Page 258 - Boiler plant and distribution system optimization manual

Waste Heat Recovery 243

b. bypassing part of the inlet air around the Table 12.4—Cold-end air preheater temperature
heating surfaces. material selection guide.
———————————————————————
Incoming
c. using a recuperating steam coil in the air
Fuel Air Temp Material
duct upstream of the air preheater. Type Range Specification
———————————————————————
Oil 190-205°F Carbon steel components.
With high sulfur fuel, the expected life of an
air preheater is short. The strategy is to extract as Corrosion resistant low alloy steel
much energy as possible from the flue gas regard- for the cold-end element.
less of the corrosion potential and to specify ma-
Oil 155-190°F Corrosion resistant low alloy steel
terial to keep the problem to a minimum (Table intermediate element
12.4).
Enameled cold-end element.
Combustion Air Preheating
Low alloy steel for rotor and
Although preheating of combustion air is structural parts in the cold-end to
not generally recommended for smaller boilers, the same level as the enameled
it could be the only way to significantly improve elements.
boiler efficiency. As a retrofit, the cost may be
Oil Below 175°F Enameled intermediate element.
high because of bulky ductwork, and possibly a
long distance between the stack and air inlet. Enameled intermediate element.
Increasing combustion-air temperature
Corrosion resistant low allow
worsens emissions of NO , typically from 20 to steel rotor and supports to same
x
100 ppm for a 100°F rise. Reductions in SO , car- level as enameled elements.
x
bon monoxide and particulate emissions result Below 155°F
Bituminous
from improved combustion.
Coal Carbon steel components.
In addition to making use of waste heat from
flue gasses, lower excess air can be maintained Corrosion resistant low allow
owing to improved combustion at the high inlet steel cold-end element.
Below 150°F
air temperature. Higher combustion tempera-
Pulverized Carbon steel components
tures also increases heat transfer and reduces Anthracite
sooting. Corrosion resistant low allow steel
cold-end element.
Gas-to-air inefficient heat exchangers are very
Gas Fuel
in comparison to gas-to-water exchangers. This is Sulfur Free
due thermal characteristics comparison to air. ———————————————————————
In summary, unless corrosion resistant ma-
terials are used, combustion air entering the pre-
heater must be suitably high to avoid cold-end This system can be applied to transfer heat to
corrosion. A steam coil heater, on the cold side, to combustion air, process air, or building air. Since
raise the temperature of the incoming fresh air is the heat exchanger requires some temperature
recommended. differential to transfer heat to or from the inter-
mediate fluid, it is inherently less efficient than
Run-around Coil a direct exchange between two primary fluids.
The typical run-around coil system is com- However this system is relatively simple and
posed of two heat exchangers coupled together more compact than a direct air/fuel gas system. A
by the circulation of an intermediate fluid (Figure run-around system eliminates the problem of the
12.18). The circulating fluid is heated by the hot close proximity of exhaust and inlet ducts. It is
stream and then piped to the second heat exchang- able to transfer heat from one location to another
er where its heat is given up to a cold stream. without great retrofit costs.