HIGH PRESSURE
BOILERS:
A boiler
is a closed vessel in which water or other fluid is heated. The
heated or vaporized fluid exits the boiler for use in various processes or
heating applications.
Most boilers produce steam to be
used at saturation temperature; that is, saturated
steam. Superheated steam boilers vaporize 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
because the higher steam temperature requires a higher flue gas exhaust
temperature. There are several ways to circumvent this problem, typically by
providing an economizer that heats the feed water, a combustion air
heater in the hot flue gas exhaust path, or both. There are advantages to
superheated steam that may, and often will, increase overall efficiency of both
steam generation and its utilisation: 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 entrained
condensation droplets will damage turbine blades.
Superheated steam presents
unique safety concerns because, if any system component fails and allows steam
to escape, the high pressure and temperature can cause serious, instantaneous
harm to anyone in its path. Since the escaping steam will initially be
completely superheated vapor, detection can be difficult, although the intense
heat and sound from such a leak clearly indicates its presence.
Superheater operation is
similar to that of the coils on an air conditioning unit, although for a
different purpose. The steam piping is directed through the flue gas path in
the boiler furnace. The temperature in this area is typically between
1,300–1,600 degrees Celsius.
Some superheaters are radiant type; that is, they absorb heat by radiation.
Others are convection type, absorbing heat from a fluid. Some are a combination
of the two types. Through either method, the extreme heat in the flue gas path
will also heat the superheater steam piping and the steam within. While the
temperature of the steam in the superheater rises, the pressure of the steam
does not: the turbine or moving pistons offer a continuously expanding space
and the pressure remains the same as that of the boiler.
Almost all steam superheater system designs remove droplets entrained in the
steam to prevent damage to the turbine blading and associated piping.
கருத்துகள் இல்லை:
கருத்துரையிடுக