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Intermittent Boiler Efficieny

Reference data and engineering information about intermittent boiler efficieny for combustion applications.

intermittentboilerefficieny

Overview

Engineering reference data for Intermittent Boiler Efficieny in combustion engineering.

Key Formulas

Heat Release

Q=m˙⋅HVQ = \dot{m} \cdot HV

Fuel energy release rate.

Air-Fuel Ratio

AF=mairmfuelAF = \frac{m_{air}}{m_{fuel}}

Mass of air per mass of fuel.

Excess Air

EA=O221−O2×100%EA = \frac{O_2}{21 - O_2} \times 100\%

From flue gas oxygen measurement.

Variables

SymbolDescriptionUnit
QQHeat release rateW
m˙\dot{m}Mass flow ratekg/s
HVHVHeating valueJ/kg
AFAFAir-fuel ratio—

Effects of Intermittent Operation

Intermittent boiler operation significantly impacts overall combustion efficiency through several energy loss mechanisms:

Primary Energy Losses

  • Flue Gas Losses: Energy is lost through exhaust gases in two ways:

    • Excess fuel condition: Unburned fuel escapes with flue gases
    • Excess air condition: Additional air beyond stoichiometric requirements absorbs heat and carries it out the stack

  • Radiation and Convection Losses: Heat dissipates from the boiler's exterior surfaces during all operating conditions

Impact of Cycling Frequency

Operating ConditionRadiation/Convection Loss Impact
Continuous operationBaseline losses
Frequent cyclingIncreased losses per unit of useful heat
Short firing periodsHighest relative losses

Key Principle

ηintermittent=ηsteady−Δηcycling\eta_{intermittent} = \eta_{steady} - \Delta\eta_{cycling}

Where:

  • ηintermittent\eta_{intermittent} = efficiency during intermittent operation (%)
  • ηsteady\eta_{steady} = steady-state combustion efficiency (%)
  • Δηcycling\Delta\eta_{cycling} = efficiency penalty from cycling losses (%)

As combustion time decreases and cycling frequency increases, the ratio of standby/radiation losses to useful energy output increases, reducing overall system efficiency. This effect is particularly pronounced during low-load conditions and frequent on/off cycling scenarios common in seasonal heating applications.

References

Source: engineeringtoolbox.com