Chimney Draft Calculator and Stove Modelling Spreadsheet

Crispin Pemberton-Pigott, New Dawn Engineering, October 28, 2004

Dear Stovers

I have adjusted the file to meet your Excel format and I think it will do as

an introduction for stove students. Obviously I can't guarantee the results

are error free nor its accuracy in the real world. If you get a real

measurement you can use it as a fudge factor on the final output. IT is

made available in the hope that it is a useful contribution to the on-going

development and better understanding of combustion and stove design. It is

a basic and theoretical model.

If you work out how to use the stove modeller, you may find very interesting

things about your own stove or one of its components. For example, in the

sample included, the velocity of the secondary air through the middle of the

combustion chamber walls of a Vesto is higher than any other place. This

means that draft energy available is applied to making the secondary air

reach the middle of the chamber to ensure good secondary burning.

If you want to add an internal chimney to the stove, put in the height and

average temperature and see the difference it makes to the gas velocity at

critical points.

If you use the Draft Calculator for a material purpose, please remember to

reference the author, Nigel Pemberton-Pigott who is

presently studying Materials Science (in Chinese) at the University of

Dalian. He is not able to find time to deal with much in the way of

recommendations or suggestions, as you can imagine. If you can't imagine,

go to a Chinese website, look at the characters and think about trying to

learn partial differential equations and the thermodynamics of crystals in

Chinese. You will quickly get the idea. In other words please don't bother

him, bother me at . Kevin suggested some colour coding

to make things easier to comprehend. You are welcome to paint away!

Look for the file draft.xls or draft.wb2 at:

Stove Burning, Boiling and Efficiency Tests,

Helping you burn 'em better!


MANAGER - New Dawn Engineering / Appropriate Technology Exchange (ATEX)

October 26, 2004

Dear Stover Developers

Below is the sort of input and output that comes from the Chimney Draft Calculator.

If you are interested I will place the file in two formats (it will take me a little while to write the page) on the New Dawn Engineering website in the Stove Testing area. The Quattro Pro file has a picture (floating object) of the Vesto with the different flows from the temperatures you enter. Excel users may find that stove draft section difficult to understand without the graphic but give it a try with your stove - the results are still useful.

There is a separate portion of the sheet (not shown below) for placing your stove 'sections' in it where you can put the temperatures as the air or gases as they pass from one place to another. This means you can sum the different temperatures/drafts to add up to a total draft for the stove. It allows negative drafts as are used in the Vesto pre-heating.

At this time it does not account for excess air however that is easily added manually at the end as it only changes the gas velocity in the chimney. The advantage of leaving it out is that you can work out the actual air flow and gas velocity within the stove, or through a stove component.

Undoubtedly this Chimney Draft Calculator can be improved. It was not really written to be a general case true-to-life predictor of reality. It is however good for explaining the nature of the problem.

There is enough information in the outputs to calculate the power available from the chimney which might be used to turn a fan or perform some other mechanical work.

Good luck creating Cleaner Cookers!





Chimney Draft Calculator is copyrighted. (c) 2003 Nigel Pemberton-Pigott

Copies of the file should include this brief notice and calculations made using it should credit the author.



All cells with a pale blue background are for your value/variable entries. Do not edit the other cells as they probably contain formulas.

The spreadsheet is designed to calculate the burning of wood and the resulting draft. It requires that the chimney temperature be measured. It does not account for thermal losses to the chimney itself so the temperature should be taken approximately 1/2 along its length.

'AIR' means the normal air around you.

'GAS' means the gases that are considered in the flame.

'Gases' means all the gases including the 'GAS' and excess 'AIR'.

Assume that no CO is being produced and combustion is complete.

Center line of three holes is gas/air boundary

Formula for burning wood: CH1.4O0.6 + 1.05(O2 + 3.76N2) => CO2 + 0.7H2O + 3.95N2

AIR FACTOR (actual air flow/ideal air flow) 1.250

Diameter of chimney 72.800 mm

Time to burn fuel 3600 seconds

Mass of wood used in time above 1000 g

Temp inside chimney 325 C

% carbon in the wood 50 %

% water contained in wood 12 %

Height of chimney 8.000 Metres

Heat content of wood/biomass 19.3 MJ/Kg

Mass of actual fuel 880.000 g

Rate of fuel burn /min 14.667 gm/min

Heat produced 4717.778 Watts

Moles of actual fuel 38.261 moles

Mass of carbon 459.130 g

Moles of CO2 created during burning 38.261 moles

Mass of CO2 produced during burning 1683.478 g

Volume of CO2 produced during burning 0.857 m^3

Mass of O2 need to burn carbon 1285.565 g

Moles of O2 needed to burn carbon 40.174 moles

Volume of O2 needed to burn carbon 0.900 m^3

Volume of air needed to supply the O2 at 273K 4.285 m^3

Mass of H2O vapor created in burning 482.087 g

Moles of H2O vapor created in burning 26.783 moles

Volume of H2O vapor created during burning 0.600 m^3

Moles of N2 entering/exiting the flame 149.217 moles

Mass of N2 entering/exiting the flame 4178.087 g

Volumes of N2 exiting the flame (273) 3.342 m^3

Volume of CO2 already in the incoming 0.001 m^3

Mass of H2O in the wood 120.000 g

Moles of H2O from wood 6.667 moles

Volume of H2O vapor from wood itself 0.149 m^3

Volume of GAS exiting the flame (273) 4.950 m^3

Volume of gases exiting the flame (273) air factor 6.021 m^3

Volume of extra air in air factor 1.071 m^3

Volume of GAS in chimney (no air from air factor) 10.843 m^3

Volume of Gases in chimney with extra air factor 13.190 m^3

Real air volume from outside with air factor 5.357 m^3

Air flow per second 0.004 m^3/sec

Cross-sectional area of chimney 0.004 m^2

Velocity of flue gases 0.880 m/sec

GAS after burning

Density (g/m^3) Proportion after flame

CO2 1964.286 0.173

H2O 803.571 0.151

N2 1250.000 0.675

Total density of GAS at (STP) 1306.293 1.000

Total density of GAS at flue temp 596.351 g/m^3

Gases after burning

Density of extra AIR at flue temp 0.593 kg/m^3

Density of GAS at flue temp 0.596 kg/m^3

Average density of all gases in flue 0.538 kg/m^3

Draft of chimney 59.766 Pascals