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Old 02-01-13, 11:06 AM   #391
AC_Hacker
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Quote:
Originally Posted by ham789 View Post
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.
If this was a single cross flow core, it wouldn't make any difference.

But with multiple cores, you are correct.

Best,

-AC

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Old 02-01-13, 04:23 PM   #392
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Ham789, Thanks for the precious comments.
I saw many HRV with similar ports positions, some of them do this to use a single motor instead of two, I wasn't aware they'd loose that much efficiency that way.If I got this right it would be enough switching positions between the motor n12 and the plenum n.11, right?
You are also right about the freezing issues but as I said that's why I'll use the pre heating exchanger. If only I could think of a system that regulates the heat only to prevent hxs from freezing, something like an electronic valve attached to Arduino... But that's as far as I can think for now.

@piwoslaw: thanx for cleaning up my post's mess
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Old 02-01-13, 04:45 PM   #393
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Originally Posted by kostas View Post
6. Post heating exchanger (as n.3 but with more power, like 2Kw or so)
I think 2kW or so is too much.

Asuming 70 m^3/h and 80% efficiency and inside temperature of 20C, fresh air 0C you need to heat the air something like 4C (7F). (You intend to preheat the incoming air, I asume the lowest setting of fanspeed and low pressuredrop.)

70 m^3 of air is about 70 *1.2 = 84kg.
The specific heat of air is around 1,0 kJ/(kg*K) (ignoring moisture here)

Now to heat 1 hour worth of airexchange you need roughly:
84kg/h * 4K * 1,0kJ/(kg*K) = 336kJ/h.
This translates to 336000J/3600s = 93J/s or 0,1kW of heating capacity.

You are just heating up 20 liters per second of air a few degrees, not an entire room at once.

And indeed swapping #11 and #12 will fix it. You always want the two warm flows on one end and the two cold flows on the other end.
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Old 02-01-13, 05:15 PM   #394
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Yes, to be honest I assumed 2Kw simply because I can easily find one of these from a friend (plumber). I understand that heating it with 200% more power won't make my HRV a central heating system, right?
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Old 02-02-13, 03:47 AM   #395
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In theory nothing is holding you back to use the system for heating your entire house.

Especially when your central heating is already using low temperature water (40C instead of 80C) it would be doable. the 2kW element doesn't need to output 2kW, it is just capable of doing so in certain conditions.

In your current design you intend to heat all the air leaving the HRV. So the same temperature air will go to the bedrooms but most people like it a bit cooler there than in the livingroom. Also you don't need to heat those rooms all day when nobody is there.

You could place the heating element in the ducts toward your livingroom/kitchen and just ventilate the rest of the house with cooler air.

Would this be the only source of heat or is it intended to assist your main heating?
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Old 02-02-13, 04:49 AM   #396
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I have a traditional heating system, it puts out 70C hot water. I certainly cannot replace it by my HRV but it could be complementary. But I guess that the air volume handled is too low to work as a real heating system, usually that kind of systems move a lot of air to heat a whole house. Your suggestion to separate the heated flows is valid, though
I must see how this would change the whole design of the unit as the space is not that big down there.
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Old 02-02-13, 12:19 PM   #397
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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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Old 02-28-13, 10:46 AM   #398
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I will say that I like counter flow design as well .

Won't be able to finish my project any time soon, but I keep thinking about the design. I thought to go for an upscaled version of Ham789's counter-current core. Scaled up to measure something like 1.6m in length, with 30 or so frames of 3.5 mm thick (times two counting the framed space between two plates). At least that's the length and thickness of the plates I have been able to buy cheap. I like to keep the width of the frames limited (say 30cm) and make that up by making lots of plates. That's more work, but I would otherwise be afraid that the air would search the shortest route between the plates and some parts would hardly get any air.

A choice that keeps me awake at night is whether the benefit of having both fans on the "hot" side of the core makes up for the inconvenience that for the fan blowing air into the core is "centralised", I you catch my drift, blowing very strongly into a narrow area. In stead, I could have both fans sucking air out of the core, which would distribute the pressure more evenly... but then I have to place on of them on the cold humid side, which the fan won't like too much. Am I exagerating these concerns? Perhaps that is simply a disadvantage of axial fans which I did not consider. I am installing this in a close space, there is no way to get the fan installed further away from the core.

I am still tempted to glue up this entire core up and just take it out and hose it in the shower with some soap, every other year or so. As i was looking for glue, I ran accross this which I wanted to share with you as it seems more affordable than lots of glue I see.

technicoll® 9110 - Ihr Online-Shop für technische Klebstoffe, Kleber, Dichtstoffe, Schraubensicherung und mehr
(fluid, for glueing surfaces)

technicoll® 9310 Heißklebesticks - Ihr Online-Shop für technische Klebstoffe, Kleber, Dichtstoffe, Schraubensicherung und mehr
(for heatgun)
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Old 03-01-13, 11:04 AM   #399
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Quote:
Originally Posted by pladijs View Post
...A choice that keeps me awake at night is whether the benefit of having both fans on the "hot" side of the core makes up for the inconvenience that for the fan blowing air into the core is "centralised", I you catch my drift, blowing very strongly into a narrow area. In stead, I could have both fans sucking air out of the core, which would distribute the pressure more evenly... but then I have to place on of them on the cold humid side, which the fan won't like too much. Am I exagerating these concerns? Perhaps that is simply a disadvantage of axial fans which I did not consider. I am installing this in a close space, there is no way to get the fan installed further away from the core....
Regarding your concern for fans, here are some links to a kind of centrifugal fan, similar to the type that I have acquired for the HRV that I am currently assembling parts for.

Link #1

Link #2

Axial computer fans are a wonderful bargain because they are manufactured in such huge quantities. These are more expensive that axial computer fans, but they are quite efficient. The feature that they have that might interest you is that the air-stream is diffuse and exits in a 360 degree pattern, so your HX cores would have an even and non-directed air flow.

To use the fan, a well-fitted round shroud will need to be fabricated and the fan mounted close to, but not touching the shroud.

I searched for a long time to find these fans at a reasonable cost. You might have a better chance to find them in Europe. You might be able to find these being used in large computer installations, so look to computer scrap.

Best,

-AC
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Old 03-24-13, 06:39 AM   #400
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Quote:
Originally Posted by kostas View Post
I was working on the operational part of my DIYHRV.
....
What do you guys think?
Hi, I am also very interested in home made ​​heat exchangers.

Due to the fact that you are using Sketchup, are you know about - Khamsin plugin (shortlink to website - goo.gl/1Aqvo ) for simulating Computational Fluid Dynamics (CFD)?


Last edited by tapa; 03-24-13 at 06:44 AM..
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