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Boiler Efficiency and Heat Loss

Boiler combustion efficiency calculation, stack loss, and efficiency improvement methods.

boilerefficiencyCalculator

Overview

Boiler efficiency compares the useful heat delivered to water or steam against the chemical energy released by burning fuel. Three measures are commonly used:

  • Combustion Efficiency — how completely the burner consumes fuel, gauged by unburned fuel and excess air in the flue gas.
  • Thermal Efficiency — how effectively the heat exchanger transfers combustion energy to the working fluid, excluding radiation and convection losses.
  • Fuel-to-Fluid Efficiency — the overall ratio of energy absorbed by the fluid to energy supplied in the fuel, accounting for all losses.

In practice, "boiler efficiency" usually refers to thermal or fuel-to-fluid efficiency depending on context.

Key Formulas

General efficiency

η=QoutQin×100%\eta = \frac{Q_{\text{out}}}{Q_{\text{in}}} \times 100\%

Heat exported by a liquid (water) heat-transfer fluid

Qout=m˙⋅cp⋅ΔTQ_{\text{out}} = \dot{m} \cdot c_p \cdot \Delta T

Heat exported by a steam boiler (evaporation at saturation)

Qout=m˙⋅heQ_{\text{out}} = \dot{m} \cdot h_e

Net Calorific Value (approximate)

NCV≈0.90×GCV\text{NCV} \approx 0.90 \times \text{GCV}

Variables

SymbolDescriptionTypical Unit
η\etaBoiler efficiency%
QoutQ_{\text{out}}Heat exported to fluidkW
QinQ_{\text{in}}Heat supplied by fuelkW
m˙\dot{m}Mass flow ratekg/s
cpc_pSpecific heat capacity of fluidkJ/(kg·°C)
ΔT\Delta TTemperature difference (outlet − inlet)°C
heh_eEvaporation energy at saturation pressurekJ/kg
GCVGross calorific value of fuelkJ/kg or Btu/unit
NCVNet calorific value of fuelkJ/kg or Btu/unit

Boiler Efficiency Calculator

Boiler Fuel-to-Fluid Efficiency

Unit Converter

Boiler Energy Unit Converter

Fuel Energy Content

9 rows
Approximate gross calorific values of common boiler fuels (Natural Gas listed at midpoint of 950–1050 Btu/cu ft)
Fuel
Unit
Gross Energy(Btu)
No. 1 OilGallon137400
No. 2 OilGallon139600
No. 3 OilGallon141800
No. 4 OilGallon145100
No. 5 OilGallon148800
No. 6 OilGallon152400
Natural Gascu ft1000
Propanecu ft2550
Butanecu ft3200

Source: engineeringtoolbox.com

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

The original fuel-energy table has one header row and 9 data rows: No. 1 Oil through No. 6 Oil, Natural Gas, Propane, and Butane. All 9 data rows are preserved below.

Fuel Oil - Energy Content

9 rows
Fuel Oil - Energy Content
Fuel
Unit
Energy(Btu)
No. 1 OilGallon137400
No. 2 OilGallon139600
No. 3 OilGallon141800
No. 4 OilGallon145100
No. 5 OilGallon148800
No. 6 OilGallon152400
Natural Gascu. ft.950 - 1050
Propanecu. ft.2550
Butanecu. ft.3200

Source: engineeringtoolbox.com

Engineering Notes

  • Source unit notes: 1 Btu = 1055.06 J, 1 U.S. gallon = 3.785e-3 m3 = 3.785 dm3 (liter), and 1 ft3 = 0.02832 m3.
  • Air-fuel ratio matters most. Too much excess air cools the furnace and carries enthalpy up the stack; too little air causes incomplete combustion, unburned fuel, and visible smoke. Modern burners with O₂ trim controls can push combustion efficiency above 85 %.
  • GCV vs NCV. Most published fuel heating values are GCV. Because boiler flue gases are almost never condensed, the useful heat available corresponds to NCV. For hydrocarbon fuels NCV ≈ 90 % of GCV; for hydrogen-rich fuels (e.g., natural gas) the gap can be closer to 10–11 %.
  • Radiation and convection losses are typically 1–3 % of full-load input for well-insulated, water-tube units and can be higher for older fire-tube designs, especially at low load.
  • Blowdown and soot reduce effective output. Regular blowdown to control water chemistry and periodic soot blowing on the fire-side are essential for maintaining rated efficiency over time.
  • Part-load operation degrades efficiency because fixed standby losses (radiation, convection, auxiliary power) represent a larger fraction of input at reduced firing rates.
  • The natural-gas entry in the table is a rounded midpoint; pipeline gas composition varies by region and season.

References

Source: engineeringtoolbox.com