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Old 02-01-13, 07:24 AM   #390
ham789
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Join Date: Dec 2011
Location: tigard, oregon
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Very nice work. What tools did you use to make the drawings?

I think you have a fundamental air flow problem.
You want the ports exposed to the outside to be
on the same end of the exchanger. If the air doesn't
flow in opposite directions, you're giving up the efficiency
improvement due to dual cores. You want the greatest
possible temperature difference everywhere in the system.
Then drive that temperature difference toward zero across
the board.

I'll say the same thing differently.
If you have a counter flow heat exchanger, you do just that.
With a long path, you can approach 100%.
If you reverse one air flow so they are in the same direction,
your output temperatures will be half way between the two
temps at the other end. You can't do any better than 50%.

On the outside end, you want the outgoing air to flow upward
through the wide-open channels so the water can condense
and drain back into the warm area instead of toward the
freezing end. This won't work so well if you put the air thru
smaller coroplast channels because of surface tension and you
don't have enough air pressure to blow the water uphill.
If all of your channels are small, you'll want the air to flow
downward to blow out the water...but I think that will have
more freezing issues. Would be interesting to experiment
with the plate spacing and surface tension to see what the
minimum effective spacing might be.

A counter-flow exchanger is intuitive because you can predict
the temperature at various places in the core.
For cross-flow, it's not so simple.
In the output end of the system, the coldest part of the core
is at the corner where the coldest incoming air is. Since there
is a differential in the average temperatures at the ports,
we know that the other corner has to be warmer.

The objective is to not freeze up. The warmer air exiting
is waste. And it's higher than the colder corner of the exchanger.
We need some way to get that heat transferred back into
the cold corner. That might be done with a temperature
controlled path from the hot part back into the cold part.
If you allow ALL the air to flow back, the system temperature
will rise to the interior air temperature. So, there's some amount of flow
that keeps the coldest corner of the core above freezing.
This also affects where you put the fans so the pressures
are right to make the air go where you want it.

If you live in an area with moderate cold, that's the simplest
and most efficient thing I can think of. You trade fresh air
for defrosting. Do the math to see if it works in your climate.

If you have to add heat, that's also the place to add it.
Any more heat than the amount to keep the output port
minimum temperature above freezing is wasted.
But it's worth a lot of careful thought to verify that.

I'd be interested in opposing views on optimum defrosting.
I really have little more than intuition to support my claims.
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