[Stoves] Heat Exchanger in Pots

[Crispin Pemberton-Pigott]

Dear Frank

The first thing to assess is why the simmering is done and what numbers are expected from it. There is a 'thermal efficiency' number. The thermal efficiency is not obtainable from the things measured so that is a loser from the get-go.

If you want to measure the heat gained by the pot, that is different from keeping a hot pot hot. The heat exchanger is a way to extract heat continuously‎. There is no influence of altitude. No water evaporates during a test lasting hours. We found (not knowing in advance) that the line pressure is very constant all day. There was no guarantee of that. A pressure regulator could be used if it is an issue.

The inlet temperature in January was variable ‎but amazingly constant in June.

The bending was done with a tuning bender, sort of a cheap one, the I filed and polished carefully to keep the bends from pinching. The tube was really thin - from the refrigeration people. I recommend anything thicker!

I will send a pic of the ‎bender in action. There is a sort of a trick to it and took photos to illustrate. We got almost six metres into that coil. It has a lot of cooling potential, maybe five kW.

We are thinking that for the fifteen litre pots it should be a 10mm tube instead of 8.

Regards
Crispin
‎
Hi Crispin,

Nice bends in the copper tubing. Whats the trick to doing that?
It looks like a very good system and so simple. The water flow could be by constant head to keep it constant going in, perhaps. So after the experiment with flowing water is completed you remove the coils and mix and determine the heat energy in the water left?

If I am thinking that elevation (done at sea level or 12,000 ft) will not be of concern as you are looking at relative heat changes(?) and that is not dependent on altitude? That saves a LOT of potential error from the standard WBT and trying to maintain a temperature at simmer.

Frank




Frank Shields
franke at cruzio.com


> On Jun 22, 2015, at 11:55 AM, Crispin Pemberton-Pigott <crispinpigott at outlook.com> wrote:
>
> Dear Friends
>
> Here is a new example of a copper tube heat exchanger in a pot:
>
> <image007.jpg>
>
> This was made at the YDD Lab in June.
>
> <image008.jpg>
>
> <image009.jpg>
>
> <image011.jpg>
> The white cable ties were covered with epoxy to seal them.
>
> This is a smaller pot that normal and holds 5 litres. Now we have two and can test a two pot stove.
>
> Here is a set of temperature profiles during a test.
> <image015.jpg>
> The top Cyan line is the water temperature in the pot not rising higher than 60 degrees.
> The second brown line is the temperature at the gas sample point.
> The 3rd green line is the heat exchanger (water out)
> And the 4th line, light blue, is the inlet side of the heat exchanger. The difference can be read to 0.01 degrees and the noted drift between the two RTD’s was 0.004 max.
>
> The two low lines, red and blue, are the chiller temps – the gas is chilled using a vortex tube and compressed air to drop the water vapour before going to the gas analyser. This approach is very simple, cheap, and can be used in a high humidity environment for continuous testing even with very high water vapour loads. By changing the compressed air pressure, the chiller can be brought down to 2 C or even lower. The gas warms on the way to the analyser and the relative humidity drops dramatically ensure the cells are not contaminated. The system has no moving parts after the compressor.
>
> The calculated heat gain by the pot is
> Water mass x 4.186 x Delta T in the pot,
> plus the flow rate though the heat exchanger x Delta T x 4.186.
>
> The shorter 3-metre tube in the large pot is able to handle more than 3 kW of heat gain. The second pot has about 6m of tubing.  The flow rate is typically 34-35 cc/second. This is calibrated before and after each test using a bucket and a scale and has proven to be very constant. The 10 second resolution is a few tens to a few hundreds of Joules.
>
> Any burn cycle can be conducted on the stove and the heat gained by the pot determined for any selected interval. It can faithfully report the heat transfer efficiency at any time when combined with a gas analyser that detects water vapour, O2, CO2 and CO (at a minimum).
>
> Regards
> Crispin
>
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