Page 308 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 293

stub end welded to the more brittle superheater tubes.
Those stub ends are protected from the heat by baffles
or refractory coatings.
To prevent problems with water depositing in
them many superheaters are designed to drain com-
pletely by installing the headers at the bottom with
the tubes extending up from the headers. We call them
“drainable” superheaters. Boilers in most utility plants
are of a construction that doesn’t drain, the tubes hang
down from the headers into the furnace or flue gas pas-
sages and they’re called “pendant” type superheaters.
Some superheaters are separately fired. Boilers
on ships of the Navy usually have two furnaces, one
before the superheater and one after it so the superheat
temperature can be controlled. In shoreside applications
there’s frequently a requirement for small quantities of
Figure 10-27. Flexitube tube to driving joint
superheated steam so a separately fired superheater is
installed to boost the temperature of that steam.
become more prevalent after the writing of this book Large power generating boilers can also have re-
because the deregulation of electricity has finally forced heaters. They’re the same as a superheater in construc-
utilities to become more efficient so more distributed tion but steam passing through a reheater will be steam
generating systems will be built. Your boiler plant will that has passed through part of the steam turbine after
eventually become a power generator as well as a steam leaving the boiler outlet. To ensure the steam remains
generator unless it’s a very small boiler plant or has a superheated in the lower pressure stages of the turbine
very inconsistent load. it is reheated in the reheater. Construction is about the
Since steam can only be superheated when there same as a superheater.
is no water left around to evaporate, any superheated Boilers with superheaters will always have a safety
steam boiler takes steam at the boiler outlet to super- valve at the outlet of the superheater and a valved vent
heat. The steam flows through a connecting pipe to a line to atmosphere for ensuring flow through the su-
header where it’s distributed through a number of par- perheater during startup and upset conditions. Another
allel tubes exposed to the furnace (radiant superheater) pressure gauge and a thermometer are also standard
or flue gases after they pass through the screen tubes trim items.
(convection superheater). There the steam temperature
is increased as it absorbs heat from the flue gas. Steam Drum Internals
Since the heat transfer rate is not as efficient as All that steam and water entering the drum needs
boiling water the steam velocity is rather high in the to be separated so the steam can go out the steam nozzle
superheater to ensure turbulent flow for the best pos- and the water can drop down the downcomers. To aid in
sible cooling of the metal tubes. The full load pressure separating the steam and water parts are installed in the
drop in a superheater is typically 10 psi because it takes steam drum. Everything that’s installed inside the boiler
a lot of pressure drop to create the turbulence for good is described as “internals” and that includes steam and
heat transfer. The thin gas film that makes a conven- water separating devices. Most steam drum internals
tional boiler tube much cooler than midway between are something like the details shown in Figure 10-28.
the furnace gas and boiling water temperature when Baffles deflect the steam and water mixture entering the
boiling water is repeated on the inside of a superheater drum to prevent water splashing up to the outlet. They
so the tube metal in a superheater is considerably spread the water and steam out over the surface so it can
hotter. The superheater materials of construction are separate by gravity (heavier water falls, lighter steam
designed for those higher temperatures. Many of them rises).
use tube metal that is not as malleable (easy to mold or The steam then has to go up over the top of the dry
bend) as normal boiler tubes; in fact they’re so brittle pipe and down through the holes in it to get inside the
that they can’t be rolled. A short piece of malleable tube dry pipe which is connected by a tee to the boiler steam
that’s rolled into the header is frequently provided as a outlet. Small pipes connected to each end of the dry pipe