Yield wood stove and flue gas temperature

Heating, insulation, ventilation, VMC, cooling ... short thermal comfort. Insulation, wood energy, heat pumps but also electricity, gas or oil, VMC ... Help in choosing and implementation, problem solving, optimization, tips and tricks ...
dedeleco
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by dedeleco » 12/11/10, 02:29

Okay with chatelot16, if we want to be precise, we're not done !!
but we can look at different air flow rates the temperature of the fumes before and after the exchanger (before adding draft air) and therefore with the max measured (better combustion) have an idea to refine by measuring all CO2, H2O, CO etc.

But in my opinion it is necessary to measure in the flue the smoke temperature and not outside on the stove tube at T very lower in my house !!
We can take a thermocouple with fine wires !! (or Pt probe)
A fine copper wire welded at one point to a fine iron wire makes a thermocouple at 0 € with millivolmeter which must be calibrated at a few points of T.

Finally I would like to have the texts of standards to measure the performance of wood stoves !!
In German it must be found free on the internet !!
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Philippe Schutt
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by Philippe Schutt » 12/11/10, 08:33

The text of the standard is subject to copyright. It is impossible to have it legally without buying it, from the afnor for example.

I have not found it in any European country.
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Christophe
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by Christophe » 12/11/10, 09:24

dedeleco wrote:But in my opinion it is necessary to measure in the flue the smoke temperature and not outside on the stove tube at T very lower in my house !!


I have always been told (engineering course) that with a fluid flowing in a "" metallic "container in a stabilized state, the temperature of the walls was very close to that of the fluid.

Ok we have a gas but still, can we have your figures?

For me this method is quite good except for the emissivity which I have not corrected. I will look to correct it.

Anyway, what we are looking for with this method is not precision (see remark by chatelot but these are orders of magnitude and in my opinion, a T ° of smoke lower than 100 ° C (80 ° C in this case) is a pledge of good performance right?
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chatelot16
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by chatelot16 » 12/11/10, 09:56

for me a temperature of 80 ° at the exit of a wood stove is a missed sign of measurement! the real temperature is certainly higher

the air is poor conductor of heat: the sheet of a furnace pipe takes advantage of the heat only a small thickness of the smoke that passes through it

worse the air outside the pipe has the same properties: conclusion the sheet takes an intermediate temperature between inside and outside ... may be closer to the average of 2 than the inside temperature

a first precaution would be to put a good layer of glass wool around the furnace pipe ... but damage it reduces re performance ... the furnace pipe participates in the heating

the right solution is a termometer that plunges into the pipe

for an infrared thermometer the good solution is to pierce the pipe and weld a piece of tube closed at the end which enters the furnace pipes: this cavity being entirely in the furnace pipe will necessarily be at the temperature of the smoke and the infrared thermometer is very precise for measuring in a cavity
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by pb2488 » 12/11/10, 10:03

Hello,
Is thermostatic control for a wood stove useful and effective?
Cdlt
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by Christophe » 12/11/10, 10:08

Houlala, from wiki: http://fr.wikipedia.org/wiki/%C3%89missivit%C3%A9 the emissivity of stainless steel varies in great proportion according to its state ...

In my case you think I should take which case?

Stainless steel (bolt): 0.32
Stainless steel oxidized at 800 ° C: 0.85
Polished stainless steel: 0.075
Grinded stainless steel: 0.11
Sandblasted stainless steel: 0.38 - 0.44
301 type stainless steel: 0.54 - 0.63


Our nozzle is rather matte (see photo it is not 316 L with chrome appearance), so I think its emissivity is between 0.4 and 0.5 ... what do you think?

By default (yesterday's measurements) the emissivity is set to 0.95, I think the black font is close to this, we must be close to the case:

0.97 matt black lacquer


It is therefore necessary to make the correction on the stainless steel.
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boubka
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by boubka » 12/11/10, 10:19

Hello Christophe
the stove is made of cast iron, the nozzle made of stainless steel.

most infrared thermometers (can be even all) do not work on stainless steel, the displayed values ​​are not reliable at all.
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by chatelot16 » 12/11/10, 10:21

with a deep cavity it is always equivalent to an 1 emissivity even if the materials have an average emissivity

drilling and putting a piece of plunger at a time solves all the problems
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by Christophe » 12/11/10, 10:43

chatelot16 wrote:for me a temperature of 80 ° at the exit of a wood stove is a missed sign of measurement! the real temperature is certainly higher


Not as bad as that in my opinion ... to have an order of magnitude is enough, if the wall is less than 100 ° C, the T ° of the fumes in the heart certainly does not exceed 150 ° C. You can touch the nozzle where I take measurements without getting burned (well, not too long anyway ...). An oven at 150 ° C you don't touch it without burning yourself ...

Urban artist the T ° gradient between the top, the side and the bottom of the nozzle tends to prove that the measured T ° is close to the container ...

Measurements on the nozzle with emissivity of 0.45:

- High: 88 ° C
- side: 61 ° C
- down: 52 ° C

If one measures these differences of T ° on the nozzle, it is good that these measurements are significant of the T ° of the fumes (I say not in the middle of the nozzle but at least on the circumference of the gas vein ...) ...

And finally, I just remade 2 measurement series (10 min of difference) with the emissivity corrections:

a) 0.45 emissivity

1 nozzle (top): 88 ° C
1 stove: 215 ° C (for info)

2 nozzle (top): 72 ° C
2 stove: 200 ° C (for info)

b) 0.95 emissivity

1 nozzle (top): 62 ° C (for info)
1 stove: 148 ° C

2 nozzle (top): 48 ° C (for info)
2 stove: 128 ° C

We would therefore have with correction of the emissivities this:

1 measurement: 88 ° C nozzle for 148 ° C stove
2 measurement: 72 ° C nozzle for 128 ° C stove


I obviously measured at the same points and without changing the draw. The difference in T ° just indicates that the fire is in the downward power phase ...

After yes if we want more precision, an internal measurement would be necessary, I could do it with the gas / liquid probe of the same thermometer which measures up to 200 ° C ... but I don't really want to pierce the nozzle ... even for you : Cheesy:
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dedeleco
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by dedeleco » 12/11/10, 13:28

We calibrate the thermometer with a real thermometer against the wall (insulated bulb) or thermocouple well supported, otherwise mystery ??
I agree with chatelot16, common sense !!
Simply the wall is halfway between the opening and that of the smoke.
Otherwise potash and assimilate a free book like:
A Heat Transfer Textbook
http://www.onlinefreeebooks.net/go.php? ... ttv124.pdf
or the different chapters of:
http://www.sft.asso.fr/document.php?pag ... roject=sft
(less detailed) and all the references and scientific papers in these books !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!

So roughly agree that the fumes are close to 130 at 150 ° C in the center of the duct!

Be very careful that on many stoves, inserts, boilers, there are several air inlets and even one in outlet after exchanger called regulator (my case at home) which cool the fumes well and distort everything!
T can be halved !!
It is necessary to measure the battery at the outlet of the exchanger and before the regulator air inlet (or other name) which sucks in air draft !!
This is why it is necessary to measure the rate of CO2 to correct !!

Otherwise on the internet, in German or Dutch one must find free European standards detailed in my opinion !! and paying in France !!
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