TM 5-810-15
(3) Another factor to be considered in super-
firing lignite or subbituminous coal or any coal that
heater design is the flue gas velocity over the out-
has an inherent moisture content greater than 15
side of these tubes. This flue gas velocity should
percent. Air heaters are normally not used on
normally be in the range of 55 to 60 feet per second
stoker fired boilers where the coal being burned has
(fps) in order to avoid external tube erosion from
a caking tendency. Coal of the caking type is
the fly ash particles entrained in this gas.
normally associated with areas such as the midwest.
(4) Another factor to be considered for units
Air heaters are required on pulverized coal and
with superheaters is the outlet steam piping and the
ACFB fired boilers to heat the combustion air and
shutoff valves. As the temperature approaches 700
the primary air. The use of either a recuperative or
degrees F at the superheater outlet, the steam
regenerative type air heater may be determined by
piping material can be either SA-106B or 106C.
space requirements, desired exit gas temperature,
This material has an allowable limit of 800 degrees
maintenance and related costs and other factors
F and a maximum allowable stress of 10,800 and
involved in a LCCA. The air heater reclaims some
12,000 pounds per square inch (psi), respectively.
of the heat energy in the escaping flue gas and adds
The higher the steam temperature, the better and
that heat to the air required for the combustion of
more expensive the steam piping material should
the fuel. This not only decreases the heat loss to the
be.
stack, but also decreases the excess air required at
b. Air heater. Either recuperative (tubular) or
the burner. Each 100 degrees F increase in air
regenerative (rotating plate type) air heaters may be
temperature represents an increase of about 2
used to heat combustion air on stoker fired boilers
percent in boiler unit efficiency.
Example:
Input--MB/hr
Effcy.
Load
Fuel Cost
Fuel Cost
Increase
Factor
Dollars
Savings
x Percent
x Hours/year
x Per MB
Per Year
Air Heater example*
140
x 0.10
x 7,000
x 2.00
= 6,000
*Example assumes 100,000 lb/hr capacity, 140 MB/hr full load input, 10 percent efficiency increase with air heater, fuel cost
.00/MB, and MB = Million Btu*s. Refer to figure 3-14 for fuel savings that can be realized by preheating combustion air.
Regenerative type air heaters are suitable for a lower exit gas temperature dependent upon the dewpoint of the fuel to be burned.
However, they are more susceptible to pluggage and the probability of requiring water washing should be built into the design of the
unit. On the other hand, tubular heaters are also difficult to keep clean and in order to prevent excessive maintenance costs, retubing
or manual cleaning, usually an exit gas temperature in the order of 300 to 350 degrees F is preferred.
c. Steam coil air preheaters. Steam coil air pre-
(1) The source of the steam is normally low
heaters are used to heat air entering the air heater,
pressure, 100 psig or less, and it is frequently the
recuperative or regenerative type, in order to raise
exhaust steam from some other piece of equipment
the average cold end temperature to prevent acid
such as a steam turbine drive or other process that
dewpoint corrosion. This type of equipment is
exhausts steam at a pressure of 15 psig or higher.
normally incorporated into the design of a boiler
(2) When justified by a LCCA, the steam
unit for low load operation and startup operation
coil preheater drains may be individually piped to
particularly in those areas with low ambient air
another receiver tank from which the condensate
temperatures. They are desirable in that the main
can be recovered. Drains to this receiver tank may
air heaters, recuperative or regenerative, have
need to be heat traced to prevent freezing. If a
corrosion sections that are more readily maintained.
receiver tank is not justified by the LCCA, the air
This type of air heater uses extended surface,
preheater drains will be piped to wastewater drains.
normally referred to as fins, to reduce the overall
The piping arrangement should conform to the
steam coil manufacturer*s recommendations.
size of this air preheater. The air pressure drop
through the steam coil air preheater is generally
However, it should be noted that this is treated
limited to about 1 inch of water. It is generally
water and the cost for treating should be included
located in the duct between the FD fan and the
in the cost analysis.
d. Economizers. Only bare tube economizers
main air heater. However, in those areas that have
extremely low ambient air temperatures, it is not
should be used on any coal fired boiler. Finned tube
uncommon to have an air preheater ahead of the
(extended surface) economizers should not be used
FD fan that could preheat cold winter air up to
on coal fired boilers as they are more susceptible to
about 40 degrees F.
both pluggage and corrosion when used in
3-21
Previous Page
