Boiler

A boiler is a closed vessel in which water or other fluid is heated under pressure. The

steam or hot fluid is then circulated out of the boiler for use in various process or

heating applications. A safety valve is required to prevent over pressurisation and possible

explosion of a boiler.

Overview

Construction of boilers is mainly limited to copper, steel and cast iron. In Live steam

toys, brass is often used.

Sources of heat for the boiler can be the combustion of fuels such as wood, coal, oil or

natural gas. Electric boilers use resistance or immersion type heating elements. Nuclear

fission is also used as a heat source for generating steam. Waste-heat boilers, or HRSGs use

the heat rejected from other processes such as gas turbines.

Boilers can also be classified into fire-tube, water-tube boilers or cast iron sectional

depending on whether the heat source is inside or outside the tubes or in the case of the

cast iron sectional the design and manufacture of the boiler. The goal in all cases is to

maximize the heat transfer between the water and the hot gases heating it. For example,

steam locomotives have fire-tube boilers, where the fire is inside the tube and the water on

the outside. These usually take the form of a set of straight tubes passing through the

boiler through which hot combustion gases flows.

In water-tube boilers the water flows through a large number of narrow tubes around the

fire. 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.

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.

There are other types of boilers, largely of historical interest. For example, the Cornish

boiler developed around 1812 by Richard Trevithick for generating steam for steam engines.

This was both stronger and more efficient than the simple boilers which preceded it. It was

a cylindrical water tank around 27 feet long and 7 feet in diameter, and had a coal furnace

placed in a single cylindrical tube about three feet wide which passed centrally along the

long axis of 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 of the boiler before being expelled via the chimney. This was later improved upon in

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 whilst the other was operating. These designs are really primitive fire tube

boilers, and led on to the Scotch boiler which was a popular fire tube design.


Supercritical boilers
Supercritical boilers are used for the generation of electric power. They operate at

"supercritical pressure". In contrast to a "subcritical boiler", a supercritical boiler has

no water - steam separation. There is no generation of steam because the pressure is

regulated above the "critical pressure" at which steam bubbles can form. Thus, the fluid

generated is called "supercritical fluid". It passes below the critical point as it does

work in the high pressure turbine and enters the generator's condensor. This is more

efficient resulting in slightly less fuel use and therefore less greenhouse gas production.


Hydronic boilers
Hydronic boilers are used in generating heat typically for residential uses. They are the

typical power plant for central heating systems fitted to houses in northern Europe, as

opposed to the forced air furnaces or wood burning stoves more common in North America. The

hydronic boiler operates by way of heating water/fluid to a preset temperature and

circulating that fluid throughtout the home typically by way of radiators, baseboard heaters

or through the floors. The fluid can be heated by any means....gas, wood, fuel oil, etc, but

in built-up areas where piped gas is available, natural gas is currently the most economical

and therefore the usual choice. The fluid is in an enclosed system and circulated throughout

by means of a motorized pump. These hydronic systems are being used more and more in new

construction in North America as they are more economical than forced air furnaces and make

it easier to construct smaller diameter water pipes as it is the larger ventilation piping.

Most new systems are fitted with condensing boilers for greater efficiency. "Boiler" is

clearly a misnomer for this kind of device, which is really nothing but a large water heater

in which the water is never intended to boil; but the name is universal and unlikely ever to

change.


Accessories
Boiler fittings

* Safety valve
* Water column: to help the operator tell if there is a satisfactory level of fluid in

the boiler, a water gauge or water column is provided.
* Bottom blowdown valves
* Surface blowdown line
* Circulating pump


Feedwater accessories

* Feedwater pressure regulator
* Vacuum pump
* City water makeup
* Low water cut-off switch
* Backflow preventer
* condensat



Steam accessories

* Main steam stop valve
* Steam traps



Combustion accessories

* Fuel oil system
* Gas system
* Coal system
* Automatic combustion systems



Controlling draft