No room for squirrel cage

Mercury · #1 2006-10-08, 11:43am

I'm upgrading the ventilation system in my shop which is built into a 6x10 trailer. I'm using a domestic kitchen hood 30" wide with side baffles. It sits right against the wall and is attached to the ceiling which is quite low. I would like to get about 700 CFM of flow and the air would be vented directly through the wall behind the hood outside so there's no static pressure to deal with.

I've been looking at forums and ventilation web sites for nearly a week now and am having the hardest time finding a fan that will work for me. Unless it is pretty small it won't fit under the hood which is only 6" deep so that leaves mounting it either in the wall in the case of a standard axial fan or having a weatherproof unit I can mount outside. I've also looked at inline duct fans and found the GroTek Hurricane fan made for greenhouse venting that can withstand moisture and continuous operation. It pushes 630 CFM through a 6" duct and is near the top of my list of possibilities. I've also looked at restaurant style ventilators and exterior powered vents for domestic range hoods. Some of them create some pretty good flow (up to 1200 CFM) but they are very large and not cheap.

Does anyone have any good ideas or input on what fan I should consider?

Mark

MikeAurelius · #2 2006-10-09, 5:51am

It isn't totally necessary to have a completely weather proof fan to mount outside.

You can create your own weather proof housing for the fan by building a plenum box that the fan is mounted in.

It should be made of either galvanized or painted metal to prevent rusting.

It should have an access door so you can get to the fan for servicing and cleaning.

All seams should be sealed with silicone sealant, then covered in metallic duct tape.

The fan can be mounted so the exhaust points out of the box, then put a round duct adapter on one end of the plenum box and mount your ducting to that.

Be sure to use flexible waterproof conduit and the proper fittings to ensure that moisture cannot enter the fan through the wiring - don't use just plain rubber coated wiring - you do need moisture proof conduit.

6" is pretty small diameter for most ventilation ducting, there's no rule that says you cannot use a size increaser to go from 6" to 8", just don't go from 8" to 6"!

Mercury · #3 2006-10-09, 10:44am

Thanks for your input Mike. I'd still prefer not to build such an enclosure. Is there any reason why you'd recommend a squirrel cage blower over another fan type given that there's no ducting or static pressure to consider?

MikeAurelius · #4 2006-10-09, 10:54am

Cost basically. Squirrel cage fans are much less expensive and easier to maintain than axial fans.

Comparing prices for two identical output fans, an axial fan is going to cost on the order of 2 to 3 times what a squirrel cage would. Additionally, squirrel cage fans are typically going to have better performance across the range of static pressures than an axial fan is - and that is because of the design.

Mercury · #5 2006-10-09, 1:15pm

Okay, makes sense although the squirrel cage fans I was looking at were about the same price as the centrifugal inline fans. I'm not looking at a traditional axial fan. It's meant for pushing air with a significant duct run and/or static pressure.

One I was considering can be seen here: http://cgi.ebay.com/CAN-FAN-8-INCH-H...QQcmdZViewItem

The other fan I'm considering is actually a squirrel cage fan in a weatherproof housing for exterior mounting. It is meant for kitchen vent hood exhaust and is rated at 600 CFM which I think should be sufficient for my hood size. One thing that I'm still not sure of though is the duct size needed to establish a good airflow velocity. The Broan fan mentioned above has a duct diameter of 7". More information about this fan can be found here: http://broan.com/display/router.asp?ProductID=1251

Mark

MikeAurelius · #6 2006-10-10, 6:05am

The other problem with axials is that the motors are typically AO - air over cooling. Meaning that the air stream is what keeps them cool. If you have a large torch (Redmax, Tigershark, CC, Phantom or larger) you are going to have a fairly warm airstream. Most of these fan are not designed to work with airstream temperatures greater than 140 degrees. Temps in excess will lead to motor failure from overheating.

The Broan looks good, but keep in mind that it is not designed for high static pressure applications. I realize you indicated that the run will be very short (essentially through the wall), but if you ever change the set up or move to a different location, this fan will probably not work for you.

Mercury · #7 2006-10-13, 1:12pm

Okay, thanks to the suggestions I've received on this board I've finally settled on an 800 CFM squirrel cage fan which I'll be installing tomorrow. I'll be hooking it up to a solid state variable speed control so I can dial in my optimum speed.

If anyone else is interested in a solid low cost squirrel cage fan, you can find the one I got that lists for $229 but is available for $99 here: http://www.mdhydro.com/squirrel-cage-fans.html The variable speed controler is also available on this site here: http://www.mdhydro.com/sostspco.html

I'd like to run my numbers by the folks on this board before I do the duct work just to make sure I'm on the right track.

Since I already have a 10" x 3.25" standard kitchen hood duct installed in the wall I'd like to stay with this if possible. Since the duct is going through the wall with no additional duct run I shouldn't incur much of a static pressure volume loss but I do plan to have one 90 degree bend in the ductwork outside.

So, the system I envision is an 800 CFM squirrel cage fan pulling through a 10" x 3.25" duct, 90 degree elbow that also goes into 8" round duct before connecting to the fan.

I figure my air velocity would be 3555 at full speed so I'd probably run it at more like 600 CFM most of the time unless fuming or dealing with other heavier dust or fumes. According to my calculations that would give me an air velocity of more like 2666 which is about the airspeed that Mike has suggested in his Ventilation 101 article. The one thing I'm not sure about is how much of a drop in CFM I could expect from pulling air through the 10" x 3.25" duct?

Any other suggestions or comments are always appreciated too.

Mark

GlassyEyedGirl · #8 2008-12-02, 1:59pm

I know you already bought a fan, and I wish I would have seen your post before I got mine, but I got this 10" axial fan that comes with a shutter that opens when the fan is turned on for $162 (free shipping):

http://store.h-mac.com/tpishmodidre1.html

I have it mounted in a window, and a box-type workstation that is pushed all the way up against the wall that has the window in it so it exhausts directly from my workspace. It is listed as 1325 - 2280 cfm, which sounds like a lot, and I thought it would suck the dots right off my beads, but it really doesn't. I use a bobcat, so I am thinking heat won't be much of an issue, either. Anyone with an opinion is free to chime in if I am way off-base!

Hope your fan works out for you Mark!

Beth