Page 304 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 289

spread over the front wall to help protect the refractory. The predominant design is the D type (Figure 10-
The rear wall, on the other hand, is usually fit- 21) which has only one drawback and that’s the problem
ted with bent tubes spread out to cover it. The wall is with transporting and supporting something with most
typically much lighter in construction than the front of the weight on one side. The D tubes extend out of the
wall, an allowance partially provided by the tubes and drum to form the roof of the furnace, drop to form the
distance from the heat of the flame. Frequently the rear furnace side wall, and return under the furnace to the
wall is called the target wall because the flame is shoot- mud drum. It has one convection bank of tubes centered
ing straight at it and the tubes against the rear wall are between the drums to limit sootblower requirements.
called target tubes. The tubes form a framework of steel This construction makes it possible for the flue gas to
that helps to hold the rear wall in place, especially dur- leave the boiler via the front or side. A more detailed
ing shipment of the boiler and that’s a major consider- diagram (Figure 10-22) will help you identify some of
ation in the wall thickness. the standard features of this construction.
The O type boiler (Figure 10-20) is similar to the A There are many modifications to this design with
while eliminating one header by providing a drum in different manufacturers featuring different details. D
the bottom center just like the top. The headers required type boilers are also manufactured in semi-shop fabri-
many handholes for rolling the tubes in an A type boiler cated form where the furnace portion is shipped as an
so the single drum eliminated that expense but pro- independent assembly from the convection bank with
duced a boiler with a smaller furnace cross section. the two drums. Another arrangement is the D tubes
The single bottom drum saved one longitudinal and casing are shipped loose for installation in the field.
weld as well. All the longitudinal welds in modern boil- These may still be referred to as “packaged” boilers
ers are X-rayed making them more expensive to form. despite final field assembly. Shipping the furnace or its
The O type boiler is only manufactured by Erie City components separately allow for larger capacity boilers
Iron Works of Erie, Pennsylvania (now Zurn), and is the without the restraints of shipping clearances and still
only boiler I know of where the feedwater line enters the retaining most of the advantages of a package boiler.
bottom drum. Some of the same difficulties experienced Unlike the scotch marine firetube and other smaller
with the A boiler are associated with the O design. This boilers “package” doesn’t clearly describe the assembly
boiler is not a good candidate for firing solid fuels or for water tube boilers. A package boiler can be shipped
heavy fuel oil because it’s almost impossible to remove without any burner or connecting piping. Almost any
the soot and ash from the bottom of the boiler. It does package water tube boiler with a capacity over 25,000
work well on gas. pph is not ready for connecting pipe and wire and start-

Figure 10-20. “O” type boiler Figure 10-21. “D” type boiler