Page 153 - Boiler_Operators_Handbook,_Second_Edition

138 Boiler Operator’s Handbook

special requirements for one of those boilers. ONCE THROUGH BOILERS
A waste heat boiler will always have a lot more
heat exchange surface than a fired boiler because there I am aware of one manufacturer of once through
is no radiant heat transfer. It’s safe to assume a waste boilers in the commercial to industrial range and many
heat boiler will have twice the heating surface of a con- electric utility boilers are designed for once through op-
ventional boiler for the same capacity. It isn’t uncom- eration. The boiler feed pump(s) or a boiler water circu-
mon to encounter a waste heat boiler with finned tubes lating pump is used to produce the differential pressure
to provide additional heating surface so you will not required to force the water and steam through the boiler.
necessarily encounter boilers with twice the number of Similar to HTHW generators these boilers can contain
tubes. Depending on the source of the heat the boiler can multiple circuits of tubing with inlet and outlet headers
incorporate an economizer section to preheat the feed- but many have one single circuit to maintain a point of
water and can be of once through design. The materials conversion from saturated to superheated steam. Feed
of construction may include materials that don’t con- water is forced into the tubes and steam which may be
form to the requirements of the Rules for Construction saturated or superheated leaves the tubes. All electric
of Heating Boilers (Section IV) or Rules for Construc- utility boilers of this design generate superheated steam.
tion of Power Boilers (Section I) because the liquids or Needless to say, those boilers must use absolutely pure
gases that are the source of the heat would destroy those water because any dissolved solids would build up in the
materials. In those cases the boilers are constructed in boiler tubes as scale.
accordance with the Rules for Construction of Pressure Once through boilers that generate saturated steam
Vessels (Section VIII) as an “unfired boiler” which al- also incorporate a steam and water separator at the outlet
lows use of exotic materials including stainless steels, so only steam is delivered to the facility. The water that
Inconel, and others. exits the boiler with the steam is separated and returned
The largest, physically, waste heat boiler I ever back to the inlet of the circulating pump. The one boiler
encountered was one I helped design and, as far as I design I know of has only one tube formed into a long
know, is still in service in Wilmington, North Carolina. coil that surrounds the boiler furnace and may have addi-
It is twenty-four feet in diameter, over ninety feet tall tional tubing extending the coil as a convection section. A
and generates about 25,000 pph of low pressure steam. safety feature on these boilers, that doesn’t exist on other
The largest in capacity is a unit that looks more like an types, consists of a switch activated by the thermal expan-
economizer and only preheats boiler plant makeup wa- sion of one of the boiler tubes. If there is insufficient water
ter, 120 million Btuh. They come in a variety of sizes and flow through the tube its coil expands as it is heated to
configurations that are so variable that there’s no de- trip the switch.
scribing them all and their operation varies significantly Flow has to be proven before lighting a fire in one of
depending on the conditions of the fluid flow stream the these boilers so treat the low water flow switch with the
heat is coming from. same concern and testing as a low water cutoff.
A low water cutoff is a required element for any
boiler and they should always be provided on waste
heat boilers unless the temperature of the fluid stream ENGINES AND EMERGENCY GENERATORS
is less than about 750°F where the metal will not
overheat. I can recall one system where a contractor At the risk of being called overly emphatic I will
installed a waste heat boiler connected directly to the repeat that you should never operate equipment until
exhaust of a steel annealing furnace which exhausted you’ve read the instruction manual. These guidelines
a heating stream at about 1800°F. The new boiler was only apply if you can’t find the manual, something that
melted down two days after installation because the happens with surprising regularity.
water source failed. If the temperature is high enough The most common engine in a boiler plant is nor-
there should always be a way of diverting the waste mally the emergency generator. For reasons that still
heat stream to prevent overheating the boiler. In some escape me ships, from other nations, which I visited
cases there is no diversion of the waste heat stream but during my tenure in the merchant marine were almost
it’s possible to add air to dilute it until the boiler metal always powered only by engines. I enjoyed taking tours
can withstand the temperature. With those exceptions of those ship’s engine rooms to see diesel engines so
any waste heat boiler should be treated like a normal large that you could stand inside one of the cylinders
boiler. and fuel injectors that were so large it took two men to