- 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.
Draught is defined as the pressure difference that causes air or gas flow within a boiler system. It is needed mainly for two reasons.
- To supply sufficient air for completing the combustion.
- To remove flue gases from the system after combustion and the heat exchange.
There are two types of draught applied to the boiler system.
- The natural draught
- The forced draught
We will discuss here in this article about natural draught. The natural draught is always preferred since it does not need any running cost although it has a big initial cost. Natural draught allows natural circulation of air through the boiler system. The natural draught mainly depends upon the height of the chimney.
To calculate the required chimney height for natural draught in a boiler system, we use two basic gaseous pressure equations. The equations are:
Where, “P” is the pressure of air or gas, “ρ” is the density of the air or gas, “g” is the gravitational constant, and “h” is the height of the head.
Here “V” is the volume of the air or gas, “m” is the mass of the gas or air, “T” is the temperature measured in kelvin scale and “R” is the gas constant.
Equation (2) can be rewritten as
During the combustion process in the furnace, carbon reacts with oxygen (O2) to form carbon dioxide (CO2). The volume of solid carbon is negligible compared to the air needed for the reaction. Therefore, we assume the volume of air needed for combustion is equal to the volume of flue gases produced, if the temperature before and after combustion is the same. However, this isn’t accurate because air entering the combustion chamber increases in volume due to combustion heat, matching the volume of flue gases produced.
Let us assume, ρo is the density of the air at 0oC or 273 K, and say it is To
Here, P is the pressure of air at 0oC or 273 K, that is at To K.
If the pressure P remains constant, the relationship between the density and temperature of air or gases can be written as:
Where, ρa and ρg is the density of the air at temperature Ta and Tg K respectively.
From, equation (1) and (5) we can write the expression of pressure at point “a” outside the chimney, as
The volume of the air at the temperature Tg would be
Let us assume, m kg of air is required to burn 1 kg of carbon then the density of the flue gas would be
The pressure of the flue gas inside the cheimny from equation (1) and (8), would be
The pressure difference between the inside and outside of the chimney can be calculated using equations (6) and (9):
Here, “h” is the minimum height of the chimney to be constructed for the draught ΔP. The flue gas will flow upwards through the chimney due to this pressure difference. So, by calculating this pressure difference one can easily calculate the approximate height of the chimney to be constructed. The pressure difference can be represented as a formula for calculating the height of the chimney for a natural draught.
