Page 292 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 277

The water flow tended to be so low in those tubes that
scale formed in them, you might still run into one of
those boilers and be told that there are certain steaming
rates you want to avoid to prevent scaling problems in
portions of the boiler. A number of designs were modi-
fied to include “unheated downcomers,” tubes or pipes
installed between the top and bottom drums (or head-
ers) on the boiler to provide an unheated path for the
water to circulate through.
We actually added some unheated downcomers to
a boiler in an effort to correct a problem with overheat-
ing of the boiler’s roof tubes despite the fact that I didn’t
agree with the solution. Sometimes unheated downcom-
ers aren’t obvious, they’re buried in a tube bank where
Figure 10-1. Steam flow pattern in firetube boiler flue gas can’t get at them.
Okay, some wise guy is asking “what does this
have to do with hot water boilers?” The truth is that
there is some steam generation to force circulation in
most hot water boilers; there has to be. Maybe there
isn’t at low loads but the differences in density of heated
water are not enough to produce the rapid flow of water
needed to carry the heat away from the heat transfer
surfaces. The steam that’s generated condenses again
when the bubbles separate from the heat transfer surface
and find their way to colder (by a few degrees) water in
the boiler.
There are some hot water boilers, HTHW gen-
erators for example, that are designed to force the water
along and absorb the heat fast enough to prevent steam
formation but I’m willing to bet that you would find
steam bubbles forming and collapsing in any conven-
tional hot water boiler. If you watched that pot on the
stove while the water was heating up you probably no-
Figure 10-2. Steam flow pattern in water tube boiler
ticed signs of movement which was due to differences
in density of water heated at the bottom and the colder
the water drops are called “downcomers.” It stands to water on top (cooled some by the air) and along the
reason that all the tubes that face the furnace of a boiler sides. You also should have noticed that bubbles formed
must be risers. Remember that 60% of heat transferred on the bottom of the pan and lifted off then disappeared
by radiation? When the boiler is operating at low loads before reaching the surface. I’m certain that must hap-
only a few of the tubes, those along the sides of the pen in most hot water boilers.
boiler that are heated on one side only (and don’t face Keep in mind that circulation is absolutely neces-
the furnace) will be downcomers. As the boiler load sary to prevent scale formation and blocking of tubes
increases even the downcomers will have some steam to the degree they overheat and fail. If bottom blows
bubbles forming in them because they’re absorbing aren’t adequately removing the accumulating sludge in
heat and more tubes will have to become downcomers a boiler the normal circulation can sweep some of that
in order to move all the water that has to circulate in sludge into some risers with almost instantaneous fail-
the boiler. Some tubes will always be risers, some will ure a certainty.
always be downcomers, but many of them switch back Growth of scale on tubes will restrict flow in the
and forth. boiler and accelerate the scale formation as a result. If
Some water tube boiler designs encountered prob- you have scale in your boiler its demise is only a ques-
lems with the translation from risers to downcomers. tion of timing. Loose drum internals that will break