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Safety -- Make sure you are safe!

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  #1  
Old 2009-09-29, 7:37pm
Mel. Mel. is offline
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Join Date: Feb 04, 2009
Posts: 8
Default Help? Calling all experts, I still have ventilation setup questions

My current ventilation system is really inadequate......I've been reading thru all the ventilation threads for the last few months now and I want to make sure I'm (or my husband rather) setting up my ventilation system correctly but I still have several questions. I've attached a picture of Lenora's system that I really like but I'm going to do some size modifications which are highlighted in red. I've also attached a link with specs and picture of a Grainger squirrel cage fan that I would like to purchase but need some clarification on before I spend the $200 on it.

My questions are:

1. Based on my numbers (28 x 24 / 144 * 125 = 583), I need a min of 583 cfm fan. Do ya'll agree?

2. Grainger link.......The exhaust part of this fan is rectangular, can I put an 'adaptor' on it that turns it into a circular 8 inch exhaust? (I hope that made sense!)

3. Concerning this squirrel fan, is the 6 inch intake adequate? I am planning on using 8 inch exhaust ducts, non flex of course.

4. What is this thing circled in green? It looks like a mounting area but I'm not really sure because this fan model is a little different from the other fans on the Grainger website.

5. Some fans have an 'open' motor design which requires a separate plenum box built. Is this one of those fans?

6. Is my hood big enough? I am using a Cricket with a 5 LPM oxycon.

7. Am I asking too many questions?


I want to thank everyone for their help in advance!!!!

Regards - Mel



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  #2  
Old 2009-09-30, 7:01am
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JoeDeM JoeDeM is offline
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Location: Barrie, Ontario
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how long is your duct run?

This is from the fan table you linked to
CFM @ 0.000-In. SP 595 free air
CFM @ 0.125-In. SP 560
CFM @ 0.250-In. SP 515
CFM @ 0.500-In. SP 420
CFM @ 0.750-In. SP 300

I would say from this data that this fan is on the border line depending on your duct length. The Box on the side is the electrical connection box.

example 8" @ 3 ft would be .02-In SP which would put it close 595.
8" @ 20ft would be .13-In sp which would be less than 560

Last edited by JoeDeM; 2009-09-30 at 7:12am.
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  #3  
Old 2009-09-30, 7:53am
Dale M.'s Avatar
Dale M. Dale M. is offline
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Location: A Little Bit West of Yosemite Valley
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The open motor design that requires a plenum box will depend on how the motor is connected to the fan assemble through the fan housing.... If there is a opening (large) where motor shaft goes through housing yes it requires a plenum box, if the opening is sealed to air flow (on motor side), it does not require a plenum box..... Considering the fan intake opening has ring on it to accept duct work My guess is it does not require a plenum box, and you can direct connect duct work to fan housing... If the fan has 6 inch adapter ring for duct work on intake side its probably adequate for getting duct from hood to fan housing, as for exhaust side adapting it down to a round opening and using 8 inch duct is probably best solution.... Static pressures is resistance to air flow, so any intake and exhaust ducting does have effect on air flow, so using larger duct were practical is best....

The boxed in area (green circle) is probably for electrical connection AND IS ALSO the support structure for the fan motor....

As for fan specifications its better to go a little stronger fan than minimal size dictated by calculations.... So your your calculation come out to 585CFM I would use a little "fudge factor" in there for static pressure (duct resistance to air flow) and consider something in the 650 or a tad bit larger size....

Dale
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  #4  
Old 2009-10-01, 6:42pm
NMLinda NMLinda is offline
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Location: Herndon, VA
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As JoeDeM and Dale allude, it's important to know the static pressure contributed by the ducting in/out of your fan. If you mount the fan with the input directly at your hood opening, you need only calculate the static pressure caused by the ducting at the output. If you're going straight out the wall behind your hood, you won't likely have much, but will have at least one bend (bends are the biggest contributors). The bigger the ducting you use, however, the lower the static pressure. A good resource to understand how to calculate the static pressure of your ducting is

http://mikeaurelius.wordpress.com/ventilation-primer/

Once you calculate it, you can compare it to the cfm vs static pressure numbers JoeDeM posted for you. As JoeDeM and Dale indicate, you might be on the low side.

Very nice looking hood, by the way. Once you get the right fan matched up to it, you should have an excellent system.

Linda
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