Efficiency

Chapter from the Fired Heaters Design Guide.

HEATER EFFICIENCY CALCULATION

Efficiency Calculation Methods

DIRECT METHOD (Input-Output):
η = Q_absorbed / Q_fired × 100%
• Q_absorbed = process duty (BTU/hr) from heat balance
• Q_fired = fuel flow × NHV (or LHV)
• Simple but requires accurate fuel flow measurement

INDIRECT METHOD (Heat Loss — preferred for field evaluation):
η = 100% - ΣLosses
Losses include:
• Stack loss (largest): sensible heat in flue gas above ambient
• Radiation & convection loss from casing (1-3% typical)
• Unaccounted losses (0.5-1%)

Stack Loss Calculation:
Q_stack = m_flue × Cp × (T_stack - T_ambient)
• m_flue from combustion stoichiometry (fuel composition + excess air)
• Cp of flue gas ≈ 0.25-0.27 BTU/lb·°F
• T_stack from stack thermocouple

Efficiency Improvement Opportunities

Opportunity	Typical Savings	Implementation	Risk/Limitation	Payback
Reduce excess air	1-2% per 5% excess air reduction	Burner tuning, O2 trim control	CO formation if too low	Immediate (operational)
Add convection surface (APH)	5-10% efficiency gain	Air preheater installation	Acid dew point corrosion, cost	1-3 years
Install economizer	3-5% efficiency gain	Water/oil preheating coil	Flue gas ΔP, acid dew point	1-3 years
Repair/replace insulation	0.5-2% per heater	Refractory and casing repair	Access during turnaround	<1 year
Optimize draft	0.5-1%	Stack damper adjustment	Flame instability if too low	Immediate
Combustion air preheat	1% per 40°F air preheat	Recover stack heat to combustion air	Burner design must accommodate	2-4 years

Source: Fired_Heaters_Design_Guide_v1.xlsx · sheet “Efficiency”

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