16-05-2014, 12:15 PM
Fire-tube boiler
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A fire-tube boiler is a type of boiler in which hot gases from a fire
pass through one or more tubes running through a sealed container of
water. The heat of the gases is transferred through the walls of the
tubes by thermal conduction, heating the water and ultimately creating
steam.
The fire-tube boiler developed as the third of the four major historical
types of boilers: low-pressure tank or "haystack" boilers, flued boilers
with one or two large flues, fire-tube boilers with many small tubes,
and high-pressure water-tube boilers. Their advantage over flued
boilers with a single large flue is that the many small tubes offer far
greater heating surface area for the same overall boiler volume. The
general construction is as a tank of water penetrated by tubes that
carry the hot flue gases from the fire. The tank is usually cylindrical for
the most part—being the strongest practical shape for a pressurized
container—and this cylindrical tank may be either horizontal or
vertical.
Operation
In the locomotive-type boiler, fuel is burnt in a firebox to produce hot
combustion gases. The firebox is surrounded by a cooling jacket of
water connected to the long, cylindrical boiler shell. The hot gases are
directed along a series of fire tubes, or flues, that penetrate the boiler
and heat the water thereby generating saturated ("wet") steam. The
steam rises to the highest point of the boiler, the steam dome, where
it is collected. The dome is the site of the regulator that controls the
exit of steam from the boiler.
Schematic diagram of a "locomotive"
In the locomotive boiler, the saturated steam is very often passed into
type fire-tube boiler
a superheater, back through the larger flues at the top of the boiler,
to dry the steam and heat it to superheated steam. The superheated
steam is directed to the steam engine's cylinders or very rarely to a turbine to produce mechanical work.
Exhaust gases are fed out through a chimney, and may be used to pre-heat the feed water to increase the
efficiency of the boiler.
Draught for firetube boilers, particularly in marine applications, is usually provided by a tall smokestack. In all
steam locomotives since Stephenson's Rocket, additional draught is supplied by directing exhaust steam from
the cylinders into the smokestack through a blastpipe, to provide a partial vacuum. Modern industrial boilers use
fans to provide forced or induced draughting of the boiler.
Another major advance in the Rocket was large numbers of small-diameter firetubes (a multi-tubular boiler)
instead of a single large flue. This greatly increased the surface area for heat transfer, allowing steam to be
produced at a much higher rate. Without this, steam locomotives could never have developed effectively as
powerful prime movers.
Scotch marine boiler
For more details on this topic, see Scotch marine boiler.
The Scotch marine boiler differs dramatically from its predecessors in
using a large number of small-diameter tubes. This gives a far greater
heating surface area for the volume and weight. The furnace remains a
single large-diameter tube with the many small tubes arranged above
it. They are connected together through a combustion chamber – an
enclosed volume contained entirely within the boiler shell – so that the
flow of flue gas through the firetubes is from back to front. An
enclosed smokebox covering the front of these tubes leads upwards
to the chimney or funnel. Typical Scotch boilers had a pair of
furnaces, larger ones had three. Above this size, such as for large
steam ships, it was more usual to install multiple boilers.[3]
Washout
The working life of a locomotive boiler is considerably extended if it is
spared from a constant cycle of cooling and heating. Historically, a
locomotive would be kept “in steam” continuously for a period of
about eight to ten days, and then allowed to cool sufficiently for a hot-
water boiler washout. The schedule for express engines was based on
mileage.[6] Today's preserved locomotives are not usually kept
continuously in steam and the recommended washout interval is now
fifteen to thirty days, but anything up to 180 days is possible.