The expression of the pressure difference between inside and outside of a chimney shows that it has a term which is the height of the chimney. To achieve sufficient pressure difference between two points or to attain enough natural draught we have to increase the height of the chimney which is not always practical in the view of cost-effectiveness. In those cases, we have to think about artificial draught instead of natural draught. We can achieve artificial draught, mainly with two methods. One gets produced by steam jet, and forced air produces another. Introducing artificial draught one can significantly reduce the height of the chimney to fulfil the same purpose removing flue gasses to the atmosphere.
Here, an air fan is attached to the base of the chimney or nearer to it. When the fan rotates, it sucks the flue gases from the boiler furnace system. The suction of flue gases from the furnace producers a pressure difference between the outside air and inside flue gases, and which creates a draught. Due to this draught, fresh air enters the furnace. As the draught gets induced due to the suction of gases, we call the method as induced draught. ID fan or induced draught fan sucks the flue gases from the boiler system and pushes to atmosphere through the chimney along its height. In natural draught, the temperature of the flue gases plays a vital role in traveling the gases through the chimney to the atmosphere. But in the case of induced draught, the temperature of flue gas is not an essential parameter. We can utilize the heat energy of the flue gases as much as possible. In induced draught after extracting almost entire heat from the flue gases, the cold flue gases, we remove the gases to the atmosphere forcefully. So we can achieve the purpose of a very tall chimney with much shorter one which is a significant saving of money for the system.
Theoretically induced draught and forced draught are more or less same. The only difference is that in induced draught the suction fan is used and in case of forced draught the blower fan is used. In case of forced draught system, we fit the blower fan before the coal bed. The blower blows the air from the atmosphere to the coal bed and grate where flue gases get created after combustion. The fresh air (preheated) coming to the furnace pushes the flue gases inward. The flue gases then pass through the economiser, air preheater, etc. toward the stack of the chimney. The forced draught creates positive pressure inside the system. Because of that, we should take special care to protect the system from any leakage otherwise the performance of the system gets affected.
Balanced draught is a combination of forced draught and induced draught. Here we attached blower fans at the entry point of the furnace and induced fans at exit point as well. Here, we utilize benefits of both forced and induced draught. In the balanced draught system, we use forced draught to push the air to the coal, to the grate and later to air preheater. We use the induced draught to withdraw the gases from the economiser and air preheater etc., and then ultimately to discharge the gases to the top of the stack of the chimney.