DIY ventilation heat exchanger

[Piwoslaw]

Originally Posted by AC_Hacker

As I recall, the increase in efficiency for the 180 degree counter flow was small (5% to 8%). The 90 degree design seems to dominate the market, my guess is that it is pretty efficient and easy to make.

For me, the lesson to be learned is that China is making the cross-flow cells and selling them to OEMs. If we can discover the source of the cells, we can build the box ourselves.

From what I read somewhere, the cross flow's efficiency is around 60%-70%, while the counter flow can get over 90%, so that may be worth fighting for. I think the differences in our sources come from different designs being tested.

I'm all for finding a source of cheap cross flow cells. Using 2 or 3 to make a counter flow set up (like in the picture in my second post) would give killer efficiency. I tried googling in both English and Polish for just the guts, but no luck, only "recuperators with alu cell". I'll ask at a local builder's forum.

Re heat pump: In my climate it's very hard to find an old A/C unit, air conditioning is just starting to be mainstream. Old refrigerators, now that's a different story. I can find one in any town, or even in any roadside ditch or forest:/ On the other hand, taking a fridge apart is illegal in Europe (environmental protection laws), so hush-hush

EDIT: I found this page on different types of heat exchangers. It's in Polish, but maybe Google Translate will help. It confirms that the cross flow exchanger's efficiency is 60%-70%, and adds that using 2 of them can be 95% efficient. Also, there is a spiral heat exchanger, which appears to be just aluminum foil rolled up into a spiral tube. The benefits are 85% efficiency and frost resistance (frost is the main enemy of cross- and counter-flow cells, requiring heating elements to keep the airflow from getting restricted).

[1blueox]

These are fascinating posts. Somebody asked about plans for 180 degree counter-flow exchangers so I thought I'd post.

I'm making a counter-flow air to air exchanger out of plywood and aluminum foil as an experiment. The design is very simple: imagine a piece of plywood as wide as the foil and as long as you want. Along the long edges are 1x2's to support the foil. Then the foil is laid on them. On top of the foil at its edges are another pair of 1x2's, with another sheet of plywood on top. Imagine folding the whole thing back on itself every couple of feet giving you two channels for the air, separated by foil (obviously you can't fold the plywood, that's just to give the concept). Add two small fans to move the air, some duct work at each end, and there you have it!

Those of you with far more experience than I have can improve on materials and dimensions, but if the foil is 18 inches wide and maybe 30 feet long, it should do a pretty good job. Humidity is not much of a factor where I live, but if condensation becomes a problem I'll clearly have to modify things a bit. This version is just for my own fun.

[AC_Hacker]

1bluebox,

Originally Posted by 1blueox

I'm making a counter-flow air to air exchanger out of plywood and aluminum foil as an experiment...

This sounds like a worthy project.

From my Internet research, it learned that there are two types of air energy recovery schemes. One uses non-porous membrane material, like aluminum foil, and the other uses a porous material that will allow condensing water to re-humidify incoming air. This might suggest other materials for you to try.

Also, it would seem to me that you'd want to go much thinner than 1x2 for your membrane separator. I'd think that membrane spacing on the order of 1/8 inch would be more in the ball park.

I also came across a DIY ERV discussion wherein someone was considering using a material like plastic food wrap for the membrane material. Seems like such a material would be cheap, easy to work with, quiet, and unaffected by condensation.

As my home insulation and infiltration project progresses, I'm beginning to notice that the controlled admission of new air is now required keep the comfort level at an acceptable level.

Regards,

-AC_Hacker

[Piwoslaw]

Originally Posted by AC_Hacker

If we can discover the source of the cells, we can build the box ourselves.

I found a (Swedish?) company that is a global supplier of heat exchangers:
Heatex AB

No idea of prices, but 'm sure that if we group-ordered a few hundred, then we could probably negotiate a deal

[AC_Hacker]

Originally Posted by Piwoslaw

I found a (Swedish?) company that is a global supplier of heat exchangers:
Heatex AB

Piwoslaw,

Don't know if you saw it, but the company has some interesting ERV design software (more properly, sizing software). So if one were going to try to fabricate their own Energy Recovery Ventilator, You could get a good idea where to begin.

The link is here:

Heatex AB - The No.1 Air-to-Air Heat Exchanger Company: Thanks!

... If this doesn't work for you,maybe go to the root URL and look for "software".

Regards,

-AC_Hacker

[Daox]

I was reading on the Canadian Passive House Institute website and found this:

Although heat recovery ventilation units (HRVs) were pioneered in Canada and the US, over the past decade European manufacturers have taken HRV performance levels well beyond anything achieved in the NorthAmerican market, and have re-designed heat exchanger cores completely. Passive House-compliant units are certified by the Passivhaus Institut, which in 1996 developed a more stringent HRV testing protocol to that used in North America. As a result, the best European HRVs achieve over 92% heat exchange, as measured from the inflow and outflow air streams.

Due to the much smaller size of their heat exchanger cores and lower insulation and air sealing levels, most Canadian or US models likely do not exceed 50% efficiency under the same measurement criteria.



Passive House-compliant Heat Recovery Ventilator (HRV)



The best-performing European HRVs are significantly larger in size than conventional North American models, and all incorporate 'counter-flow' heat exchange cores, developed specifically for Passive Houses, which provide much greater heat transfer area and therefore improved performance over the 'cross-flow' cores used here in Canada

Sounds like the US/CAD models are just too small! 92% is amazing. The ones I was looking at the other day you were lucky to see much more than 60%! Sounds like a bigger DIY option would be far superior.

[Xringer]

Those things do look large. But, it seems logical that a smaller unit would work
for smaller houses, if you used a slower air-flow.
Since slower air will get more of a chance to give up it's heat before leaving the unit.

[Piwoslaw]

Originally Posted by Xringer

Those things do look large. But, it seems logical that a smaller unit would work for smaller houses, if you used a slower air-flow.
Since slower air will get more of a chance to give up it's heat before leaving the unit.

I believe it was in that MAKE article which AC_Hacker linked (I can't get it to open this morning) it was said that the lowest speed setting for the fans was not the most efficient, that it seemed to exchange more heat with a higher speed. Maybe with more air coming through there is a larger DeltaT, increasing the amount of heat absorbed?

I've noticed that the main two issues when constructing recuperators are

1. Cross-section, which limits the amount of air that can be pushed through the unit,
2. Surface area, which limits the amount of heat that can be transfered from one airflow to the other.

If the unit is very small, then either the surface area is puny, or it can't handle a large volume of air (pumping losses), or both. The big DIY counter-current unit in one of the pictures I posted previously had its largest dimension probably around 150cm, the other two below 100cm, and its exchange surface area is about 25m2.

[Daox]

Yeah the fan speed thing is odd. I'd tend to think that slower would be better as Xringer said, but their findings show otherwise. In any case, it definitely looks like larger is the way to go.

[AC_Hacker]

Originally Posted by Daox

Yeah the fan speed thing is odd. I'd tend to think that slower would be better as Xringer said, but their findings show otherwise. In any case, it definitely looks like larger is the way to go.

In Fluid Mechanics, there is the issue of laminar vs. turbulent flow.

This graphic illustrates a couple of aspects of the difference:

In laminar flow mode, the friction of the surface of the pipe or in our case, the surface of the heat exchanger, causes the flow rate to become slower at the boundary between pipe and fluid. The maximum flow rate is at the center of the pipe and it decreases toward the pipe surface. This means that if there is a temperature difference between the fluid and the pipe, the slower moving layers near the pipe edge change temperature more readily than the layers near the center.

If the flow rate is increased, there comes a point where the fluid no longer flows in regular stratas, but changes flow mode and the flow becomes chaotic, the fluid tumbles as it flows.

In turbulent flow mode, the layers are continuously and chaotically changing, The graphic suggests that some of the fluid flows backward, which it does not, but some flows forward more slowly.

Since heat transfer happens as a function of temperature difference, the turbulent flow mode is more efficient than laminar flow, because the 'particles' of fluid get exposed to particles of pipe and are more subject to temperature change.

Also, the idea that slowing down the the fluid flow rate will result in increased heat transfer doesn't hold up either. While it is true that if the fluid flows more slowly, it's temperature will be higher when it exits, there is less total heat exiting with a lower speed.

To appeal to intuition, if you had been working in the hot sun and had gotten dangerously overheated and needed to cool down, would you rather sit by a window through which the air was barely moving, or would you rather sit in front of a window where there was a fan turned to high speed?

That's the difference that increased flow rate, and turbulent flow makes.

Regards,

-AC_Hacker

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