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Radiators and convectors are types of heat exchangers designed 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.

One might expect the term "radiator" to apply to devices which 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.

Automobiles coolant radiatorIn automobiles, with a liquid-cooled internal combustion engine, a radiator is connected to channels running through the engine and cylinder head, through which is pumped a liquid (coolant). This liquid is typically a mixture of water with ethylene glycol (a.k.a. Antifreeze (coolant)) and a little corrosion inhibitor.

Fluid cycle The fluid is pumped in a closed system from the radiator to the engine, where it absorbs and conducts heat away from the engine parts and carries the heat, primarily, to the radiator. The radiator is, on many conventional vehicles, typically mounted behind the vehicle's grille, with outside air driven through the radiator by the vehicle's forward motion.

There are other mounting options for the radiator, especially in mid- and rear-engined vehicles, and some vehicles have characteristic body ducts and vents to ensure that air is conducted through the radiator core.

Movement of the air through the radiator usually needs to be supplemented by a Fan (mechanical) so that the engine can be cooled while stationary or working hard. However the fan will absorb excessive power at speed, so variable pitch, or friction drive, mechanical fans and electric fans are normally used on faster vehicles such as cars. Some vehicles, such as liquid-cooled motorcycles, do not have any supplementary feature for inducing airflow through the radiator.

The radiator transfers the heat from the fluid inside to the air outside, thereby cooling the engine.

Heater A system of valves or baffles, or both, is usually incorporated to simultaneously operate a small radiator inside the car. This small radiator, and the associated blower fan, is called the heater core and serves to warm the cabin interior. (Heating the interior of the car also helps to cool the engine, this is the reason for the mechanic's instruction to turn on the heating system if the car is overheating.)

Temperature control The engine temperature is controlled by the thermostat, a wax-pellet type temperature operated valve.When the engine is cold the thermostat is closed. Coolant bypasses back to the inlet of the circulating pump and is returned directly to the engine. Directing water to circulate only through the engine, and the heater core, allows heat to build up in the engine. When the coolant reaches the thermostat's activation temperature it begins to open, allowing water into the radiator. The thermostat will open as far as needed to maintain the design temperature by metering the amount of water going to the radiator.

How far the thermostat opens depends on many factors such as engine power demand, heat produced, the air flow through the radiator, outside air temperature and cleanliness of the radiator itself. The basic process is entirely mechanical, the only electrically controlled part is the electric cooling fan. Some, mostly older, cars do not use an electric fan but one driven by the engine directly, or via a fan clutch or a simple fluid coupling, as part of the water-pump drive from the crankshaft pulley.

The size of the radiator is usually chosen so that it can keep the engine at the design temperature under the most extreme conditions a vehicle is likely to encounter (such as climbing a mountain while fully loaded on a hot day).

Coolant pressure Because the thermal efficiency of internal combustion engines increases with internal temperature the coolant is kept at higher-than-atmospheric pressure to increase its boiling point. A calibrated pressure-relief valve is usually incorporated in the radiator's fill cap.

As the coolant expands with increasing temperature its pressure in the closed system must increase. Ultimately the pressure relief valve opens and excess fluid is dumped into an overflow container. Fluid overflow ceases when the thermostat modulates the rate of cooling to keep the temperature of the coolant at optimum. When the coolant cools and contracts (as conditions change or when the engine is switched off) the fluid is returned to the radiator through additional valving in the cap.

Boiling or overheating On this type system, if the coolant in the overflow container gets too low, fluid transfer to overflow will cause an increased loss by vaporizing the engine coolant.

Severe engine damage can be caused by overheating, by overloading or system defect, when the coolant is evaporated to a level below the water pump. This can happen without warning because, at that point, the sending units are not exposed to the coolant to indicate the excessive temperature.

To protect the unwary the cap often contains a mechanism that attempts to relieve the internal pressure before the cap can be fully opened. Some scalding of one's hands can easily occur in this event. Opening a hot radiator drops the system pressure immediately and normally causes a sudden eruption of super-heated coolant which can cause severe burns (see geyser).

History The invention of the automobile water radiator is attributed to Karl Benz. Wilhelm Maybach designed the first honeycomb radiator for the Mercedes 35hp .

Supplementary radiators Some engines have an oil cooler, a separate small radiator to cool the motor oil. Cars with an automatic transmission often have extra connections to the radiator, allowing the transmission fluid to transfer its heat to the coolant in the radiator.

turbo charger or supercharger engines may have an intercooler, which is an air-to-air or air-to-water radiator used to cool the incoming charge—not to cool the engine.

Road-vehicle air conditioning also uses a form of cooling radiator and, in some vehicles, it may be closely integrated with the engine's coolant radiator so that they appear to be a single unit.

== Buildings ==In buildings a radiator is a HVAC device, which is warmed by steam from a boiler, or by hot water being pumped through it from a water heater (usually, if not quite accurately, referred to as a "boiler").

Such radiators transfer the majority of their heat by thermal radiation and by convection.

Conventional radiators A conventional hot-water radiator consists of a sealed hollow metal container, usually flat in shape. Hot water enters at the top of the radiator by way of pressure, from a pump elsewhere in the building, or by convection.

As it gives out heat the hot water cools and sinks to the bottom of the radiator and is forced out of a pipe at the other end. The pipe either has a large surface area or attached fins to increase its surface area and therefore contact with surrounding air. The air near a radiator is then heated and produces a convection current in the room drawing in cold air to heat.

If set up improperly, radiators, and their supply and return pipes, can make loud banging noises like someone hammering on the pipes. This is due to either the pipes rubbing on surrounding surfaces while Thermal expansion due to heat changes or to sudden fluctuations of the supplied water pressure. Proper mounting of the radiators and supply pipes will reduce expansion noises, while upward-mounted stub ends with a trapped bubble of air (not interfering with flow, as would an un-bled radiator) will provide a cushion against pressure fluctuations, an Water hammer device.

Stereotypical cast iron radiators (as pictured) are no longer common in new construction, replaced mostly with copper pipes which have aluminum fins to increase their surface area. In the U.K., modern domestic radiators tend to be of sheet steel construction (often with steel fins), though copper/aluminium is often found in industrial Air Handling System heat exchangers.

The radiator was invented in 1855 by Franz SanGalli. He was the first to produce a system of central heating and patented his invention in Germany and the US.

These days there are many designs and varieties of radiators, from conventional designs to the latest in vogue. Indeed in some places radiators are actually seen as an artform, much like a sculpture. Designer radiators are testemant to the new wave in thinking, and changing customer attitudes.

Steam Steam has the advantage of flowing through the pipes under its own pressure without the need for pumping. For this reason, it was adopted earlier, before electric motors and pumps became available. Steam is also far easier to distribute than hot water throughout large, tall buildings like skyscrapers. However, the higher temperatures at which steam systems operate make them inherently less efficient, as unwanted heat loss is inevitably greater.

Steam pipes and radiators are also prone to producing banging sounds (known as "water hammer") if condensate fails to drain properly; this is often caused by buildings settling and the resultant pooling of condensate in pipes and radiators that no longer tilt slightly back towards the boiler.

Fan assisted radiators A more recent type of heater used in homes is the fan assisted radiator. It contains a heat exchanger fed by hot water from the heating system. A thermostat switch senses the heat and energises an electric fan (mechanical) which blows air over the heat exchanger.

Advantages of this type of heater are its small size and even distribution of heat around the room. Disadvantages are the noise produced by the fan, and the need for an electricity supply.

Underfloor heating layer.

The current trend in radiant heating is towards underfloor heating, where warm water is circulated under the entire floor of each room in a building. A network of pipes, tubing or heating cables is buried in the floor, and a gentle heat rises into the room. Because of the large area of this type of radiator, the floor only needs to be heated a few degrees above the desired room temperature, and as a result, convection is almost non-existent. These systems are reputed to have a high level of comfort, but are generally difficult to install into existing buildings.For best results, a floor covering that conducts heat well (such as tiles) should be used.

The hypocaust was a Roman heating system using a similar principle of operation.

Bleeding All "radiant" (ie. heat radiates from hot water) systems need to be bled, or purged of air, on occasion.

If there is air (or other gases such as Hydrogen) trapped inside the radiator, then the water cannot rise to the top, and only the bottom area gets hot. A bleed screw near the top of the radiator allows the trapped air to be 'bled' from the system, and thus restore correct operation. Often radiators located on upper floors will accumulate more air than ones on lower floors as the air will tend to rise to the topmost point in the system. These may have to be bled more often. Usually radiators are bled once or twice per season, or as needed. Another reason to exclude air is to minimise corrosion of the steel pressed radiators. Note that most central heating systems need a corrosion inhibitor added into the circulating hot water, so that the production of Hydrogen is minimised. This is created in untreated systems, by the action of the hot water on the iron in the absence of air (stripping off the oxygen atom to leave hydrogen as H2 when iron oxide is created). Note that if air is getting into the radiators frequently, this may be the sign of a leak somewhere, such as a dripping valve, or loose joint.

Electronics In electronics, a radiator is also known as a radiating electrical element. Radiating elements are a basic subdivision of an Antenna (electronics). Radiating elements are capable of transmitting or receiving Electromagnetism energy.

See Also

Heater core
Intercooler
Coolant
Internal combustion engine

Sources

Haynes Opel Omega & Senator Service and Repair Manual, 1996, ISBN 1-85960-342-4

External links

Site about construction of heated floors in buildings (in Dutch language)
How Car Cooling Systems Work

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