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 that transfer heat primarily by thermal radiation, while a device that 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.
Buildings
A cast iron household radiator
In buildings a radiator is a heating 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 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 the contact with surrounding air. The air near a radiator is then heated and produces a convection current in the room that draws 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 expanding and contracting 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 anti-hammer device.
Stereotypical cast iron radiators (as pictured) are no longer common in new construction; they have been replaced mostly with copper pipes that have aluminum fins to increase their surface area. In the UK, modern domestic radiators tend to be of sheet steel construction (often with steel fins), though copper and aluminium are 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.
There are many designs and varieties of radiators, from conventional to modern style. Radiators are sometimes seen as an art form, much like sculpture.
Steam
Single-pipe steam radiator
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 problem is often caused when buildings settle and the condensate pools in pipes and radiators that no longer tilt slightly back toward 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 turns on 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
During construction, tubing is placed on the floor throughout the room and later covered with a concrete layer.
The current trend in radiant heating is toward underfloor heating, in which 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 needs to be heated only a few degrees above the desired room temperature, and as a result, convection is almost nonexistent. 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.
Bleeding
All radiant systems (systems in which heat radiates from hot water) 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, thus restoring correct operation. Often radiators located on upper floors will accumulate more air than ones on lower floors because 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 minimize 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 minimized. In untreated systems, the action of the hot water on the iron in the absence of air will strip off the oxygen atom to leave hydrogen as H2 when iron oxide is created. Note that if air is getting into a radiator frequently, there may be a leak somewhere, such as a dripping valve or a loose joint.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Radiator."
