Building Homes or Extensions/building services


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.



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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Dan Griffin


I can answer almost all questions related to the total construction process. My expertise is in commercial construction, though I can field most any residential question. I have hands on experience in concrete, heavy equipment, masonry, all phases of carpentry, interior finishes, and I am fairly strong in mechanical and electrical.


I have over 20 years experience as a commercial carpenter and commercial construction superintendent. I have another 20 years experience in facility management for a major school district.

My favorite hobby for he past 12 years has been singing bass in a The OkChorale men's barbershop chorus and the Mature Moments quartet.

I hold a Bachelor's degree in English and Math. I have completed many continuing education hours in the building trades. I hold a Master Carpenter card from the AGC, Associated General Contractors.

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