Chimney Sizing

Overview

Engineering reference data for Chimney Sizing 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

Chimney Draft Calculator

The chimney draft pressure difference can be calculated using the derived formula based on the Ideal Gas Law. This tool calculates the draft for a chimney given its height and temperatures.

The formula used is: $dp_{ch} = h \cdot \rho_o \cdot \left(1 - \frac{T_o}{T_i}\right) \cdot a_g$ where:

• $T_o$ and $T_i$ are absolute temperatures in Kelvin (K).

Chimney Sizing Guidelines

Beyond calculating the draft, proper chimney design requires sizing the cross-sectional area to handle the flue gas volume.

The required chimney area $A$ is calculated as: A = \frac{Q}{v} \tag{4} where:

• $A$ is the cross-sectional area (m²).
• $Q$ is the volume flow rate of flue gases at chimney temperature (m³/s).
• $v$ is the gas velocity (m/s).

Key Design Constraints:

• Small Furnaces: Flue gas velocity should not exceed 2 m/s.
• Large Furnaces: Design velocity should not exceed 10 m/s.

General Sizing Rule of Thumb: For a boiler with a heat output $P$, the required chimney area $A$ can be approximated as: $A \approx \frac{P}{1 \text{ kW}} \times 1100 \text{ mm}^2$ This means 1 kW of boiler heat input generally requires 1100 mm² of chimney cross-sectional area.

References

• Original Source