Supercritical steam generator

Steam generation power plant.

Supercritical steam generators are frequently used for the production of electric power. They operate at supercritical pressure. In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3,200 psi/22.06 Mpa or 3,200 psi/220.6 bar) that actual boiling ceases to occur, the boiler has no liquid water - steam separation.
There is no generation of steam bubbles within the water, because the pressure is above the critical pressure at which steam bubbles can form. It passes below the critical point as it does work in a high pressure turbine and enters the generator's condensor. This results in slightly less fuel use and therefore less greenhouse gas production. The term "boiler" should not be used for a supercritical pressure steam generator, as no "boiling" actually occurs in this device.

Water tube boiler

Diagram of a water-tube boiler

Another way to rapidly produce steam is to feed the water under pressure into a tube or tubes surrounded by the combustion gases. The earliest example of this was developed by Goldsworthy Gurney in the late 1820s for use in steam road carriages. This boiler was ultra-compact and light in weight and this arrangement has since become the norm for marine and stationary applications. The tubes frequently have a large number of bends and sometimes fins to maximize the surface area. This type of boiler is generally preferred in high pressure applications since the high pressure water/steam is contained within narrow pipes which can contain the pressure with a thinner wall. It can however be susceptible to damage by vibration in surface transport appliances. In a cast iron sectional boiler, sometimes called a "pork chop boiler" the water is contained inside cast iron sections. These sections are mechanically assembled on site to create the finished boiler.

High pressure water tube boilers generate steam rapidly at high temperatures that can be increased by lengthening the tubes. The superheater section of the tubes.

Superheater

A superheated boiler on a steam locomotive.

L.D. Porta gives the following equation determining the efficiency of a steam locomotive, applicable to steam engines of all kinds: power (kW) = steam Production (kg h^-1)/Specific steam consumption (kg/kW h).

A greater quantity of steam can be generated from a given quantity of water by superheating it. As the fire is burning at a much higher temperature than the saturated steam it produces, far more heat can be transferred to the once-formed steam by superheating it and turning the water droplets suspended therein into more steam and greatly reducing water consumption.

The superheater works like coils on an air conditioning unit, however to a different end. The steam piping (with steam flowing through it) is directed through the flue gas path in the boiler furnace. This area typically is between 1,300–1,600 degree Celcius (2,372–2,912 F). Some superheaters are radiant type (absorb heat by thermal radiation), others are convection type (absorb heat via a fluid i.e. gas) and some are a combination of the two. So whether by convection or radiation the extreme heat in the boiler furnace/flue gas path will also heat the superheater steam piping and the steam within as well. It is important to note that while the temperature of the steam in the superheater is raised, the pressure of the steam is not: the turbin or moving pistons offer a "continuously expanding space" and the pressure remains the same as that of the boiler. The process of superheating steam is most importantly designed to remove all droplets entrained in the steam to prevent damage to the turbine blading and/or associated piping. Superheating the steam expands the volume of steam, which allows a given quantity (by weight) of steam to generate more power.

A steam turbine with the case opened

When the totality of the droplets are eliminated, the steam is said to be in a superheated state.In a Stephensonian firetube locomotive boiler, this entails routing the saturated steam through small diameter pipes suspended inside large diameter fire tubes putting them in contact with the hot gases exiting the firebox; the saturated steam flows backwards from the wet header towards the firebox, then forwards again to the dry header. Superheating only began to be generally adopted for locomotives around the year 1900 due to problems of overheating of and lubrication of the moving parts in the cylinders andsteam chests. Many firetube boilers heat water until it boils, and then the steam is used at saturation temperature in other words the temperature of the boiling point of water at a given pressure (saturated steam); this still contains a large proportion of water in suspension. Saturated steam can and has been directly used by an engine, but as the suspended water cannot expand and do work and work implies temperature drop, much of the working fluid is wasted along with the fuel expended to produced it.

Firetube Boiler

The next stage in the process is to boil water and make steam. The goal is to make the heat flow as completely as possible from the heat source to the water. The water is confined in a restricted space heated by the fire. The steam produced has lower density than the water and therefore will accumulate at the highest level in the vessel; its temperature will remain at boiling point and will only increase as pressure increases. Steam in this state (in equilibrium with the liquid water which is being evaporated within the boiler) is named "saturated steam". For example, saturated steam at atmospheric pressure boils at 100 °C (212 °F).

Saturated steam taken from the boiler may contain entrained water droplets, however a well designed boiler will supply virtually "dry" saturated steam, with very little entrained water. Continued heating of the saturated steam will bring the steam to a "superheated" state, where the steam is heated to a temperature above the saturation temperature, and no liquid water can exist under this condition.

Physical layout of the four main devices used in the Rankine cycle

Most reciprocating steam engines of the 19th century used saturated steam, however modern steam power plants universally use superheated steam which allows higher steam cycle efficiency.

Solid fuel firing

In order to improve the burning characteristics of the fire, air needs to be supplied through the grate, or more importantly above the fire. Most boilers now depend on mechanical draft equipment rather than natural draught.

The stack effect in chimneys: the gauges
represent absolute air pressure
and the airflow is indicated with light grey arrows.
The gauge dials move clockwise with increasing pressure.

This is because natural draught is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as chimney height. All these factors make effective draught hard to attain and therefore make mechanical draught equipment much more economical. There are three types of mechanical draught:

1. Induced draught: This is obtained one of three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler. The denser column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet or ejector oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draught fan (ID fan) which sucks flue gases out of the furnace and up the stack. Almost all induced draught furnaces have a negative pressure.
2. Forced draught: draught is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draught furnaces usually have a positive pressure.
3. Balanced draught: Balanced draught is obtained through use of both induced and forced draft. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric.

Combustion

The source of heat for a boiler is combustion of any of several fuels, such as wood, coal,oil, or natural gas. Nucleas fission is also used as a heat source for generating steam.

Nuclear fission

Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas trubines.

Multi-tube boilers

A significant step forward came in France in 1828 when Marc Ssguin devised a two-pass boiler of which the second pass was formed by a bundle of multiple tubes. A similar design with natural induction used for marine purposes was the popular “Scotch” marine boiler. Prior to the Rainhill trials of 1829 Henry Booth, treasurer of the Liverpool and Meanchester Railway suggested to George Stephenson, a scheme for a multi-tube one-pass horizontal boiler made up of two units: a Firebox surrounded by water spaces and a boiler barrel consisting of two telescopic rings inside which were mounted 25 copper tubes; the tube bundle occupied much of the water space in the barrel and vastly improved heat transfer.

The firebox of a coal-fired train steam engine.


Section of typical boiler and firebox

Old George immediately communicated the scheme to his son Robert and this was the boiler used on Stephenson's Rocket, outright winner of the trial. The design was and formed the basis for all subsequent Stephensonian-built locomotives, being immediately taken up by other constructors; this pattern of fire-tube boiler has been built ever since

Cylindrical fire-tube boiler

An early proponent of the cylindrical form, was the American engineer, Oliver Evans who rightly recognised that the cylindrical form was the best from the point of view of mechanical resistance and towards the end of the 18th Century began to incorporate it into his projects. Probably inspired by the writings on Leupold’s “high-pressure” engine scheme that appeared in encyclopaedic works from 1725, Evans favoured “strong steam” i.e. non condensing engines in which the steam pressure alone drove the piston and was then exhausted to atmosphere.
The advantage of strong steam as he saw it was that more work could be done by smaller volumes of steam; this enabled all the components to be reduced in size and engines could be adapted to transport and small installations. To this end he developed a long cylindrical wrought iron horizontal boiler into which was incorporated a single fire tube, at one end of which was placed the fire grate. The gas flow was then reversed into a passage or flue beneath the boiler barrel, then divided to return through side flues to join again at the chimney (Columbian engine boiler). Evans incorporated his cylindrical boiler into several engines, both stationary and mobile. Due to space and weight considerations the latter were one-pass exhausting directly from fire tube to chimney. Another proponent of “strong steam” at that time was the Cornishman, Richard Trevithick. His boilers worked at 40–50 psi (276–345 kPa) and were at first of hemispherical then cylindrical form. From 1804 onwards Trevithick produced a small two-pass or return flue boiler for semi-portable and locomotive engines.

Trevithick's engine of 1806 is built around

an early example of a flued boiler (specifically, a return-flue type)

The Cornish boiler developed around 1812 by Richard Trevithick was both stronger and more efficient than the simple boilers which preceded it. It consisted of a cylindrical water tank around 27 feet (8.2 m) long and 7 feet (2.1 m) in diameter, and had a coal fire grate placed at one end of a single cylindrical tube about three feet wide which passed longitudinally inside the tank. The fire was tended from one end and the hot gases from it travelled along the tube and out of the other end, to be circulated back along flues running along the outside then a third time beneath the boiler barrel before being expelled into a chimney. This was later improved upon by another 3-pass boiler, the Lancashire boiler which had a pair of furnaces in separate tubes side-by-side. This was an important improvement since each furnace could be stoked at different times, allowing one to be cleaned while the other was operating.

Lancashire boiler, from Fairbairn's lecture

Railway locomotive boilers were usually of the 1-pass type, although in early days, 2-pass "return flue" boilers were common, especially with locomotives built by timothy Hackworth.

Haycock and wagon top boilers

Haycock and wagon top boilers

Newcomen steam engine.
– Steam is shown pink and water is blue.
– Valves move from open (green) to closed (red)

For the first Newcomen engine of 1712, the boiler was little more than large brewer’s kettle installed beneath the power cylinder.

Kettle from a Korean tea house.

Because the engine’s power was derived from the vacuum produced by condensation of the steam, the requirement was for large volumes of steam at very low pressure hardly more than 1 psi (6.9 kPa) The whole boiler was set into brickwork which retained some heat.

Pump to demonstrate vacuum

A large vacuum chaber

A wall built in Flemish bond

A voluminous coal fire was lit on a grate beneath the slightly dished pan which gave a very small heating surface; there was therefore a great deal of heat wasted up the chimney. In later models, notably by John Smeaton, heating surface was considerably increased by making the gases heat the boiler sides, passing through a flue. Smeaton further lengthened the path of the gases by means of a spiral labyrinth flue beneath the boiler. These under-fired boilers were used in various forms throughout the 18th Century. Some were of round section (haycock). A longer version on a rectangular plan was developed around 1775 by Boulton and Watt (wagon top boiler). This is what is today known as a three-pass boiler, the fire heating the underside, the gases then passing through a central square-section tubular flue and finally around the boiler sides.

Component of prime mover in boiler machine

The steam generator or boiler is an integral component of a steam engine when considered as a prime mover; however it needs be treated separately, as to some extent a variety of generator types can be combined with a variety of engine units. A boiler incorporates a Firebox or Furnace in order to burn the fuel and generate heat; the heat is initially transferred to water to make steam: this produces saturated steam at ebullition temperature saturated steam which can vary according to the pressure above the boiling water.

The higher the furnace temperature, the faster the steam production. The saturated steam thus produced can then either be used immediately to produce power via a turbine and alternator , or else may be further superheated to a higher temperature; this notably reduces suspended water content making a given volume of steam produce more work and creates a greater temperature gradient in order to counter tendency to condensation due to pressure and heat drop resulting from work plus contact with the cooler walls of the steam passages and cylinders and wire-drawing effect from strangulation at the regulator. Any remaining heat in the combustion gases can then either be evacuated or made to pass through an economiser, the role of which is to warm the feed water before it reaches the boiler.

Boiler Machine (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

(1–300 psi/0.069–20.684 bar; 6.895–2,068.427 kpa), but at pressures above this it is more usual to speak of a steam generator.

An industrial boiler, originally used for supplying steam to a stasionary steam engine

A boiler or steam generator is used wherever a source of steam is required. The form and size depends on the application: mobile steam engines such as steam liocomotives, portable engines and steam-powered road vehicles typically use a smaller boiler that forms an integral part of the vehicle; stasionary steam engines, industrial installations and power stations will usually have a larger separate steam generating facility connected to the point-of-use by piping. A notable exception is the steam-powered fireless locomotive, where separately-generated steam is transferred to a receiver (tank) on the locomotive.