MIL-HDBK-1003/6
d)
Temperature and pressure indicators on flue gas outlet.
e)
Temperature and pressure indicators on flue gas inlet.
f)
Manual shutoff and bypass piping control valving for low load
operation.
g)
Low point drain on boiler feedwater with drain valves.
h)
Means to wash down the economizer when the unit is taken out of
service.
i)
Drainage for washdown water in gas ducts.
j)
Access doors for cleaning and observation.
k) Feedwater heater for startup or low load operation. Feedwater
coils can be located in the mud drum for preheating water at low loads.
l)
Flyash hoppers.
8.6
Air Heaters
8.6.1
General. Air heaters are installed to recover waste heat leaving
the boiler or economizer and add the heat to incoming combustion air.
Application. Condensing type air preheaters should be used if
8.6.2
economically justifiable, otherwise the minimum flue gas temperature exiting
the heater must be controlled so that the metal temperatures are kept above
the dew points of the gases. See Figure 37 for minimum metal temperatures for
heaters. Depending on the fuel used and method of firing the nominal lower
limit for flue gas would be between 300 to 350 degrees F (149 to 177
degrees C). The higher limit of the preheated air may range up to 600
degrees F (315 degrees C). In cases of high steam pressure and/or high flue
gas temperature, sufficient heat may remain in the flue gas after an
economizer to warrant both the installation of an economizer and an air
heater. In all cases however, the high and low limits as well as the overall
plant heat balance must be taken into consideration.
Design Considerations. The size of air heater to install is based
8.6.3
on engineering and economic considerations. The increase in efficiency is
balanced against the original cost of the air heater, the cost of additional
fan power to overcome increased draft losses, and operating and maintenance
costs of the fans. The efficiency increases approximately 1 percent for a 40
degrees F (22.2 degrees C) decrease in outlet gas temperature or for a 50
degrees F (27.8 degrees C) increase in combustion air inlet temperature. An
air heater not only increases the efficiency of the boiler at all loads, it
also permits a reduction in the physical size of the boiler.
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