Using exhaust air after heat exchange (or not) as heat pump input?

[Garald]

  On 16/05/2024 at 17:21, JohnMo said:

The only work being done is done by the fans to move the air.

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.3.2

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Sure, in the physical sense - I believe it. But surely something non-trivial is going on if the air going out is at 4 C and the air going in is at 20 C, say. If any fool (read: me) builds a heat exchanger, the air going out and the air going in will be at about the same temperature.

 

[Garald]

(I mean "work in the physical sense".)

 

[SteamyTea]

  On 16/05/2024 at 19:21, Garald said:

work in the physical sense

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[Garald]

  On 16/05/2024 at 19:38, SteamyTea said:

 

 

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This is a bit confused :).

[Garald]

But seriously - yes, the first and second laws are respected, but I'm still mystified by the mechanism. Presumably it is not some sort Rube-Goldbergesque contraption where a steam engine powers a heat pump.

 

[Garald]

Just asked an applied friend, and he has a conjecture. Is this it?

 

https://www.heatpipe.com/engineering-manual/heat-pipe-principle-and-applications/?fbclid=IwZXh0bgNhZW0CMTAAAR3xQrm_78fPi0UOOwRZvEYPSuy8r1Atsg9g5ALqL1trhn65aqvViZ6tgQk_aem_ASjRtEBhO0MTBUHkK_BqoJqJDEcObo2s0pIrxF4ZWED0yDAxIgtn3VCF2OWCIEW27dVGLj4-U9egT6L7gMAfaLax

[JohnMo]

  On 16/05/2024 at 21:21, Garald said:

Just asked an applied friend, and he has a conjecture. Is this it?

 

https://www.heatpipe.com/engineering-manual/heat-pipe-principle-and-applications/?fbclid=IwZXh0bgNhZW0CMTAAAR3xQrm_78fPi0UOOwRZvEYPSuy8r1Atsg9g5ALqL1trhn65aqvViZ6tgQk_aem_ASjRtEBhO0MTBUHkK_BqoJqJDEcObo2s0pIrxF4ZWED0yDAxIgtn3VCF2OWCIEW27dVGLj4-U9egT6L7gMAfaLax

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No, it's just a cross flow heat exchanger. Hot air from house goes in one end of the exchanger (end A) and cooled air come out the other end (end B). Cold air from outside enters at end B and come out to feed the house at end A.

 

Imagine a tube, inside a tube, the inside tube is the outgoing flow, the annulus formed between the tubes has the incoming flow. Both flow in opposite directions. If the tubes were long enough the temperature of the air at both ends of inner and outer/inner tube annulus would end up at the same temperature. As heat always moves to cold.

 

If the heat exchanger was 100% efficient the mid point between end A and B would be at half the difference between inside air temperature and outside air temperature for both incoming and outgoings air streams.

 

But because they are not 100% efficient you get a slight difference in temperature at end A and B between the incoming and outgoing flows. No magic going on.

[Garald]

  On 16/05/2024 at 22:06, JohnMo said:

No, it's just a cross flow heat exchanger. Hot air from house goes in one end of the exchanger (end A) and cooled air come out the other end (end B). Cold air from outside enters at end B and come out to feed the house at end A.

 

Imagine a tube, inside a tube, the inside tube is the outgoing flow, the annulus formed between the tubes has the incoming flow. Both flow in opposite directions. If the tubes were long enough the temperature of the air at both ends of inner and outer/inner tube annulus would end up at the same temperature. As heat always moves to cold.

 

If the heat exchanger was 100% efficient the mid point between end A and B would be at half the difference between inside air temperature and outside air temperature for both incoming and outgoings air streams.

 

But because they are not 100% efficient you get a slight difference in temperature at end A and B between the incoming and outgoing flows. No magic going on.

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But this is not what the diagram you post yourself shows!

 

 

If it were just what you say, the exhaust temperature would be a little higher than 12.5C, and th supply temperature a little lower than 12.5C. That's not at all what the diagram depicts.

Edited May 16, 2024 by Garald

[Tumble]

These heat exchangers aren’t working on the bulk energy of the air, but rather the local energy at each point in the exchanger. As these are contra-flow exchangers the hot extract air entering is exchanging heat with the supply air right as it exits. As the extract air progresses through the exchanger its temperature will drop as the energy is extracted until it’s basically at the external air temperature. Does that help?

 

I’ve also found this diagram, which shows the difference between a parallel-flow and contra-flow exchanger:

[Garald]

  On 18/05/2024 at 07:48, Tumble said:

These heat exchangers aren’t working on the bulk energy of the air, but rather the local energy at each point in the exchanger. As these are contra-flow exchangers the hot extract air entering is exchanging heat with the supply air right as it exits. As the extract air progresses through the exchanger its temperature will drop as the energy is extracted until it’s basically at the external air temperature. Does that help?

 

I’ve also found this diagram, which shows the difference between a parallel-flow and contra-flow exchanger:

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Ah, that's very nice! Thank you!

[zzPaulzz]

Apologies for resurrecting this post, but my question is similar.  From what I read here,  there is no downside to routing my MHVR exhaust to flow out through my ASHP (vent out behind the ASHP). There might a small marginal gain but if the effort/cost is minimal then I might as well do it.  Correct? 

[ToughButterCup]

  On 15/05/2024 at 15:56, SteamyTea said:

Negative exergy

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I suffer from that after a skin full and a kebab 

[JohnMo]

  On 09/08/2024 at 06:12, zzPaulzz said:

Apologies for resurrecting this post, but my question is similar.  From what I read here,  there is no downside to routing my MHVR exhaust to flow out through my ASHP (vent out behind the ASHP). There might a small marginal gain but if the effort/cost is minimal then I might as well do it.  Correct? 

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You will at best make no difference, at worst impede the flow through the ASHP. A small 5-6kW ASHP takes in 250m³/hr of air a big one 7500m³/h.

 

You will be providing air from the MVHR slightly higher than ambient and around 1/10 of the air or way less if you have a bigger. So could make points of degrees heat increase. So in effect around 0.01 adder to CoP optimistically. 

 

You would be better off spending the time making you heat pump run as efficiently as possible.

[zzPaulzz]

Fair point. I have a plant room for my MHVR and DHW tank with the ASHP attached to the wall outside, so I have plenty of options for venting the MHVR. Just asking if I need to avoid venting over the ASHP so I have my answer, thanks!