Yet ANOTHER beginner tries to figure out the mysteries of ventilation...

[Judy D.]

I have a small area (8' x 8') in a much larger wood shop (I'm a full-time professional woodturner) where I am building a bead studio. There are three walls and I will curtain off the open side to keep dust accumulation to a minimum in the bead space.

I've built a small bench about 3' wide x 2' deep, and plan to put a hood over it. The hood will vent to a nearby (abandoned) door to outside, about 8' to the door (I'll cut a hole for fan/ducting). I will bring in makeup air from another room, actually the garage my shop is on the back of. I'll raise the large garage door a bit to allow plenty of airflow from that area. I'll either make an insert to place in the opened small door to shop (which is right beside the bench) with a hole in it for the duct, or bring it from an opening that exists in the masonry wall between the shop and the garage about 9' away (three bends?). Either way I plan to have a duct for the makeup air come into the side of the bench and thence to an opening in the benchtop at the back of the bench.

I'll be using a Minor torch, with natural gas and an oxycon. Was figuring on 10" duct, and found the fan below on eBay.



If someone conversant with the nuts and bolts of the ventilation issues could comment on this I would be grateful. I have read LOTS of threads here and other places, and seem to be stuck at that point where you have way more information than you can digest into some specific plan. So comments on what I've come up with so far would help me to see where I'm ok and/or where I'm deficient in my understanding of things. Is the basic plan reasonable? Is this the type of fan I should be looking at?

I'm figuring some small amount of air would be pulled from the larger shop area to create flow from behind me into the hood/exhaust ducting. I just don't want all the heated air going out so fast; it sometimes gets very cold here in Ohio.

Thank you so much for any help you can offer.

[cheng076]

See the reply to your other post.

From your description here's what I would do. I would build my hood about 30" to 36" wide X 20" deep and hang it over the bench at forehead height or mount it to the wall in front of the torch. Slightly lower if you plan on working seated. The fan you picture would then be adequate IMHO. This is a very good fan type as it is one of the quietest types in use. Try to run your ducting with as few bends as possible and as short as possible. I would mount the fan at the far end from the bench for sound considerations but physical and electrical considerations may be a factor. Unless you enclose the entire room and bring in makeup air to the torch or bench top then there is almost nothing you can do to prevent room air from being sucked out and room temps dropping as it will be diluted by makeup air and sent outside. I use a radiant heater in my studio when working even though it does not get as cold as in Ohio by a long shot.

Keep us posted on your progress and if you need any more help.
PJH

[Judy D.]

Thanks for your response. I am planning to bring in makeup air as described, through the side of the bench and up through the benchtop. It would be helpful to have some idea of the best way to do that--I'm thinking about 5" deep and 16" long, at the very back, so the torch flame would be just in front of it? And should I put screen over it or something? I thought I'd make an enclosure below the surface out of cement board, just large enough for the duct to come into it. That way if any hot glass did go down into the opening, it would land on flame proof surface. Also thought to put up a piece of that vertically at the back of the bench. The makeup air would come from the garage--in hopes that it will provide close to enough, and I won't draw too much from the heated room air.

Don't know if this is clear, I hope so.

Also, I've read somewhere that it's not good to use that accordion-type duct? That would be so easy...too much drag? On the other hand, I've seen pics of it in use. So it would help me to know whether that would work or not.

Thanks so much. Your comments are so helpful and appreciated. I know the nuts and bolts of it won't be that big of a deal, I just need to get it in my head what I need to do.

[Judy D.]

Here's a pic of that ducting, in case I was unclear.

[LarryC]

Originally Posted by Judy D.

Thanks for your response. I am planning to bring in makeup air as described, through the side of the bench and up through the benchtop. It would be helpful to have some idea of the best way to do that--I'm thinking about 5" deep and 16" long, at the very back, so the torch flame would be just in front of it? And should I put screen over it or something? I thought I'd make an enclosure below the surface out of cement board, just large enough for the duct to come into it. That way if any hot glass did go down into the opening, it would land on flame proof surface. Also thought to put up a piece of that vertically at the back of the bench. The makeup air would come from the garage--in hopes that it will provide close to enough, and I won't draw too much from the heated room air.

Don't know if this is clear, I hope so.

Also, I've read somewhere that it's not good to use that accordion-type duct? That would be so easy...too much drag? On the other hand, I've seen pics of it in use. So it would help me to know whether that would work or not.

Thanks so much. Your comments are so helpful and appreciated. I know the nuts and bolts of it won't be that big of a deal, I just need to get it in my head what I need to do.

Work is not really the right word to use in this case. Flexible ducting will effect how the system works. If you have a large enough fan and the run of ducting is short it may be just fine. Keep it to short distances. I use short runs of it where it really helps with routing the duct.

[Judy D.]

Larry, I think it would be about ten or 12 feet to go up from the hood and then over/down to the door, and would only need one bend (from vertical to horizontal above the hood), an could have a pretty shallow curve down to the opening in the door. What do you think? I don't want to use it if it's going to mean a much bigger/more expensive fan. But wow, it would be easier than rigid ducting. I was planning on 10" diameter.

The makeup air duct could easily be very short, if I bring it from the door to the garage. I figure I can play around with that, and check for the results (adequate exhausting)...

[Dale M.]

Flex duct will cut the efficiency of you system to below workable standards.... All ventilation calculations are based on smooth wall "steel" type of ducting with engineered turns... IF you plan to use flex you probably need to upgrade fan CFM by 25%... Which further causes a problem by increasing CFM rate of fan you create more turbulence and there for effect flow capacity even more...

Here is something you may want to play with....

http://www.artglassanswers.com/calc2/test2.html

Bill and I started developing it to get people off spread sheets (mine) and pencil and paper calculations and on to real world calculations on the fly... Unfortunately we never quite finished project (so some "cells" do not display- But overall its very accurate though) ....

Dale

[Judy D.]

Ok, then, rigid it is. *sigh*

On your calculator there, Dale--should I be aiming at some particular velocity figure there in the last column? And THANK YOU, too; I don't completely understand every part of that but it does help, esp. after trying it two or three ways. It seems to show that the 780 cfm fan would be more than adequate.

I wonder if I could get away with that flex stuff for the primary makeup air supply??

And, to reiterate, if anyone has thoughts or comments or pictures of a work surface with the hole(s) for through-the-bench makeup air, that would help me. I'm a whole-to part person; if I get the big picture, as it were, I can easily figure out the details.

Thanks!

[Dale M.]

A velocity below about 2,500 cfm will allow toxic particle to drop out of the air flow and contaminate the duct work... Flows above about 4,500 cfm will probably cause excessive excess noise in vent system...

Actually the length of duct work used for make up air is effected by all the same characteristics of exhaust duct... So if you have 12 feet of exhaust duct, and 15 feet of make up air duct you actually have 25 feet of duct...

Dale

[cheng076]

I did some small scale experiments a year or so ago that showed makeup air openings at the back of the bench, as against the wall, were as effective as openings on each side of the bench but slightly less effective than makeup air from behind the worker. remember that the purpose of a vent system is to supply you with clean breathable air as well as remove flame/glass generated contaminants.

[Judy D.]

Originally Posted by Dale M.

A velocity below about 2,500 cfm will allow toxic particle to drop out of the air flow and contaminate the duct work... Flows above about 4,500 cfm will probably cause excessive excess noise in vent system...

Actually the length of duct work used for make up air is effected by all the same characteristics of exhaust duct... So if you have 12 feet of exhaust duct, and 15 feet of make up air duct you actually have 25 feet of duct...

Dale

Now I'm confused all over again. Assuming (possibly incorrectly?) you meant a velocity of 2500 fpm, not a volume of 2500 cfm, I still can't figure out any numbers at all to put into that calculator that come up with anything like 2500 fpm. If I put in the numbers PJH mentions above, with 25' total of duct (half for intake and half for exhaust), 10" diameter duct, and 5 90-degree bends, I get a minimum fan cfm of 625, which gives a velocity of 1145 fpm. How would I more than double that, and why is the calculator giving me a minimum fan volume that can't possibly produce that? Is PJH THAT far off when he says the 790 cfm fan would be adequate for my situation? I thought I sort of understood all of this but now I'm back to square one. Please, what am I missing?

[Dale M.]

Originally Posted by Judy D.

Now I'm confused all over again. Assuming (possibly incorrectly?) you meant a velocity of 2500 fpm, not a volume of 2500 cfm, I still can't figure out any numbers at all to put into that calculator that come up with anything like 2500 fpm. If I put in the numbers PJH mentions above, with 25' total of duct (half for intake and half for exhaust), 10" diameter duct, and 5 90-degree bends, I get a minimum fan cfm of 625, which gives a velocity of 1145 fpm. How would I more than double that, and why is the calculator giving me a minimum fan volume that can't possibly produce that? Is PJH THAT far off when he says the 790 cfm fan would be adequate for my situation? I thought I sort of understood all of this but now I'm back to square one. Please, what am I missing?

Oops... My bad yes its fpm when working with duct velocity... To get velocities in ball park, you have to change duct diameters... The cfm of the fan is determined by hood opening (face opening) and desired volume of flow at face (100 to 125 cfm) ...

For playing with calculator try these numbers...

24x36 hood opening
25 ft duct
8 inch diameter
125 cfm
5 90° bends

Results should be around 2150 fpm velocity factor with a 750 cfm fan which is probably ok...

Dale

[Judy D.]

Ok, that works out. And is it ok to use the 10" dia. fan and just bump up the duct at the end to accomodate it, if I don't find an 8" fan of the proper CFM?

[Dale M.]

Diameter of fan is not critical its the flow (cfm) of fan that is critical, yes you can use adapters to change diameter of duct at fan...

Dale

[Judy D.]

The 8" ones all seem to be 720. Which would make the numbers more marginal, right?

[Judy D.]

Thank you so much for your help, Dale.

[cheng076]

I got you and BeadsBlossum mixed up somewhat but a lot of wwhat I said to her applies so read it with that in mind. Your situations are somewhat the same.

OK here we go......

1. Make a scale drawing of your room on graph paper. Put in windows and doors. Mark where your bench is going to be. Mark where you will mount the fan and hood, usually right above the bench. Mark where the kiln will be and think about how it will be supported, cart, shelf, or stand. Optional - mark where shelves, glass storage, a desk & chair might go, stereo, etc. Might try laying out the room size wise and cut little squares out of construction paper so you can lay them on the drawing and move them around for best fit (layout). I use one of those cheap, free, house drawing programs for this sort of thing. This is also a good time to select where you want to put electrical outlets around the room. I like to put them above counter height except where my desk is so I don't have cords hanging over the front edge of my bench even for temporary stuff, and I don't have to crawl under the bench to plug/unplug.

2. Room size has no bearing on vent fan size. Doing the drawing you probably already decided on a bench size even if you didn't realize it at the time. Decide what type of hood you want. There are quite a few. a. sheet metal fabricated to your design by a commercial shop. b. home made from metal flashing and wooden frame. c. wooden frame with plywood sides covered with metal or not. d. reuse of something such as a metal wash tub. e. large funnel shape usually fabricated by a commercial shop. And others. Look at the photos in the forum for ideas. Make a decision and stick to it unless it becomes impossible to impliment given other choices. It's really a balancing game.

3. Think about and decide where and how the hood will be mounted and where and how the ducting will be run. Your vent guy can be a big help here. Explain that you need to remove the gasses from your torch flame plume as if the hood was a large vacumn cleaner hose opening. Now is the time to also think about where you want to get makeup air. You mentioned wanting to have it come in at the back of your bench in front of the torch. If this is your choice then you will need to decide where the ducting will run and from where. Your idea of a flameproof box under the bench is good ( and use a screen on the opening).

4. You now have the basic mechanical items designed and selected. Now you have to select the proper fan. Given that you want to use the axial fan pictured at 750 cfm then your hood is limited to about 6 square feet. So that could be 2 ft X 3 ft or whatever combination stays about 6 sqr ft and within your budget. The hood doesn't have to cover the whole bench just the torch flame plume area of your torch.

5. At this point you have all the elements designed and selected so it's a matter of installing them and hooking it all up.

6. Think about where and how many lights you will need. Usually at least one over the bench, maybe another in the center of the ceiling, one over the desk or seating area. I like a fluorescent tube light over my torch as it allows me to see imperfections in my marbles but I like incandescents over the desk for reading and crafting otherwise I have fluorescents as general lighting...they're cheaper to operate. I use daylight tubes in them for colors.

Most people in my opinion think about ventilation backwards. The most important thing is that it function in such a manner that it supplies YOU with clean contaminate free air to breath; coincidentily but secondarily is that it remove those contaminates from your work space.

Keep at it, the light bulb will come on soon.

If I recall correctly you mentionedd your space was three sided, one end/side completely open. That will make directing makeup air into ducts under the bench impossible so in order to do that you will have to close off that open end wall and possibly add a door. Makeup air is usually passive, drawn in by the 'suction' of the vent fan. Unless the room is closed more or less air tight then your makeup will come from the open wall. It always takes the path of least resistance.

[Judy D.]

That is a VERY helpful post (sorry, should I call you PJ? Cheng?), thank you so much for putting the time into it! I promise if I ever know anything much about glass bead making and studio-setting-up, I shall pass on your generosity.

Happily I've got pretty much everything thru 5 figured out except the precise design of the hood and then just doing it all. The size of room and placement of things is not very flexible but I already did that layout thing right off, I always do that because it will sometimes make you see something you would not in person.

I have a couple of questions about the hood if I may impose on you again. First, is there any reason not to make it out of cement board, or at least the enclosure (vertical) part, before the trapezoid shapes that go up to the actual round duct? Here's my thinking: I believe I saw somewhere (so many links to keep track of, I've lost a few) of your hood with the triangular pieces on the sides...very tickled to see that as I had wondered if that would work. Two reasons, one to allow easier access without having to push out to access kiln (which will be on a small cart to the left of my bench), and two purely visual, so I don't feel so claustrophobic. My thinking is that if this part (the part closest to the torch, and the part likely to have hot glass hitting it) is cement board, then maybe I could make those trapezoids out of the foam-backed insulation board--would be so much easier to cut them, and they would be lighter. I can attach the lower "box" part (back and two triangles on point) to the bench, which is quite sturdy.

And is there a particular size the front vertical piece should be? Obviously long enough to go all the way across, but I mean top to bottom? That will then determine the exact shape/size of the other pieces.

I will go with the 8" duct as Dale suggested.

And here's what I was thinking about the whole makeup air thing. If it's totally ridiculous or def. won't work, I can make a wall and put a door in if I need to. What I'd hoped to do is this: I can run a very short duct through the door to the front (garage) part of the building into the bench box thing. When working, I could open the large garage door just a little bit to allow plenty of air in. That space is unheated except for residual heat from a furnace that heats an upstairs apartment above the garage.

Then I thought if I cover the end opening of the space with heavy plastic curtains (I'd really like to be able to keep the space more open when I'm not torching as it will continue to be my office as well), then the exhaust would pull primarily from the duct to the garage, since that air is right below it. Some air would be pulled from the larger shop beyond the curtain, but if I can control how much (I can figure out a way to have it mostly sealed by overhang at the top and maybe some kind of weight or sill at the bottom) air comes from there, then there would also be air coming from behind me to get sucked past my head and into the exhaust. So, really two questions: aside from the curtain vs wall issue, wouldn't I want SOME air to come from behind me? and then, is there any hope of the curtain working or is that just nuts? It's not a huge thing if I do have to build a wall but would rather not for a couple of reasons. So if the curtain idea might work, then I may just try it and see if I can control how much air comes that way.

Thanks again!

Judy


(Sorry, quote button not working, but probably no posts in between...)

[cheng076]

PJ is fine. Cheng076 is a carry over from my shipping days when I was the Ch ief Eng ineer of vessel 076, the Chief Gadao, a 989 ft Matson container ship.

No reason at all. There are several posts from people who have done just that. I have tested my hood with the torch running and the fan on normal low speed. I can not feel any temperature rise, actually it cools down. Even with the fan turned off and the torch running for an hour or more the sides of the hood barely get warm. The volumn of air flowing past the sides cools the sides extremely well.

The front verticle piece on my hood is mostly there so I can mount a fluorescent light on the inside surface and tools on the outside and so I don't bang my head on a sharp edge. I guess I need to take some new pics of the hood as it is now. The first hood I built for a studio in my garage was a simple 'V' of sheet metal with the ends closed off with triangles of plywood and hung open end down. The fan was mounted inside the hood up near the bend in the metal since it was a double ended suction squirrel cage type. That eliminated a plenum. Then it was ducted out thru a hole in the garage wall.

Makeup air. As I mentioned the makeup air will be drawn to the hood due to the vent fan suction by the path of least resistance. Now think of a sail on a sail boat. The pressure of the wind is very very small as you can just barely feel it. Now put that pressure against a large area and you have a tremendous force. Example...an 8' X 10' wall has 11520 sqr inches of surface area. The average person can produce 2 to 3 psi of pressure with their lungs. Actual breathing pressure is maybe 0.2 psi. That breathing pressure equates to over 2300 pounds (more than a ton) of force on the wall. Your plastic screen or whatever will come billowing into the room like a sail. Perhaps the simplest solution to having the area remain somewhat open and having the makeup air ducted to the bench and keep room air warm is to put a forced makeup system in; that is to install another fan in the makeup air duct and use a damper on either the exhaust or makeup side to balance the two fans for the correct flow. Best of both worlds and probably less expensive than a new wall and door.

[Judy D.]

Not a bad idea, thanks, I'll consider that instead of wall. But can always put up a wall. Won't be a pretty one, but the ones in here aren't either...

I shall post again when I have more to tell. Or ask. Again my sincere appreciation.

Judy

[cheng076]

Might as well make it a pretty one if you do it. Nice to have some 'pretty' around when you work. LOL

[Judy D.]

HA! Good one! See, the problem with that kind of thinking is, if I DID put up a REALLY nice, flat, perfectly finished wall...why then, everything ELSE would suddenly look WAY worse than it does now. This could lead to an infinite regression through SO many layers of prep and repair, repair and prep...I'd NEVER get around to making beads. I think it best to stick with pretty raw function, and leave pretty itself to the beads I hope to be making, eventually!

J.

[cheng076]

I hear ya!

[laserglass]

Originally Posted by cheng076

I did some small scale experiments a year or so ago that showed makeup air openings at the back of the bench, as against the wall, were as effective as openings on each side of the bench but slightly less effective than makeup air from behind the worker. remember that the purpose of a vent system is to supply you with clean breathable air as well as remove flame/glass generated contaminants.

make up air ducts at the back of the bench make no sense at all. your ventilation fan ends up pulling in a lot of clean air from the ducts leaving less capacity for the ventilation fan to remove combustion byproducts. yes you do not get as cold because you ventilate the cold air back outside, but your system does not work as well.

[cheng076]

Mark,
I beg to differ with you on this.

My experiments indicate that the efficiency of the 'back of the bench' makeup ducts are at least 90 plus percent as good as other supply systems. Several people here have installed such systems and are very satisfied with their performance. I intend to redo my system to use this method this summer.

PJH

[Lorraine Chandler]

PJH, Mark is a rocket scientist or something of that nature ( he is brilliant) and what he says makes sense. He has been a Guru here and on Wetcanvas for many many years and has always been able to explain his answers so we can understand it all and has never steered us wrong...

I have thought about his answer and it makes pefect sense when I put it with the other infomation that has been gathered about how torching will send the combusted fuels into all directions and some of it rolls back into the face area and breathing area of the lampworker..

If the replacement air is at the back of the bench and the exhaust fan over the middle of the bench I can, in my minds eye see the extracted fumes heavily pulled from the back of the bench curving upward to the middle of the bench and extracted, but I do not see a good pull of replacement air over the shoulders from behind to be extracted from the breathing area directly in front of the lampworker to the hood extraction point.

Mark, thank-you for this info, as I was going to do the back of the bench replacement air. I have reconsidered now. Would you please tell me what would be the ideal replacement air situation? We are setting up my new Tuff Shed and I want it to be as clean airwise as possible. Also am I understanding what you said correctly?

So would make up air be best coming from back of bench and over the shoulders from behind?

http://www.artglassanswers.com/forum...keup+air#p2256

[cheng076]

Lorraine,
I do not doubt that Mark is well qualified to provide good answers to questions posed here on LE. I have always found his posts to be clear, concise, and accurate also. While my own qualifications may be less lofty I have had training in HVAC, pressure vessels, and other disciplines in my 53 years in the working world. In this instance I built a scale model of a studio and bench setup and ran experiments with various supply and makeup system configurations that indicated two things in regard to a 'back bench' make up air supply; 1. That it worked effectively to remove torch generated contaminants with very little to no back flow or leakage into the room air. 2. That it provided an adequate supply of fresh air that actually came from behind the workers head to supply that all inportant fresh breathing air. I attribute this breathing air flow to induced air currents. The purpose of these experiments had less to do with torch contaminants and more to do with breathable fresh air supply. I do not/did not have the resources to do a full on technical scientific study but I am personally confident enough in my results to post them.

I find your second paragraph confusing as for that to happen the contaminants would have to flow against a considerable current of air flowing at a considearble speed into the hood if your vent fan is functioning adequately. My experience also indicates that torch contaminants are wholy(sp) generated within the torch plume and do not disperse significantly until the torch plume is well up into the hood capture area further negating the possibility of mixing with the room air.

I respect Marks opinion but my own differs from his and I disagree with his comments as I stand by mine.

ETA - I am confident enough that I will be modifying my own system to a 'back bench' system this spring/summer when I have collected and fabricated the necessary components.

[Judy D.]

I know some very experienced, long-term torch workers who use through the back of the bench make-up air. It seems to me that unless the rest of the room was 100% sealed, which would be nearly impossible, that SOME air HAS to be pulled in from behind. It seems to make sense to me that if the air flow is sufficient, that you would want a fair amount of it to come from below/beyond the flame, where the toxins are produced. I'll be doing some careful testing of whatever I end up with to see that I am getting fresh air and that the torch fumes are being exhausted. I'll consider that if I do a smoke test and can't smell anything, that is a good result. I'm still interested in what everyone has to say, but I'll share something from my history in woodturning that I tell my students sometimes when there is a heated dispute among experts about some technical point...if the experts can't agree, it's possible that the distinction from the two possibilities they are arguing for is not of significance for a beginner. I don't know if that applies here but it could--perhaps both systems remove the contaminants and supply fresh air to the breather well enough? It's fine with me if you both (or anyone else) continue to comment, but please keep it as courteous as PJ's reply just above.

I have another question, specifically about fans. I had to have a small garage furnace replaced last week, and not having yet purchased a duct fan, my frugal nature has kicked in and I am considering whether or not I could use the salvaged squirrel-cage fan and relatively-new-motor as my exhaust fan. It's all in pretty good shape, but I have no idea how to determine the CFM. The motor is 1750 rpm, and the rotating part of the fan, whatever it's called, is, I think, about 8" inside diameter, about 10" outside diameter; it is 9 1/2" across (the fins themselves are 9" long and about an inch wide), with a disc in the center that goes only from the shaft to the inside of the fins.

Assuming there is no big intrinsic reason not to use it (I can figure out the logisitics of plugging one side and bringing my 8" duct into the other side, fastening the thing to the wall, etc.) is there anyone who can help me figure out what size pulleys to put on the motor shaft and the fan shaft to produce the desired CFM (something between 750 and 1,000, I think is what I'd want)? I could do the math if I had a clue how to set up the problem...but I do not.

[Dale M.]

If you have make and model number of old furnace, you might get fan capacities from its "specifications"...

Dale

[Judy D.]

I'm afraid that info was no longer legible. I'll check around online, but I think this furnace was so old I doubt I'll find anything.