Steam generator (boiler)
Encyclopedia
A steam generator is a form of low water-content boiler
, 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
,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
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 according to the pump throughput, or by a closed-loop control to maintain the measured temperature.
They are used as auxiliary boilers on ships.
. 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
.
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.
Boiler (steam generator)
A boiler or steam generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure but, at pressures above this, it is more...
, similar to a flash steam boiler. The usual construction is as a spiral coil of water-tube
Water-tube boiler
A water tube boiler is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which heats water in the steam-generating tubes...
, 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
Boiler explosion
A boiler explosion is a catastrophic failure of a boiler. As seen today, boiler explosions are of two kinds. One kind is over-pressure in the pressure parts of the steam and water sides. The second kind is explosion in the furnace. Boiler explosions of pressure parts are particularly associated...
,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
William Fairbairn
Sir William Fairbairn, 1st Baronet was a Scottish civil engineer, structural engineer and shipbuilder.-Early career:...
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
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
. The burner output required varies according to the quantity of water being evaporated: this can be either adjusted by open-loop control according to the pump throughput, or by a closed-loop control to maintain the measured temperature.
They are used as auxiliary boilers on ships.
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 separatorSteam separator
A Steam separator, sometimes referred to as a moisture separator, is a device for separating water droplets from steam. The simplest type of steam separator is the steam dome on a steam locomotive...
. 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
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 of the system...
.
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 spiralSpiral
In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point.-Spiral or helix:...
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.