NATURAL DRAUGHT

NATURAL DRAUGHTShri Sad Vidya Mandal Institute of technology, Bharuch

Subject : Power plant engineering

Enrollment no : 130450119043 130450119047 130450119054

Guided by : Ass. Professor, Mr. Mayur Makwana SVMIT, Bharuch

Content

Introduction to draught systemFunctions & Classification of DraughtsIntroduction to natural draughtEstimate the height & diameter of chimneyCondition for maximum discharge through a chimneyEfficiency of chimneyAdvantages and Limitations of chimney or natural draught

Draught

Draught is defined as the pressure difference (between absolute gas pressure at any point in a gas flow passage and the atmospheric air outside the boiler) required to maintain flow of air and exhaust gases through the boiler setting.

Function of draught:To supply required quantity of air to the boiler furnace for the combustion of fuel.To remove exhaust gases from the boiler passage to maintain flow of air and gases.To create pressure difference against pressure losses in the flow passage.To discharge the exhaust gases such that they will not be objectionable or injurious to the surroundings.

Losses in the air gas circuit of Boiler:

Classification of Draught

Natural draught

It produces draught which required to force air and gases through the furnace, boiler accessories & setting.It carries the product of combustion to such a height that they will not be objectionable or injurious to surrounding.The chimney is vertical tabular structure build either of masonry concrete or steel.The draught produces by the chimney is due to density difference between the column of hot gases inside the chimney & cold air outside the chimney & also on the height of the chimney above the level of the furnace grate.The density difference depends on the temperature of flue gases passing in chimney & temperature of atmospheric air.

Estimate the height & diameter of chimneyNatural Draught

Natural Draught = Difference of pressure due to cold air column and hot gas column

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Efficiency of Chimney

AdvantagesChimney draught does not require any external power to produce draught.Simple in construction, less cost& has long life.No mechanical parts & hence maintenance cost is negligible.Chimney keeps flue gases at a high place in the atmosphere which prevents contamination of atmosphere & maintains the cleanliness.

LimitationsThe draught produced by chimney depends on height of chimney, atmospheric air tem & tem of flue gases leaving through the chimney. Hence in order to get appropriate draught at given height off chimney, the flue gases have to be discharged at comparatively high temp resulting in the reduction in overall efficiency of plant.The maximum pressure available for producing natural draught by chimney is hardly 10 to 20 mm of water.Draught created by chimney is dependent on atmospheric air &flue gases conditions, hence there is no flexibility to create more draught under peak load conditions because draught available is constant for given height of chimney.Due to low velocity of air, the mixing process of air & fuel is not proper & hence combustion is very poor. This increases the specific fuel combustion of boiler.

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