material to be processed is gradually fed through the furnace by water-cooled beams that lift and move materials in short steps.
The drive system of the furnace is protected from scales by sealing materials and an arrester. The cooling of the beams is to ensure a long life of usefulness. The door to the furnace opens as the beam enters and closes automatically.
Blast Furnaces
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.
The parts of a blast furnace are the hopper, adjustable gates, rotating chute that blends the materials, fire brick, combustion chamber, gas burner, carbon brick, tap hole, and tuyere to supply air. The raw materials are loaded in the stack zone and progress to the barrel zone or reduction zone where the chemical reaction takes place.
Process Furnaces
Process furnaces are 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.
Process Electric Furnaces
Electric process furnaces are used to heat a gas stream. Attached to the inner walls of the furnace’s insulation are electric elements, which surround a process coil and radiantly pass heat though the coil to the fluid. All parts of the surface of the furnace are heated evenly, but heating time zones can be included when specific temperatures are necessary. Process electric furnaces are used in the refining, petrochemical, and chemical industries.
Fired Process Furnaces
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. Fired process furnaces are widely used by refineries, petrochemical plants, the chemical industry, gas processing, ammonia plants, olefin plants, and the fertilizer industry.
Several names are given to fired process furnaces, including feed preheaters, cracking furnaces, fractionator heaters, steam reforming heaters, and crude heaters. Fired process furnaces can reach temperatures of 3500 °F (1926 °C). The created heat is released into an open space, where it is transferred to tubes containing the fluid. The tubes are placed along the walls and roof of the open space. Then, the heat is transferred by direct radiation convection or from refractory wall linings in the open chamber.
Oil Refinery Furnaces
Oil refinery furnaces are an essential part of the 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 to 878 °F (470 °C) before it enters the distillation tower. The furnaces burn off waste gasses from the refining process and use energy-efficient heat exchangers.
Crude oil contains a mixture of hydrocarbons that have to be separated into parts referred to as fractions. Lighter fractions boil off and leave heavier fractions to produce bitumen, fuel oil, diesel and jet fuel, petrol, and petroleum gasses.
Chapter Four – Methods for Heating Furnaces
Heat is generated in a furnace using a variety of methods, including the burning of a fuel or the conversion of electricity to heat. The most common type of furnace is fuel-powered due to the expense of electricity. Though various forms of fuel are the most economical, there are processes where electricity has an advantage over fuels.
How a furnace is powered makes a difference in its design. Though uncommon, there are designs that still use solid fuels. Furnaces can be further classified by where the heat process takes place, with electric furnaces using resistant or induction heating.
Electric-Powered Furnaces
Electric furnaces use a heating element to convert electricity to heat. A variety of materials are used to produce heating elements, with iron chrome aluminum and nickel chrome alloys being the most common. In the glass industry and in research and development, precious metals are used as elements but are not used for industrial purposes due to their cost.
In some electric processes, various gasses are added to the heating process of the furnace to improve efficiency and the distribution of heat.
Liquid-Fueled Furnaces
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.
Electric Arc Furnaces
Electric arc furnaces are used to produce carbon steel and alloy steel by recycling ferrous scrap. Scrap is melted and converted to steel by high-powered electric arcs that are formed by a cathode and one or more anodes. The scrap is loaded into a basket with limestone for slag formation, then charged in the furnace. The energy required to melt the scrap and heat is approximately 350 kWh to 370 kWh. The amount of energy necessary to power the arc depends on the mix of scrap and its composition.
Electric High Frequency Induction Furnaces
Electric induction furnaces work on the same principles used to design transformers. The primary winding of an induction furnace is wound around the furnace and connected to an AC electrical supply. The charge inside the furnace acts as the secondary winding and uses induced current to heat up the charge. The primary coils are made of hollow tubes through which water circulates to keep the coils cooled to the appropriate temperature limits.
Heat is generated by eddy currents flowing concentrically, producing a high frequency supply of 500 Hz to 1000 Hz. A laminated core is used to protect the furnace's structure. Energy is transferred to the heated object through electromagnetic induction.
The benefits of high frequency induction furnaces are:
Decreased melting time
Precision temperature control
Simple design of crucible and container
Automatic stirring with eddy currents
Lower overall cost
Gas-Powered Furnaces
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. For environmental protection, gas-fired furnaces use oscillating combustion technology (OCT) to reduce nitrous oxide (NOx), a waste product from burning natural gas.
Chapter Five – Furnace Regulations
The major concern with industrial furnaces is their emissions, which are regulated by the Environmental Protection Agency (EPA). The federal New Source Performance Standards (NSPS) have stipulations regarding the size, function, and construction of industrial furnaces. The emissions of greatest concern are listed as Hazardous Air Pollutants (HAP).
Furnace operations are differentiated by furnaces designed for processing new products and those for heating. In 2011, the EPA published a list of pollutants and limits for each type. The publication was in compliance with part 60 of the Clean Air Act.
The International Organization for Standardization (ISO) has developed a set of specific regulations regarding industrial furnaces, which are found in ISO 13574, 13577, 13578, 13579, and 23459. They were first introduced in 2008 as ISO/TC 244 and have been progressively adjusted to include arc furnaces with ISO 13578: 2017.
ISO 13574: 2017 – outlines the vocabulary associated with industrial furnaces
ISO 13577 – has stipulations regarding safety standards for combustion and the handling of fuel, use of gasses, and required protective systems
ISO 13579 – outlines energy measurement and efficiency
ISO 23495 – was enacted in 2021 regarding the requirements for converters and similar equipment
The American National Standards Institute (ANSI) has standards and codes regarding the safety, reliability, quality, and performance of industrial heating equipment.
The American Society of Mechanical Engineers (ASME) inspects and approves of industrial furnaces that are in compliance with their standards. The main concerns of the organization are safety and quality.
The United States Department of Energy (DOE) has established energy efficiency standards regarding industrial manufacturing equipment.
National Fire Prevention Association (NFPA)
The NFPA works to prevent injury and property and economic loss from fire and electrical hazards. The organization has specific guidelines in reference to furnaces under NFPA 86.
NFPA 86 – The purpose of the NFPA 86 is to minimize explosions or fire hazard risks by outlining safeguards in the event of an explosion. The idea is to avoid cases where explosive limits of fuels exist, whether they are from the fuel being used or from products being heated. Included in the guidelines are pre-startup sequences since most accidents occur during startup. Although the standards are very broad and all-inclusive, the many concepts can be categorized as:
Location
Construction
Heating systems
Electrical management
Operation
Maintenance
Inspection
Testing
The main focus of NFPA 86 is the avoidance and control of risk and harm caused by poor management of furnace operations. The basic principle is that risk leads to injury, damage, and endangerment.
Conclusion
A furnace is a direct-fired device used to provide heat for industrial processes that require heat in excess of 752 °F (400 °C).
Through the combustion of fuels and gasses, raw materials and products are heated by direct or indirect contact.
The wide selection of furnaces have different methods of performing their functions and use different fuels.
Regardless of the differences in operation, all furnaces serve the primary purpose of providing heat.
Industrial uses of furnaces tend to center around the annealing, melting, tempering, and carburizing of metals.
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