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Posted by Bob D. on June 25, 2012 at 09:33:06:
In Reply to: Boiler posted by Mark on June 19, 2012 at 20:29:43:
In the overwhelming majority of cases, it isn't; and it's usually a bad idea to try (excepting very well managed and designed utility sets). The concept probably evolves from the "good idea" of pre-heating combustion air. Pre-heating combustion air using a source that's usually going to be thrown away (flue gas, or hot air from the upper clerestory of old plants, etc.) salvages some BTUs that have to be added to heat up the nitrogen (80% of combustion air) that passes through the furnace pretty much unchanged, since it doesn't contribute to the combustion process (let's ignore NOx for the moment, which doesn't really "contribute" anyway). As a counter-point, in gas turbines, when a "peak" power boost is needed, water is injected into the process to cool (increase density) of the air constituent: more air allows more fuel (at the same ratio) to get more power out. Then again, I could be wrong, again.
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