Boiler Efficiency: Key Concepts and Calculation Methods
Boiler performance, including efficiency and evaporation ratio, declines over time due to poor combustion, fouling of heat transfer surfaces, and inadequate maintenance. Even new boilers can underperform due to deteriorating fuel or water quality. Boiler efficiency tests help identify deviations from optimal performance and pinpoint areas for corrective action.
What is Boiler Efficiency?
Boiler thermal efficiency is the percentage of heat input effectively used to generate steam. It can be assessed using two methods:
1. Direct Method (Input-Output Method):
Compares the energy gained by the working fluid (water/steam) with the energy content of the fuel.
Formula
Boiler Efficiency= Steam Generation×(hg−hf) ×100÷Fuel Consumption×GCV
Where:
hg: Enthalpy of saturated steam (kcal/kg)
hf: Enthalpy of feed water (kcal/kg)
GCV: Gross Calorific Value of fuel (kcal/kg)
Parameters to Monitor:
- Quantity of steam generated per hour (kg/hr)
- Quantity of fuel used per hour (kg/hr)
- Working pressure and superheat temperature
- Feed water temperature
- Fuel type and GCV
Advantages:
- Quick evaluation of boiler efficiency
- Requires minimal parameters and instruments
Disadvantages:
- Does not identify reasons for efficiency loss
- It does not account for individual losses
- Indirect Method (Heat Loss Method):
- Efficiency is calculated by subtracting heat losses from 100%.
Standards: British Standard BS 845:1987 and ASME PTC-4-1.
Major Heat Losses:
- Dry flue gas loss
- Moisture loss from fuel and combustion air
- Hydrogen combustion loss
- Radiation loss
- Unburnt fuel loss
Data Required:
- Ultimate fuel analysis (H2, O2, S, C, moisture, ash)
- Flue gas composition (% O2 or CO2)
- Flue gas and ambient temperatures
- GCV of fuel and ash
- Combustible content in ash (for solid fuels)
Calculation Steps:
- Calculate the actual mass of air supplied per kg of fuel.
- Determine heat losses due to dry flue gas, moisture, hydrogen combustion, radiation, and unburnt fuel.
- Sum up losses and subtract from 100% to find efficiency.
Example Loss Calculations:
Dry Flue Gas Loss:
=m×Cp(Tf−Ta)×100÷GCV of fuel
Where:
m: Mass of dry flue gas (kg/kg fuel)
Cp: Specific heat of flue gas (0.23 kcal/kg°C)
Tf: Flue gas temperature (°C)
Ta: Ambient temperature (°C)
Moisture Loss:
Loss=M×[584+Cp×(Tf−Ta)]÷GCV of Fuel
Where:
M: Moisture in fuel (kg/kg fuel)
Cp: Specific heat of steam (0.45 kcal/kg°C)
Radiation and Unaccounted Losses: Typically, 1-2% for small boilers and 0.2-1% for large boilers.
Conclusion
Boiler efficiency is critical for optimal performance. The direct method offers quick efficiency evaluation, while the indirect method provides detailed insights into heat losses. Regular monitoring and maintenance are essential to sustain boiler efficiency and address performance issues.
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