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Old 01-27-13, 05:08 PM   #371
pladijs
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Ham, Kostas, your posts will greatly help me with the construction of my unit, many thanks for your input...

Ham, you are very right about the 80-20 rule. But our goals were quite different. Mine was "get rid of peaks of humidity", due to no proper kitchen hood, no bathroom exaust, and clothes drying inside in winter time. All of that in a tiny appartment; not good. Given my inefficient heat exchanger that required some simple humidity based fan control. I does not make sense to have that unit on full power 24/7 and i dont want to switch it on and off every time i cook something or take a shower. Now adding 3 more sensors and hooking it up to wifi, that was just for fun. But it will allow me collect observations on hard-to-precict relationship between, say, fan speeds and efficiency, which might indeed deviate strongly in practice from what one might expect.

But now i decided to go for a larger unit after all... I think my control then will become as simple as "run at some fixed speed, unless RH<30 (switch to min) or RH>60 (switch to max)". Confession, I will add a mode for summertime, and an antri-freezup cycle when its gets really cold). Further confession: perhaps I will add a physical switch giving a choice between "auto" (the above rules), min (holidays), max (party mode, or badly burned food alert). My internal nerd is really taking over lately, and he looks forward bigtime to estimating the optimal fixed speeds of both fans using the collected data through running some regression analysis.

I won't pre or post heat, that question was just out of interest; why one would want to pre-heat rather than post-heat.

For filtration, I went for oversizing to limit the pressure drop. Those pocket filters have square square meters of filter surface. Their advertised pressure drops are very small at the flows our units are running at. They are a bit expensive; but I'd rather pay for a more expensive larger filter (giving clean air) rather than paying for electicity to push air through a smaller filter. I also live in a city, some quite busy roads near the house, I want to keep fine particles out.

I really want to avoid going for an oversized and possibly inefficient fan; but I'd need a ballpark figure of how much pressure there is to overcome. For the ducts, I can calculate a guesstimate using for example http://storedproducts.okstate.edu/Ae...Duct_Loss.aspx or similar tools

For a flow of 60CFM, for my immaculate straight tubing of about 16inch tubes with 0.11m(4.33 inch) diameter, that tool estimates 7Pa of pressure drop. Double that for the bends going into and out of the HRV itself. For my ridiculously oversized filter, I expect 5-10Pa at the max. The big unknown is the pressure drop to expect from a HR-unit like Kosta's. 10Pa, 20Pa, 50Pa, 200Pa? Beats me... But it hugely affects the choice of fan, obviously. I would really love to hear some of your insights on this before deciding on a fan; I am still a bit puzzled on the size of some of the fans you are using. Is it simply due to the fact that you have more air to push around, and some more relevant lenght of ducts?

I thought to go for a double "DISTANCED PP CORE", but I am surprised as well that it comes out so badly out of your tests, Kostas. Is this related to the flow resistance being much lower in one direction in that setup, and you happened to be sending the cold air through that direction in your tests? I am going to have a close look at all of your pictures and steal a lot of your ideas!

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Old 01-27-13, 05:11 PM   #372
kostas
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Quote:
Originally Posted by ham789 View Post
Very nice work.
Which is the Solid PP core and which is distanced????
Maybe I did not explained it properly.
The Solid PP is the one with no distance between the sheets.
You can see it on this picture.

The distanced is the one with the PP sticks glued on both sides and center (the first pictures I posted).


Quote:
How are you measuring efficiency?
I used the following formula:

Efficiency (%)= (T4-T1) / (T3-T1)x100 where,

T1 is the external fresh air coming into the HX
T3 is the internal stale air coming into the HX (extracted from the house)
T4 is the internal fresh air getting out of the HX (pulled into the house)

Now, the first time I made the measurements I got as follows:

Aluminum core: 83,7%
PP solid core: 88,2%
PP distanced core: 78%


I then tried the dual core solution excluding the PP distanced core:

Aluminium dual core: 97,6%
PP solid dual core: 99,2%
Hybrid (aluminium - PP solid) dual core: 87%


But, I repeat, I had really small fans that means that air moves slowly into the HXs and it has more time to exchange heat.

Calculating the temperatures of my previous post using the ultimate version of the HRV (aluminum and PP solid dual core, at least double size than previous but with 220V fans) I have 81,1% efficiency.

Quote:
Dual core rates quite the same, but allows much more air to pass trough.
The last dual core photo was a combination of PP and aluminum cores, same results (but that was quite obvious..)

I'm confused by that paragraph.
Two equal cores in series should have twice the resistance
to air flow. If e is the efficiency of one core, I'd expect
n cores in series to be something like 1-(1-e)^n...but it's
too early in the morning to try to prove that.

What am I missing?
Maybe I was wrong about that last statement.
The numbers above prove that the dual core configuration has better performance as it offers double exchanging surface.
The resistance to air flow can be solved increasing the fan speed, but more resistance means also more turbulence, hence more heat exchange.
At least that's what I understand by observing the data.
I'm going to do some more measurements these days and try to be more precise.
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Old 01-27-13, 06:49 PM   #373
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Ok, I'm still a little confused. Sounds like the solid PP
is identical sheets of coroplast rotated 90 degrees.
There are two problems with that. First, the thickness
of the conductive path is double. But, I think an even
bigger problem is the bad contact between the sheets
over their full area. Air in the middle is a killer.

Efficiency (%)= (T4-T1) / (T3-T1)x100
My formula works out to be
100x(T4-T1) / (T3-T1) which is the same as yours
with an additional parenthesis pair.

I think your efficiency measurements are way optimistic
for that configuration. Having very low air flow certainly
contributes, but even so...

I have myself convinced that the dual-core cross flow
exchanger is more efficient, but not because of more area.
Yes, for imperfect materials, more area helps, but the
theory suggests that a dual core of the same effective
surface area as a single core will still be significantly
more efficient...at the cost of higher back pressure for the
same flow.

I gave up on any configuration of cross flow and went
straight for the counterflow. It's easier to construct
and has potentially better performance.

I'll post some pictures.
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Old 01-27-13, 07:57 PM   #374
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This is my current configuration.
I slid out the end fan, raised the top fan slightly
and slid out the core to give some sense of the
construction.

It all fits together like a Chinese jigsaw puzzle.
The plastic is either press-fit or stitch welded.
Never found any affordable glue for coroplast.
The outer shell is one piece of coroplast, origami-style.

The only tape on the system seals the holes
in the end of the coroplast shell. And duct tape
holds the filter material on the outside end.

I wouldn't even need the tie-wrap if I hadn't
accidentally cut the plastic where I shouldn't have.

This one is 4" square and two feet long. Meets my needs.
No reason it couldn't be arbitrarily large.
I poked it thru a hole in the window because it was easy.
And it saved another right angle bend in the air flow.
I thought about sizing it to fit a square-to-round furnace
duct adapter, but I'm too cheap to spend $7 each
on them.

I keep buying fans and duct work when I find 'em cheap
at garage sales, but I just can't get past "if it works,
don't fix it."

Right now, it's 42F outside, 66F inside and I'm losing
5.9F across the heat exchanger. Meets my 80-20
criteria.

I live alone and don't cook much. Most people
would want more air flow, but this should scale nicely.
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Old 01-27-13, 10:06 PM   #375
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There's a lot of engineering data here
Klingenburg GmbH - Energy Recovery - Counterflow plate heat exchanger
that might address the pressure drop questions.
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Old 01-28-13, 01:03 AM   #376
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Thanks a lot, really, I will surely have a look at it, but I have some urgent work to do the next few days/weeks so you wont be hearing from me.

I was just trying out this software, which claims to allow to calculate pressure drops specifically for their heat exchangers. I should hope that if i pick on of their models with similar dimensions, it should match with an error of say 50%.

Software Tools - Heatex

I'm still waiting for my download link. clicking "download (0)" on this suspicious looking site worked as well, however. HEATEX Select 4.9 Download Freeware by Heatex AB

I must say I am kind of hesitating now between cross and counter flow. My initial plan with the alu-frames was quite similar to the one of Fornax, so essentially counter-flow. As that turned into a big mess with the alu-foil, I gave up on that; but I always liked the concept of counter flow, given the clear advantages. If I build a counter-flow one, I will probably recycle the "box" I have already finished. It's quite big, 0.42mX0.8mX0.4m (or so, this is by heart), that would give a huge surface.

But one of the reasons I am a bit afraid of going for a counter flow version is that it would be harder to clean, having to take out all these 0.42mX0.8m frames becomes a mess, how do you handle them, how do you clean them; quite a bit of work. I guess if one lets the fresh air flow between the plates (and not in the small channels), one can more easily clean the most important surface. But still, considering the smaller cross-flow cores, if you would glue them up completely, it seems to me you could simply pop pop those into a dishwasher once a year or so and have it done with. Seems like a major advantage. In that sense your model might not be so scaleable, unless i'm missing something. But in all honesty, I doubt that when using some minimal filtering, it would be a serious health risk not clean the core for a year or 2, no? Please tell me if that is a crazy statement.

Kostas, hope you are reading this, is it me, or is there a difference in the height of your cores in the inital pictures you sent and the latest ones? seems like they shrunk? is there a reason for that? Could just be my eyes of course.

I am slowly shifting to somewhat larger fans, now also looking at this one
DC axial compact fan - 4412 /2HHP by ebm-papst

For 13 Watts, that seems quite powerfull with 60CFM@100Pa. (But I realise 60CFM might be a joke to many here).
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Old 01-28-13, 03:30 AM   #377
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I think cleaning is a good idea, but don't think you need to go
overboard.
I had a window A/C for 40 years and never cleaned the coils/fins etc.
Every summer evening, I open the windows and suck 3500CFM of
unfiltered air into the house. I don't get too worried about a few
extra spores coming in thru the HRV.

Since my coroplast is welded together, it ain't coming apart.
I just stick it in a bucket of dilute bleach and hoze it out.
That's one advantage of having a small core that just slides out.
And if I pop out the fans, I can dunk the whole shebang in the bucket.

I live in about 8000 cubic feet of my house. 60CFM would give
an air change in just over 2 hours, plus the other leakage. That's
plenty for me.

If you have 3 kids, six cats and glue plastic model airplanes for a hobby,
you'll want a lot more.

I have one of those blowers that are used for blow-up bouncy-castle
things. And a smaller centrifugal blower with a 6" output port
and stalls at 250Pa. And I saved the old furnace blower that can do
1000CFM. All my experiments have convinced me that I want the
back pressure to be as low as possible. It costs a lot of energy/money
to move air at high pressure.

Keep the pressure low and the volume low. Don't make the air change
direction any more than you have to.

If you fart, open a window.
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Old 01-28-13, 03:54 PM   #378
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Quote:
Originally Posted by ham789 View Post
I think your efficiency measurements are way optimistic for that configuration. Having very low air flow certainly
contributes, but even so...
No, it's not. It's real data from real measurements with real instruments. For instance here are the temperatures of tonight with my current configuration (the finished one):
Ext T: 3C
Internal stale air: 21,5 C
Fresh air inlet from HRV: 18,9 C
Unless my math is wrong this gives me a 85,95% efficiency. Note that I no longer have small pc fans but (at least) 180m3/h 220V ones. I could definitely declare the system as "efficient"!
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Old 01-28-13, 06:54 PM   #379
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Didn't mean to upset you...
It's hard to argue with numbers. Sounds like yours is working
fine.

I was reacting to:
Now, the first time I made the measurements I got as follows:

Aluminum core: 83,7%
PP solid core: 88,2%
PP distanced core: 78%

I then tried the dual core solution excluding the PP distanced core:

Aluminium dual core: 97,6%
PP solid dual core: 99,2%
Hybrid (aluminium - PP solid) dual core: 87%

I wouldn't argue with an 86% number for a dual-core
cross-flow system.

I was skeptical about 99.2%
I wouldn't even represent that I could measure three
air temperatures, do the math and expect 0.8%
accuracy. It took some doing to select two thermocouples
that gave the same reading on two ports of the same meter.
We're subtracting large numbers with small differences.
A degree of error makes a lot of difference.

I think it would be educational if you posted the six
numbers for your HRV. There are 4 in/out ports.
And two chambers between the cores. Would be interesting
to know the inside dewpoint and how much water is coming out of each core, at least
qualitatively. I theorize that getting the water out early
reduces the freeze-up problem in cold climates.
That should also correlate with the efficiency measurements
on each core. The one doing most of the condensation
oughta be more efficient. We need those
two internal temperatures to calculate all that.

What did you use to measure air flow?
I looked at the manual for the propeller gauge you mentioned.
This is one of the times I wish I were multi-lingual.
I've been using a hot-wire anemometer. The big thermistor
is about the size of a gnat. The small one is...well...
I think I can see something there....
I easily get 50% variation in the air flow depending
on where I stick the probe. I experimented with
long tubes with air straighteners, but didn't make much
progress. Closest thing I made was a quart-size
yogurt tub with a big hole cut in the end. Gives more repeatable
results, because I have a stable air flow number
and a defined area, but there's back pressure that skews the measurement.

Somewhere in the attic, I have an ancient propeller
with a turns counter on it. You measure the revolutions
over a fixed time period and calculate flow from there.
Measurements are more repeatable because there's a
zillion times larger area being sensed, but I'm not sure
that means it's accurate. I still need to come up with
an area number to calculate mass flow.
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Old 01-29-13, 07:39 AM   #380
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Ham789 no upset at all, I apologize if I gave that idea

You are right about the measurements, I have a fairly good semi-professional thermometer / hygrometer (analog, not IR) but getting the right values is quite a task. But anyway I do not pretend to be 1000% precise, a 10% or slightly more error threshold is acceptable for my needs.
The data you see there were taken last October. The weather was mild at the time and there wasn't a big difference between internal and external temperature, so I used a hair dryer as hot air inlet and ambient air as the stale air. Regarding the PP solid core I've had these values:

T3 (hair dryer): 49
T4 (HRV exit air): 45,7
T2 (hair dryer outlet): 38,5
T1: (ambient air inlet): 21

PP solid core efficiency: 88,2%



I do not have anything to measure the airflow yet, so I reported the fan's manufacturing values hoping that the're close to reality

Regarding the internal temperatures and dewpoint values I am planning to install 4 T/RH sensors cabled to an Arduino in order to log the report.
As far as I had already seen there was low condense inside, and that's because the exhausting stale air is quite dry (38% @21).
For instance, yesterday's measurements were:
External fresh air: 3 @ 87% RH
Internal stale air: 21 @ 38%RH
Fresh air from HRV inlet: 18,9 @ 31% RH
No condense in the chambers.

Hope that helps.

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