DIY ventilation heat exchanger

[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

* * * * *

[Daox]

Thanks for the explanation AC Hacker. I am familiar with laminar and turbulent flow. I just didn't expect that a higher speed created that much more turbulence. Very interesting!

[Piwoslaw]

I think this forum needs a Thanks button!

Originally Posted by Daox

I just didn't expect that a higher speed created that much more turbulence.

I recently read that an airflow speed of at least 5m/s in ventilation ducts creates unacceptable noise. This may be the speed at which laminar turns to turbulent flow.

[AC_Hacker]

Originally Posted by Piwoslaw

I recently read that an airflow speed of at least 5m/s in ventilation ducts creates unacceptable noise. This may be the speed at which laminar turns to turbulent flow.

That's it exactly.

-AC_Hacker

[Daox]

Originally Posted by Piwoslaw

I think this forum needs a Thanks button!



I recently read that an airflow speed of at least 5m/s in ventilation ducts creates unacceptable noise. This may be the speed at which laminar turns to turbulent flow.

I'd give you a thanks, but... We'll get on it. Its a nice feature.


Thanks for that tidbit of info too!

[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