Water is most often heated electrically by putting a heating element in direct contact with it. Such an element is therefore called an immersion heater. The water to be heated is usually enclosed inside an insulated cylinder: a hot water tank. Such a heating element consists of a thin metal tube that contains an electrical resistance heater; this, in turn, is encased in an insulating material. To control the temperature of the water, a probe inserted alongside the element triggers a switch according to the temperature of the water, the system as a whole constituting a Thermostat.
Domestic immersion heaters edit
Domestic immersion heaters, usually rated at 3 kilowatts and on a 1.5" British Standard Pipe screwplug in the UK, run on the normal domestic electricity supply, but consumers may also take advantage of a cheaper, off-peak electricity tariff such as Economy 7 (in the UK). In a typical off-peak installation, a lower immersion heater is connected to the separately switched off-peak heating circuit and an upper heater is connected to the normal circuit via its own switch. The consumer then has the option to top-up the available hot water supply at any time, rather than waiting for the cheaper supply to turn on (typically after midnight). A poorly insulated hot water cylinder will increase running costs because a consumer must pay for electricity used to generate lost heat.
Electric shower and tankless heaters also use an immersion heater (shielded or naked) that is turned on with the flow of water. A group of separate heaters can be switched in order to offer different heating levels. Electric showers and tankless heaters usually use from 3 to 7.5 kilowatts.
Industrial immersion heaters edit
Industrial immersion heaters can be either screwed or flanged. Screwed industrial immersion heaters, in the UK usually on a 2.25" British Standard Pipe are usually only rated up to approximately 24kW, with 6kW being considered the very top end that can be accommodated safely on a single phase supply. Flanged immersion heaters (such as those used in electric steam boilers) can be rated at up to 2000 kilowatts, or more, and require a three-phase supply.
Against other means of heating water, electrical immersion heaters have a notable disadvantage: water immediately adjacent to the heating element is heated to a temperature which is high enough to promote the formation of scale in hard water areas. This accumulates on the element, but over time, as the element expands and contracts through its heating cycle, the scale cracks off and drops to the bottom of the tank, progressively filling up the tank. This reduces the tank's capacity and, where the immersion heater is secondary to the heating of the water by a coil fed from a gas or oil-fired boiler, can reduce the efficiency of the primary heating source by covering that other coil and in turn reducing its efficiency. Regular flushing-out of accumulated sediment can reduce this problem.
Such problems can be avoided at the design stage, by maximising the amount of hot element in the liquid, thus reducing the watts density. This reduces the working temperature of the surface of the element, reducing the build up of limescale. Watts density should be 40w/in² or below in hard water areas, but can safely be 60w/in² where hard water is not an issue.
Electrode heater edit
With an electrode heater, there is no wire-wound resistance and the liquid itself acts as the resistance. This has potential hazards, so the regulations governing electrode heaters are strict.
See also edit
- Electric heating
- Heater (types of heaters)
- Solar combisystem
- Energy conservation
References edit
