Page 325 - Boiler_Operators_Handbook,_Second_Edition

310 Boiler Operator’s Handbook

tion that there were several attempts to use heat pipes for CHX. By using city water you’re running high tempera-
air preheaters in the 1980’s, usually without success. At ture differentials (city water is normally between 40 and
least I don’t recall a successful application. They should 70°F) so the poor heat transfer capability of the corrosion
work but there are times when technology isn’t applied resisting materials is overcome. The typical applications
correctly and it results in failures. Heat pipes contain a right now use high grades of stainless steel for gas fired
refrigerant (not the typical ones used for freezers and applications and Teflon coated copper for more acidic
air conditioners but something capable of operation at flue gases.
high temperatures) and employ a simple concept. The To withstand the corrosive properties of the flue
pipes are sloped so the liquid refrigerant flows down gas after passing through the CHX the exhaust duc-
the pipe to the end exposed to flue gas where the hot twork is constructed of corrosion resistant materials,
flue gases heat the refrigerant until it boils. The gaseous usually FRP (fiberglass reinforced plastic piping). Those
refrigerant then flows up the sloped pipe to the end ex- materials are not common to boiler plants despite the
posed to the colder combustion air where the refrigerant fact they’ve been used in some cooling tower operations
condenses, giving up its heat to the combustion air. The for several years now. They’re not difficult to deal with
tubes are sealed in a casing where they penetrate a wall in operation or maintenance, they just require learning
separating the hot flue gas and cold combustion air. If about them. It’s best to read the instruction manuals for
you encounter a working one I would love to have some the materials your plant may have because there are
information on it. considerable variations in capability and handling.
The condensate from a CHX has a low pH because
Condensing Heat Exchangers the condensed water will absorb the CO and SO in
2 2
A condensing heat exchanger (CHX for short) the flue gas to form acids. The drain piping should be
could be an economizer or an air preheater as well as FRP to a point where the condensate can be neutralized.
other devices. What makes a CHX a CHX is the use of Mixing the acid condensate of a CHX with the caustic
materials of construction that are corrosive resistant, blowdown from the boiler can produce a mixture that
allowing the heat exchanger to operate at temperatures may meet the local jurisdiction’s requirements for sani-
below the acid dewpoint. Condensation of acidic flue tary sewage. If it doesn’t you’ll have to add caustic soda
gas components is expected and accounted for. to neutralize the mix before it is dumped to the sewer.
There’s a definite difference between a CHX and A final consideration for heat traps is they don’t
the other heat traps because the others aren’t designed to have to be used on boilers or trap the heat from boilers.
recover the latent heat in the flue gas. When the hydrogen I’ve had some very successful projects that saved cus-
in the fuel burns to form water it normally leaves the tomers a lot of money by using these devices to recover
boiler as steam. With natural gas firing the energy that heat lost up the stack from process operations. What is
could be recovered by condensing that steam amounts to an economizer, for all practical purposes, sits in a steel
about 11% of the total heat input. A CHX is designed to mill recovering an average 75 million Btuh (120 million
condense as much of that water as possible to recover an peak) and all it’s doing is preheating boiler plant make-
additional 970 Btu per pound of water condensed. up water. Many a boiler plant can save a fair amount of
The additional heat that can be recovered by a CHX energy in the winter if normal building exhaust can be
helps pay for the exotic materials of construction but trapped and used as combustion air. In those cases the
many of the materials that can withstand the corrosive building and its occupants preheat the air.
acids can’t tolerate the high temperature of the flue gas.
Metals that can handle both haven’t been proven as of the
writing of this book but they may be. In the next ten years BURNERS
so condensing air preheaters and other CHX options will
become standard boiler plant devices. Right now they Most boilers get their heat for the hot water or
aren’t because of many unsuccessful applications and, steam from the combustion of fuel which requires a
to be perfectly honest, operators not understanding the burner. There are some devices for combustion that
benefits of them and how to operate them properly. aren’t called burners, including stokers but all of them
The current common application is a CHX used combine the fuel with air to form a combustible mixture
for preheating boiler water makeup and service water so the air and fuel react to produce combustion prod-
independent of the boilers. Flue gas is drawn from the ucts and heat. The purpose of the burner is to control
boiler stacks by an induced draft fan downstream of the the mixing of fuel and air so the combustion occurs