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A '''furnace''' is a semi-open container system used to generate and distribute heat.
{{Other uses}}
{{Citations missing|date=July 2007}}
[[File:Piec krepa.JPG|thumb|Industrial Furnace from 1907]]
A '''furnace''' is a device used for [[heat]]ing. The name derives from [[Latin]] ''fornax'', [[oven]].
In [[American English]] and [[Canadian English]], the term ''furnace'' on its own is generally used to describe [[house]]hold 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 [[industry|industrial]] furnaces which are used for many things, such as the [[Resource extraction|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 language|English]] names for many metallurgical furnaces worldwide.
The heat energy to fuel a furnace may be supplied directly by fuel [[combustion]], by electricity such as the [[electric arc furnace]], or through [[Induction heating]] in [[induction furnace]]s.
==Household furnaces==
[[image:Condensing furnace diagram.png|thumb|250px|A condensing furnace]]
A [[household]] furnace is a [[major appliance]] that is permanently installed to provide heat to an interior space through intermediary fluid movement, which may be [[air]], [[steam]], or hot [[water]]. The most common [[fuel]] source for modern furnaces in the United States is [[natural gas]]; other common fuel sources include [[liquefied petroleum gas|LPG]] (liquefied petroleum gas), [[fuel oil]], [[coal]] or [[wood]]. In some cases [[electrical resistance]] heating is used as the source of heat, especially where the cost of electricity is low.
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 gas|exhaust]]. Modern high-efficiency furnaces can be 98% [[Thermal efficiency|efficient]] and operate without a chimney. The small amount of waste gas and heat are mechanically ventilated through a small tube through the side or roof of the house.
Modern household furnaces are classified as ''[[Condensation|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 boiler|condensing furnace]]s. 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|Annual Fuel Utilization Efficiency]] (AFUE) range.
===Modern furnace components===
The furnace components can be divided into three categories.
#The burners, [[heat exchanger]], draft inducer, and venting.
#The controls and safety devices.
#The blower and air movement.
The flame originates at the burners and is drawn into the heat exchanger by the negative pressure produced by the draft inducer. The hot gases produced by the combustion of the flame pass through the chambers of the heat exchanger and heat the metal walls of the heat exchanger. The gases cool as they transfer the heat to the heat exchanger and are at about {{convert|120|F|C|-1|abbr=on}} as they exit on a high efficiency furnace. The cooled gases then enter the draft inducer blower and are pushed into the venting pipes. The exhaust gases then are directed out of the house through the vent pipes.
The controls include the gas valve, ignition control, ignitor, flame sensor, transformer, limit control, blower control board, and flame roll out switch. The transformer provides 24 volts of electricity to power the controls. 24 volts is applied to the thermostat that is installed in the living space.
The thermostat is basically an automatic switch that closes and completes the electrical circuit when the room temperature drops below the heat setting. This then allows 24 volts to the circuit board which initiates the heat sequence. The circuit board has a relay that closes to power up the motor on the draft inducer blower. Then the circuit board ignitor relay is energized which sends 120 volts to the hot surface ignitor and makes it glow bright and get extremely hot.
Next the gas valve relay in the circuit board is energized. This allows voltage to the gas valve and energizes a solenoid coil in the gas valve which opens the valve to allow gas to flow to the burners. The gas flows into the burners and is ignited by the hot surface ignitor. The ignition control circuit board applies an AC voltage to the flame sensor which is just a stainless steel rod. An interesting thing occurs inside a burning flame, which is called ionization. That is, free electrons are produced which can conduct electricity through the flame itself. The electrons will normally flow from the flame sensor, through the flame when present, and back to ground through the grounded burners.
The ignition system must prove that a flame is present to continue the gas flow, or if there's no flame, then shut off the gas flow through the gas valve to prevent a possible explosion. It also must not be fooled into thinking there is a flame present by a flame sensor that is touching the ground from being broken or bent. The way it does this is by a diode effect where the sensor surface area is less than 10% of the ground surface area. This produces a half-wave of electrical current from each full wave. The ignition control circuit detects the half-wave to determine if the sensor is merely touching ground. If the ignition control receives this half wave signal from the flame sensor then combustion will continue.
Now the circuit board timer counts a determined amount of time and energizes the blower relay. This relay powers up the blower motor and air is then pushed over the heat exchanger where it removes the heat from the hot metal and enters the ductwork to go to the various rooms in the house. The limit control is a safety device that will open the electrical circuit to the ignition control and stop the gas flow if the furnace overheats. The flame roll-out switch does the same thing if the flame was rolling out of the heat exchanger instead of being completely induced into it by the draft inducer.
The blower creates a negative pressure on the intake side which draws air into the ductwork return air system and blows the air out through the heat exchanger and then into supply air ductwork to distribute throughout the home.
===Heat distribution===
The furnace transfers heat to the living space of the building through an intermediary distribution system. If the distribution is through hot water (or other fluid) or through steam, then the furnace is more commonly termed a [[Boiler#Hydronic boilers|boiler]]. One advantage of a boiler is that the furnace can provide hot water for bathing and washing dishes, rather than requiring a separate [[water heater]]. One disadvantage to this type of application is when the boiler breaks down, both heating and domestic hot water is not available.
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 (mechanical)|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 [[Friction|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 [[Duct (HVAC)|ductwork]], which may be made of sheet metal or plastic "flex" duct and insulated or uninsulated. Unless the ducts and [[Plenum space|plenum]] 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.
==Metallurgical furnaces==
[[Image:The Manufacture of Iron -- Filling the Furnace.jpg|thumb|right|250px|''The Manufacture of Iron -- Filling the Furnace'', an 1873 [[wood engraving]]]]
In [[metallurgy]], several specialised furnaces are used. These include:
*Furnaces used in [[smelter]]s, including:
** The [[blast furnace]], used to [[redox|reduce]] [[iron ore]] to [[pig iron]]
** [[Steelmaking]] furnaces, including:
*** [[Puddling furnace]]
*** [[Reverberatory furnace]]
*** [[Bessemer converter]]
*** [[Open hearth furnace]]
*** [[Basic oxygen furnace]]
*** [[Electric arc furnace]]
*** [[Electric induction furnace]]
*Furnaces used to remelt metal in [[foundry|foundries]].
*Furnaces used to reheat and [[heat treatment|heat treat]] metal for use in:
** [[Rolling mill]]s, including [[tinplate]] works and [[slitting mill]]s.
** [[Forge]]s.
*[[Vacuum furnace]]s <!-- Found at [[WP:DEP]], probably needs to be sorted into some category of furnaces? -->
==Industrial process furnaces==
[[Image:furnace2-en.svg|thumb|350px|Schematic diagram of an industrial process furnace]]
An industrial furnace or direct fired heater, is an equipment used to provide heat for a process or can serve as [[chemical reactor|reactor]] which provides heats of reaction. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. However, most process furnaces have some common features.
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 (steam engine)|firebox]]. In this chamber where combustion takes place, the heat is transferred mainly by [[radiant energy|radiation]] to tubes around the [[fire]] in the chamber. The [[heat transfer fluid|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 [[Earth's atmosphere|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 (chemistry)|cracking]] at the furnace temperature.)
===Radiant section===
[[Image:Middle rad sect.jpg|thumb|450px|Middle of radiant section]]
The radiant section is where the tubes receive almost all its heat by [[Thermal radiation|radiation]] from the flame. In a vertical, cylindrical furnace, the tubes are vertical. Tubes can be vertical or horizontal, placed along the [[refractory]] wall, in the middle, etc., or arranged in cells. Studs are used to hold the [[Thermal insulation|insulation]] together and on the wall of the furnace. They are placed about 1 ft (300 mm) apart in this picture of the inside of a furnace. The tubes, shown below, which are reddish brown from [[corrosion]], are [[carbon steel]] tubes and run the height of the radiant section. The tubes are a distance away from the insulation so radiation can be reflected to the back of the tubes to maintain a uniform tube wall temperature. Tube guides at the top, middle and bottom hold the tubes in place.
===Convection section===
[[Image:Conv sect.gif|thumb|450px|Convection section]]
The convection section is located above the radiant section where it is cooler to recover additional heat. [[Heat transfer]] takes place by [[convection]] here, and the tubes are finned to increase heat transfer. The first two tube rows in the bottom of the convection section and at the top of the radiant section is an area of bare tubes (without fins) and are known as the shield section, so named because they are still exposed to plenty of radiation from the firebox and they also act to shield the convection section tubes, which are normally of less resistant material from the high temperatures in the firebox. The area of the radiant section just before flue gas enters the shield section and into the convection section called the bridgezone. Crossover is the term used to describe the tube that connects from the convection section outlet to the radiant section inlet. The crossover piping is normally located outside so that the temperature can be monitored and the efficiency of the convection section can be calculated. The sightglass at the top allows personnel to see the flame shape and pattern from above and visually inspect if flame impingement is occurring. Flame impingement happens when the flame touches the tubes and causes small isolated spots of very high temperature.
===Burner===
[[Image:Burner1.jpg|thumb|450px|Furnace burner]]
The burner in the vertical, cylindrical furnace as above, is located in the floor and fires upward. Some furnaces have side fired burners, such as in train [[locomotives]]. The burner [[tile]] is made of high temperature refractory and is where the flame is contained. Air registers located below the burner and at the outlet of the air blower are devices with movable flaps or vanes that control the shape and pattern of the flame, whether it spreads out or even swirls around. Flames should not spread out too much, as this will cause flame impingement. Air registers can be classified as primary, secondary and if applicable, tertiary, depending on when their air is introduced. The primary air register supplies primary air, which is the first to be introduced in the burner. Secondary air is added to supplement primary air. Burners may include a premixer to mix the air and fuel for better combustion before introducing into the burner. Some burners even use steam as premix to preheat the air and create better mixing of the fuel and heated air. The floor of the furnace is mostly made of a different material from that of the wall, typically hard castable refractory to allow technicians to walk on its floor during maintenance.
A furnace can be lit by a small [[Pilot light|pilot flame]] or in some older models, by hand. 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 fuel|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 manual ignition method (like a match).
===Sootblower===
Sootblowers are found in the convection section. As this section is above the radiant section and air movement is slower because of the fins, [[soot]] tends to accumulate here. Sootblowing is normally done when the efficiency of the convection section is decreased. This can be calculated by looking at the temperature change from the crossover piping and at the convection section exit.
Sootblowers utilize flowing media such as water, air or steam to remove deposits from the tubes. This is typically done during maintenance with the air blower turned on. There are several different types of sootblowers used. Wall blowers of the rotary type are mounted on furnace walls protruding between the convection tubes. The [[lance]]s are connected to a steam source with holes drilled into it at intervals along its length. When it is turned on, it rotates and blows the soot off the tubes and out through the stack.
===Stack===
[[File:Stack damper mechanism.svg|thumb|350px|right|Stack damper]]
The flue gas stack is a cylindrical structure at the top of all the heat transfer chambers. The [[Flue|breeching]] directly below it collects the flue gas and brings it up high into the atmosphere where it will not endanger personnel.
The stack [[Damper (architecture)|damper]] contained within works like a [[butterfly valve]] and regulates [[Stack effect|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.
===Insulation===
Insulation is an important part of the furnace because it prevents excessive heat loss. Refractory materials such as [[firebrick]], castable refractories and [[High temperature insulation wool|ceramic fibre]], are used for insulation. The floor of the furnace are normally castable type refractories while those on the walls are nailed or glued in place. Ceramic fibre is commonly used for the roof and wall of the furnace and is graded by its [[density]] and then its maximum temperature rating. For example, 8# 2,300 °F means 8 lb/ft<sup>3</sup> density with a maximum temperature rating of 2,300 °F. An example of a castable composition is kastolite.
===First fire===
{{unrefsection|date=October 2006}}
The '''first fire''' is the moment when a furnace or another heating device (usually for industrial use such as metallurgy or ceramics) is first lit after its construction. The [[refractory]] of the furnace walls should be as dry as possible and the first fire should be done slowly with a small flame as the refractory of the still unfired furnace has a minimal amount of moisture. Gradually or during subsequent firings, the flame or heat source (e.g. [[Kanthal]] heating elements) can be turned up higher.
After first fire some adjustments should be done usually to fine-tune the furnace. Despite this, a first fire is always a moment of great excitement for the people who designed and built the furnace.
==Outdoor wood-fired boilers==
{{Cleanup|section|date=January 2008}}
===Description===
An [[outdoor wood-fired boiler]] (OWB) also known as a waterstove or outdoor wood furnace or simply a wood boiler, is a heating technology that has grown in popularity in the northern United States and Canada. OWBs in most cases look like a small shack with metal siding. They are self-contained, and are only connected to the building they heat through underground insulated water pipes. OWBs contain a metal combustion chamber for a wood fire, which is surrounded by a water tank or water jacket. The fire heats the water, which is then circulated through the insulated water pipes into the heated building. Once the hot water from the boiler reaches the building, the heat from the hot water can be transferred to most existing heating systems and the building's hot water supply.
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.
===Benefits===
OWBs have several benefits that increase their popularity. Their large combustion chamber accommodates more fuel than many other forms of wood heat, decreasing the number of times an owner has to add fuel to the fire. Home insurance may cost more for people who heat with an indoor form of wood heat than with an OWB. Finally, for people with a large supply of free wood and willing to invest the time to prepare the wood and stock the OWB, an OWB can be less expensive than heating with gas, oil, or electricity.<ref>[http://today.msnbc.msn.com/id/26738221/ MSNBC]</ref>
===Controversy in the United States===
OWBs are not without controversy, as their emissions sometimes bother neighbors. Some states and municipalities have regulated the devices.<ref>[http://www.anr.state.vt.us/dec/air/owb/regl-other.html ANR]</ref> They are not currently regulated by the United States [[Environmental Protection Agency]] (EPA), unlike other forms of wood heat. However, recently the EPA has worked with manufacturers to develop a method for manufacturers to identify OWBs that meet a voluntary emissions standard.<ref name="epa.gov">[http://www.epa.gov/woodheaters/what_epa_doing.htm EPA]</ref> Studies conducted on OWBs suggest that these devices may produce more emissions, most notably [[particulate]] matter under 2.5 micrometers (PM2.5) than other heating technologies, though manufacturers dispute these assessments [http://www.nescaum.org/documents/assessment-of-outdoor-wood-fired-boilers]. Exposure to elevated levels of PM2.5 has been associated with cardiopulmonary health effects and premature death.<ref>[http://www.epa.gov/oar/particlepollution/health.html EPA]</ref>
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".<ref>[http://www.hpba.org/index.php?id=154 HPBA]</ref> 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<ref name="epa.gov"/> 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.<ref>[http://www.epa.gov/woodheaters/partners.htm EPA]</ref>
The EPA now publishes a list of all OWHH / OWB units that pass the new voluntary program.<ref>[http://www.epa.gov/owhh/models.htm EPA]</ref> These furnaces come with either an orange EPA tag, signifying Level 1 certification, or a white EPA tag, signifying Level 2 certification, to notify the customer of the units particular emission level output. (One beneficial aspect of this process to consumers is that outdoor wood boilers that are EPA-certified are usually more energy efficient than those that are not, extracting more energy per unit of wood, and thus reducing costs to the owner. Plus, consumers benefit by knowing that such boilers are far less likely to annoy their neighbors.)
Boilers that do pollute enough to cause a [[public nuisance]] (such as by smoke wafting into the house of a neighbor) can be subject to lawsuits by nearby people who are impacted by the smoke nuisance in question, an ancient right under the [[common law]] for the [[abatement]] of [[nuisance]]. This is in addition to local and state regulations, laws, or ordinances that cause restrictions on operation to or even compel removal of excessively polluting boilers. For example, the [[Commonwealth of Massachusetts]] [[Massachusetts Department of Environmental Protection|Department of Environmental Protection]] has barred the sale, installation, or use of new outdoor wood boilers that are not Level 2 certified by the EPA, though old boilers remain grandfathered so long as they do not cause a public nuisance or manifestly impact health and safety.
==See also==
{{colbegin}}
*[[Blast furnace]]
*[[Electric arc furnace]]
*[[Basic oxygen steelmaking]]
*[[Open hearth furnace]]
*[[Russian oven]]
*[[Shell boiler]]
*[[Jetstream furnace]]
*[[HVAC]]
*[[Humidifier]]
*[[Solar power]]
*[[Cremation]]
*[[Fan heater]]
*[[Geothermal heat pump|Geothermal systems]]
*[[Fire test|Fire test furnaces]]
*[[Batch oven]]
*[[Forced-air gas]]
{{colend}}
==Notes==
{{Reflist}}
==References==
{{Refbegin}}
*{{Cite book|author=Gray, W.A. and Muller, R|title=''Engineering calculations in radiative heat transfer |edition=1st|publisher=Pergamon Press Ltd|year=1974|isbn=0-08-017786-7 or ISBN 0-08-017787-5}}
*{{Cite book|author=Fiveland, W.A., Crosbie, A.L., Smith A.M. and Smith, T.F. (Editors)|title=Fundamentals of radiation heat transfer |edition=|publisher=American Society of Mechanical Engineers|year=1991|isbn=0-7918-0729-0}}
*{{Cite book|author=Warring, R. H|title=Handbook of valves, piping and pipelines|edition=1st|publisher=Gulf Publishing Company|year=1982|isbn=0-87201-885-7}}
*{{Cite book|author=Dukelow, Samuel G|title=Improving boiler efficiency|edition=2nd|publisher=Instrument Society of America|year=1985|isbn=0-87664-852-9}}
*{{Cite book|author=Whitehouse, R.C. (Editor)|title=The valve and actuator user's manual|edition=|publisher=Mechanical Engineering Publications|year=1993|isbn=0-85298-805-2}}
*{{Cite book|author=Davies, Clive|title=Calculations in furnace technology|edition=1st|publisher=Pergamon Press|year=1970|isbn=0080133665}}
*{{Cite book|author=Goldstick, R. and Thumann, A|title=Principles of waste heat recovery|edition=|publisher=Fairmont Press|year=1986|isbn=0-88173-015-7}}
*{{Cite book|author=ASHRAE|title=ASHRAE Handbook. Heating, ventilating and air-conditioning systems and equipment|edition=|publisher=ASHRAE|year=1992|id=ISSN 1078-6066|isbn=0910110808}}
*{{Cite book|author=Perry, R.H. and Green, D.W. (Editors)|title=[[Perry's Chemical Engineers' Handbook]]|edition=7th|publisher=McGraw-Hill|year=1997|isbn=0-07-049841-5}}
*{{Cite book|author=Lieberman, P. and Lieberman, Elizabeth T|title=Working Guide to Process Equipment |edition=2nd|publisher=McGraw-Hill|year=2003|isbn=0-07-139087-1}}
{{Refend}}
==External links==
{{Commons|Furnace}}
* [http://www.heaterdesign.com/design0.htm Industrial Fired Heaters]
* [http://highperformancehvac.com/gas-furnace-types/ Gas Furnaces for HVAC Applications]
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