This article takes an in-depth look at industrial furnaces.
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A furnace is an industrial device that delivers direct electric or fired heat for processes requiring temperatures above 752°F (400°C). Various industrial processes necessitate heating to prepare materials for production or to finalize applications. The reliability and robustness of electric and fired industrial furnaces ensure precise temperature control and dependable performance essential for successful manufacturing operations.
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 design of furnaces varies based on the fuel source and furnace type, with significant differences between electric and fuel-fired models. All furnaces are constructed from materials designed to endure extreme temperatures without failure or damage. Key design considerations include process temperature, furnace height, outer diameter (OD), length, and the required pressure range.
Furnaces operate differently and utilize various types of fuel. Historically, furnaces were powered by wood or coal, necessitating frequent refueling to maintain consistent heat. Today, modern furnaces use fuels that are delivered automatically, enhancing efficiency and reducing the need for manual refueling.
The key to an efficient and cost-effective furnace is its fuel source. Historically, coal and wood were used, but they caused environmental pollution, required constant replenishment, and made it challenging to maintain a consistent temperature. Modern furnaces have automated fuel feed systems and use electricity to ensure stable temperatures.
Fuel-fired furnaces are the most commonly used type. The choice of fuel influences the furnace’s design, but modern furnaces can accommodate various fuels. As with any heat-generating device, adequate oxygen supply is crucial for the furnace’s efficient operation.
Electric furnaces are both efficient and environmentally friendly as they do not emit flue gases. However, they are costly to operate. Electric furnaces can be categorized into two types: induction and resistance heating.
Resistance heating is the most costly form of electric furnace heating. These furnaces use a circulating fan to ensure even temperature distribution. Resistors in these furnaces can be constructed from various materials, and sometimes the load itself acts as a resistor.
Induction heating, on the other hand, targets specific areas of a workpiece. It involves passing electricity through a coil surrounding the load, with the current frequency determined by the load type. To prevent overheating, the coils are cooled with water.
Fuel is delivered to the burners, where it is combusted to generate heat. Furnaces typically feature multiple burners that can be positioned in various sections depending on the furnace's design. Each burner includes an oxidizer to convert chemical energy into thermal energy. The choice of fuel for a furnace is influenced by the burners, which mix the fuel with air and ignite it. Burners must be stable, cost-effective, reliable, and energy-efficient, with precise flame dimensions.
Burner components include the nozzle, mixing tube, downstream connection, and air-fuel ratio control. The fuel and air are mixed to achieve optimal flame quality, with forced air facilitating the mixing process.
Burners are capable of producing six different types of flames: A, C, E, F, G, and H.
Heat transfer in a furnace occurs through three primary mechanisms: radiation, convection, and conduction.
In a furnace, the burners serve as the primary heat source and are positioned within a chamber surrounded by tubes on all sides. When the burners are ignited, they emit heat that radiates to the fluid contained within the tubes.
Convection involves the transfer of heat through the movement of gas or liquid. In a furnace, tubes positioned above the heating chamber capture heat as it exits before it is released through the stack. This process helps to enhance furnace efficiency by minimizing heat loss.
Conduction involves the transfer of heat through a solid material. In a furnace, heat conduction occurs when heat is transferred to the tubes, which serve as the surfaces through which the heat is conducted.
The diagram below illustrates the processes of radiation and convection.
The stack, which functions as a chimney or vertical pipe, vents hot air or flue gases from the furnace. Emissions from the stack are carefully regulated and monitored to prevent the release of harmful gases into the environment. Flue gases typically include carbon dioxide (CO2), water vapor, nitrogen, and oxygen. In response to environmental concerns, many stacks now include filtration systems, such as air scrubbers, to remove contaminants before the gases are released. The high pressure outside the stack helps to expel the flue gases.
Some stack configurations are equipped with damper blades, which are thin metal plates used to control air flow. In larger furnaces, these dampers may have multiple blades to safeguard the stack and prevent unwanted materials from entering the furnace. Dampers are sized to match the diameter of the stack.
Furnaces have the dual functions of generating heat and aiding in production processes. In industrial settings, they are commonly used for tasks such as annealing, melting, tempering, and carburizing metals. While these are essential roles, furnaces are versatile and come in various designs to accommodate a wide range of functions.
Residential furnaces are designed to provide adequate heating and are generally straightforward in their design. In contrast, industrial furnaces are more complex, engineered to deliver significantly higher amounts of heat. Although the fundamental design principles are similar, industrial furnaces incorporate additional features to meet their more demanding applications.
Choosing a residential furnace is relatively straightforward, given its primary function of heating. Industrial furnaces, however, require careful consideration of several factors, including their specific role in production. Industrial furnaces can be categorized into those that provide direct contact and those that offer indirect contact.
Ashing is a technique used to measure the weight change of a sample as its components are burned off. This process eliminates organic material from the sample before further analysis. Ashing involves the complete combustion of the tested material. Given the complexity of ashing, furnaces used for this purpose are specifically designed to handle the samples being analyzed.
An ashing furnace typically includes a heating element enclosed within a chamber, and it is equipped with a scale to measure the sample's weight before, during, and after combustion. These furnaces are relatively compact and contain components such as a chamber, heating element, heat plate, and chamber threshold, all of which are replaceable.
Ashing furnaces are employed in the food industry to determine the mineral content of food by heating samples to 1112 °F (600 °C). The remaining dry weight of the ash reveals the mineral concentration. Additionally, the petroleum industry utilizes ashing furnaces to assess the combustibility of products and crude materials.
Calcination is a thermal treatment process in which samples are heated to temperatures just below their melting point to induce thermal decomposition or remove volatile components. This process is essential for extracting metals from ores, particularly those mined as carbonates or sulfates, through a reduction method carried out in a calcination furnace.
During calcination, the ore is subjected to high temperatures in the absence of air or oxygen, which helps to eliminate moisture from the ore. Sometimes, calcination is also known as purification, as it involves removing volatile and oxidizing elements from the ore.
A tempering furnace is used to heat treat metal products to enhance their toughness and hardness. This process improves the metal's ability to absorb energy and resist deformation before cracking. By optimizing the beneficial properties of the metal, a tempering furnace significantly enhances its mechanical characteristics.
Tempering furnaces utilize ceramic and quartz heating elements equipped with electrical coils to ensure even heating throughout the chamber. The heating temperature range varies based on the material being processed, with tempering typically occurring at temperatures between 542 °F and 1382 °F (300 °C and 750 °C).
Annealing is a heat treatment process used to soften metals, making them more amenable to cold working and enhancing their mechanical, electrical, and other physical properties. Annealing furnaces are designed to relieve internal stresses in materials by heating them to their recrystallization temperature, thereby increasing their ductility for subsequent machining. Post-annealing, the workpiece can undergo various processes such as rolling, drawing, forging, extrusion, heading, or welding, which may otherwise induce internal stress.
Sintering is a heat treatment technique used to convert loose or fragmented materials into a solid mass. The process involves heating the material to a temperature just below its melting point, with the exact temperature varying depending on the material. During sintering, the porous spaces within the material are reduced as it is compressed and shaped under high temperatures and pressures. The aim is to enhance the material's properties, including thermal and electrical conductivity, strength, and translucency.
Tensile testing is a method used to evaluate materials by applying tension until they fracture or fail. This process assesses properties such as strength, elongation, and reduction in cross-sectional area. As a destructive testing technique, it helps determine the material's failure point and overall durability. Tensile testing is one of several evaluations used on products, and most tensile testing furnaces are compact, designed to fit within a laboratory setting.
A rotary tube furnace is a type of heat treatment furnace that operates with a rotating circular chamber. As the materials move along a circular path through the furnace, they are exposed to heat in various thermal zones. This continuous processing method involves applying heat to a rotating tube, ensuring uniform treatment of the materials.
Bell furnaces are batch-type heat treatment units designed for processes such as sintering or drying. These furnaces operate with the load placed inside a sealed dome, providing high-capacity processing for large workloads. They are not suited for small batches and can be powered by electricity, gas, or thermal circulation systems.
Box furnaces are versatile units used for various heat treatment processes, including calcining, curing, annealing, stress relieving, preheating, and tempering. Their straightforward design makes them adaptable and reliable. They can feature single or multiple heating zones and operate at temperatures between 1800 °F and 3100 °F (1000 °C and 1700 °C). Box furnaces are available in a range of configurations and sizes, from compact tabletop models to large, heavy-duty multi-level units.
Pit furnaces are situated at floor level and feature top-loading designs. The workpieces are placed in fixtures, baskets, or directly at the base of the furnace. These furnaces are ideal for heating long shafts, tubes, and rods, and are primarily used for melting small quantities of metal for casting purposes. Pit furnaces are traditionally fueled by coke.
Quenching is a process that rapidly cools a workpiece from a high temperature, commonly used to form martensite in steel. The cooling medium can be water or oil. Quenching furnaces are often used in conjunction with batch furnaces, roller hearths, or pusher furnaces. These furnaces are specifically designed to meet the needs of various applications. Precision control of temperature is essential in quenching furnaces to prevent uneven heating and overheating.
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 well-suited for quenching materials as they use an inert gas to rapidly cool the treated item. These furnaces are equipped with a vacuum chamber, hydraulic system, and a dedicated cooling system.
Walking beam furnaces are effective for processing large, heavy parts. They are commonly used for annealing, forging, heating, stress relieving, quenching, and tempering, with a maximum temperature of 2012 °F (1100 °C). In this type of furnace, the material is gradually moved through the furnace by water-cooled beams that lift and transport the materials in incremental steps.
The furnace's drive system is safeguarded from scale buildup by sealing materials and an arrester. The beams are cooled to ensure their longevity. The furnace door opens automatically as the beam enters and closes as it exits.
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 components of a blast furnace include the hopper, adjustable gates, rotating chute for blending materials, fire brick, combustion chamber, gas burner, carbon brick, tap hole, and tuyere for supplying air. The raw materials are introduced into the stack zone and then move to the barrel or reduction zone, where the chemical reactions occur.
Process furnaces are crucial in various industrial operations for preparing fluids. The two primary types of process furnaces are electric and fired. While electric process furnaces are more costly to operate, they offer the benefit of not generating pollutants.
Electric process furnaces are employed to heat a gas stream. Electric elements are attached to the inner walls of the furnace’s insulation, surrounding a process coil and radiating heat through the coil to the fluid. While all surfaces of the furnace are heated uniformly, heating time zones can be incorporated when specific temperatures are required. Process electric furnaces find applications in the refining, petrochemical, and chemical industries.
Fired process furnaces serve the same purpose as electric process heaters, which is to heat a fluid to the desired working temperature. In these furnaces, the fluid flows through tubes that are heated by combusting fuel. Fired process furnaces are commonly used in refineries, petrochemical plants, the chemical industry, gas processing, ammonia plants, olefin plants, and the fertilizer industry.
Fired process furnaces are known by various names, such as feed preheaters, cracking furnaces, fractionator heaters, steam reforming heaters, and crude heaters. They can achieve temperatures up to 3500 °F (1926 °C). The generated heat is released into an open space, where it is transferred to tubes containing the fluid. These tubes are situated along the walls and roof of the open space. Heat transfer occurs via direct radiation, convection, or from refractory wall linings within the chamber.
Oil refinery furnaces are crucial to the refining process, heating crude oil to the required inlet temperature for the distillation column. They are employed before the preflash and at the atmospheric and vacuum columns. Crude oil must be heated to 878 °F (470 °C) before entering the distillation tower. These furnaces also burn off waste gases from the refining process and use energy-efficient heat exchangers.
Crude oil is a mixture of hydrocarbons that must be separated into different fractions. Lighter fractions boil off, leaving heavier fractions to produce bitumen, fuel oil, diesel and jet fuel, petrol, and petroleum gases.
Heat in a furnace can be generated through various methods, including burning fuel or converting electricity into heat. Fuel-powered furnaces are more common due to the high cost of electricity. However, there are processes where electricity provides advantages over traditional fuels.
The power source of a furnace influences its design. Although less common, some designs still use solid fuels. Furnaces can also be classified by the location of the heat process, with electric furnaces utilizing either resistance or induction heating.
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 certain electric processes, various gases are introduced into the furnace heating process to enhance efficiency and improve heat distribution.
Liquid fuels generate combustible fumes. Most liquid fuels are derived from fossil fuels, though alternatives include hydrogen, ethanol, and biodiesel. Oil is the most commonly used liquid fuel for heating and reheating materials during treatments. For a liquid-fueled furnace to operate efficiently, the fuel must combust completely without leaving any residue.
Electric arc furnaces are utilized to produce carbon steel and alloy steel through the recycling of ferrous scrap. In these furnaces, scrap is melted and converted into steel using high-powered electric arcs created by a cathode and one or more anodes. The scrap, along with limestone for slag formation, is loaded into a basket and then charged into the furnace. The energy required for melting the scrap and heating ranges from approximately 350 kWh to 370 kWh. The exact amount of energy needed depends on the composition and mix of the scrap.
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 ranging from 500 Hz to 1000 Hz. A laminated core is utilized to protect the furnace's structure, while energy is transferred to the heated object through electromagnetic induction.
The benefits of high-frequency induction furnaces include:
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.
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) include stipulations regarding the size, function, and construction of industrial furnaces. The emissions of greatest concern are categorized as Hazardous Air Pollutants (HAP).
Furnace operations are categorized into those designed for processing new products and those used for heating. In 2011, the EPA published a list of pollutants and their limits for each type, in compliance with part 60 of the Clean Air Act.
The International Organization for Standardization (ISO) has established specific regulations for industrial furnaces, detailed in ISO 13574, 13577, 13578, 13579, and 23459. Introduced in 2008 as ISO/TC 244, these standards have been progressively updated to include arc furnaces, with the latest revision being ISO 13578: 2017.
The American National Standards Institute (ANSI) has established standards and codes related to the safety, reliability, quality, and performance of industrial heating equipment.
The American Society of Mechanical Engineers (ASME) inspects and approves industrial furnaces to ensure compliance with their standards, with a primary focus on safety and quality.
The United States Department of Energy (DOE) has set energy efficiency standards for industrial manufacturing equipment.
The NFPA works to prevent injury, property damage, and economic loss due to fire and electrical hazards. The organization has specific guidelines for furnaces under NFPA 86.
NFPA 86 aims to minimize explosion and fire hazard risks by outlining safeguards for explosive conditions. It addresses the potential explosive limits of fuels, whether from the fuel itself or from the products being heated. The guidelines include pre-startup sequences, as most accidents occur during startup. While the standards are comprehensive, they cover several key concepts:
The primary focus of NFPA 86 is on preventing and managing risks associated with furnace operations. The core principle is that inadequate risk management can lead to injury, damage, and safety hazards.
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