- Natural Draught Definition: Natural draught is the air movement in a boiler system caused by pressure differences, without mechanical assistance.
- Purpose of Natural Draught: It supplies air for combustion and removes flue gases, ensuring efficient boiler operation.
- Chimney Height Importance: The effectiveness of natural draught relies heavily on the chimney height.
- Combustion and Volume Change: Combustion increases gas volume, affecting the draught system.
- Pressure Difference Calculation: Calculating the pressure difference helps determine the necessary chimney height for optimal natural draught.
Natural draught is the pressure difference that moves air and flue gas through a boiler when a chimney, not a fan, creates the flow. Draught is needed for two jobs.
- To supply enough air to complete combustion.
- To take flue gases out of the plant after combustion and heat exchange.
Boiler draught is often split into these two groups on teaching pages. Mechanical plants also use induced draught and balanced draught, which this page does not derive.
- The natural draught
- The forced draught
This page covers natural draught. A tall chimney can avoid fan power, but it costs more to build and gives only a small pressure difference. Large plants usually add fans. The available natural draught rises with chimney height and with the density difference between hot flue gas and outside air.
Chimney height for natural draught is estimated from two gas-column relations:
Here “P” is the pressure of the air or gas, “ρ” is the density, “g” is the acceleration due to gravity, and “h” is the height of the column.
Here “V” is volume, “m” is mass, “T” is temperature on the kelvin scale, and “R” is the gas constant.
Equation (2) can be rewritten as
In the furnace, carbon reacts with oxygen (O2) to form carbon dioxide (CO2). The volume of the solid carbon is tiny next to the air used in that reaction. If the gas temperature did not change, the volume of air supplied would nearly equal the volume of dry flue gas formed from that carbon burn. The furnace heat raises the gas temperature, so the actual flue-gas volume is larger than the cold incoming air volume.
Take ρo as the density of air at 0oC or 273 K, and call that temperature To
Here, P is the pressure of air at 0oC or 273 K, written as To K.
If pressure P stays constant, density and temperature of the air or gas are related by
Here ρa and ρg are the densities at temperatures Ta and Tg K.
From equations (1) and (5), the pressure at point “a” outside the chimney is
The volume of the air at temperature Tg is
If m kg of air is needed to burn 1 kg of carbon, the density of the flue gas is
The pressure of the flue gas inside the chimney, from equations (1) and (8), is
The pressure difference between the inside and outside of the chimney follows from equations (6) and (9):
Here “h” is the theoretical chimney height that would produce draught ΔP if friction and velocity head were zero. Hot flue gas then rises through the chimney. A built chimney must be taller than this hydrostatic estimate so it can also overcome those losses. The same pressure difference is the starting formula for natural draught chimney height.
