More venting questions!

[Diane (clarus)]

First of all I want to thank everyone who has contributed to all of the wonderful information regarding how to properly set up ventilation for lampworking!

A few more questions:

Where is the optimum place to put the blower? My setup is going to be a hood over my bench, going up two feet to a 90 degree turn, then a 10 foot run to the outside wall. Is it better to put the blower at the end of the run on the outside wall, or should I put it at the 90 degree angle above the hood?

What is the best angle for the main part of the hood? Mine will be wall mounted 30" above the bench with the hood being at an angle from the face opening up to the entrance to the vent pipe. Is a sharp angle or a shallow angle better for capturing the air, or does it matter?

Is there a particular number range that I should be looking for when calculating the static pressure number? For instance, I'm looking at a face opening of about 9 sq ft, a 980 CFM blower, 12 linear feet of 8" duct, 1 turn, and a loss factor of about .0143 which gives me a static pressure of 0.70858634. Is this what it should be, or should I try tweaking the numbers using different blowers, different duct size, etc.?

With blowers, should I look for one that has an outlet that matches the duct size; e.g., if I'm using an 8" duct should the outlet be 8", or would a 5-9/16" x 7-3/16" outlet be sufficient? The blower I'm looking at is the Dayton 4C054 from Grainger.

Thank you in advance for sharing your wisdom!

-Diane

[MikeAurelius]

Diane - good questions.

The hood should be parallel to the work surface, always. By tilting it you are going to be sucking more general room air than contaminated air.

The fan can be mounted anywhere it is convenient to do so. There are no restrictions or requirements on where it can go. Factors to take into account: fan mounting needs, noise (very important), nearest power etc.

An SP of .70 is out of range for a 4C054. If you look at the SP table for this fan, it shows a -- for .70 SP. This means the fan cannot operate with this high of pressure.

When you interpret the SP table, your final CFM is listed under the appropriate SP, for example, the fan is rated at 980 CFM at 0.0 SP, but only 655 CFM at .60 CFM. This fan does not move sufficient air for your particular design.

I just ran your base numbers (980, 12', 1 bend, 8" duct) through my calculator, and I think you made a mistake somewhere in your calculations. The loss factor is actually 0.03, and your total system pressure is 0.24.

Using these numbers instead, you will get a final fan CFM of about 875 CFM, about 100 CFM less than the design. Whether this is adequate for your installation or not I can't determine without a better idea of the layout.

The blower outlet does not have to match the duct size. You are essentially dumping the outlet either into a larger duct or directly outside, so it doesn't matter.

[Diane (clarus)]

Thanks for your answers. A few clarifications requested:

Regarding the angle of the hood - you said it should be parallel to the work surface. Does that mean the entire surface should be flat? Most hoods I see have angles. For instance, if you look at the Glasscraft hood, it has flat perpendicular sides, but the front and back are partially perpendicular to the worksurface with the rest being about a 45 degree angle between the flange and the top. That's what I was asking - should it have angles in the top, and/or flanges on the side?

Regarding placement of the fan, that's still open for discussion. It's good to know we have options. I presume that it would be much quieter if it were mounted on the wall at the end of the run either inside or outside, as opposed to right above the hood? Power isn't an issue - the main line comes in on the wall with my bench.

Regarding static pressure - you were correct that I had some miscalculations in my numbers. I had put together a simple Excel spreadsheet and had forgotten to put in a couple of values. Now I verified it using the numbers in your wonderful writeups over on the other forum and everything adds up correctly.

When you ran my numbers you said:

I just ran your base numbers (980, 12', 1 bend, 8" duct) through my calculator, and I think you made a mistake somewhere in your calculations. The loss factor is actually 0.03, and your total system pressure is 0.24.

Using these numbers instead, you will get a final fan CFM of about 875 CFM, about 100 CFM less than the design. Whether this is adequate for your installation or not I can't determine without a better idea of the layout.


Where did you come up with the loss factor of 0.03? I have your table and couldn't figure out where that had come from. Was I reading it wrong?

I'm open to whatever CFM of fan is necessary - I'm asking these questions before we even attempt to build the thing or buy any of the components because I want to get it right the first time!

For what it's worth, I'm working in our garage that is 20x20 square feet and plan to have vents behind the work bench to draw in fresh air. I live in California, so heating/cooling is less of an issue than elsewhere.

Your wisdom is much appreciated!

[MikeAurelius]

Ah, the dreaded clarifications...LOL!!

Ok, what I meant by parallel to the work surface: The hood itself does have many angles to it, however, it has straight sides that terminate as a "face" or opening. This face or opening is what should be parallel to the work surface.

If you lay the hood on the bench opening down, this is how it should be mounted above the bench, as level as possible.

Yes, it will certainly be quieter mounted away from the hood area. One of the things people have done is to create a plenum or suction box that the fan is mounted in. This does have certain drawbacks and I don't particularly recommend it - the plenum should be constructed of metal, not wood (in case the motor overheats and catches on fire), it should have an access door so that the fan can be serviced (cleaned) or replaced, etc. Mount the fan solidly so it does not vibrate (this is usually the cause of most noise issues). All pretty common sense items...

Loss factor: no, sorry, the 0.03 is an intermediate calculation number, don't worry about it.

Based on your description, I think the fan will work quite well for you. The only suggestion I'd make is to see if you can possibly shorten the duct run - the ducting will need lots of support to keep it from buckling.

One other suggestion is that as you are assembling the duct "structure" is to use the metal foil tape instead of the plastic duct tape. The metal foil tape maintains its adhesive properties for a very long time - 4-5 years, instead of 6-9 months for the plastic duct tape.

[Diane (clarus)]

Mike, I think I'm almost there.

Yes, I understand what you're talking about the face opening of the hood needs to be parallel to the surface. What I was asking is if there was any benefit to having the hood itself be built with a sharp angle between the face and where the duct run begins, meaning a taller hood? Or is a shallower hood, like the one from Glasscraft, better? If that's not clear I'll try to do a drawing.

With the fan, I was planning to mount it inside a box, and your suggestion that it should be made of metal and have an access panel makes total sense. My husband is a master at mounting things securely, so I'm sure he'll be able to do it without any issues. In my case I think mounting it on the outside wall would be ok, since we don't have any freezing or extreme heat issues, and if we're going to build the box anyway it might as well be in a place where it's going to be quieter. Do you know if they make boxes for mounting these or should we put one together ourselves?

So you think the 875 CFM fan would be the best one in this situation? I'm having it vent out the back of the garage because if it went out the same wall as the hood the exhaust air would contaminate the makeup air that will be coming from the vents behind my bench. Venting out the roof isn't an option because my husband doesn't want to cut a hole up there.

Thanks for the recommendation about the foil tape. That's what I was planning to use, but it's nice to know it is the right thing. It shouldn't be an issue as far as supporting the run - there are rafters every 18 inches, so I can put a strap on each one if necessary.

Thanks again for your help!

-Diane

[MikeAurelius]

Ah, I see what you are talking about - no - there's really no major issues about the angles on the hoods themselves - many times they are there to extend the hood out with a narrow profile for looks only, it doesn't affect the actual air movement qualities, it is just another way to increase the overall hood coverage area.

In most cases, you will have to build the plenum box yourself. They typically are site-built by HVAC folks anyway.

Yes, given the set up you've described, I think the 875 net CFM should work very well for you. If there are any issues, you can always use some additional baffles to bring the sides of the hood down towards the bench top.

[Diane (clarus)]

Cool! Thank you again for your help, Mike!

-Diane