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.
In automobiles with a liquid-cooled internal combustion engine a radiator is connected to channels running through the engine and cylinder head, through which a liquid (coolant) is pumped. This liquid is typically a half-and-half mixture of water and ethylene glycol or propylene glycol (with a small amount of corrosion inhibitor) known as antifreeze.
The radiator transfers the heat from the fluid inside to the air outside, thereby cooling the engine. Radiators are generally mounted in a position where they will receive airflow from the forward movement of the vehicle, such as behind the grill. Where engines are rear- or mid-mounted, it's usually still necessary to mount the radiator behind the front grill, so as to achieve sufficient airflow, even though this requires long coolant pipes.
An earlier construction method was the honeycomb radiator. Round tubes were swaged into hexagons at their ends, then stacked together and soldered. As they only touched at their ends, this formed what became in effect a solid water tank with many air tubes through it.
Vintage cars may also have used radiator cores made from coiled tube, a less-efficient but simpler construction.
All automobiles for many years have used centrifugal pumps to circulate their coolant, driven by geared drives or more commonly by a belt drive. This "fan belt" has a well-established reputation for being slightly unreliable, a failure being rapidly obvious as the engine overheats. Despite the name though, it's the coolant pump's failure that causes the overheating, not the fan.
The engine temperature is primarily controlled by a wax-pellet type of thermostat, a valve which opens once the engine has reached its maximum operating temperature. When the engine is cold the thermostat is closed. Coolant flows to the inlet of the circulating pump and is returned directly to the engine, bypassing the radiator. Directing water to circulate only through the engine allows heat to build up, whilst avoiding localised "hot spots". Once the coolant reaches the thermostat's activation temperature it opens, allowing water to flow through the radiator. Optimum operating temperature is maintained by the cyclic opening and closing of the thermostat valve.
Airflow speed through a radiator is a major influence on the heat it loses. Vehicle speed affects this, in rough proportion to the engine effort, thus giving crude self-regulatory feedback. Where an additional cooling fan is driven by the engine, this also tracks engine speed similarly.
Engine-driven fans are often regulated by a viscous-drive clutch from the drivebelt, which slips and reduces the fan speed at low temperatures. This improves fuel efficiency by not wasting power on driving the fan unnecessarily. On modern vehicles, further regulation of cooling rate is provided by either variable speed or cycling radiator fans. Electric fans are controlled by a thermostatic switch or the engine control unit. Electric fans also have the advantage of giving good airflow and cooling at low engine revs or when stationary, such as in slow-moving traffic.
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.
Development in high-performance aircraft engines required improved coolants with higher boiling points, leading to the adoption of glycol or water-glycol mixtures. These led to the adoption of glycols for their antifreeze properties too.
Since the development of aluminium or mixed-metal engines, corrosion inhibition has become even more important than antifreeze, and in all regions and seasons too.
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 may cause a sudden ebullition of super-heated coolant which can cause severe burns (see geyser).
As they are so dependent on airspeed, surface radiators are even more prone to overheating when ground-running. Racing aircraft such as the Supermarine S.6B, a racing seaplane with radiators built into the upper surfaces of its floats, have been described as "being flown on the temperature gauge" as the main limit on their performance.
Pressurized cooling systems operate by adding heat to the coolant fluid, causing it to rise in temperature in inverse proportion to its specific heat capacity. With the need to keep the final temperature below boiling point, this limits the amount of heat that a given mass-flow of coolant can dissipate.
Attempts were made with aero-engines of the 1930's, notably the Rolls-Royce Goshawk, to exceed this limit by allowing the coolant to boil. This absorbs an amount of heat equivalent to the specific heat of vaporization, which for water is more than five times the energy required to heat the same quantity of water from 0°C to 100°C. Obviously this allows the necessary cooling effect with far less coolant requiring to be circulated.
The practical difficulty was the need to provide condensers rather than radiators. Cooling was now needed not just for hot dense liquid coolant, but for low-density steam. This required a condenser far larger and with higher drag than a radiator. For aircraft, especially high-speed aircraft, these were soon realised to be unworkable and so steam cooling was abandoned.
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").
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 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, 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.
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 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.
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.
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.
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.