I took a crack at analyzing the freeze problem.
I assumed 75% overall efficiency and 50% for each core
and worked the problem backwards. I assumed a lot
of model parameters I can't really justify. would be interesting
to see some measurements on a prototype.
Started with 70F inside temp and 20F outside temp.
I assumed that the input air can never condense and freeze.
This analysis shows that the output temp at C is always
above freezing.
If the inside humidity is lower than 50%, you don't get much
condensation in the left-hand core.
You really would like
the temperature at F to be low enough to condense
as much water as possible in the core that can't freeze.
But that's the opposite of what you want for best efficiency.
Problem is that the temperature is not uniform across the
width of the core.
In this case, the far right end of the core is gonna be at 20F
and will freeze. That restricts the hot air flow so the
frozen area will spread.
The 'hot' area of the output stream will be above freezing
and is all wasted heat. If it could be rerouted to the cold
intake area, you could trade some freeze resistance for
recycling some of the stale air.
Wonder if you couldn't add a fan to blow some air
from the output plenum back into the coldest part of the
output port
to keep the lowest temperature above freezing and not
waste any energy?
As the efficiency increases, the temperature spread
decreases and the problem gets worse fast.
My point is that even if you have to add heat, you can heat
only the area marked 'cold' and probably don't have to heat
the air above freezing.
All you need is that the output temperature
stays above freezing at all points in the core.
The whole thing is very sensitive to your actual efficiency
numbers and the details of the climate.
I have no idea how to analyze it. The thought of two-dimensional
integral equations makes my brain freeze.
Did I mention that I really like counter flow heat exchangers
where you can assume constant temperature across the
width so the math becomes liner and one dimensional?
I'm continuing to ramble...better get some sleep...
Feedback???
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