melamine fume hood build more space? or better air flow?

[skinny]

alright i have a choice between making my fume hood 4 feet wide by 2 feet high and 2 feet deep with a slanted front sash and a 6 inch gap at the front bottom for air flow or 3 feet wide. what are your thoughts on this? 4 feet allows me to mount a shoplight i already own for lighting but i would have to cut my front swinging doors in half and having hinged front sash but at 3 feet i can make a sliding sash with a single piece of panel. i also have a 800 cfm blower that will vent the hood i dont know how to calculate cfm needed for my measurements especially with a slanted front. i want to be able to swap the blower for an air scrubber in the future but the one id get is only 550cfm. ill be using a gas scrubbing setup to catch most of the fumes so calculating the cfm doesnt need to be exact i just want good airflow incase the gas scrubber fails or i need to remove a seeing glass during a reaction. i did a quick search but wanted to start a discussion about my choices before im stuck with them for my particular build, i read a bunch of threads that helped me pick materials and that was super helpful i feel like this might be an easy answer i would think the more space the better and im hoping you guys have some more educated input.



heres some day photos of the build considering all i had was hand tools and some drills i think it came out pretty well got a light mounted inside just have to hook up the blower and ducting and install the front sash clean it up and maybe give it a layer of paint before staining the shit out of it with acid haha

 

[eaglekeeper]

Even if you intend to use a scrubbing setup, that light fixture won't last long. Those exposed contact points for the bulbs will start to corrode and require constant cleaning.

 

[Yggdrasil]

alright i have a choice between making my fume hood 4 feet wide by 2 feet high and 2 feet deep with a slanted front sash and a 6 inch gap at the front bottom for air flow or 3 feet wide. what are your thoughts on this? 4 feet allows me to mount a shoplight i already own for lighting but i would have to cut my front swinging doors in half and having hinged front sash but at 3 feet i can make a sliding sash with a single piece of panel. i also have a 800 cfm blower that will vent the hood i dont know how to calculate cfm needed for my measurements especially with a slanted front. i want to be able to swap the blower for an air scrubber in the future but the one id get is only 550cfm. ill be using a gas scrubbing setup to catch most of the fumes so calculating the cfm doesnt need to be exact i just want good airflow incase the gas scrubber fails or i need to remove a seeing glass during a reaction. i did a quick search but wanted to start a discussion about my choices before im stuck with them for my particular build, i read a bunch of threads that helped me pick materials and that was super helpful i feel like this might be an easy answer i would think the more space the better and im hoping you guys have some more educated input.



heres some day photos of the build considering all i had was hand tools and some drills i think it came out pretty well got a light mounted inside just have to hook up the blower and ducting and install the front sash clean it up and maybe give it a layer of paint before staining the shit out of it with acid haha

As Eaglekeeper says those won't last long.
That is why most of us make a plexi glass opening in the hood and keep the light on the outside.
And if you make the glass front with hinges, make sure it is of plastic material.
Or better a sliding mechanism without hinges.

 

[skinny]

Even if you intend to use a scrubbing setup, that light fixture won't last long. Those exposed contact points for the bulbs will start to corrode and require constant cleaning.

thanks for that input dave, i already figured as much but i didnt figure it would be that fast like i mentioned i already owned the light so it doesnt really cost me, but i do want to upgrade when this does go out. whats your recommendation for lighting? i was thinking i could build a sheild of some type around it.

 

[skinny]

As Eaglekeeper says those won't last long.
That is why most of us make a plexi glass opening in the hood and keep the light on the outside.
And if you make the glass front with hinges, make sure it is of plastic material.
Or better a sliding mechanism without hinges.

i addressed the light not lasting in my reply to dave so i wont repeat it here but as for replacing it i saw some folks use led strips would that be a better option? good point too with plastic hinges i am trying to use as little metal as possible i didnt think about those. but i did make sure my design will include the hinges if any on the out side. the only problem i have with a sliding sash is the sash is already 2 piece which means id have to install some kind of channel in the center of the hood and have two separate sashes which i guess itll already have. im limited though on what i can do i built and did all the cuts here with a hand circulating saw with a laser and tape measure im lucky it came out as good as it did. i cleaned up later in the evening and caulked all the cracks it looks alot better than in the pictures here

 

[FrugalRefiner]

For the lighting, cut an opening in the top of the hood a little bigger than the light. Then cut a piece of plexiglass a little bigger than the opening to fit over it, and silicone it on top. Then you can put your light on top of it (leave a little space to allow for ventilation) outside the hood.

Dave

 

[BGDOCK]

I think this will get you close
Area of Face of hood 4 x 2 x 100
Inches to sq feet wold be
Area /144 ( to get sq. Ft. ) x100 for cfm
So if you have bends I think that reduces the pressure and the length as well .
Good luck I’m also working on a hood
Going to harbor Frieght now for a 1500 cfm 8” fan

 

[skinny]

 

[Shark]

Looking good.

 

[skinny]

Looking good.

thank you shark, ive got a quick question im gonna attach a drawing for reference but i still need to vent and duct the hood and because i went so big id need a really big fan what i was thinking is going for two vent holes on the box with a double venturi so the big blower would be pushing horizontal across the top of the box over the top vent hole the second would be pushing vertically across the side of the box and side vent hole together hopefully pulling all the air from the front of the box to the back. what do you think of this design do you think it would work?
i forgot to mention some details and include them in my drawings. the big blower on top is 8" just before the venturi ill squeeze down to 6" the side is a 6 inch booster fan variable speed and itll be 6 inches all the way up

 

[Shark]

I have no idea really. I have never quite understood how they calculate the flow and allow for elbows and Venturi’s and all. I mainly used a draft style system and just used longer or bigger pipe until it seemed to work good.

 

[BGDOCK]

I believe the cfm stays the same it’s the pressure that changes , I find my self in a similar situation ( hood is a little big but the perfect size to work in ) instead of two ducts on the top and bottom of the hood , I was going to create a false wall in the back with opening at the top and bottom to pull fumes and airflow through , I believe it called throw , I have a floor hvac floor grille duct piece ( 4x8 ) that I was thinking would work . The duct will be painted w/ POR-15 inside and flex seal outside .Harbor Freight fan motor totally enclosed 1550 cfm 8” probably have to use 6x8 reducers .

 

[4metals]

I think where your horizontal airflow intersects your vertical airflow it will cause back pressure issues. I would angle the horizontal up at a 45 a few feet past where it picks up the hood air and insert a second TY a few feet above where the vertical TY picks up the hood air. This way you would have both airstreams well on their path and not interfere with each other.

You realize you have to compress the airflow from the blower and allow it to decompress and draw air from the hood just as it passes the hood opening. Your sketch does not show that compressing.

 

[BGDOCK]

Top view

 

[4metals]

I just quickly whipped this up to try to make sense out of the sketch


One thing this does not show is above the TY where it goes off the top of the page I would increase the pipe size because the pipe needs to accommodate the airflow of 2 fans so a wider duct up there will not cause turbulence and cost efficiency. The white lines show the tapers inside the pipe to make the venturi effective. Note the tee's are true wye's not a 90º tee with a 45 glued on.

 

[BGDOCK]

Skinny Im sorry. I posted in the wrong thread . My bad guys really .

Jeff

 

[snoman701]

You want the 6" all the way outside with that blower. You are already necking it down a lot just going to the 6" Learned that one the hard way.

I actually go out through the wall, because it's too hard to find a good way to run 6" pvc through the roof.

 

[snoman701]

Oh, your blower will last about a year and a half, and will get increasingly loud. If you want, I can tell you what the part number is for the rubber reducer you want.

 

[BGDOCK]

yes sir that would be nice to have , I was considering getting the extended Warrenty for 14 dollars .
There is a 6 “ duct that was run thru the roof . It may have been an exhaust for a furnace ( former owner made jewelry ) so i was thinking of sleeving it w/ 4 “ triple walled pvc landscaping or drainage pipe

 

[snoman701]

Fernco 1056-86. I ordered from SupplyHouse

I had went vertical through the roof, but I got sick of the rain leakage. I had a galvanized cap on top that lasted about a year. If I was going through the roof again, which I wont, I'd fabricate everything from polypropylene. Plastec offers a 6" weather cap as well, but I have no clue how much it costs. Their stuff tends to be pricy.