Intermittent Boiler Efficieny

Overview

Engineering reference data for Intermittent Boiler Efficieny in combustion engineering.

Key Formulas

Heat Release

$Q = \dot{m} \cdot HV$

Fuel energy release rate.

Air-Fuel Ratio

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

Mass of air per mass of fuel.

Excess Air

$EA = \frac{O_2}{21 - O_2} \times 100\%$

From flue gas oxygen measurement.

Variables

Symbol	Description	Unit
$Q$	Heat release rate	W
$\dot{m}$	Mass flow rate	kg/s
$HV$	Heating value	J/kg
$AF$	Air-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 Condition	Radiation/Convection Loss Impact
Continuous operation	Baseline losses
Frequent cycling	Increased losses per unit of useful heat
Short firing periods	Highest relative losses

Key Principle

$\eta_{intermittent} = \eta_{steady} - \Delta\eta_{cycling}$

Where:

• $\eta_{intermittent}$ = efficiency during intermittent operation (%)
• $\eta_{steady}$ = steady-state combustion efficiency (%)
• $\Delta\eta_{cycling}$ = 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

• Original Source