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Diagram of a water-tube boiler.
Applications
Boilers have many applications. They can be used in stationary
applications to provide heat, hot water, or steam for domestic use,
or in generators and they can be used in mobile applications to
provide steam for locomotion in applications such as trains, ships,
and boats. Using a boiler is a way to transfer stored energy from
the fuel source to the water in the boiler, and then finally to the
point of end use.
Materials
The pressure vessel in a boiler is usually made of steel (or alloy
steel), or historically of wrought iron. Stainless steel is virtually
prohibited (by the ASME Boiler Code) for use in wetted parts of
modern boilers, but is used often in superheater sections that will
not be exposed to liquid boiler water. In live steam models, copper
or brass is often used because it is more easily fabricated in smaller
size boilers. Historically, copper was often used for fireboxes
(particularly for steam locomotives), because of its better
formability and higher thermal conductivity; however, in more
recent times, the high price of copper often makes this an
uneconomic choice and cheaper substitutes (such as steel) are
used instead.
Cast iron may be used for the heating vessel of domestic water
heaters. Although such heaters are usually termed "boilers", their
purpose is usually to produce hot water, not steam, and so they
run at low pressure and try to avoid actual boiling. The brittleness
of cast iron makes it impractical for high pressure steam boilers.
Fuel
The source of heat for a boiler is combustion of any of several
fuels, such as wood, coal, oil, or natural gas. Electric steam boilers
use resistance- or immersion-type heating elements. Nuclear fission
is also used as a heat source for generating steam. Heat recovery
steam generators (HRSGs) use the heat rejected from other
processes such as gas turbines.
Configurations
Boilers can be classified into the following configurations:
• "Pot boiler" or "Haycock boiler": a primitive "kettle" where a
fire heats a partially-filled water container from below. 18th Century
Haycock boilers generally produced and stored large volumes of
very low-pressure steam, often hardly above that of the
atmosphere. These could burn wood or most often, coal. Efficiency
was very low.
• Fire-tube boiler. Here, water partially fills a boiler barrel with a
small volume left above to accommodate the steam (steam space).
This is the type of boiler used in nearly all steam locomotives. The
heat source is inside a furnace or firebox that has to be kept
permanently surrounded by the water in order to maintain the
temperature of the heating surface just below boiling point. The
furnace can be situated at one end of a fire-tube which lengthens
the path of the hot gases, thus augmenting the heating surface
which can be further increased by making the gases reverse
direction through a second parallel tube or a bundle of multiple
tubes (two-pass or return flue boiler); alternatively the gases may
be taken along the sides and then beneath the boiler through flues
(3-pass boiler). In the case of a locomotive-type boiler, a boiler
barrel extends from the firebox and the hot gases pass through a
bundle of fire tubes inside the barrel which greatly increase the
heating surface compared to a single tube and further improve heat
transfer. Fire-tube boilers usually have a comparatively low rate of
steam production, but high steam storage capacity. Fire-tube
boilers mostly burn solid fuels, but are readily adaptable to those of
the liquid or gas variety.
• Water-tube boiler. In this type,the water tubes are arranged
inside a furnace in a number of possible configurations: often the
water tubes connect large drums, the lower ones containing water
and the upper ones, steam and water; in other cases, such as a
monotube boiler, water is circulated by a pump through a
succession of coils. This type generally gives high steam production
rates, but less storage capacity than the above. Water tube boilers
can be designed to exploit any heat source and are generally
preferred in high pressure applications since the high pressure
water/steam is contained within small diameter pipes which can
withstand the pressure with a thinner wall.
Boiler for steam locomotive[3]
Flash boiler. A specialized type of water-tube boiler.
Fire-tube boiler with Water-tube firebox. Sometimes the two
above types have been combined in the following manner: the
firebox contains an assembly of water tubes, called thermic
syphons.
The gases then pass through a conventional firetube boiler.
Water-tube fireboxes were installed in many Hungarian locomotives,
but have met with little success in other countries.
Sectional boiler. In a cast iron sectional boiler, sometimes
called a "pork chop boiler" the water is contained inside cast iron
sections. These sections are assembled on site to create the
finished boiler. |
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Safety
Historically, boilers were a source of many serious injuries and
property destruction due to poorly understood engineering
principles. Thin and brittle metal shells can rupture, while poorly
welded or riveted seams could open up, leading to a violent eruption
of the pressurized steam. Collapsed or dislodged boiler tubes could
also spray scalding-hot steam and smoke out of the air intake and
firing chute, injuring the firemen who loaded coal into the fire
chamber. Extremely large boilers providing hundreds of horsepower
to operate factories could demolish entire buildings.[4]
A boiler that has a loss of feed water and is permitted to boil dry
can be extremely dangerous. If feed water is then sent into the
empty boiler, the small cascade of incoming water instantly boils on
contact with the superheated metal shell and leads to a violent
explosion that cannot be controlled even by safety steam valves.
Draining of the boiler could also occur if a leak occurred in the
steam supply lines that was larger than the make-up water supply
could replace. The Hartford Loop was invented in 1919 by the
Hartford Steam Boiler and Insurance Company as a method to help
prevent this condition from occurring, and thereby reduce their
insurance claims.[5]
Superheated steam boilers
A superheated boiler on a steam locomotive.
Main article: Superheater
Most boilers heat water until it boils, and then the steam is used at
saturation temperature (i.e., saturated steam). Superheated steam
boilers boil the water and then further heat the steam in a
superheater. This provides steam at much higher temperature, but
can decrease the overall thermal efficiency of the steam generating
plant due to the fact that the higher steam temperature requires a
higher flue gas exhaust temperature. There are several ways to
circumvent this problem, typically by providing a feedwater heating
"ecomomizer", and/or a combustion air heater in the hot flue gas
exhaust path. There are advantages to superheated steam and this
may (and usually will) increase overall efficiency of both steam
generation and its utilisation considered together: gains in input
temperature to a turbine should outweigh any cost in additional
boiler complication and expense. There may also be practical
limitations in using "wet" steam, as causing condensation droplets
will damage turbine blades.
Superheated steam presents unique safety concerns because, if
there is a leak in the steam piping, steam at such high
pressure/temperature can cause serious, instantaneous harm to
anyone entering its flow. Since the escaping steam will initially be
completely superheated vapor, it is not easy to see the leak,
although the intense heat and sound from such a leak clearly
indicates its presence.
Supercritical steam generators
Supercritical steam generators (also known as Benson boilers) 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 3,200 psi/220.6 bar that actual boiling
ceases to occur, and the boiler has no 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 the
high pressure turbine and enters the generator's condenser. This is
more efficient, resulting in slightly less fuel use. The term "boiler"
should not be used for a supercritical pressure steam generator, as
no "boiling" actually occurs in this device.
Hydronic boilers
Hydronic boilers are used in generating heat for residential and
industrial purposes. They are the typical power plant for central
heating systems fitted to houses in northern Europe (where they
are commonly combined with domestic water heating), 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 (or sometimes in the case of
single pipe systems, until it boils and turns to steam) and circulating
that fluid throughout 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. The name
can be a misnomer in that, except for systems using steam
radiators, the water in a properly functioning hydronic boiler never
actually boils. Most new systems are fitted with condensing boilers
for greater efficiency. These boilers are referred to as condensing
boilers because they condense the water vapor in the flue gases to
capture the latent heat of vaporization of the water produced
during combustin.
Hydronic systems are being used more and more in new
construction in North America for several reasons. Among the
reasons are:
They are more efficient and more economical than forced-air
systems (although initial installation can be more expensive,
because of the cost of the copper and aluminum).
The baseboard copper pipes and aluminum fins take up less
room and use less metal than the bulky steel ductwork required for
forced-air systems.
They provide more even, less fluctuating temperatures than
forced-air systems.
The copper baseboard pipes hold and release heat over a
longer period of time than air does, so the furnace does not have to
switch off and on as much. (Copper heats mostly through
conduction and radiation, whereas forced-air heats mostly through
forced convection. Air has much lower thermal conductivity and
higher specific heat than copper; however, convection results in
faster heat loss of air compared to copper. See also thermal mass.)
They do not dry out the interior air as much.
They do not introduce any dust, allergens, mold, or (in the
case of a faulty heat exchanger) combustion byproducts into the
living space.
Forced-air heating does have some advantages, however. See
forced-air heating.
Accessories
Boiler fittings and accessories
Safety valve: It is used to relieve pressure and prevent
possible explosion of a boiler.
Water level indicators: They show the operator the level of
fluid in the boiler, also known as a sight glass, water gauge or water
column is provided.
Bottom blowdown valves: They provide a means for removing
solid particulates that condense and lay on the bottom of a boiler.
As the name implies, this valve is usually located directly on the
bottom of the boiler, and is occasionally opened to use the pressure
in the boiler to push these particulates out.
Continuous blowdown valve: This allows a small quantity of
water to escape continuously. Its purpose is to prevent the water
in the boiler becoming saturated with dissolved salts. Saturation
would lead to foaming and cause water droplets to be carried over
with the steam - a condition known as priming.
Hand holes: They are steel plates installed in openings in
"header" to allow for inspections & installation of tubes and
inspection of internal surfaces.
Steam drum internals, A series of screen, scrubber & cans
(cyclone separators).
Low- water cutoff: It is a mechanical means (usually a float
switch) that is used to turn off the burner or shut off fuel to the
boiler to prevent it from running once the water goes below a
certain point. If a boiler is "dry-fired" (burned without water in it) it
can cause rupture or catastrophic failure.
Surface blowdown line: It provides a means for removing foam
or other lightweight non-condensible substances that tend to float
on top of the water inside the boiler.
Circulating pump: It is designed to circulate water back to the
boiler after it has expelled some of its heat.
Feedwater check valve or clack valve: A non-return stop
valve in the feedwater line. This may be fitted to the side of the
boiler, just below the water level, or to the top of the boiler.
Top feed: A check valve (clack valve) in the feedwater line,
mounted on top of the boiler. It is intended to reduce the nuisance
of limescale. It does not prevent limescale formation but causes the
limescale to be precipitated in a powdery form which is easily
washed out of the boiler.
Chemical injection line: A connection to add chemicals for
controlling feedwater pH. |
