Building a Fume Hood

[Chaotic42]

I'm getting into a few hobbies that generate some obnoxious gases and debris, like plastic lure making, soldering, painting, etc. I'm also going back to school as an old man to study chemistry and want to do some basic stuff like titrations, distillation, etc. to get back into the swing of things. As such, and as part of my wanting to learn basic woodworking, I decided to build a fume hood. I thought you all might find it interesting and I'd like to get your thoughts on the design. I'll post updates and pics here as I get it going, but for now I just have some basic plans.

This will go in my basement on a cart with wheels that I'm going to make first. It's made out of 3/4" MDF and Lexan, and I'm going to use silicone to seal corners and then add some PTFE liquid coating. I'll paint the whole thing in a 500-degree resistant engine paint. I don't plan on doing anything truly dangerous, but I might as well do it right the first time. Here's a quick plan I made in SketchUp (note, files are fairly large):

Hood Front
Hood Side
Hood Back
Hood Top
Hood Corner

Link to MDF
Link to Lexan

For the vent, I'm using a Jabsco drip-proof plastic blower:
Link to Blower

For now, I'm going to build a little outlet I can stick into an open window when I'm using it. Eventually I'll install something more permanent. The Lexan will have handles in it so I can move it all the way down in case I need to by moving those metal clips out of its path. It can slide up and down in an emergency.

I'm curious what you all think. Has anyone here done this? Any and all comments are appreciated.

 

[sdifox]

Why not more of a woodworking shop vac setup with a hose instead of a hood? That should have more concentrated suction.


Never mind you have an enclosure. That should work. What are you doing with the fume though?

 

[Chaotic42]

The fumes may be flammable and/or corrosive, so I have to make sure the contaminated air isn't exposed to the motor. I'll be blowing the fumes out the back of the house towards the woods.

 

[esquared]

Why not just buy one? Seems time and parts may cost you more. Unless you feel like you want to build it.

They seem cheap enough here:

Amazon.com

I worked in labs most of my life and when working in start ups, we bought a lot of used equipment from labs that were shutting down
and even a lab equipment company that specialized in used equipment.

I worked a lot in BSL2 labs and the hoods were fairly cheap. Getting them re-certified with new HEPA filters were generally more expensive than buying the hood outright.

 

[Chaotic42]

Why not just buy one? Seems time and parts may cost you more. Unless you feel like you want to build it.

They seem cheap enough here:

Amazon.com

I worked in labs most of my life and when working in start ups, we bought a lot of used equipment from labs that were shutting down
and even a lab equipment company that specialized in used equipment.

I worked a lot in BSL2 labs and the hoods were fairly cheap. Getting them re-certified with new HEPA filters were generally more expensive than buying the hood outright.

Eh, I want to build it, and the materials will be a good bit less than that. Seems like a good way to learn some basic woodworking and make something I need at the same time. I'll be in it for... maybe $500. Not too bad.

 

[Paperdoc]

The item linked above is designed mainly as a DUST-free work cabinet.It has HEPA filters in the exhaust stream that simply goes into the room air. Not what OP is doing.

OP, a couple notes.
1. Do NOT design for sealing in the fumes. A good commercial lab fume hood has air INLET SLOTS, typically at the worktable surface at the back and the front so that air FLOWS through the cabinet and outside. What goes out MUST be replaced by fresh air or it does NOT work!
2. That motor may be a bit small. You want LOTS of air flow and that may require a larger fan / motor.

 

[Chaotic42]

Yeah, what he linked would blow stuff back into my face, but I figured it was more of a general "just buy something you know works" thing.

My thinking was that the air would all come in through the 1'x4' work opening so there would be no likelihood of fumes coming back out at me. Maybe a hole in the bottom/back with a flap so I could close the front and open the flap in an emergency?

 

[Paperdoc]

On the commercial ones I used, at the front of the work surface there was a curved lip extending outward under the front panel window (toward the user) and down that left under it an open channel (hood-wide) for air to enter at worktable level. At the rear a hood-wide slot at the bottom of the rear panel did the same. It has a sliding panel held in position by two screw knobs to alter the height of the slot. To be honest, I never knew whether that was for fresh air INTAKE or was connected to the EXHAUST ductwork to remove heavy vapours. The main point is that the exhaust air flow was NOT solely the gases produced in the hood. LOTS of fresh air from the room was drawn by the fan though the hood to dilute the gases etc., discharging outside a dilute mixture of fresh air and toxic materials.

Four additional thoughts.
3. Where the air vents out of the house, install a pipe of 3 feet or more out to get the air flow away from the house wall. This will avoid having those fumes trapped close to the house by normal wind flow around the building.
4. Use a good-sized vent "pipe" from your hood to the outside to ensure max airflow. For example, a 4" diameter flexible dryer vent hose, or 4" galvanized ductwork pipes.
5. If your house is well-sealed to minimize drafts, make sure to open a nearby window when using this. You cannot exhaust a good air flow via the fan of this hood without replacing it with fresh air inflow from outside.
6. A closed box normally has terrible lighting. Commercials use lights in a sealed fixture up in the top. Maybe a simple cheap alternative these days would be a couple of Christmas strings of clear LED's. Look for ones with permanently-installed non-replaceable lamps so all the wiring is sealed in with no exposure of metal parts to gases.And get one spare string for when an original string goes bad. You do not want to be trying to repair and re-seal lights if you can avoid.

 

[Greenman]

I did something similar on a smaller scale for my laser. Since I was only dealing with smoke and occasional caustic fumes, it's a pretty simple setup. Plywood box with amber acrylic windows in the top so I could keep an eye on the process without opening the lid, and a 4" inline fan that vents through the exterior wall. Works like a charm.

 

[bbhaag]

I don't have anything constructive to add but it sounds like a cool project! When you get going on building be sure and post some pics because I'd love to see them.

 

[Chaotic42]

I appreciate the input, everyone. Good point about having airflow into the house. My basement is actually about 9 feet off the ground, I live on a hill, and it has nothing but windows facing the downhill area, so the plan is to pipe the outflow out one of the windows and several feet out into the yard with PVC.

I need to build some tables and work surfaces first - I'm starting from zero. I'll document everything and post it here, but it may be a bit.

As far as lighting goes, I was thinking about routing some holes in the top - maybe 4" down to 1/8" with a little lip - so I can drop round glass/lexan in and have lights above them. I'm also going to build it with a double wye so I can hook up to three blowers in parallel if I need to, then join the airflow back together before exhausting it. I think 4" PVC should be fine with that, unless you all think having multiple exhausts is better.

I was also thinking about maybe sanding the inside of the top down so it's slightly bowl shaped toward the exhaust port, but that may be excessive.

 

[Greenman]

Three blowers seems like overkill to me. One will completely vent the hood in less than 30 seconds. If whatever you have going on is actually producing enough fumes or smoke to require moving 450cfm, I'd step up to a 6 or 8 inch duct and a higher volume blower.
Three blowers into on outlet will also require flapper valves or manual gates to prevent backflow when two aren't in use.

 

[Chaotic42]

So the great ice storm of the 21st century is over here in STL and the Amazon/Home Depot deliveries are flowing like wine. I'm thinking I should make the hood 3 feet deep instead of 4. That will have the benefit of meaning more air replacement per minute and prevent me from having to kind of lean over to reach stuff in the back. It's extra MDF cuts, but I think it'll be better in the long run. It will also cut down on the weight. I think 3/4" MDF was overkill.

I'm also thinking about putting either ceramic tile all around the inside, or maybe doing that and putting a thin PTFE sheet down at the bottom of the hood. I need to build a couple of work tables to get some practice in and have a place to really work with this stuff.

 

[Chaotic42]

So I've got all of my stuff in save the fume hood glass shield. I decided I'm going to go with a 3'x6' hood that's 4' tall after doing some dry runs on a 4'x4' surface. Here's what I'm thinking for the support bench under it. I've got some woodworking experience under my belt, and based on stuff I've made, I want the floor of the hood to be about 45" tall, so I worked backwards. The table will be 4'x8' to give me room behind and to the sides of it, and it's on very robust 8" casters.

I'm taking 2" PVC and cutting it in half while tapering the cut slightly so I can have drainage gratings on the floor of the hood. If something bad happens, the badness can flow down the grates, into the sloped pipe, and then into a borosilicate holding bottle I'll attach underneath.

I also decided to make the hood itself out of birch plywood since MDF crumbles and is a pain. I want the hood to be fairly easy to repair, so I'm using the MDF on the table where spillage is less likely and flatness is important. I increased the vent fan to a 4' non-sparking 12V DC blower that can do 250 CFM, so that's 3.5 replacements per minute at full blast. I'll have a second lower capacity fan installed in parallel in case things get really bad. The blowers are on variable output DC controllers, so I can run them at a minimum of about 8V.

As always, any and all feedback is welcome. I'm not sure if I'm going to do a facing on that middle shelf incase I need to put something right at 15" under there that I can't sort of tuck under a facing board.

 

[Chaotic42]

This is a neat little board for the Raspberry Pi, but man are thermocouple wires tiny.

 

[Chaotic42]

Modified the table plans a little. LMK what you think.

Red = 35" 4x6
Green = 36 13/16" 4x6
Yellow = 89" 2x4
White = 96" 2x4

The surfaces are 3/4" MDF which has been cut into 2x4'x4' squares for ease of moving and the white cubes below represent the wheels which are 9.2" tall. Ignore the snapping and intersection errors, the bottom shelves will have the corners cut out so the fit around the 4x6s. The only weirdness is the bottom shelf which has a full 2x4 underneath the MDF. It'll be attached up through the bottom of the MDF and into the green 4x6. I'm not sure if it's enough support. Screws will also be driven up through the MDF into the green 4x6 itself.

 

[Greenman]

It's both over and under engineered. I'd use a design more like this.

 

[Chaotic42]

Good call on it being too complex. Went with something simpler and I think it will work out ok. It's definitely solid.


That 4x6 was juicy enough that my house will smell like pine for a month. Stickiest of the icky.

 

[Chaotic42]

Well, it's finally a fume hood. I want to cut out the top and sides and add lexan for lights, and I have some through hose adapters so I can pump in water, argon, and a vacuum line. Not sure how much it weighs, but I can't even slide it around on the table.

 

[Chaotic42]

Thank you, Aziz

I think she's finally ready for some work. This light structure was a pain to make. I'm still trying to decide where to put the gas lines, so I'll play with it a bit to see how it goes. I appreciate the feedback and the likes. This was a lot of fun to make. Also, we tested, the GF does easily fit inside.

 

[sdifox]

So what exactly are you going to do to your girlfriend that you need a fume hood that big?

 

[Greenman]

So what exactly are you going to do to your girlfriend that you need a fuke hood that big?

What happens in the fume hood stays in the fume hood.

 

[Chaotic42]

So what exactly are you going to do to your girlfriend that you need a fuke hood that big?

When I showed it to her, she said you could fit a person in there, so I told her to climb in.

 

[Chaotic42]

 

[sdifox]

I am too lazy to go through all that just for coffee.