Boilers
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.
Construction of boilers is mainly in steel, stainless steel, and wrought iron. In
live steam models, copper or brass is often used. Historically copper was often
used for fireboxes (particularly for steam locomotives), because of its better thermal
conductivity. The price of copper now makes this impractical. Cast iron is used
for domestic water heaters. Although these are usually termed "boilers", their purpose
is 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 steam pressure
vessels.
Superheated steam boilers boil the water and then further heat
the steam in a superheater. This provides steam at much higher
temperature, and can decrease the overall thermal efficiency of the steam plant
due to the fact that the higher steam temperature requires a higher flue gas exhaust
temperature. However, there are advantages to superheated steam.
For example, useful heat can be extracted from the steam without causing
condensation, which could damage piping and turbine blades.
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 (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. Most new
systems are fitted with condensing boilers for greater efficiency. 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. 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 combustion. |