I was wondering about the physics for mechanical ventilation with heat recovery. My understanding is that it’s a passive system that has a head exchanger formed of many thin layers of a metal (presumably aluminium) which then alternate airstreams entering and leaving a house to heat up the incoming air
The diagrams you’ll look at will show an example like it being 0º outside and 20º inside, then after the heat exchange, it heats up the incoming air to 18º
Is this not bad science? The pressure of the house has to remain constant, so the incoming volume of air has to equal the outgoing volume of air. At best - if the air had infinite time to exchange heat, the best you’d achieve is 10º for the incoming air.
In the real case, I’d assume your heat exchanger would reach 10º, and the incoming air would interact with it for at most a few seconds. I just can’t see any real heat transfer happening here
What’s your thoughts? A scam, or something that has actual benefits?
Edit - I’ve left the original post in tact - but I did find an answer. It’s a real phenomena called countercurrent flow/countercurrent heat exchange. It’s very important that the flows are in opposite directions - if they’re not, you’ll just reach the equilibrium temperature. But when they flow in the opposite directions, it is possible exchange nearly all the heat. The phenomena also shows up in nature - ducks have a countercurrent heat exchange system in their legs so the are able to recover heat losses from their feet being in water
I’m more of a chemist than a physicist but I had a similar difficulty understanding fractional distillation.
If the concept maps, the key thing is that the length of the path through the heat exchanger means a lot here, and that different parts of the heat exchanger can have different temperatures.
With short path lengths/little exchange time you’d have a heat exchanger like you describe - It just reaches a temp half way between the two regions it connects. But, imagine you replaced the single heat exchanger in this setup with two heat exchangers hooked up in series with a unheated middle room connecting them - Then the outside temp could be 0C, the first heat exchanger temp would be 5C, the small room’s temp would be 10C, the second heat exchanger would be 15C, and the heated room would be 20C.
In this situation, the air coming out of the second heat exchanger and into the heated room is 15C instead of 10C.
There’s no actual need for the middle room, you could directly attach the two heat exchangers end-to-end to achieve a similar result.
In essence, a heat exchanger that heats the incoming air to 18C is just a bunch of the heat exchangers that you are thinking of stacked so that the output of one is the input of the next, with a temperature gradient that forms between them.