Page 248 - Boiler plant and distribution system optimization manual

Waste Heat Recovery 233

conditions. The outlet water temperature Table 12.3.
may be the dominant limitation in selecting Typical design parameters for a
economizer size. conventional economizer.
————————————————————————
Maximum
g. Acid formation and condensation on the gas
Minimum Exit Flue Fin
side of the heating surface is determined
Fuel Inlet Water Temperature Density
solely by the temperature of the surface
Type Temperature (°F) (Fins/In)
which is essentially the same as the water ————————————————————————
temperature. Natural
Gas 210 300 5
h. A minimum gas temperature of 250°F in the
stack is desirable. Basically, this is to assure No. 2
that the flue gas will be sufficiently buoyant Fuel Oil 220 325 4
to escape into the atmosphere and not mush-
room around the stack and cause smoke or No 5 & 6
acid rain nuisance. Fuel Oil 240 350 3

Coal 240 350 2
COLD-END CORROSION IN ECONOMIzERS ————————————————————————

The major portion of sulfur in fuel is burned feedwater preheat system. The system is illustrat-
and forms sulfur dioxide (SO ) in the flue gas; a ed in Figure 12.6. It is essentially a feedwater pre-
2
small portion, 3 to 5 percent, is further oxidized heater with sensors controlling the steam admis-
to sulfur trioxide (SO ). These oxides combine sion valves to the heater.
3
with moisture to form sulfurous (H SO ) and sul- The steam admission valve sensors measure
3
2
furic acid (H SO ) vapors (Figure 12.2). When in the water temperature entering the economizer
4
2
contact with a surface below the Acid Dew Point and the temperature of the stack metal. Both of
(ADP), condensation takes place. The ADP is di- these surfaces are subject to corrosion from tem-
rectly related to the amount of sulfur in the fuel as perature exposure below the ADP. Using the heat-
shown in Figure 12.3. er insures neither surface will cool to that point.
Because of the higher heat transfer coeffi- As an additional protection, the portion of the
cient in liquid-metal than in gas-metal heat trans- stack exposed to very cold weather conditions
fer, the gas side metal temperature of an econo- could be insulated to keep the metal temperature
mizer is closer to the water temperature than the from becoming too cold, approaching the ADP.
gas side temperature.
Design parameters for a conventional econ- Reduce Excess Air
omizer are shown in Table 12.3, illustrating the Reducing excess air raises the dew point
minimum recommended feedwater temperature temperature in the stack gas (Figure 12.7). Re-
and flue gas exit temperatures. search has shown a direct relationship between
excess air and the formation of sulfur trioxide.

CONTROLLING ACID FORMATION Use Corrosion-resistant Materials or Sleeves
Corrosion-resistant alloy steels can be used
Controlling Economizer Inlet in preventing corrosion, however their high cost
Temperature normally prohibits their actual use. Interlocking
The most efficient and effective means of con- cast iron sleeves over carbon steel tubes can also
trolling economizer metal temperature is with a be used where severe acid conditions are antici-