Furnace used for smelting, refining, or melting in which the fuel is not in direct contact with the contents but heats it by a flame blown over it from another chamber. Such furnaces are used in copper, tin, and nickel production, in the production of certain concretes and cements, and in aluminum recycling. In steelmaking, this process (now largely obsolete) is called the open-hearth process. The heat passes over the hearth and then radiates back (reverberates) onto the contents. The roof is arched, with the highest point over the firebox. It slopes downward toward a bridge of flues that deflects the flame so that it reverberates.
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Chamber heated with electricity to very high temperatures, for melting and alloying metals and refractories. Modern electric furnaces generally are either arc furnaces or induction furnaces. Arc furnaces produce roughly two-fifths of the steel made in the U.S. In the induction furnace, a coil carrying alternating electric current surrounds the container or chamber of metal; circulating eddy currents induced in the metal produce extremely high temperatures.
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Vertical shaft furnace that produces liquid metals by the reaction of air introduced under pressure into the bottom of the furnace with a mixture of metallic ore, fuel, and flux fed into the top. Blast furnaces are used to produce pig iron from iron ore for subsequent processing into steel; they are also employed in processing lead, copper, and other metals. The current of pressurized air maintains rapid combustion. Blast furnaces were used in China as early as 200 BC, and appeared in Europe in the 13th century, replacing the bloomery process. Modern blast furnaces are 70–120 ft (20–35 m) high, have 20–45-ft (6–14-m) hearth diameters, use coke fuel, and can produce 1,000–10,000 tons (900–9,000 metric tons) of pig iron daily. Seealso metallurgy, smelting.
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Type of electric furnace in which heat is generated by an arc between carbon electrodes above the surface of the material (commonly a metal) being heated. William Siemens first demonstrated the arc furnace in 1879 at the Paris Exposition by melting iron in crucibles; horizontally placed carbon electrodes produced an electric arc above the container of metal. The first commercial arc furnace in the U.S. (1906) had a capacity of four tons (3.6 metric tons) and was equipped with two electrodes. Modern furnaces range in heat size from a few tons up to 400 tons (360 metric tons), and the arcs strike directly into the metal bath from vertically positioned, graphite electrodes to remelt scrap steel or refine briquettes of direct-reduced iron ore.
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In American English, the term furnace on its own is generally used to describe household heating systems based on a central furnace (known either as a boiler or a heater in British English), and sometimes as a synonym for kiln, a device used in the production of ceramics.
In British English the term furnace is used exclusively to mean industrial furnaces which are used for many things, such as the extraction of metal from ore (smelting) or in oil refineries and other chemical plants, for example as the heat source for fractional distillation columns.
The term furnace can also refer to a direct fired heater, used in boiler applications in chemical industries or for providing heat to chemical reactions for processes like cracking, and is part of the standard English names for many metallurgical furnaces worldwide.
Combustion furnaces always need to be vented to the outside. Traditionally, this was through a chimney, which tends to expel heat along with the exhaust. Modern high-efficiency furnaces can be 98% efficient and operate without a chimney. The small amount of waste gas and heat are mechanically ventillated through a small tube through the side or roof of the house.
Modern household furnaces are classified as condensing or non-condensing based on their efficiency in extracting heat from the exhaust gases. Furnaces with efficiencies greater than approximately 89% extract so much heat from the exhaust that water vapor in the exhaust condenses; they are referred to as condensing furnaces. Such furnaces must be designed to avoid the corrosion that this highly acidic condensate might cause and may need to include a condensate pump to remove the accumulated water. Condensing furnaces can typically deliver heating savings of 20%-35% assuming the old furnace was in the 60% Annual Fuel Utilization Efficiency (AFUE) range.
Air convection heating systems have been in use for over a century, but the older systems relied on a passive air circulation system where the greater density of cooler air caused it to sink into the furnace, and the lesser density of the warmed air caused it to rise in the ductwork, the two forces acting together to drive air circulation in a system termed "gravity-feed; the layout of the ducts and furnace was optimized for short, large ducts and caused the furnace to be referred to as an "octopus" furnace.
By comparison, most modern "warm air" furnaces typically use a fan to circulate air to the rooms of house and pull cooler air back to the furnace for reheating; this is called forced-air heat. Because the fan easily overcomes the resistance of the ductwork, the arrangement of ducts can be far more flexible than the octopus of old. In American practice, separate ducts collect cool air to be returned to the furnace. At the furnace, cool air passes into the furnace, usually through an air filter, through the blower, then through the heat exchanger of the furnace, whence it is blown throughout the building. One major advantage of this type of system is that it also enables easy installation of central air conditioning by simply adding a cooling coil at the exhaust of the furnace.
Air is circulated through ductwork, which may be made of sheet metal or plastic "flex" duct and insulated or uninsulated. Unless the ducts and plenums have been sealed using mastic or foil duct tape, the ductwork is likely to have a high leakage of conditioned air, possibly into unconditioned spaces. Another cause of wasted energy is the installation of ductwork in unheated areas, such as attics and crawl spaces; or ductwork of air conditioning systems in attics in warm climates.
The following rare but difficult-to-diagnose failure can occur. If the temperature inside the furnace exceeds a maximum threshold, a safety mechanism with a thermostat will shut the furnace down. A symptom of this failure is that the furnace repeatedly shuts down before the house reaches the desired temperature; this is commonly referred to as the furnace "riding the high limit switch". This condition commonly occurs if the temperature setting of the high limit thermostat is set too close to the normal operating temperature of the furnace. Another situation may occur if a humidifier is incorrectly installed on the furnace and the duct which directs a portion of the humidified air back into the furnace is too large. The solution is to reduce the diameter of the cross-feed tube, or install a baffle that reduces the volume of re-fed air.
In metallurgy, several specialised furnaces are used. These include:
Fuel flows into the burner and is burnt with air provided from an air blower. There can be more than one burner in a particular furnace which can be arranged in cells which heat a particular set of tubes. Burners can also be floor mounted, wall mounted or roof mounted depending on design. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace known as the radiant section or firebox. In this chamber where combustion takes place, the heat is transferred mainly by radiation to tubes around the fire in the chamber. The heating fluid passes through the tubes and is thus heated to the desired temperature. The gases from the combustion are known as flue gas. After the flue gas leaves the firebox, most furnace designs include a convection section where more heat is recovered before venting to the atmosphere through the flue gas stack. (HTF=Heat Transfer Fluid. Industries commonly use their furnaces to heat a secondary fluid with special additives like anti-rust and high heat transfer efficiency. This heated fluid is then circulated round the whole plant to heat exchangers to be used wherever heat is needed instead of directly heating the product line as the product or material may be volatile or prone to cracking at the furnace temperature.)
A furnace can be lit by a small pilot flame or in some older models, matches... Most pilot flames nowadays are lit by an ignition transformer (much like a car's spark plugs). The pilot flame in turn lights up the main flame. The pilot flame uses natural gas while the main flame can use both diesel and natural gas. When using liquid fuels, an atomizer is used, otherwise, the liquid fuel will simply pour onto the furnace floor and become a hazard. Using a pilot flame for lighting the furnace increases safety and ease. (compared to using a match)
The stack damper contained within works like a butterfly valve and regulates draft (pressure difference between air intake and air exit)in the furnace, which is what pulls the flue gas through the convection section. The stack damper also regulates the heat lost through the stack. As the damper closes, the amount of heat escaping the furnace through the stack decreases, but the pressure or draft in the furnace increases which poses risks to those working around it if there are air leakages in the furnace, the flames can then escape out of the firebox or even explode if the pressure is too great.
A damper and fan on the boiler interacts with a thermostat inside the building. If the building's temperature falls, the thermostat will trigger the damper to open, letting oxygen enter the combustion chamber, which causes the fire to burn more intensely. The fire will then raise the temperature of the water which increases the heat supplied to the home.
As of July 2006, the HPBA, along with many of the major OWB manufactures, have requested users of their products follow the "Outdoor Wood Furnace Best Burn Practices" These guidelines have been set up by the HPBA to help cut down on problems associated with OWBs.
Early in January 2007, the United States Environmental Protection Agency (EPA) initiated a voluntary program for manufacturers of outdoor wood furnaces. The EPA's primary intent is to encourage manufacturers to produce cleaner Outdoor Wood-fired Hydronic Heaters (OWHH) models. The EPA also wants those who buy an OWHH / OWB to buy the cleanest models available, which are those that meet EPA performance verified levels. To participate in this program, manufacturers commit their best efforts to develop cleaner models with goals of distributing their units starting in April 2007.
The EPA now publishes a list of all OWHH / OWB units that pass the new voluntary program. These furances come with the 'orange EPA OWHH tag' to notify the customer of the units particular emission level output.