Simple Air Heat Exchanger
Simple Air Heat Exchanger
(OP)
Can someone please give a basic formula for calculating the practicality of using a concentric tube counterflow heat exchanger for building heat recovery purposes.
A friend would like to bring in fresh air and exhaust stale air from a room of around 250m2 size. To save the cost of a heat recovery unit, I suggested the above using standard galvanised steel thin wall ventilation duct with outer insulation (cold intake through the inner).
At times the intake air can be sub zero, even sub -10 C, and the room (and hence initial exhaust air temp,) needs to be kept in the 20 to 25 C range. There would obviously be a lot of wasted heat energy without an exchanger. There is the possibility for a fairly long linear stretch of pipe.
I thought about packing and supporting the inner within the outer with some material, e.g. chicken wire / other metallic packing, etc, which will help to mix the outgoing air and conduct the heat onto the inner.
I need to calculate the effects of flow rate, initial hot and cold temps, and pipe cross sections and lengths on the final incoming air temp. Ideally all in metric units.
Obviously there is no need for great accuracy in the calculations. Many thanks in advance.
A friend would like to bring in fresh air and exhaust stale air from a room of around 250m2 size. To save the cost of a heat recovery unit, I suggested the above using standard galvanised steel thin wall ventilation duct with outer insulation (cold intake through the inner).
At times the intake air can be sub zero, even sub -10 C, and the room (and hence initial exhaust air temp,) needs to be kept in the 20 to 25 C range. There would obviously be a lot of wasted heat energy without an exchanger. There is the possibility for a fairly long linear stretch of pipe.
I thought about packing and supporting the inner within the outer with some material, e.g. chicken wire / other metallic packing, etc, which will help to mix the outgoing air and conduct the heat onto the inner.
I need to calculate the effects of flow rate, initial hot and cold temps, and pipe cross sections and lengths on the final incoming air temp. Ideally all in metric units.
Obviously there is no need for great accuracy in the calculations. Many thanks in advance.





RE: Simple Air Heat Exchanger
Air is not very good at exchanging heat.
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: Simple Air Heat Exchanger
I presumed (perhaps erroneously?) that a long enough counterflow concentric anulus with moderate flow rate would give a satisfactory efficiency at lower cost (the customer has surplus thin wall steel tubing to hand).
RE: Simple Air Heat Exchanger
The ones you see advertised, if they are providing realistic values, are probably a design that will provide you with more area than the tube in tube concentric heat exchanger.
Something like finned tubes or something like that. Which means that you idea is worth exploring a little more in depth, just think a little more about the exchanger's design.
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: Simple Air Heat Exchanger
RE: Simple Air Heat Exchanger
RE: Simple Air Heat Exchanger
I presume the low inefficiency, given the poor conductivity of air, is due to classic laminar flow in the tubes. This was the reason I was proposing some packing to cause turbulent flow and better heat transfer. Vortex generators or aerofoils could alternatively be installed. Have no idea, hence my posting, whether this makes a materially more practical arrangement.
To narrow the question down, if it helps: using 150/200 mm diameter tubes, external insulation, packing and/or turbulence generation measures in both tubes, ca. 10 metres of run, what flow rate would we need with 30 C temp diff to reach around 50% efficiency. If that's impractical, and the efficiency is still far off, would doubling the run to around 20m help (it's possible).
Larger diameters are not impossible, but presumably we would then put several smaller tubes in the larger? This would though increase costs.
For ref, here is a link to one of the many manufacturers of heat recovery units:
http://www.nuaire.co.uk/Product/Commercial_Products/Heat_Recovery
Efficiencies up to 90% are claimed by some manufacturers.
Bribyk: Appologies for the second post, but after a couple of days and looking at the date of the last post on this group, it appeared that there was a low prospect of a timely answer here. Obviously I was wrong. I would not object to admin redirecting or removing the other post.
RE: Simple Air Heat Exchanger
RE: Simple Air Heat Exchanger
Ask yourself how you are going to maintain this equipment, and whether the energy savings will pay for fabrication costs and maintenance issues..