Radiator

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.

The heating radiator was invented by Franz San Galli, a Polish-born Russian businessman living in St. Petersburg, between 1855–1857.^[1][2]

Contents 1 Radiation and convection 2 Heating 3 Engine cooling 4 Electronics 5 Theory of operation 6 References

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.

Heating

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, chiefly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plants or any similar use of such an engine.

To cool down the engine, a coolant is passed through the engine block where it is heated, is fed into the inlet tank of the radiator, which distributes the coolant across the radiator core, then cools down as it circulates through the radiator tubes to the opposite tank, the cold coolant is fed back to the engine, and the cycle repeats.

This coolant is usually water-based, with addition of glycols to prevent freezing and other additives to limit corrosion, erosion and cavitation, but may also be an oil. The first engines used Thermosyphon to circulate the coolant, however nowadays all engines but the smallest ones use a pump.

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 increased 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 has increased, to take over the vast majority of vehicular applications. The main driver for these application is reduced weight and cost, and the increased focus has led to equal heat-performance with copper/brass. For stationary applications, in particular MW-class installations, copper-brass constructions are still dominant.

Air having both a lower heat capacity and density than 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. For this purpose, radiators are often paired with a fan that blows cooling air through the radiator. Fans can consume a significant amount of power, so to minimize their use radiators are often behind the grille at the front end of a vehicle, so that ram air can give a portion or all of the necessary cooling air flow, and the fan remains disengaged.

Electronics

As electronic devices become smaller yet more capable, 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 sinks, which dissipate thermal energy, should not be confused either with electric radiators or electromagnetic radiator elements, a subdivision of antenna in electronics which transmit or receive electromagnetic energy.

Theory of operation

From an engineering perspective, a radiator varies from an ideal black body by a factor, , called the emissivity, which is a spectrum-dependent property of any material. Commonly, a fluid thermal mass, containing the heat to be rejected, is pumped from the heat source to the radiator, where it conducts to the surface and radiates into the surrounding cooler medium. The rate of heat flow depends on the fluid properties, flow rate, conductance to the surface, and the surface area of the radiator. Watts per square metre are the SI units used for radiant emittance. If the system is not limited by the heat capacity of the fluid, or the thermal conductivity to the surface, then emittance, M is found by a fourth-power relation to the absolute temperature at the surface. The Stefan-Boltzmann constant is used to calculate it, as . Since heat may be absorbed as well as emitted, a radiator's ability to reject heat will depend on the difference in temperature between the surface and the surrounding environment. For particular operating temperatures, a system's overall heat flow may be given in thermal watts, abbreviated W_t.

References