Page 307 - Boiler_Operators_Handbook,_Second_Edition

292 Boiler Operator’s Handbook

tube doesn’t. Many hot oil heaters and some steam and the ease of removing and replacing tubes those ques-
hot water boilers consist of one coil of tube or two coils to tions are a little moot. There is a question in my mind
produce a furnace and convection pass. A boiler consist- as to whether higher efficiency, ease of repair, and other
ing of one continuous tube or several tubes connected in price advantages can compensate for lower reliability
parallel are called once-through boilers. If they generate that may be associated with these units because they
steam the water used is ultra pure or some water leaves have a wider range of thermal cycling under normal
the boiler with the steam and is separated from it to operation due to the small volumes of water.
remove the solids and impurities. Such boilers have no I have discovered that there are problems with the
controllable steam and water line so other means are field erection of flexitube boilers because I served as an
necessary to ensure they aren’t dry fired. Some are fitted expert witness in an arbitration case where a contractor
with temperature sensors that can identify conditions by had installed the tubes improperly. While it’s practically
superheat. One uses the coil of tube itself, when the tube impossible to mis-align the tubes on the sides where the
gets hotter than saturation temperature its thermal ex- length of tube fixes their position it is possible to mis-
pansion trips a limit switch. Should you encounter one align the tubes where they form the baffles that separate
of those boilers in your plant the best thing to do, once the passes. That’s what the contractor did and the leak-
again I say it, is to read the instruction manual. age of flue gases from the furnace into the second pass
New in my time is the “flexitube” boiler (Figure before combustion was completed resulted in very noisy
10-26 being one example) which has taken advantage operation and regular explosions.
of the bent tube construction to produce a boiler that
is lighter, easy to repair, easy to field erect, and highly Superheaters
efficient. The only disadvantage of these boilers is their Most commercial and industrial boilers produce
very low water content. Tubes in these boilers are bent to saturated steam only. Superheaters associated with elec-
very small radii to achieve the form that allows them to tric power generation and driving large equipment will
use the tubes as baffles and produce a five pass
boiler. In order to comply with code restrictions
on bending of tubes (which makes the wall at
the outside of the bend thinner) they are con-
structed using 3/4- or 1-inch tubes compared to
the typical water tube boiler that is principally
2-inch tubes.
An additional feature of the flexitube de-
sign includes a new way of connecting the tubes
to the drums or headers; that construction is
shown in Figure 10-27. The ferrule is a forged
tapered plug bored to accept the tube and the
tube is rolled into the ferrule instead of into
the drum. They can also be welded together. To
install the tube the ferrule is driven into a cor-
respondingly tapered hole punched or drilled
and reamed into the drum or header. Precise
machining of the ferrule and drum provides a
tight fit and the dog is used to clamp it in posi-
tion for added security.
I haven’t seen this method used on high
pressure boilers but it makes field erection of
low pressure boilers much simpler. There are
some questions about the long term operation
of these boilers because thermal cycling could
loosen the ferrules and movement could wipe
out the ceramic fiber insulation used to seal the
ends of the passes but when weighted against Figure 10-26. Flexitube boiler