Another question for Dale
 

I am finally putting in proper ventilation in my studio and there seems to be a debate brewing over ducting size for cfm's.

Here are some stats.

My hood will be 24" deep and 36" wide.
It will be 23" above the table top at the bottom of the hood
There will be a solid back down to the table top and side baffles all the way down as well
The fan I am looking at is 790cfm 10" vortex fan.
The ducting run will be approx 8" vertical, one 90deg and then about 12" to outside. A very short run.

Here is the million dollar question. Is the cfm rating of the fan good enough if I take the ducting down to 8" or 7"?

It will be one heck of a lot more affordable to go to bigger ducting than to go to a higher cfm fan.

The next size up in these fans is 1140 cfm's and about $100 more.

Here is a drawing of the hood itself....


Opinions, advise always accepted gratefully.

Well you will have a face opening (about 23x36) of 828 sq-in or 5.75 sq-ft...

5.75 sq-ft multiplied by 125 cfm=718 cfm DESIRED.....

I would say you 790 cfm fan is adequate, and with only about 18 inches of ducting and 1 - 90° turn, either 7 or 8 inch duct would be adequate, so you can probably build on what parts cost not what size you have to have....

Dale

Thank you so much for your speedy answer Dale. I appreciate it.

Dale, does the reducer to change the 10" fan to 8" ducting affect the efficiency of the fan? Or is it so insignificant that it doesn't factor in the overall calculations?

ETA: Would it be better to start with an 8" fan, thus eliminating the reducer?

Hayley, the 8" fan is only 747cfms which is cutting it a bit close....that being said, I see your point of view on it. According to the calculations I need 718 cfms for a straight run.

Now while my ducting run will be very short there is that one 90 deg in it and I'm not sure if that will reduce the cfm's enough to be not enough if you know what I mean.

Off to do more research. Good thing I am still in the planning stages. I am also consulting with a HVAC guy on Monday about fabricating the hood and am curious as to what he will recommend.

Fans have a outside diameter that are dictated by the construction methods of the fan, this is not necessarily the size of the duct required for fan to operates efficiently.... Fans operate against what is called "static pressure" or resistance to flow... Long duct runs have more static pressure that short runs, small diameter duct runs have more static pressure than larger diameters duct runs.... In the end, if you have fan that is operating efficiently against low static pressures and a test of your ventilation proves it is working effectively you are good....

Dale

Thank you for the explanation, Dale - I guess Leslie's system would work if her the actual fan size is more in line with the 8" ducting since she has such a short duct run. I have read quite a few older posts against reducing ducting, that's why I asked.