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Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics. The radiator is always a source of heat to its environment, although this may be for either the purpose of heating this environment, or for cooling the fluid or coolant supplied to it, as for engine cooling. Despite the name, radiators generally transfer the bulk of their heat via convection, not by thermal radiation, though the term "convector" is used more narrowly; see radiation and convection, below.
The Roman hypocaust, a type of radiator for building space heating, was described in 15 AD. The heating radiator was invented by Franz San Galli, a Polish-born Russian businessman living in St. Petersburg, between 1855 and 1857.
Radiation and convection 
One might expect the term "radiator" to apply to devices that transfer heat primarily by thermal radiation (see: infrared heating), while a device which relied primarily on natural or forced convection would be called a "convector". In practice, the term "radiator" refers to any of a number of devices in which a liquid circulates through exposed pipes (often with fins or other means of increasing surface area), notwithstanding that such devices tend to transfer heat mainly by convection and might logically be called convectors. The term "convector" refers to a class of devices in which the source of heat is not directly exposed.
Radiators are commonly used to heat buildings. In a central heating system, hot water or sometimes steam is generated in a central boiler, and circulated by pumps through radiators within the building. There are two types: Single-pipe and double-pipe. The single-pipe radiators work with steam, while the double-pipe radiators work with steam or hot water.
Engine cooling 
Radiators are used for cooling internal combustion engines, mainly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plants and other places where such engines are used.
To cool down the engine, a coolant is passed through the engine block, where it absorbs heat from the engine. The hot coolant is then fed into the inlet tank of the radiator and distributed across the radiator core. As the coolant circulates through the radiator tubes on its way to the opposite tank, it cools again. The cold coolant is fed back to the engine, and the cycle repeats.
This coolant is usually water-based, with the addition of glycols to prevent freezing and other additives to limit corrosion, erosion and cavitation. However, the coolant may also be an oil. The first engines used thermosiphons to circulate the coolant; today, however, all but the smallest engines use pumps.
As it circulates through the tubes, the coolant transfers its heat to the tubes which, in turn, transfer the heat to the fins that are lodged between each row of tubes. The fins then release the heat to the ambient air. Fins are used to greatly increase the contact surface of the tubes to the air, thus increasing the exchange efficiency.
Up to the 1980s, radiator cores were often made of a copper (for fins) and brass (for tubes, headers, side-plates, while tanks could be made also of brass or of plastic, often a polyamide). Starting in the 1970s, use of aluminium increased to take over the vast majority of vehicular applications. The main drivers for aluminum are reduced weight and cost. However, the superior cooling properties of Copper-Brass over Aluminium makes it preferential for high performance vehicles or stationary applications. In particular MW-class installations, copper-brass constructions are still dominant (See: Copper in heat exchangers). CuproBraze is a copper-alloy heat exchanger technology for harsh temperature and pressure environments such as those in the latest generations of cleaner diesel engines mandated by environmental regulations. Its performance advantages over radiators made with other materials include better thermal performance, heat transfer, size, strength, durability, emissions, corrosion resistance, repairability, and antimicrobial benefits.
Since air has a lower heat capacity and density than liquid coolants, a fairly large volume flow rate (relative to the coolant's) must be blown through the radiator core to capture the heat from the coolant. Radiators often have one or more fans that blow air through the radiator. To save fan power consumption in vehicles, radiators are often behind the grille at the front end of a vehicle. Ram air can give a portion or all of the necessary cooling air flow, and the fan remains disengaged.
As electronic devices become smaller, the problem of dispersing waste heat becomes more difficult. Tiny radiators known as heat sinks are used to convey heat from the electronic components into a cooling air stream. Heat is transferred to the air by conduction and convection; a relatively small proportion of heat is transferred by radiation owing to the low temperature of semiconductor devices compared to their surroundings.
- "Family Sangalli / San Galli". Gruner-fam.de. Retrieved 2011-09-20.
- The hot boxes of San Galli (Russian)
- Vehicle radiators: Can CuproBraze turn copper into a bona fide contender?; American Metal Market September 2008; http://dl.dropbox.com/u/46572847/Perspectives-radiators.pdf
- Partanen, Juho (2011). Hot property: Heat exchangers that optimize product reliability, decrease lifecycle costs and improve profitability are just the ticket for increasing the lifespan and performance of off-highway machinery; Industrial Vehicle Technology; March 2011; http://viewer.zmags.com/services/DownloadPDF
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