Radiator
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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. This article discusses radiators that 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.
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[edit] 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 outside air driven through the radiator by the vehicle's forward motion, often supplemented by a fan. The radiator transfers the heat from the fluid inside to the air, thereby cooling 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.)
The engine temperature is controlled by the thermostat, a wax-pellet type temperature operated valve. When the engine is cold, the thermostat is completely closed and allows heat to build up in the engine, by allowing water to circulate only through the engine (and the heater core). 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, heat produced, the air flow through the radiator, outside air temperature. The basic process is entirely mechanical, the only electrically controlled part is the electric cooling fan. Some (mostly older) cars do not even use an electric fan, but use a fan driven by the engine directly or via a fan clutch.
Note that the size of the radiator is usually chosen such that it can keep the engine at the design temperature under the most extreme conditions a car is likely to encounter (such as climbing a mountain, while fully loaded, on a hot day)
As the thermal efficiency of internal combustion engines increases with internal temperature the coolant is kept at higher-than-atmospheric pressure to increase it's boiling point. A calibrated pressure-relief valve is usually incorporated in the radiator's fill cap. As the heated coolant expands the pressure relief valve opens and excess fluid is dumped into an overflow container. When the coolant cools and contracts the fluid is sucked back into the radiator through additional valving in the cap. on this type system if the coolant in the overflow container gets to low the fluid transfer will cause an increased loss by vaporizing the engine coolant. Severe damage can be caused by overheating when the coolant is evaporated to a level below the water pump. This can happen withoout warning because at that point the sending units are not exposed to the coolant to indicate the temperature. The cap often contains a mechanism to relieve the internal pressure before the cap can be opened. Opening a hot radiator can cause sudden eruption of super-heated coolant which can cause severe burns (see geyser).
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. 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 charged or supercharged engines may have an intercooler, which is an air-to-air or air-to-water radiator used to cool the incoming charge and not to cool the engine.
[edit] 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"). In a building, radiators transfer the majority of their heat by radiation and by convection.
[edit] 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/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. 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 [1] are testemant to the new wave in thinking, and changing customer attitudes.
[edit] 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 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.
[edit] 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.
[edit] Underfloor heating
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.
[edit] 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.
[edit] 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.
[edit] See Also
[edit] Sources
- Haynes Opel Omega & Senator Service and Repair Manual, 1996, ISBN 1-85960-342-4
[edit] External links
