Draught System- Natural and Artificial draught system
Posted Date: 15-Jun-2011
By this article I will explain draught system, which generally used in many industries like coal gas plants and steel industries to decrease the environmental pollution. Here I will explain different types of draught system that are generally used.
Because of the emission of large amount of flue gases and other materials environment is polluted, thus to decrease the environmental pollution some techniques and equipments are used. Generally Electrostatic precipitators and Draughts system is used by coal gas plants to decrease the environment pollution. Here we explain the brief about the Draught system.
Draught are of two types:
The natural draught is obtained with the use of tall chimney which may be sufficient or insufficient to overcome the losses in the system. Its usefulness depends upon the capacity of the plant and duct work. This system of producing the draught is useful for small capacity boilers and it does not play much important role in the present high capacity thermal power plants. A chimney is a vertical structure of masonry; brick, steel or reinforced concrete built for the purpose of enclosing a column of hot gases to produce the draught and discharge the gases high enough which will prevent an air pollution the draught produced by the chimney is due to the temperature difference of hot gases in the chimney and cold air outside the chimney. Consider the height of the chimney above the grate level is ‘H’. The pressure acting on the grate from the chimney side
P1 = Pa +Wg H
And the pressure acting on the grate from the atmospheric side
P2 = Pa +WaH
Where Pa is the atmospheric pressure and Wa, Wg are the weight densities of the atmospheric air and hot gases passing through the chimney. The gas density varies along the height of the chimney as part of the heat is lost by the gas to the chimney. Therefore the average density of the gas should be taken for circulation.
The net acting pressure on the grate of the combustion chamber due to the pressure exerted by gas column and air column is given by:
P=P2 – P1
Artificial draught can be further classified as:
In a forced draught system, a lower is installed near the base of the boiler and air is forced to pass through the furnace, flues, economizer, air-preheater and to the stack. This draught system is known as positive draught or forced draught system because the pressure of air throughout the system is above atmospheric draught system or forced draught system because the pressure of air throughout the system is above atmospheric pressure and air is forced to flow through the system. A stack or chimney is also used in this system but its function is to discharge gases high in the atmosphere to prevent the contamination. It is not much significant for producing draught therefore height of the chimney may not be very much.
In this system, the blower is located near the base of the chimney instead of near the grate. The air is sucked in the system by reducing the pressure through the system below atmosphere. The induced draught fan sucks the burned gases from the furnace and the pressure inside the furnace is reduced below atmosphere and induces the atmospheric air to flow through the furnace. The action of the induced draught is similar to the action of the chimney. The draught produced is independent of the temperature of the hot gases therefore the gases may be discharged as cold as possible after recovering as much heat as possible in the air-preheater and economizer.
This draught is used generally when economizer and air-preheater are incorporated in the system. The fan should be located at such a place that the temperature of the gas handled by the fan is lowest. The chimney is also used in this system and its function is similar as mentioned in forced draught but total draught produced in induced draught is the sum of the draught produced by the chimney and the fan.
It is always preferable to use a combination of forced draught and induced draught instead of using any one of these system alone. If the forced furnace is used alone, then the furnace cannot be opened either for inspection or for firing because the high pressure air inside the furnace will try to blow out suddenly and there us every chance of blowing out the fire completely and furnace stops. If the induced draught is used alone, then also furnace cannot be opened either for firing or inspection because the cold air will try to rush into the furnace as the pressure inside the furnace is below the atmospheric pressure. This reduces the effective draught and dilutes the combination. To overcome both the difficulties mentioned above either using forced draught or induced draught alone, a balanced draught is always preferred. The balanced draught is a combination of forced and induced draught. The forced draught overcomes the resistance of the fuel bed therefore sufficient air is supplied to the fuel bed for proper and complete combustion. The induced draught fan removes the gases from the furnace maintaining the pressure inside the furnace just below atmosphere. This helps to prevent the blow-off of flames when the doors are opened as the leakage of air is inwards. The pressure inside the furnace is near atmospheric so there is no danger of blowout of flames or there is no danger of inrushing the air into the furnace when the doors are opened for inspection. The pressure of air below the grate is above atmosphere and it helps for proper and uniform combustion. The pressure of air above the grate is just below the atmosphere and it helps to remove the exhaust gases as quick as possible from the combustion zone.
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