# Designing and building my fume hood



## mbrolin (Dec 7, 2019)

Planning out how I want to build my fume hood. It is cold here in Central Indiana. Planning on doing my refining indoors. I have 2 barns. One of the barns is heated and cooled and broken up in two parts, a garage part and an office part. I plan on creating the fume hood in the office part. There are 2 windows in this room. I want to use these for the air intake and the exhaust. I don't want to be cutting holes in the walls or roof at this time. It is about 20 feet between the 2 windows. I plan on having 3 bays in the hood, so I have plenty of room to have many different projects going on. I also want to keep my stock pot and waste bucket in the fume hood. I imagine each bay being about the width of a mortar tub, so 25 to 30 inches. 

Some ideas right now, for the fan I am thinking of either https://www.harborfreight.com/13-gallon-industrial-portable-dust-collector-31810.html or https://www.harborfreight.com/2-hp-industrial-5-micron-dust-collector-97869.html. For most of the hose I will use Dust collector hosing, https://www.harborfreight.com/dust-collector-accessory-kit-93601.html. It is 4", is that wide enough, or do you think it would be need to be bigger? I'm liking the Dust collector hosing because of it's flexibility and the fixtures in the kit would be useful for connecting to the hood and the board to the outside that will be int he window. I'm sure I will need some rigid PVC pieces, but I think i can do most of it in the flex hose. 

For the body of the fume hood, instead of wood, I'm thinking of using this, https://www.menards.com/main/buildi...8-c-13411.htm?tid=1664057452212021911&ipos=13. I was thinking with this, it should be more durable to the fumes than wood, plus can use screws and glue like wood, but also could use a plastic welder on the bottom to make sure it will not leak if there are spills or other disasters. I plan for it being closed most of the time relying on outside air for the make up air, except when working. Other wise the openings will be closed.

What are your thoughts and suggestions so far? Will try to add some pictures of the area.


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## butcher (Dec 7, 2019)

The ideas look good to me, have you calculated your desired CFM through your fume hood with that of the fan, are How are you planning to use the fans in the system?

I can also tell, you have been doing your research.

Nice thread, I am looking forward to watching your build.

My one concern is a garage anywhere near, But if the system is built, operated, and maintained properly, and you were not just blowing acidic fumes out of the window for the cold north wind to carry around to your open garage door, then the office may not be a bad lab


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## mbrolin (Dec 10, 2019)

Here is a picture where I plan to put the fume hood from outside and inside. 

Outside View



Inside View



It is a metal pole barn, so the the outside walls are metal. How much of a concern is that do you think? The roof are normal asphalt shingles. The inside walls are dry walled, as seen in the picture That outside wall faces east, the one I want to place the fume hood. Here the wind blows from the west most of the time. The driveway and cars are on the south side. On the south side there is a utility room that sticks out from the main barn, you it in the outside photo where the roof doesn't go all the way up like the rest, and in the inside photo it is the open door at the end. The pole barn was built before the house, so all utilities, well pump, and such come to the property here, then branched to the house and other barn. The unit next to the North window in the outside picture is a heat pump for additional heating and cooling. The barn is also heated by radiant heating, and has a train air conditioning unit on the North side. Never actually tried it yet. Just moved into the property 2 years ago. There's a bathroom behind me in the back of the barn. The garage part is separated from the office part by a wall. The door to that is by the utility room. The house is about 30 feet away from the southeast corner. 

Still trying to decide which window to put the exhaust. The south window is closer to the house and cars/drive way. The north window is right next to the heat pump.


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## mbrolin (Dec 10, 2019)

butcher said:


> The ideas look good to me, have you calculated your desired CFM through your fume hood with that of the fan, are How are you planning to use the fans in the system?
> 
> I can also tell, you have been doing your research.
> 
> ...



Thanks Butcher. Not yet, haven't figured out how wide each bay will be. Probably around 25 to 35 inches. Most of the time it will be closed up, so the 4" intake air will match the 4" exhaust port and that will be it. You want about 100 cfm per sqft of opening correct? Will only have one bay open at one time, and only when working. Will have to figure how big is will be. How tall should the fume hood be total and how tall should the opening be when working to give enough room? 

Another general question, has anyone worked with pvc paneling? Can it be cut just as easily as wood as it says? Will want to use a table saw to cut it.


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## butcher (Dec 10, 2019)

You would not want it close to the heat pump coils, the other window looks far enough away.

I do not know how thick the PVC panel but if not thick it can be cut with a razor and straight edge, or a table saw...
The recognized acceptable minimum volumetric rate (CFM) of air for fume hood exhaust is typically cited from the National Fire Protection Association’s NFPA 45 recommendation of 25 CFM per interior square foot of the work area. This data is used by mechanical system designers, air balancers, Variable Air Volume (VAV) and mechanical contractors. As the sash is closed on a fume hood installed in conjunction with a VAV system, the volumetric rate of air is reduced to maintain a constant face velocity (fpm).

Fume hood exhaust airflow should not be reduced to less than the flow rate recommended in ANSI Z9.5, Laboratory Ventilation.

ANSI Z9.5 2011
The ANSI Z9.5 standard will reflect a new minimum of 150 air changes per hour inside the fume hood. As long as this new minimum is adequate for the specific application, and depending on fume hood dimensions, this is approximately a 52-60% reduction in the minimum volumetric rate of air.

Fume HoodAirflow -Cubic Feet per Minute (CFM) Calculation Explained 
The calculation for the CFM of a fume hood is obtained by multiplying the FACE OPENING (window of the fume hood)and the FACE VELOCITY. The FACE OPENING value is the maximal open Width x Height for your fume hood while FACE VELOCITY references the capture distance.e. depth of the fume hood. 

A typical FACE VELOCITY is 100 fps as many fume hoods are similar in depth to a standard desk. 

Note: For this sample calculation we are using Bel-Art –SP Scienceware fume hood catalog no. H50000-0002which is 22 inches wide by 12 inches high. FACE OPENING Calculation:

22” x 12” = 264 square inches. Divide this by 144 to get Square Feet. 264 ÷ 144 = 1.83 square foot opening 

You would now multiply this area measurement (1.83sq.ft.) by your desired FACE VELOCITY, in this case, 100fpm and that will give you the CFM requirements you need. 1.8 square feet x 100 fpm = 180 CFM So in this case, you would need your exhaust fan to be able to draw 180 CFM. Our catalog listing for 50000-0002 lists that an even 200 CFM needed, rounding up from the 180 CFM calculated here. Getting here is the easy part. To determine how powerful a fan is needed, you still need to calculate the length of the exhaust pipe(including turns) leading from the fume hood to the end exit point. The longer the distance and the more curves you have, the more powerful your in-line exhaust fan will need to be to attain the 180CFM.


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## mbrolin (Dec 11, 2019)

butcher said:


> You would not want it close to the heat pump coils, the other window looks far enough away.
> 
> I do not know how thick the PVC panel but if not thick it can be cut with a razor and straight edge, or a table saw...
> The recognized acceptable minimum volumetric rate (CFM) of air for fume hood exhaust is typically cited from the National Fire Protection Association’s NFPA 45 recommendation of 25 CFM per interior square foot of the work area. This data is used by mechanical system designers, air balancers, Variable Air Volume (VAV) and mechanical contractors. As the sash is closed on a fume hood installed in conjunction with a VAV system, the volumetric rate of air is reduced to maintain a constant face velocity (fpm).
> ...



So in addition to calculating CFM needs by hood opening, you need a minimum of 25 CMF per sq foot of internal hood working space? So in your example of the hood with an opening of 22" x 12", the calculated at 180 CFM, rounded up to 200 CFM, works if the internal work of the area of the hood is 3' x 3'. The 3' x 3' equals 9 sq ft, which would need a minimum of 175 CFM as per NPFA 45. But if the internal space of the hood was 3' x 4', then the 200 CFM result calculated by the opening wouldn't be enough. 3' x 4' = 12 would require 250 CFM by having a larger internal volume. Did I understand that correctly then?

How do you calculate for venting lengths and bends too? Is there general guidelines to go by? The windows are 20 feet apart, and on the exhaust, I want to it to bend upward after leaving the window so the exhaust exits at roof level. If I can figure out how to install a pipe going up that high and stabilize it to withstand our strong winter winds.

The PVC boards are .5 inches thick according to the web page, but I imagine just like dimensional lumber, it will be less than that. Hoping I can cut it all on the table saw just fine.

Thanks for your help Butcher.


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## anachronism (Dec 11, 2019)

You need to add something here in addition to what you have been told so far. 

Your workspace needs to be clear of fumes so your extractor fan needs to not only evacuate the fume products but also needs to cycle the air in your workspace. 

Measure the workspace area in cubic metres and decide how long it would take to change that entire air volume every 15 minutes. 

It's not as onerous as you think to do this. I use a 1500 cubic metre per hour fan and it is a lot of comfort to know that your whole environment is being changed regularly. You don't want to be worrying about running on minimum i.e. I've open my cabinet wide to change something and now I am coughing. 

Also when you design your cabinet you must use a baffle plate to collect the air at the bottom and sides of the cabinet. Top sucking only partially works.

Jon


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## kurtak (Dec 12, 2019)

Read the thread by 4metals in this section (build your own equipment) titled "Building a chemical fume hood with a plain steel blower"

It's the 18th thread down from your current thread

The blower/fans you posted (from Harbor Freight) will not work as "direct" vent fan/blowers --- in other words you can not draw the fumes directly though those fans - the fumes will ruin them in a very short time as a result of the corrosive fumes

With those fans (blowers) you need to set up a "venturi" type system 

also - because the siding on your barn is steel - you want your exhaust end of you ducting to go out above your roof - if you just go out the side of the building you will likely corrode the siding - you can go out the side but then need to go up --- I would want to go at least ten feet above the peak of the roof

Kurt


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## snoman701 (Dec 12, 2019)

The blowers are disposable. They rust out, but they work a good amount of time.

https://www.harborfreight.com/8-inch-portable-ventilator-97762.html

That powered my hood for two years. I turned it off in August or September, and it wouldn't start back up. After removing it just now, I put it in the horizontal position, spun the impeller by hand, and again tried to start it, and it powered right up. It got loud about last winter. It was exposed to the rain from above. I'm replacing this weekend. I'm going to try the next one in the horizontal position to see if not getting rained increases life span. In all honesty, I could probably just paint the motor with epoxy (or replace the bearings with a better sealed option) if I cared that much, but even at $90 every year it isn't that bad....the electric useage tops that. 

A proper Plastec (name brand) 6" vent blower costs about $1600 last I checked.

One thing to keep in mind...once you start up a hood and have any chemicals whatsoever (even capped), you have to have a source of DRY makeup air...otherwise everything will rust. It's not the acids, it's he moisture in the air coming in. By the time I'm done, my lab will have independent HVAC and be a nice little negative pressure sealed room, complete with magnahelic. Because just outside my lab is my machine shop.


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## mbrolin (Dec 13, 2019)

Thanks all for the info. 

anachronism, What do you mean by baffle plate? It sounds different then another exhaust vent on the bottom, or is that all that you mean?

Kurtak - I have read all 4metals posts on fume hoods a couple of times. Yes, I plan to use a venturi. Lets see if I can better explain my plan so far. On the air intake window, there are two vents for air. One goes to the fan and the other goes to directly to the hood. The fan exhaust will then go to a Y connector to create a venturi, hopefully, to suck the air out of the hood. The income air was going to come in the bottom side of the hood, and the hood exhaust was going to be on the top on the opposite side of the hood as the make up in air. But it sounds like air needs to be sucked up from the bottom too. Then the exhaust will go out the window. Wow, really 10 feet above the peak of the roof? Won't that greatly increase the fan you need to push air that high. How do you stabilize something that tall, especially if it is coming out of a window. That may think of another question, what shape is the top of vent where the exhaust comes out. I'm guessing it is not straight up. It would catch the rain that way and I don't think you want rain in your exhaust. I'm guessing like a U turn at the top so the fumes come out down, like a candy cane?

Thanks snoman701 for the fan selection. The reason why I was looking at the dust collector fans is because they are set up for 4" pipe. Do you think this pipe diameter is big enough for the fume hood?


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## rickbb (Dec 13, 2019)

I too use a standard "steel" squirrel cage fan. Before putting in use however, I took it fully apart and painted all interior parts that would be exposed to the fumes thoroughly with a good enamel paint. Made it last for 4 or 5 years now and still going strong. Can get about 10 of them for the price of one all plastic, made for corrosive environment models. 

But it all depends on what you are planning to do and for how long. 

I do agree you need to size the exhaust so you can change out the entire room air at least every 10 to 15 mins. Not just for the hood and ducting. And then add a few 100 CFM just for good measure.


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## snoman701 (Dec 13, 2019)

4” is not big enough if you have to ask...meaning if you aren’t doing flow and pressure drop calculations to confirm, it isn’t sufficient. 


Sent from my iPhone using Tapatalk


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## snoman701 (Dec 14, 2019)

Here ya go...

Two Fernco 1056-86 flexible PVC fittings...plugs right in to 6" PVC on either end. $45 shipped from supplyhouse.

https://www.supplyhouse.com/Fernco-1056-86-8-x-6-Flexible-Coupling-Cast-Iron-or-PVC-to-Cast-Iron-or-PVC

And one Harbor Freight 97762 8" portable ventilator, $85 picked up

6" Schedule 40 PVC was $49 at the local farm irrigation store. 10 ft length.

Good face velocity on my smallish hood. I wouldn't want more than probably 4-6 sq feet of open space. I'm building a 6-8 foot wide hood. I will do horizontal sliding windows instead of a vertical sash. 

Can't do much more of the work for you.


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## Williamjf77 (Feb 2, 2020)

I’ve been using one of those HF fans too, holding up fine, a little loud. I would definitely under rate the listed cfm specs of the fan though,

The harbor freight got me going quickly but I did score a big old 1800 cfm @ 1.5” SP. steel blower for 50$ Without motor. 

thats at 1.5hp I’m thinking of running it with a vfd. 

I was thinking of using the housing as a mold and fiberglassing a copy but the painting with an epoxy paint sounds better and more realistic.
Still in the box though.


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