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BOILER DRAUGHTS
Definition : The small pressure difference which causes a flow of gas to take place is termed as a draught.
Function of Draught
The main function of the draught, in case of boiler is to, force air to the fire and to carry away the gaseous products of combustion.
In a boiler furnace, proper combustion takes place only when sufficient quantity of air is supplied to the burning fuel.
TYPES OF DRAUGHT
It is of two types mainly
1. Natural Draught 2. Artificial Draught
Artificial Draught is classified as
1. Steam Jet 2. Mechanical Draught
a. Induced draught a. Induced draught
b. Forced draught b. Forced draught
c. Balanced draught
Necessity of Draughts in boiler are :
To force the air through the fuel bed for the fuel combustion.
To Exhaust the gases of combustion from the combustion chamber.
To discharge these gases to the atmosphere through the chimney.
1. Produced by chimney. So called as Chimney Draught.
2. Produced due to difference of densities between hot gases inside chimney & cold atmosphere outside
Artificial Draught
The draught in which it is necessary to provide some mechanical means to meet the larger draught requirement is called as artificial draught.
1. Forced draught – The fan is placed at the base of boiler grate. Also called as Positive draught as pressure through the system is above the atmospheric pressure.
2. Induced draught – Fan is placed at the base of chimney. Vacuum inside the system. Used when boiler is with air pre heater & economizer.
3. Balanced Draught – Combination of above two.
Forced Draught
In a forced draught system, a blower is fitted near the base of the boiler [before grate ] and air is forced to pass though the various elements as shown in Fig.
In a mechanical draught system, the draught is produced by a fan. It is a positive pressure draught.
As air pressure throughout the system is maintained above the atmospheric pressure, this system is known as positive system draught.
Ensure for the furnace has to be very highly sealed so that, gases from the furnace do not leak out in the boiler house.
Height of the chimney is less than the natural draught system.
Induced Draught
Working principle
In this system, a fan or blower is located at or near the base of the chimney.
The pressure over the fuel bed is reduced below that of the atmosphere by creating a partial vacuum in the furnace and flues.
The products of combustion are drawn from the main flue and they pass up the chimney.
This draught is used usually when a economizer and air preheaters are operates in the system.
The draught is similar in action to the natural draught.
BALANCED FAN DREAUGHT
Working principle
A combination of forced induced draught is employed using two fans, one, before the grate for supply of air and another, at the base of the chimney for drawing the hot flue gases. Such a system is called balanced draught.
The forced draught fan forces the air through the fuel bed on the top of the grate, and the induced draught fan sucks in gases from the boiler side and discharge them to the chimney
NATURAL DRAUGHT & ARTIFICIAL DRAUGHT
NATURAL DRAUGHT | ARTIFICIAL DRAUGHT | |
1 | It reduces the necessary height of chimney. | It does not reduces the necessary height of chimney. |
2 | It allows to burn low grade fuel. | It does not allow to burn low grade fuel. |
3 | It reduces smoke. | It does not reduce smoke. |
Difference between Mechanical draught and Natural draught
Sr. No. | Mechanical Draught | Natural Draught |
1. | The rate of combustion is more as the available draught is more. | The rate of combustion is low as the available draught is limited. |
2 | Fuel consumption per kW is 15% less than for natural draft. | Fuel consumption per kW is more. |
3. | The efficiency of artificial draught is 6 to 8%. | The efficiency of chimney draught is about 1%. |
4. | Low capital cost. | High capital cost. |
5. | Fuel burning capacity of grate is more. | Fuel burning capacity of grate is less. |
6. | Less maintenance | High running and high maintenance cost of fan. |
7. | The height of chimney used is less and independent of draught needed. | The height of chimney used is more as designed by draught’s need. |
Difference between Forced and Induced draught
Sr. No. | Forced Draught | Induced Draught |
1. | Blower is placed near the base of the boiler and forced to pass through various elements of system. | Blower is placed near the base of chimney and air is sucked to pass through various elements of the system. |
2. | Flow of air more uniform. | Air flow is not uniform. |
3. | Does not required water cooled bearing. | It requires that, water be cooled bearing for induced draught. |
4. | Small fan size is required. | Big fan size is required. |
5. | Comparatively less power is required for the forced draught fan. | More power is required for the induced draught fan. |
6. | No chance of air leakage in the furnace. | In this system, continuous air leakage takes place. |
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