On Thu, 27 Nov 2003 13:45:51 -0000, "Christian McArdle"
wrote:

The major heat recovery in terms of delivering heat to the heat
exchanger water is from the change of phase of water from gas to
liquid - latent heat of condensation.

Maybe I didn't explain very well.

The water condenses out of the gases because the gas temperature has reduced
to the extent that the air is fully saturated. As the gas temperature
reduces, it can contain less water, so the water condenses out onto the heat
exchanger, raising the temperature of the heat exchanger as it does so.

If the coldest part of the heat exchanger (which is the same as the return
temp) is 60C, the gas is capable of containing far more water than if it was
30C. Therefore, less water comes out for condensing, and the heat exchanger
benefits from less boost.

This is completely different from simply the energy lost by expelling higher
temperature air. You would even gain this energy advantage even if you
subsequently reheated the air to the same temperature. The difference would
then be that the exhaust gas would have lower humidity, water would go down
the drain and the system would be more efficient, but obviously less
efficient that had you not reheated the exhaust.

Although the plume starts happening when the boiler gets cool enough to
condense, if the return temp is very low, then the pluming may reduce again,
simply because the heat exchanger becomes efficient enough to remove most of
the water from the gases, which then don't have much further in temperature
to fall.

Christian.


Agreed. I rather suspect that the contact time and area for the
gases with the heat exchanger has an impact as well.....




..andy

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