In this article we will discuss about the meaning and types of draught.
Meaning of Draught:
In a boiler, the combustion of the fuel requires supply of sufficient quantity of air and removal of exhaust gases and this is achieved by draught system.
The circulation of air is caused by a difference in pressure, known as draught. Thus the draught is the difference in pressure between the two points i.e., atmosphere and inside the boiler. It is measured in mm of water. A differential in draught is required to cause flow of air or gas through the boiler setting. This required differential is proportional to the square of the rate of flow.
The draught may be produced either naturally or artificially (mechanical). Natural draught is produced thermally and the mechanical draught is produced by means of a fan or a steam jet. Steam jet is mostly used in steam locomotives. Draught is necessary to overcome the resistance to the flow of air and combustion gases and this resistance is contributed by ducts, chimney, fuel beds, boiler gas passages, baffles, dampers, air preheaters, economisers, deposits of soot and dust in gas passages and the required rate of combustion.
Types of Draught:
ADVERTISEMENTS:
i. Natural Draught:
The natural draught is provided by the action of chimney or stack and is used only in small boilers. Its intensity depends upon the average temperature difference between the flue gases within the chimney and the outside air (the gases within the chimney are at a higher temperature than that of the surrounding air) and also on the height of the chimney above the level of the furnace grate. Its intensity is also affected by weather conditions and boiler operating conditions. Chimney, in addition of providing natural draught, helps in reducing air pollution too, as it delivers the products of combustion and fly ash to a high altitude.
ii. Mechanical Draught:
Artificial or mechanical draught is provided when the natural draught caused by a chimney is not sufficient or where a certain draught is required to be maintained irrespective of weather conditions or boiler operating conditions. In case of large steam boilers where economisers and air preheaters are employed, the exit temperature of the flue gases is sufficiently lowered and also the volume of air required is tremendously high.
ADVERTISEMENTS:
In such cases the height of the chimney to cause the required draught may be excessive in height and cost. Under such conditions, mechanical draught is essential. The boilers with mechanical draught do not require a chimney of such a height as is necessary with natural draught.
In a mechanical draught system, the movement of air is due to the action of a fan. A mechanical draught may consist of induced draught or forced draught or both. The fans, in all these cases, have high efficiency aero foil blades inclined backward to the direction of rotation. In an induced draught system, the blower is installed near the base of the chimney and the burnt gases are sucked out of the boiler, reducing the pressure inside the boiler to less than atmospheric one.
This induces fresh air to enter the furnace. In case of a forced draught system the blower is installed near the base of the boiler. In this system of draught the air pressure throughout the system is above the atmospheric pressure and air is forced to flow through the system (furnace, economiser, air pre-heater and chimney).
In most of the cases, a forced or induced draught alone is not sufficient and to achieve advantages of both the systems a balanced draught system (combination of induced draught and forced draught systems) is used. In this system, the pressure of air below the boiler grate is above atmospheric and that above the grate is below atmospheric.
ADVERTISEMENTS:
The induced draught blower helps in sucking the products of combustion out of the system whereas the forced draught helps in overcoming the resistance inside the boiler, thus enabling supply of sufficient air to the fuel bed for proper and complete combustion.
Mechanical draught, though needs additional investment and maintenance, is essential for proper operation of modern large size boilers.