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Simple Air Heat Exchanger
3

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.

RE: Simple Air Heat Exchanger

Unless you put a very large heat exchanger your heat recovery will be minimal. If you put in a big heat exchanger, it will prove more cost effective just to heat the room with a furnance.
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

(OP)
Thanks for your post. What were you meaning by minimal? I have seen compact air 'heat recovery' units in the €700 to €1600 (approx US$1000 to $2400) range advertised, claiming around 70% efficiency.

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

What are your air exchange rates? based on those flows and the deltaT, the residence time you need to gain any significant amount of heat will force you to have a fairly large concentric tube 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

As unotec said, the concentric tube is very inefficient.  We were looking at using one of these to exchange exhaust heat (350 degC) with intake air heat (-30 degC) for an engine.  Even with the high temp on the exhaust side, we needed 20+ ft of exchanger for at 10-15 degC rise in air temp.  Not very efficient...

RE: Simple Air Heat Exchanger

PS - the same post in two forums is frowned upon.

RE: Simple Air Heat Exchanger

(OP)
The length of the tubes (at probably around 10 metres,) is less constrained than the diameter. Flow rates would likely be in the 100 to 400m3/hr range with more than one arrangement if necessary.

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

Any heat transfer book will have the formulas you require and you may be able to get the equations from a Google books preview.  They are a bit too lengthy to get into efficiently here.

RE: Simple Air Heat Exchanger

Things that will make the air swirl, thus improving convection, will also trap dirt.  If you put in metallic packing there is going to be a day of reckoning when you have to clean the heat exchanger.  I would look carefully at the relative humidity and dew point temperature to see if there is going to be condensation on the cold-side of the heated air duct.  If you have a moist surface on the galvanized for a long period of time there will be corrosion of the zinc in the form of "white rust." This corrosion will further limit the heat transfer capabilities of your heat exchanger.  If you still like the galvanized heat exchanger, you should consider installing the ducts at a slope so that condensate will drain.

Ask yourself how you are going to maintain this equipment, and whether the energy savings will pay for fabrication costs and maintenance issues..

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