BTU from Exhaust stack

[Macbeth]

I need to measure the amount of BTU's taken out of an exhaust stack by a piece of heat reclaim equipment. I need to do it on the exhaust side; the reclaim side has no way of accessing the exchange media.

I have provisions for peto tube type flow meter and temp in and out. Can some make any recommendations on some thing else.

 

[danberry]

In and out temperatures are a must but pitot tube measurements should be avoided unless there is no other way of evaluating the gas flow.

You also need the flue gas composition to determine the heat transfer.

 

[25362]

There is a graph of enthalpy vs temperature (Rosin-Fehling) for all hydrocarbon fuels with curves for different excess air %. Below 500 Celsius the % excess air, has little influence. Thus, by measuring the temperature, the enthalpy of the flue gases is directly determined.

 

[timbones]

Provided that you know the composition and flow rate of your fuel, you can determine the exhaust flow with an oxygen sensor in the exhaust gas stack. The oxygen sensor basically tells you how much excess air above stoichiomentric conditions you have (neglecting CO and NOx is usually a reasonable assumption for this calculation). This method will also give you the exhaust gas composition. You can work out the enthalpies of each constituent of the exhaust gas and then sum them together in an appropriate manner. The NIST website is useful if you need enthalpy correlations for various gases.

Tim

 

[25362]

From reading the above-mentioned R-F graph with my old eyes, the following values of kcal per cubic meter of flue gases at normal conditions (0 Celsius and 1 atm) the combustion carried out with 10-30% xs air, would be

Celsius Kcal/normal m³

500 ~180
1000 ~370

Use linear interpolations for intermediate temperatures.