It is universal law that fluid flows when there is pressure difference. If this difference is large, we will experience large flow but if is low, we will experience small flow. In boilers, we refer this pressure difference as boiler draught.
In boilers it is necessary for us to create pressure difference in order to remove the products of combustion as well as to enter fresh air inside boiler.
In boilers, we use large chimneys or stack. The main reason is that some of the combustion products are dangerous for health so we have to leaves these products at certain height where they will not affect our health.
There are two types of boiler draughts which are following
- Natural draught
- Artificial draught
In natural draught boilers, pressure difference is created naturally and we don’t need to use any external device like positive displacement device for this reason.
But in case of artificial draught boilers, pressure difference is created by using external mechanical device or steam. There are further types of artificial draught, induced type, force type or the combination of both.
There are different reasons to use draught inside boiler like to maintain the sufficient supply of fresh air for combustion process and the removal of combustion product.
Similarly there are different parameters on which draught depends upon like the type of boiler, burning fuel rate, rate of combustion products production and air requirement rate. Normally the draught value is very small so we measure it in terms of mm of water.