Efficiency, Heat Balance and Energy Conservation
Where the heat goes
A heat balance accounts for all heat supplied by the fuel. Useful heat produces steam; losses occur in dry flue gas, evaporation of moisture in fuel and air, incomplete combustion, radiation and convection, and blowdown. Boiler efficiency is useful heat divided by heat input, expressed as a percentage.
Direct and indirect methods
The direct method measures fuel input and steam output. The indirect (loss) method sums the losses and subtracts from 100%. Typical shell-boiler efficiencies are around 75–85%; water-tube units with economiser and air preheater can exceed 85%.
Improving efficiency
- Reduce excess air to the practical minimum (monitor O2 and CO).
- Keep heating surfaces clean - scale and soot are insulators.
- Use an economiser to heat feedwater and an air preheater to warm combustion air.
- Control blowdown to the minimum needed for TDS.
- Avoid steam leaks and failed-open traps.
- Match firing rate to load and avoid frequent cycling.
Traps
Questions claim excess air improves efficiency (it does not beyond the optimum), or confuse the direct and indirect methods. Remember that a high stack temperature signals lost heat, and that scale raises stack temperature and fuel consumption.
Worked rule of thumb
If flue gas temperature rises while load is steady, suspect dirty heating surfaces or excessive excess air - clean and trim combustion before adding fuel.