Thread: Industrial Fans

Originally Posted by mannum3

I read that 3,4 or 5 H.p. motors are required to shift large amounts of air at high speed to collect shaving, chips, and most importantly fine dust.

I have noticed there are industrial fans for sale, similar to an automotive radiator fan, hitched to a 3 phase electric motor.

Are these suitable to adapt or is a completely different type of fan needed?

Cheers Barry

Nope these axial type fans are unsuitable because their flow cannot be restricted in any way which is another way of saying they are High volume very low pressure devices.
As soon as ducting and machines are attached to them the back pressure generated by theses causes the fan to cavitate (just turn inside a jacket of air) and the flow will drop dramatically.

To collect fine dust about 1000 CFM is needed at the source, to collect the shavings and chips about 4000 Fpm is needed at the source of the dust making activity.

Its not just flow that that matters but what pressure the fan can generate.

To generate 1000 CFM a pressure of about 8" of water column (WC) is needed using a 6" pipe and a pressure of about 35 "WC is needed for a 4" pipe.
Small DCs can generate about 6" WC and bigger DCs can generate about 12" WC but that's about the limit for DCs.
Vacuum cleaners can generate about 30" of WC BUT only for a small flow rate. (~100 cfm)
A 30" WC fan than could move 1000 cfm will sound like a small jet engine - think ~10 vacuum cleaners.

It's not just the absolute pressure that matters but "pressure under back pressure" - this is described by something called a fan curve.

Axial type fans generate about 1" WC and have a very poor fan curve.

It's great when you can get an answer so well explained so quickly.

I had seen this type of fan on auction sites and had wondered if they would be suitable.

OK, so what is the name of the type of fan needed?

Cheers Barry

The fan needed to move larger volumes of air under some restriction (through ducting, cyclones, filters and machinery) is called an impeller.

Axial fans are useful for ventilation through large openings with no restriction like windows

In between these two are squirrel cage fans. They work like an impeller but are much quiete. but don't quite generate the pressure that a well designed impeller can generate.
Squirrels can move a lot more air than impellers but not at very high pressures so are often used for ducted air conditioning and ventilation. They work well provided large ducts and unrestrictive filters are used. At work we had a large squirrel cage fan that supplied one lab that delivered 23000 cfm through a wall (3 x 3 m) of HEPA filters. The motor that drove that fan was 12HP.

I have "googled" fan curves and have ample information to read, digest, contemplate and consider over the next few decades.

Seriously, BobL has put me on the right road with his advice, thanks again Bob.

Cheers Barry

Here's a pic of some of the types of fan you are looking for
pump%20systems.JPG
From left to right, the 2nd, 3rd and 5th are what we find in most dust collectors, the 4th is a more efficient design but restricted to cleaner air such as after a cyclone where all the big stuff has been taken out, the squirrel cage Bob mentions is the 6th and the typical vacumm cleaner design is more like the 9th.



Pete