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i have a room whcich is measuring 25*20*4m which requires ventilation by means of a fan and a duct work to provide 10 air changes per hour. the average velocity of air flow in the duct is is 3m/sec. but now the challenge is on carrying out the calaculations to determine the size of the duct which is suitable for that system.

Answer

Livison,

This is not my area of expertise.  I can get you to some of the information.

The room is 25x20x4 = 2000 cubic meters
x 10 changes per hour = 20.000 cubic meters moving through the duct work each hour.

1hr=60min x 60=3600 seconds so you need to move 20000/3600=5.5 cubic m per second

The design airflow is to be 3m/sec
5.5 mc/3= 1.85 square meters duct size.  If the duct were square the duct would need to be
1.36m x 1.36m.  The duct will need to be larger to deal with friction loss and will require additional
calculations for any turns and grille restriction.

I hope this helps.  This is sounding a bit like a homework problem.  I hope we got the right answer.
There are quite a number of duct calculators available on the internet.  Tell Google that you need a metric duct calculator.  I checked some of my calculations and they seem to be in the ball park.

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