Simple Air / Air Heat Exchanger 2

[eng3000]

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 customer would like to bring in fresh air and exhaust stale air from a room of around 250m2 size. This is not my area of expertise, but 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 21 to 26 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.

 

[MintJulep]

There are plenty of commercially available air-to-air heat recovery exchangers.

Any of them will work better, cost less, take up less space and be easier to maintain than a bunch of duct stuffed full of chicken wire.

 

[rmw]

You have got the right idea and your mind is driving in the right direction but unfortunately those of us who work with heat transfer on a regular basis recognize that what you are suggesting will amount to much of nothing.

I suggest taking your operating conditions and posting them on the HVAC thread and asking the lads there to recommend a good recuperative air-air heat exchanger. There are commercial devices that do what you want to do and are specifically designed to do so in such a way as to get the maximum bang for the minimum buck (which is USA venacular for 'biggest effect for lowest cost.'

rmw