Page 323 - Boiler_Operators_Handbook,_Second_Edition

308 Boiler Operator’s Handbook

mizers are less expensive initially but almost impossible
to repair.
Since economizers can be subjected to corrosive
conditions more frequently than the boiler the materials
of construction may be special to resist corrosion. Com-
bustion Engineering developed cast iron muffs, cast
pieces that look like finned tubes pressed over the steel
tubes, which provided a corrosion resistant covering for
excellent performance on coal and heavy fuel oil fired
boilers because the iron conducted heat well in addition
to resisting corrosion. Modern metallurgy has created
materials that permit construction of economizers that
can withstand very corrosive conditions permitting
closer operation to the flue gas acid dewpoints without
concern for serious damage.
In addition to corrosion economizers can have
problems with soot accumulation, occasional plugging
with unburned fuel, and unique situations (Figure 10-
41) with waste fuels. The tube in the top of the picture Figure 10-41. Plugged economizer, firing waste fuel
is the soot blower, you can see the coated fins in the
bottom of the picture. Soot and dirt manage to build temperature to reduce potential for corrosion in the
up between the fins of finned economizers to almost economizer and reduces the required size of later stages
completely block heat transfer. Even if there’s no rea- of the turbine for overall cost savings.
son to believe you will have problems of blockage an The best economizer arrangement (and also the
economizer should be supplied with a means to clean most expensive) is where the flue gases flow down
it or provisions to install them in the future. The com- through the economizer. For counterflow the feedwater
mon in-service method of cleaning is using soot blowers flows up through and that prevents problems with strat-
but they are ineffective when the deposits forms a hard ification and thermal siphoning. A turning box under
gelatinous mass so there should also be means to gain the economizer can also serve as a drain pan for wash
access to the economizer to clean it with water washing. water used to clean the economizer. However, those
Some economizer applications (like the one in Fig- installations introduce a hazard when the boiler is idle
ure 10-41) require regular cleaning, a tough and dirty job because any natural gas or other fuel vapors which are
for the boiler operators. The savings in fuel makes the lighter than air and get into the setting can accumulate
effort worth it. because the boiler and economizer arrangement forms a
Gas fired operations produce flue gas with very trap to hold them. I prefer to install an access door in the
low acid dewpoints so you can operate a deaerator sup- top of the ductwork between the economizer and boiler
plying economizers or low pressure boilers at lower to vent it prior to entering the setting for inspection or
feedwater temperatures when firing gas than when fir- maintenance.
ing oil or other fuels with higher carbon and sulfur con- I hate to call an economizer a heat trap because
tent. If gas is the primary fuel you can adjust (slowly) it’s so much more than that. In addition to capturing
deaerator pressures to raise the feedwater temperature heat that would be lost it provides additional heating
when firing fuels with a higher acid dewpoint and lower surface for transferring the energy that’s in the fuel into
it for firing gas. the steam and water. Adding an economizer to an exist-
An alternative to that, found in plants with auxilia- ing high pressure boiler installation can also increase
ry turbines designed for low exhaust pressures is to use the capacity of the plant because heat that was absorbed
a steam heated feedwater heater between the deaerator through the boiler surface to raise the feedwater tem-
and economizers to raise the feedwater temperature to perature is now used to generate more steam. I’ve seen
the point that corrosion will not occur when firing high capacities increased by as much as 8%. Of course the
sulfur fuels. Power generating plants normally use feed- fan has to be replaced to overcome the pressure drop
water heaters to condense some of the turbine steam and through the economizer or that added capacity is lost.
raise the feedwater temperature. It raises the feedwater Economizers require some attention when starting