A steam generator is a form of low water-content

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centra ...

, similar to a flash steam boiler. The usual construction is as a spiral coil of water-tube, arranged as a single, or monotube, coil. Circulation is once-through and pumped under pressure, as a forced-circulation boiler. The narrow-tube construction, without any large-diameter drums or tanks, means that they are safe from the effects of

explosion An explosion is a rapid expansion in volume of a given amount of matter associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Explosions may also be generated ...

,This safety from explosion is due to two causes. Firstly, small diameter tubes are inherently stronger than large tubes of the same construction, as was first appreciated by

William Fairbairn Sir William Fairbairn, 1st Baronet of Ardwick (19 February 1789 – 18 August 1874) was a Scotland, Scottish civil engineer, structural engineer and shipbuilder. In 1854 he succeeded George Stephenson and Robert Stephenson to become the third ...

in the mid-19th century. Secondly, the low water-content means that there is relatively little volume of water per area of heating surface (i.e. boiler power). As the destructive energy of a boiler explosion is largely due to the sudden release of this energy, and proportional to water volume, a low water-content boiler has a more favourable ratio of useful power (from its area) to risk (from its volume). even if worked at high pressures. The pump flowrate is adjustable, according to the quantity of steam required at that time. The burner output is throttled to maintain a constant working temperature. The burner output required varies according to the quantity of water being evaporated: this can be either adjusted by

open-loop control In control theory, an open-loop controller, also called a non-feedback controller, is a control loop part of a control system in which the control action ("input" to the system) is independent of the "process output", which is the process varia ...

according to the pump throughput, or by a

closed-loop control A closed-loop controller or feedback controller is a control loop which incorporates feedback, in contrast to an ''open-loop controller'' or ''non-feedback controller''. A closed-loop controller uses feedback to control states or outputs of a dy ...

to maintain the measured temperature. They are used as auxiliary boilers on ships.

Types

Stone-Vapor

One of the best-known designs is the ''Stone-Vapor''. The inner casing of the boiler forms a vertical bell, with an outer airtight cylindrical casing. The oil or gas burner is mounted at the top, above the coils, and facing downwards. The heating element is a single tube, arranged into a number of helical cylinders. The first helices (in the flow direction) are small-diameter tubes, wrapped in large diameter turns. Succeeding turns are coiled inside this and the tube is of progressively increasing diameter, to allow for a constant flow rate as the water evaporates into steam and forms bubbles. The steam outlet is from the final turn at the bottom of the inner helix. The outlet is approximately 90% steam (by mass) and residual water is separated by passing it through a steam-water separator. The exhaust gases turn upwards and flow over the outside of the bell, usually passing additional helices that are used as an initial

feedwater heater A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency o ...

Clayton

The Clayton steam generator is similar to the Stone-Vapor, but the burner and flow directions are reversed. The heating coil is mounted within a simple cylindrical casing. Rather than helical, cylindrical layers, the Clayton coils are arranged as layers of flat

spiral In mathematics, a spiral is a curve which emanates from a point, moving further away as it revolves around the point. It is a subtype of whorled patterns, a broad group that also includes concentric objects. Two-dimensional A two-dimension ...

s. Water is pumped into the top layers and forced downwards. Again, the tube diameter increases in steps, as evaporation takes place. The final turns form a single closely spaced helical cylinder around the burner as a water-wall furnace and is heated by radiant heat. The steam output is passed through a centrifugal separator and a dry steam quality of 99.5% is claimed.

Note

References

{{Authority control Steam boilers Boilers Marine steam propulsion Steam generators

Types

Stone-Vapor

Clayton

See also

Note

References