Everything about Furnace totally explained
A
furnace is a device used for
heating. The name derives from
Latin fornax,
oven.
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.
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 furnaces.
Household Furnaces
A
household furnace is a
major appliance that's 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
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. 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.
- "High-efficiency" in this sense may be misleading, because furnace efficiency is typically expressed as a "first-law" efficiency, whereas the exergy efficiency of a typical furnace is much lower than the first-law thermal efficiency. By comparison, cogeneration has a higher exergy efficiency than is realizable from burning fuel to generate heat directly at a moderate temperature. However, as the vast majority of consumers (as well as many government regulators) are unfamiliar with exergy efficiency, Carnot efficiency, and the Second Law of Thermodynamics, the use of first-law efficiencies to rate furnaces is well-entrenched.
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're 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.
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. 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, not only do you not have heat, you don't have domestic hot water either.
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's 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.
Metallurgical furnaces
In
metallurgy, several specialised furnaces are used. These include:
Furnaces used in smelters, including:
Furnaces used to remelt metal in foundries.
Furnaces used to reheat and heat treat metal for use in:
Vacuum furnaces
==
Further Information
Get more info on 'Furnace'.
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