Boiler Efficiency Calculation
Natural Gas - Indirect Method

Boiler efficiency can be calculated 'indirectly' using measured value of flue gas temperature and oxygen (or CO2) levels. Blowdown loss and shell heat loss are estimated. Measurements should be repeated for varying load levels during steady operation.

2. Select O2 or CO2, tick for O2  

3. Input Oxygen in Flue Gas % dry vol  
3. Input Carbon Dioxide in Flue Gas % dry vol  

4. Input Feed Water Temperature   deg C

5. Input Combustion Air Temperature   deg C

6. Input Steam Pressure   psig

7. Input Boiler Load   %

8. Estimate Blowdown   % of feedwater

9. Estimate Shell Losses   % of full load fuel use

10. Input Fuel Price   p/kWh

11. Calculate Efficiency  

12. View Results

Boiler Efficiency   %
(% of gross heat in fuel used to heat feed water and raise steam)

Cost of Steam   £ / 1000 kg

Fuel Rate   m3 / 1000 kg steam

Air Flow   m3 / 1000 kg steam

Excess Air   %

Heat Losses:
Blowdown   kWh / 1000 kg steam
Shell Losses   kWh / 1000 kg steam
Flue Gas kWh / 1000 kg steam

Total Heat Input  kWh / 1000 kg steam

13. Interpret and Investigate:

Excess air levels should be 10-15%.
Vary inputs to see effect on efficiency.