Page 302 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 287

boiler with its headers limited boiler size (it was difficult the burner, furnace, etc..
to support the tubes as they got longer) and included The first boilers were primarily induced draft de-
multiple sources for leaks (all those handholes) so, in signs because motors and fans were more expensive to
1890 a man named Sterling came up with a better con- buy and run than building a tall stack. The stack effect
cept for constructing boilers to eliminate a lot of those is also a lot more reliable but you seldom see a tall stack
problems, he decided to use bent tubes. There are partic- erected today because it’s considered an eyesore, not the
ular designs of boilers (Figure 10-17) that are identified indication of prosperity that was welcomed in the 1930’s
as Sterling boilers but for all practical purposes all bent and 1940’s. If you do see a tall stack going up its purpose
tube boilers are identified as Sterling. Bent tubes added is to disperse pollutants, not to create a draft for induced
flexibility to the design of boilers to permit hundreds of draft boiler operation.
designs and arrangements. As industry flourished the cost of fans and electric-
The evolution of bent tube water tube boilers con- ity dropped and the pressure drop across the boiler heat-
sisted of many arrangements of the sterling design, so ing surfaces increased to the point that a stack alone was
many that it would take an entire book to cover all the not sufficient and induced draft fans were developed
variations so I have no intentions of trying to describe to save on the cost of a tall stack and low pressure drop
them all. Keys to sounding intelligent about them in- boiler. Almost all of those boilers were coal fired and
clude how they’re supported (top, bottom or interme- had brick settings so use of forced draft fans was not de-
diate), drum position relative to movement of the fire sirable because pressure would force the flue gases out
(cross drum if the fire moves perpendicular to the cen- little cracks in the setting into the boiler room.
terline of the drum; we don’t say anything if it isn’t) and As boilers got larger the low furnace pressures
the pressure on the flue gas side (forced draft, balanced required to draw the combustion air into the boiler and
draft, or induced draft). mix it with the fuel also increased admission of tramp
Firetube and package watertube type boilers are air to lower the boiler efficiency. Tramp air leaks in
mostly forced draft design, the hot water heater in your after the burners. On large units it required additional
basement is most likely induced draft, and most forms structure to overcome the force of atmospheric pressure
of Sterling boilers built today are balanced draft design. on the furnace wall. To reduce the low furnace pres-
A forced draft boiler has a fan blowing air into it sures balanced draft boilers were developed where the
and the pressure produced by that fan is used to force induced draft fan, or stack, produces a slightly negative
the air and products of combustion all the way through pressure in the furnace and provides the force to move
the setting and out the stack. An induced draft boiler the flue gases out of the boiler while a forced draft fan
can use stack effect to produce the differential pressure delivers the combustion air to the furnace.
necessary to get the air and flue gases through the setting Modern fossil fuel fired electric power generat-
or the boiler can be fitted with an induced draft fan that ing boilers are all balanced draft and have significant
creates a negative pressure at the boiler outlet and forces pressure drops on the flue gas side to overcome draft
the flue gases up the stack. Induced draft methods basi- losses in the environmental controls as well as the heat
cally create a lower pressure at the outlet of the boiler so transfer elements. Some operate with induced draft fans
atmospheric air pressure can force the air and gases into capable of generating over fifty inches of water column
differential, so much that they could, if conditions were
not controlled, implode the boiler. They create so much
differential that atmospheric pressure would push in the
casing around the furnace of the boiler because it isn’t
designed to operate with that large a differential. Should
controls on those boilers fail we will get an “implosion”
the furnace walls collapse in.
I think that’s enough on Sterling design water tube
boilers, most of you will be operating other types.

Package Watertube Boilers
An interest in other watertube boiler designs can
10
be satisfied by looking up a copy of Steam but most of
Figure 10-17. Sterling boiler the watertube boilers you encounter today, except for a