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A radiator is a device designed to transfer thermal energy mechanically from one location to another in an efficient manner for the purpose of cooling and heating. In the broader sense of the word, it can also be used to describe things environmental or biological in nature, i.e. tropical cyclones or body hair, that function in exactly the same way. This article discusses radiators that are constructed to function in automobiles, buildings, and electronics.
Automobiles
In automobiles with an internal combustion engine, a radiator is connected to channels running through the engine and cylinder head, through which is pumped a liquid. This liquid is typically a mixture of water with ethylene glycol (a.k.a. antifreeze).
The fluid moves in a closed system from the radiator to the engine, where it conducts heat away from the engine parts and carries the heat primarily to the radiator. The radiator is typically mounted behind the vehicle's grille, with cold air driven through the radiator to cool the radiator, the fluid inside, and therefore the engine. A system of valves and/or baffles is usually incorporated to simultaneously operate a small radiator inside the car; this small radiator is called the heater core and serves to warm the interior cabin. (Noting that 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.)
Between the engine and the radiator lies the thermostat, a temperature operated valve. It remains closed, restricting coolant flow, until the engine reaches the thermostat's activation temperature. This arrangement allows the engine temperature to be maintained in the ideal operating range. When the engine is too cold, the thermostat closes and allows heat to build up in the engine. When the engine is too warm, the thermostat opens and allows heat to be removed by the radiator. Note that the size of the radiator is usually chosen such that, when the engine is operating under a normal load, the thermostat is constantly open. Also note that the heater core plumbing will usually bypass the thermostat controlled pipes such that the passenger cabin can be warmed regardless of the thermostat position.
The invention of the automobile water radiator is attributed to Karl Benz. Some engines have an additional oil cooler; a separate small radiator to cool the motor oil. Turbo charged engines may have an intercooler.
Buildings
In buildings, a radiator is a heating device, which is warmed by steam from a boiler or hot water being pumped into it from a water heater (usually, if not quite accurately, referred to as a "boiler"). The term is somewhat of a misnomer, as radiators in building transfer the majority of their heat not by radiation, but by convection.
Radiators are more energy efficient for heating houses than forced-air gas heating systems.
Conventional radiators
A conventional hot-water radiator consists of a sealed hollow metal container, usually flat in shape. Hot water enters at one end and rises to the top of the radiator by way of convection or by pressure from a pump elsewhere in the building.
As it gives out its heat, the hot water cools and sinks to the bottom of the radiator and then 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 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 the pipes rubbing on surrounding surfaces while expanding and contracting due to heat changes, or by 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 small trapped bubble of air (not interfering with flow, as would an unbled radiator) will provide a cushion against pressure fluctuations.
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.
The radiator was invented in 1855 by Franz SanGalli, an Italian-born German living in Russia. He was the first to produce a system of central heating and patented his invention in Germany and the US.
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 steam systems operate at make them inherently less efficient, as unwanted heat loss is inevitably greater.
Steam pipes and radiators are also prone to banging 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 thermostatic switch senses the heat and energises an electric fan 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
The current trend in radiant heating is towards floor 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 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.
Electronics
In electronics, a radiator is also known as a radiating element. Radiating elements are a basic subdivision of an antenna. Radiating elements are capable of transmitting or receiving electromagnetic energy.
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