DIY Heat Exchanger

[Onoff]

Found this:

 

http://www.builditsolar.com/Experimental/DIYHRV/DIYHRV.htm

[MikeSharp01]

Looks interesting - wonder what the efficiency would be?

[Onoff]

8 minutes ago, MikeSharp01 said:

Looks interesting - wonder what the efficiency would be?

 

Page 1 of the article says 60-70%

[SteamyTea]

Efficiency changes with airflow, the slower the better.

 

I really should get around to making my Mk3 model, was thinking of using silicone paper for the plates as it is cheap, thin and fairly rot proof.

Edited December 28, 2017 by SteamyTea

[Onoff]

13 hours ago, SteamyTea said:

Efficiency changes with airflow, the slower the better.

 

I really should get around to making my Mk3 model, was thinking of using silicone paper for the plates as it is cheap, thin and fairly rot proof.

 

Something I'm not understanding...surely the plates should be aluminium or something heat conductive?

[SteamyTea]

7 hours ago, Onoff said:

surely the plates should be aluminium or something heat conductive?

Ideally yes, but as the plates are really thin, it does not matter too much.

 

[Onoff]

3 hours ago, SteamyTea said:

Ideally yes, but as the plates are really thin, it does not matter too much.

 

As delicate as it sounds would heavy duty tin foil work?

[Jeremy Harris]

3 hours ago, SteamyTea said:

Ideally yes, but as the plates are really thin, it does not matter too much.

 

 

Yes, the thermal conductivity of the plates has almost no effect when compared to the thermal conductivity of the air flowing over them.  What's more important is slowing the air right down as it passes over the plates, which means having lots of overall flow area and very small gaps between the plates.  Ideally the air gaps should be no bigger than double the plate thickness, as that ensures that the air thermal conductivity is the limiting factor, not the thermal conductivity of the plate material. Practically it's hard to get gaps this thin though.

 

The main problem with the Coroplast and drinking straw versions is that they double the thickness of the plates and introduce a thermal barrier at the junctions.  The simplest way to make a decent home made heat exchanger might well be to make a rotating drum one.  The efficiency would be high and you can compensate for any dodgy tolerances on plate spacing by just adjusting the drum speed.  With a bit of cunning and some sensors you can automatically adjust the drum speed and fan speeds to keep the thing running in the sweet spot.  I think I'd find a rotating drum heat exchanger a fair bit easier to make than a counter flow one.

[SteamyTea]

I fancy a rotating drum one, had an oil filled (I think) one on the waste timber burner at work back in the early 90's.

It seemed pretty good.

 

@JSHarris can you knock up a quick sketch for what you have in mind?  I found making a cross flow one pretty easy but I like the idea of changing the efficiency by changing the rotation speed more (or less).

[SteamyTea]

12 minutes ago, Onoff said:

As delicate as it sounds would heavy duty tin foil work?

My very first attempt was with tin foil, was hard to work with (it tore easily) and was noisy (rustling).

I then used steel plates, but in the end I used some thin PVC as it was easy to work with.  Weighed less too.

[Jeremy Harris]

25 minutes ago, SteamyTea said:

I fancy a rotating drum one, had an oil filled (I think) one on the waste timber burner at work back in the early 90's.

It seemed pretty good.

 

@JSHarris can you knock up a quick sketch for what you have in mind?  I found making a cross flow one pretty easy but I like the idea of changing the efficiency by changing the rotation speed more (or less).

 

The ones we had on the roof our the new lab building had drums a bit like this inside (a very quick and dirty sketch):

 

 

The drum rotates very slowly, with warm, extracted, air passing through one side of the separator plate and warming up the aluminium fins.  When these rotated around to the fresh air input side, they gave up their heat to the incoming fresh air.  There are two basic topologies, the radial design like this, which is easier to seal, or a design made from a stack of thin discs, with seals between each disc.  I've seen both and from a DIY perspective I doubt there's much to choose between either.  You can make the disc type physically smaller to get the same heat exchange capacity, as you don't need such a big hub, and you may well be able to find a company that will cut out discs with central holes pretty cheaply.  Something like thin brush seals would work between the discs; the friction losses would be small because of the low speed.  You may even get away with just thin barriers not actually touching the discs and relying on the low pressure differential to adequately seal.

 

The key is to adjust the speed so that just as the drum rotates past the separator plate (and not before) the fin/disc is as close as possible to the temperature of the extracted air, that then ensures that the unit operates at the best possible efficiency.  This has to be balanced with the airflow to ensure that just as the fresh air side fin/disc crosses the separator plate it has given up as much heat as it can, i.e. there's a very small Δt between the air going to the rooms and the fin/disc temperature at that point.

Edited December 29, 2017 by JSHarris

[SteamyTea]

Seems more complicated than what I made.

I shall have to see if I can find the old pictures and drawings of it.

 

I did wonder if it would be possible to make some simple heat pipes (copper pipe and boiling water/alcohol) and stick them half in the warm airflow and half in the cold airflow (or whatever is actually needed).

But after seeing that candlestick holder I am going to make them instead and forget all about my heating bill worries

[Jeremy Harris]

Heat pipes could work.  Find a working fluid with a very low boiling point. You ideally want something where the boiling point can be adjusted (using pressure) to be around 20 degC.  Methylene chloride might work, it boils at around 40 deg C at normal atmospheric pressure, but that can easily be lowered  by reducing the pressure in the tubes. 

 

You might well find that ordinary heat pipes work well enough, and you may not need a lot of them, as even small ones have the capacity to shift a fair bit of heat.  Maybe find a few broken evacuated glass solar tubes and repurpose the heat pipes.  Add a lot of fin area to either side and it may work.  Worth an experiment, I'd have thought.

[SteamyTea]

The way I see it, the less moving parts there are, the better.

Also maintenance is something to think about.  The PVC one I made has been in the loft for a few years now, probably 4.  I have never opened it up to have a look.

I may just do that when I get a few minutes (and take some pictures).

 

I am off to look at waves now as the wind is blowing a treat (40 MPH and 10° North of West). High tide is at around 2PM on the North Coast, so may have a coffee on the way and just kill a bit of time (has to be better than going up in loft).

[joe90]

Just come back from Launceston and the road was blocked due to fallen trees and flooding.

[SteamyTea]

3 hours ago, joe90 said:

Just come back from Launceston and the road was blocked due to fallen trees and flooding.

That is your own fault for going to Launceston!!  If it is not in Bude, it is not worth having

[joe90]

1 minute ago, SteamyTea said:

That is your own fault for going to Launceston!!  If it is not in Bude, it is not worth having

HA, no screwfix in Bude but there is in Launceston ?

[SteamyTea]

There used to be a funny old hardware store, Truscott or something.  I am sure they do long nails, just like the nail bar, Tekka Beauty

[Onoff]

"If you're serious about it, I have some closed cell PE tubing left over from our MVHR installation, which might be of use as an outer housing for a tube-based unit.

 

I think I have a length of 180 and another of 200 or 220. Happy to send it your way if of interest."

 

@jack, cheers for that, Figured I'd reply in here rather than detract from the other thread! 

 

"SE England" - I could likely collect!

 

What would be the advantage of using your stuff over say soil pipe fittings for the outer tube, drain pipe up the centre and encasing the lot in "insulation". I'm thinking whether I could do a hairpin or otherwise cranked design up in the loft. NO idea btw where to start!

 

So:

 

- Single, well insulated room with vcl. 

- 150pir + 25mm eps in the floor

- 145/150mm pir between joists in the ceiling (currently 100mm with rest to be added)

- multiple downlight, speaker, body dryer penetrations in the ceiling but these will be insulated over and the vcl reinstated

- several galv conduit drops in the walls - will be intumescent or Wiska gel sealed once wiring is complete

- gap, tba under the door leading to the rest of the house

- room is nom 3m x 3m x 2.47m high so nom 22m³ volume.

 

Fan sizes etc? Vent positions?

Edited June 28, 2018 by Onoff

[PeterW]

Drain pipe has 1.5mm plastic wall -  heat transfer would be negligable. 

[Onoff]

3 minutes ago, PeterW said:

Drain pipe has 1.5mm plastic wall -  heat transfer would be negligable. 

So you need a thinner wall?  Just edited my post btw with more info.

[PeterW]

The room has a minimum fan size / extraction for building regs and you have that big F/O body fan that will over power anything in that room ... 

 

I can’t see any value adding any MVHR to the setup currently, other than a basic wall mounted one that will have much greater efficiency in a smaller box. 

 

Air to air heat exchanger membranes are measured in microns not millimeters for good reason - to maximise heat transfers.  Thick pipes would be ok with a water to water heat exchanger but still, it’s not ideal. 

[jack]

 

 

15 minutes ago, PeterW said:

Drain pipe has 1.5mm plastic wall -  heat transfer would be negligable. 

 

Just to confirm, we're talking about the housing for something like this:

 

 

Heat exchange is via the internal pipes, which would likely be the size of drinking straws.

 

110mm drain pipe was actually my first thought for the housing, and has a few additional advantages including the availability of fittings and the fact that it won't retain any condensation that might lead to mould. My initial thought was that it wouldn't be big enough, hence my suggestion of something larger, but now that I think about it, it's only for a single room, so I expect drain pipe would be fine.

 

That said, I agree with @PeterW that this is a solution looking for a problem. You'll need a lot of ductwork (two full paths: in and out), plus two fans, and somewhere to drain condensate to. Seems like overkill for a single room.

[PeterW]

4 minutes ago, jack said:

Heat exchange is via the internal pipes, which would likely be the size of drinking straws.

 

So a structure of drinking straws inside I now get, but how you would support and seal that whilst still maintaining flow would be a challenge. I’ve built a couple of contraflow copper heat exchangers and they are hard enough as you can pressure test them to find leaks ..!

[Onoff]

When do I ever listen?

 

Any excuse to play with the junk I've got here!

 

Would 15mm copper pipe do for the "drinking straws"? Appreciate they're a bit bigger! The layout and runs maybe work?

 

It'd be dead simple:

 

 

Soffit is close by, easy condensate run to.

 

Have I got the shell & tube inlet/outlets in the right position? 

 

Is it ok having the inlet and outlet in the same room? Could drop straight through the ceiling from the main tube.

 

I could load it with sensors to try and gauge the efficiency.