Combustion Analysis for HVAC Engineers
Combustion Analysis for HVAC Engineers
Combustion efficiency directly impacts operating costs and emissions. Flue gas analysis enables burner optimization, efficiency verification, and troubleshooting.
Stoichiometric Combustion
Natural gas (CH₄) complete combustion:
$$\text{CH}_4 + 2\text{O}_2 + 7.52\text{N}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + 7.52\text{N}_2$$
Theoretical air: 9.52 lb air/lb fuel (2 O₂ + 7.52 N₂ from air)
Actual air: Always exceeds theoretical due to excess air requirement
Excess Air
$$%EA = \left(\frac{\text{Actual Air} - \text{Theoretical Air}}{\text{Theoretical Air}}\right) \times 100$$
From flue gas O₂:
$$%EA \approx \frac{%O_2}{0.264 \times (20.9 - %O_2)}$$
Typical excess air:
- Atmospheric gas burner: 30-50% (4-7% O₂)
- Power burner: 10-20% (2-4% O₂)
- Too low: Incomplete combustion, CO formation
- Too high: Stack heat loss
Combustion Efficiency
Stack loss method:
$$\eta_{comb} = 100 - L_{dry} - L_{moisture}$$
Dry gas loss:
$$L_{dry} = K \times \frac{T_{flue} - T_{air}}{%CO_2}$$
Where K ≈ 0.65 for natural gas
Typical efficiencies:
- Atmospheric boiler: 75-84%
- Power burner boiler: 80-85%
- Condensing boiler: 90-98%
Flue Gas Analysis
Measured parameters:
- O₂: Indicates excess air (target: 2-6%)
- CO₂: Indicates completeness (target: 9-11% for natural gas)
- CO: Carbon monoxide (must be < 100 ppm)
- Flue temperature: Stack loss indicator
CO formation causes:
- Insufficient excess air
- Poor air-fuel mixing
- Flame impingement
- Insufficient combustion chamber volume
Practical Applications
- Annual tuning: Adjust air-fuel ratio for optimal efficiency
- Commissioning: Verify design efficiency achieved
- Troubleshooting: Diagnose sooting, high stack temperature, odors
Related Technical Guides:
References:
- ASHRAE Handbook of HVAC Systems and Equipment, Chapter 31: Combustion and Fuels
- NFPA 54: National Fuel Gas Code