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Energy Saver: Furnace, Boiler & Woodstove Combustion
(continued ...) The efficiency boost is obtained by very efficient burners, afterburners, skillful engineering and using flue gas phase change to extract an extra 10 to 15% of heat. After passing through the first heat exchanger, the hot fumes and vapors from combustion condense to a liquid simply by giving it the opportunity to do so. This is accomplished with a second heat exchanger to extract that heat released from the phase change (vapor to liquid). It is easy to describe the process but in practice it is very difficult to do. There are some very smart people out there and they have overcome these hurdles.
I mentioned phase change a few times now. Allow me to describe what that is. The various common phases of materials are solid, liquid and gaseous vapor. When a material changes phase it consumes or requires a huge amount of energy compared to other options of energy moving. This is not the magnitude of nuclear energy reaction nor some chemical energy reactions but large energy never-the-less. Melt is a word associated with solid to liquid. Evaporate is a word associated with liquid to vapor. You've seen this as water boiling in a pot on a stove *7. Condense is a word associated with vapor to liquid. Freeze or solidify is associated with liquid to solid. And sublimation is a word associated with solid to vapor. I like that word: s-u-b-l-i-m-a-t-i-o-n. You've seen sublimation in the winter time as snow disappearing while the temperature is below water's freezing point of 32 F (0 C). The snow sublimates to vapor and consumes heat energy in doing so. In general, vapor-to-liquid-to-solid moves heat energy (or it is moved) out of a material. And, solid-to-liquid-to-vapor moves heat energy (or it is moved) into a material. By controlling other things in the phase change process, one can harness heat energy.