Page 295 - Boiler_Operators_Handbook,_Second_Edition

280 Boiler Operator’s Handbook

1/2 inch thick and 650°F. The increasing thickness has
imposed a normal limit on firetube boilers of 250 psig
MAWP (maximum allowable working pressure). It’s
possible to get a firetube boiler for a higher pressure but
it’s not a common one. The other practical limit on the
size of a firetube boiler is its diameter. Anything larger
than 8 feet 6 inches in diameter will require special
permits for transporting it on our nation’s highways.
Shipping a firetube boiler without trim and panels on
the sides (but with insulation and lagging) and without
special roadway permits and escort vehicles limits the
diameter to eight feet.
To allow shipment with control panels mounted
the normal firetube boiler is limited to shell diameters of
seven feet. There’s also a limit on length which is around
twenty feet (to fit inside a low boy trailer) but longer
units are made. Since you need twice the length of the
boiler to permit replacing the tubes a twelve foot boiler
would require twenty-four feet of space and that’s the
nominal distance between building columns in average
construction. Many are built backed up to roll-up doors
so the tubes can be pulled outdoors.
Figure 10-5. Tubeless boiler All those factors place a reasonable limit on fire-
tube boilers at about 500 horsepower for a normal unit
long using 100% makeup. Their low price and vertical rated five square feet of heating surface per boiler horse-
construction allows relatively inexpensive replacement. power, 600 horsepower if all the trim is removed or the
boiler is rated at four square feet of heating surface per
boiler horsepower, and about 800 boiler horsepower if
FIRETUBE BOILERS roadway problems are not too expensive and the cus-
tomer can handle a permit load or delivery by rail. That
The firetube boiler requires a “shell” to enclose the
water and steam to complete the pressure vessel portion
of the boiler and that shell is the principal limit on the
size of a firetube boiler. To understand why the shell is
the limiting factor we have to understand some basics
about strength of materials and how we determine the
required thickness of the shell, tubes, and other parts of
a boiler. If you skipped the chapter on strength of mate-
rials you may have trouble understanding this.
You should have noticed that the required thick-
ness of the shell of a boiler or a boiler tube is a function
of the radius. As the tubes get larger the thickness has
to increase to hold the same pressure. Since the outer
shell of a firetube boiler is very large it has to be quite
thick. Thicker materials require more elaborate con-
struction practices in addition to more weight so the
price of a boiler increases proportional to its diameter
with sudden large steps in price associated with differ-
ent construction rules depending on the thickness and
temperature.
A big break point for high pressure boilers come at Figure 10-6. HRT boiler