A forced draught fan supplies air at 10 m/s against a draught of 20 mm of water across the fuel bed. Estimate the power required to run the fan if 2500 kg/h of coal is consumed and 16 kg of air is supplied per kg of coal burned. The temperature of the flue gas and the ambient air may be taken as 600 k and 300 k respectively. If the forced draught fan is replaced by an induced draught fan, what will be the power required to drive the fan?
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In power plants, are both primary air fans and forced draught fans used together, or do we just need to choose one of them?
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6 ANSWERS
Ankur Prabhakar, Piping Engineer in BHEL
Answered Aug 12, 2015
Both the fans i.e. PA Fan and ID fans are used in boiler. And both have different functions.
Primary air fan is used to transport thepulverized coal from mills to the furnace area. So its main function is the carry the fuel to the furnace for the combustion.
Whearas the FD fan provides the secondary air to the furnace area in the boiler where combustion takes place. As we all know that oxygen is required for combustion, this oxygen is provided by the air which is supplied by the forced draught fan.
Coming back to your question there is a condition where only FD fan is used in boiler and PA fan may not be used. It is possible when the boiler is fired by using only oil support that is when no coal is fired in the boiler. So in that case when coal needs not to be transported to the furnace no need of PA fan arises. In that case only FD fan runs.
Otherwise during the normal working conditions both FD and PA fans runs
==========≠===============================
Role of PA & SA Fans in Thermal Power Plants
For combustion of any fuel to take place, 3 basic ingredients are needed:
FuelAir (Oxygen to specific)and heat
Air provided for combustion reacts with the fuel according to its stoichiometric ratio. But In real some percentage of air which has been provided for combustion goes unreacted.
This generally happens because of thequantity of fuel that is fed at once and variation in the sizes of fuel particles. So, some amount of an extra air (usually some percentage of an air provided initially) must be provided to compensate for an air that went unreacted.
Read Also: Combustion Air Calculations
Primary Air Fan ( PA Fan)
Steam boilers having an external furnace are generally provided with both Primary ( pa fans ) and Secondary air inlets and both play an important role in the combustion of fuel.
Primary air is generally the basic amount of air required for complete combustion of fuel and it depends upon the composition and quantity of fuel required by the boiler. Different fuels will require a different amount of primary air.
Primary air is generally provided from the bottom through the Airbox and the fuel is fed either manually or through the screw feeder into the furnace.
Primary air fans also help in segregating the fuel particles so that each fuel particle could come in direct contact with the air resulting in efficient combustion of fuel particles.
Secondary Air Fans
Secondary air, on the other hand, is generally provided to compensate for the primary air that went unreacted. It is generally said to be an extra air that must be provided for complete combustion of fuel.
Secondary air is generally provided through the nozzles installed on the walls of the furnace.
A Forced Draft Fan designed for a particular boiler must be able to provide a sufficient quantity of air which is the sum of bothPrimary and Secondary air.
The air before entering the boiler is made to pass through an Air Preheater (APH) where the inlet air to the boiler gets heated by the outgoing flue gases and then this preheated air helps in reducing the moisture content of the fuel and further increases the boiler efficiency.
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6 ANSWERS
Ankur Prabhakar, Piping Engineer in BHEL
Answered Aug 12, 2015
Both the fans i.e. PA Fan and ID fans are used in boiler. And both have different functions.
Primary air fan is used to transport thepulverized coal from mills to the furnace area. So its main function is the carry the fuel to the furnace for the combustion.
Whearas the FD fan provides the secondary air to the furnace area in the boiler where combustion takes place. As we all know that oxygen is required for combustion, this oxygen is provided by the air which is supplied by the forced draught fan.
Coming back to your question there is a condition where only FD fan is used in boiler and PA fan may not be used. It is possible when the boiler is fired by using only oil support that is when no coal is fired in the boiler. So in that case when coal needs not to be transported to the furnace no need of PA fan arises. In that case only FD fan runs.
Otherwise during the normal working conditions both FD and PA fans runs
==========≠===============================
Role of PA & SA Fans in Thermal Power Plants
For combustion of any fuel to take place, 3 basic ingredients are needed:
FuelAir (Oxygen to specific)and heat
Air provided for combustion reacts with the fuel according to its stoichiometric ratio. But In real some percentage of air which has been provided for combustion goes unreacted.
This generally happens because of thequantity of fuel that is fed at once and variation in the sizes of fuel particles. So, some amount of an extra air (usually some percentage of an air provided initially) must be provided to compensate for an air that went unreacted.
Read Also: Combustion Air Calculations
Primary Air Fan ( PA Fan)
Steam boilers having an external furnace are generally provided with both Primary ( pa fans ) and Secondary air inlets and both play an important role in the combustion of fuel.
Primary air is generally the basic amount of air required for complete combustion of fuel and it depends upon the composition and quantity of fuel required by the boiler. Different fuels will require a different amount of primary air.
Primary air is generally provided from the bottom through the Airbox and the fuel is fed either manually or through the screw feeder into the furnace.
Primary air fans also help in segregating the fuel particles so that each fuel particle could come in direct contact with the air resulting in efficient combustion of fuel particles.
Secondary Air Fans
Secondary air, on the other hand, is generally provided to compensate for the primary air that went unreacted. It is generally said to be an extra air that must be provided for complete combustion of fuel.
Secondary air is generally provided through the nozzles installed on the walls of the furnace.
A Forced Draft Fan designed for a particular boiler must be able to provide a sufficient quantity of air which is the sum of bothPrimary and Secondary air.
The air before entering the boiler is made to pass through an Air Preheater (APH) where the inlet air to the boiler gets heated by the outgoing flue gases and then this preheated air helps in reducing the moisture content of the fuel and further increases the boiler efficiency.
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