A flue is a duct or pipe for conveying exhaust gases from a fireplace, furnace or boiler to the outer atmosphere. They usually operate by buoyancy, also known as the stack effect, or the combustion products may be 'induced' via a blower. Important criteria in ducts design are:
1.Normal gas velocity to prevent the entrained dust falling and collecting in the flue.
2.Withstand a depth of dust equal to 10-20% of the duct height.
3.Change in cross sectional area gradual
4.Gas velocity and temperature distribution must be uniform at the entry to heat exchangers and dust collectors.
5.Flow in to bend should be even
6.The inner radius of the bend should be as large as the layout permits.
7.Flow splitters should be provided at bends.
8.Flues should be as tight as possible so that
It will reduce the temperature of the metal around the leak, leading to corrosion.
With reduction in stream temp. gas density and weight both increases which increases the fan power consumption and decrease in draught.
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