Can you estimate CFM from RPM & fan size?

Question

I'm setting up a lampworking shop & need to ventilate it. Some folks are using 350cfm range hoods. The ventilator I like that is actually built for lampwork is 650cfm.

My son is going to build the hood-- and I have a squirrel-cage fan that I think will move a lot of air. Is there any way to guesstimate if I'm going to be in the 6-800 cfm range?

The motor is 1/6 hp, 2 speed- 1175rpm. [at fast speed?] The cage is about 4inches across and 12 inches in diameter.

The hood will be 40x24x18inches [40x24 opening]. The ductwork will probably[I'm open yo suggestions] be 5' of 10" flex for now. [3 horizontal and 2 vertical- no sharp bends]. In a fwew months we move it all and it will be rigid ductwork- 1' through a wall, then vertical to a cap.

Thanks, Jim

zafdor · Accepted Answer

I would not be too optimistic that you would be able to get 600 CFM outof a 1/6HP motor.

SQLit · Answer

Direct-drive, .33HP electric motor features a spark- resistant 7.625-inchimpeller wheel. Runs at 1750 RPM; provides up to 1015 cubic feet per minuteof free air displacement @ O" static pressure. Other sizes available.NOTE THE 0" static pressure!further quote from page The hood features a 10 " round discharge; this allows easy connection toyour in-house exhaust system to provide the internal negative pressurenecessary to safely remove fumes from the work surface. This hood iscompletely noncontaminating and resists a wide range of corrosive materials;select stainless or cold rolled steel.

Edwin Pawlowski · Answer

Go to   and check out the blowers on page 591.  There is a 1/6 HP and 12" dia blower that moves 185 cfm

Jim Elbrecht · Answer

Thanks - good idea.

I was real disappointed at first-- but the 12" in that case is overall dimension. The blower is only 5 1/4".

Can't find any indication of how wide those wheels are, but on p590 there's a 1150rpm that is 10 5/8" diameter and has roughly the same outlet size. It is blowing 1500-1870cfm. [and is powered by a 3/4 hp motor!] I hope mine doesn't move that much air.

Jim

Jim Elbrecht · Answer

It is a real old, bulky, constant capacitor motor. [60's?] I don't know if the rating system changed or what, but I have a slightly older 1/12 motor that seems to of the work of a more modern 1/3hp without bogging down.

That said-- should hp really have anything to do with the cfm's, or are speed and fan size the determining factors on that end?

Jim

Jim Elbrecht · Answer

-snip-That's encouraging.    I think I'll be in the ballpark.I'll have to see if the school is buying the materials.  Maybe SSwould be a good metal for my son to practice on, eh?Thanks,Jim

Can you estimate CFM from RPM & fan size?

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