Page 298 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 283

viding two passes, the flue gases are turned and return
down a portion of the tubes on their way to the stack.
Note that a pass consists of a path for flue gas to
travel from one extreme end of the flue gas containing
parts of the boiler to another. Neither of these designs
required a baffle to direct the flow of flue gas. Scotch
marine designs can have two, three, or four passes. A
two pass scotch marine boiler requires no baffles other
than means to separate the burner from the returning
flue gas. Three pass scotch marine construction requires
one baffle in the rear of the boiler to separate the first
and second pass turning box from the third pass outlet
while four pass boilers require a baffle there plus one at
the front to separate the second and third pass turning
box from the fourth pass outlet (Figure 10-13).
Figure 10-11. Baffled rear door of four pass firetube Four pass firetube boilers have a construction
boiler unique to them, the tubes at the inlet of the second past
are normally welded to the tube sheet. That’s because
turning box in the middle of the flat top which is a sur- the flue gases in the first to second pass turning box are
face that would be exposed to high temperatures if the much hotter in those boilers and the welding provides a
water level dropped enough that the only thing to cool better course for heat to pass from the metal to the water
it was steam. The plug is filled with a low melting point to prevent overheating those tube ends (however, see
metal so it would (theoretically) put the fire out with why they fail for a discussion of problems with four pass
steam if the top of the turning box was overheated. Of boilers).
course if it had to work the boiler had to be shut down, Since I mentioned that the tubes are connected
cooled down, drained and the plug replaced before differently in four pass boilers I should also explain
returning to operation. I still regret not keeping four or how they are normally connected. Whether firetube or
five that were in Power and Combustion’s stock before watertube, the normal means of connecting the tubes in
they were thrown out because nobody used them any- the boiler is by rolling. It’s a mechanical method of at-
more. They would be good for parlor games; when you tachment that is strong, watertight, and reliable but also
play what’s this? relatively easy to break so the tubes can be removed.
The locomotive boiler (Figure 10-8) is a basic single Refer to the section on maintenance for a description of
pass design. The flue gases enter the boiler proper and installing a tube by rolling.
flow through all the tubes to the outlet of the boiler. The The furnace tube is normally connected by weld-
HRT design provided improved heat transfer by pro- ing to the tube sheets. That’s because it is large and thick

Figure 10-12. Wet back scotch marine boiler Figure 10-13. Front baffle of four pass boiler