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AC> Modern central stations have "economizers"- huge rotating
rooms about 40
AC> feet diameter. The exhaust gases thus preheat the incoming
combustion
Not to be picky, but, economisers are horizontal banks of tubes
installed in the stack breaching to preheat the water coming
from the condensers, going to the feedwater heater. 
What you describe is an air heater, which is a wheel with sheets
of wavy metal in a pie sections that get heated from the flue
gas (after the economizer) and as it turns heats the intake air
from the forced air fans. The intake and outlet breeching is
usually over and under each other with seals of the pie sections
of the wheel to keep them isolated.
There is also a tube type air heater which is used in smaller
power plants.
AC> air. Don't ask me to describe the design. The design object
is, recover
AC> lotsa heat without requiring more fan power. These furnaces
usually have
AC> Forced draft fans to blow the combustion air into the flame,
and Induced
AC> draft fans to suck the burned gases out again. Any gadgets
along the way
AC> [such as precipitators, flue gas washers] demand more fan HP
thus more
AC> fuel consumed.
Very true. ....Ken
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