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Author: Subject: homebuilt fume hood
peach
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[*] posted on 4-9-2010 at 12:30


{edit x 2}By dust, I'm assuming you mean powder

The powder will be too heavy for the fan to pick up in the same way as it will fumes. It's designed to blanket puddles of things on fire (I used one of these to put my brother out when he set his feet on fire and ended up dancing around the garden - one of the funniest things I've seen). Fortunately, he did suffer some long lasting burns, so he's learnt his lesson now.

I'd expect it to work quite a bit better with something closer to a fume.

An even simpler suggestion, set fire to something. Petrol / oils / liquid lighter fuels / xylene / toluene (both gloss paint thinners) / hexanes will produce a cloud of visible smoke / soot, light solvents like ethanol, methanol, acetone, acetate etc won't. An acetylene / MAPP torch with the oxygen turned off will, a propane / butane / NG torch won't.

The hardware / plumbing stores nearby will likely sell smoke bomb pellets / matches for testing flues. You'll get lots of thick, pretty smoke and they smell nice too. Practical joke stores may sell them cheaper.

Although my 'Mr Shiterlot Sugar' didn't do a lot to it's unknowing target. Looks like I'll have to upgrade to genuine pharmaceutical capacity for a good yield.

{edit}Hopefully you still have some fire extinguisher left if you're going with the on fire route. Things that produce plumes of black smoke generally.... dislike? water.

[Edited on 4-9-2010 by peach]




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Magpie
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[*] posted on 4-9-2010 at 15:37


Open a bottle of conc HCl, or better yet conc ammonium hydroxide, in your hood. If you don't smell anything while standing in front of your hood it would seem it's doing an adequate job.



The single most important condition for a successful synthesis is good mixing - Nicodem
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cnidocyte
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[*] posted on 9-9-2010 at 07:38


I tested out my hood with a smoke bomb twice, the first time it worked well but the 2nd time the smoke bomb was near the opening and at least half of the smoke came out. I increase the face velocity somehow. I still haven't sealed up all the holes and cracks, that will definitely increase it a bit.
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peach
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[*] posted on 9-9-2010 at 08:19


Have you got some baffling in there?

The baffles, and which ones you open / close, are important in controlling where all the air is sucking from and going to. It could be that the stream is just bypassing the smoke, and that it's actually fine, just not flowing where you want it.

{People ramble on about the technicalities of fume control all the time, and lots of people have their own opinions or ways of doing things. But a universal constant is that the thing producing fumes needs to be about 6" or so AWAY from the opening. It really wants to be towards the back, near the baffles. If your smoke bomb was near the opening, that may be why. You're also supposed to stand back from the opening, as your body will create dead airspace in front of you. I work with corrosive, blistering gases quite frequently, I still don't understand the urge to stand directly in front of the hood, why you'd need to or why you'd even need to be tweaking the sash all the time. If you need to constantly play with the gear, you're doing something wrong. Shut it over, bugger off for a cup of tea. As per the solvent still rooms in universities. The still rooms on curly arrow's page, at Cambridge in their chemistry department, actually have "CLOSE THE SASH!" sticky taped to them}

[Edited on 9-9-2010 by peach]




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cnidocyte
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[*] posted on 9-9-2010 at 10:25


Quote:
Quote: Originally posted by peach  
Have you got some baffling in there?

The baffles, and which ones you open / close, are important in controlling where all the air is sucking from and going to. It could be that the stream is just bypassing the smoke, and that it's actually fine, just not flowing where you want it.

I don't actually know what baffles are yet. I've read a few definitions but I really need a picture to see what they are. If I put the smoke bomb at the back of the hood, the fan will extract all of the smoke.

Also there is not just 1 passageway for air to flow in this lab, as you can see here

the shelf space underneath the hood is open to the outside so thats an alternative route for air flow. I can easily seal that up so that the only input route is the door.
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metalresearcher
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[*] posted on 9-9-2010 at 11:47


I recently moved to a new home in which I dedicated one room as my silversmithing room and lab including a fumehood.

More pictures:

http://www.metallab.net/fumehood2.php


fumehood-finished.jpg - 39kB

[Edited on 2010-9-9 by metalresearcher]
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peach
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[*] posted on 9-9-2010 at 11:53


The baffle is simply a sheet of wood, metal, plastic, whatever you prefer, at the back of the cupboard. It covers the intake to the fan. Imagine just walling off the fan intake area. Then there's a few horizontal slits in it from the top to the bottom.

There's usually one right up at the top, then one about where a beaker or flask would be if it was sat on a plate, then one right at the bottom.

The idea is that they;

a.) Stop the air for scooting round and through the cupboard without sweeping the fumes up with them, but forcing the fan to suck a kind of virtual wall of air towards the back of the cupboard

b.) Some gases are lighter than air, some are heavier, sometimes there's a beaker spewing them off. The baffles mean the person doing the coughing can open the top one, or the bottom one, or whatever combination they want to suck the fumes away where they're pooling up. It's just like a hose on a vacuum cleaner really, or one of those fume things on a flexible light stand that electronics guys put over their boards as they do the soldering. To put the sucking where it needs sucking. The heat of a roasting hotplate can sometimes carry heavier than air gases up.

It doesn't need to be complicated, and experimenting will work as well or better than spending weeks trying to do it on paper (too many variables).

Again, some people differ on how they do it;

The big slits at the back are to create a wall moving away from the front, the fan is sucking from behind there.


Another method using sheets


They're doing a lot of chatting about the topic, here <------- :D

[Edited on 9-9-2010 by peach]




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cnidocyte
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[*] posted on 11-9-2010 at 11:09


Nice one peach. That explains it. Shit if I'd known about this I wouldn't have put my fan right in the center of the depth of the hood. Ah well I can always move it.
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peach
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[*] posted on 11-9-2010 at 11:13


No need matey, just make the top angled baffle reach further forward so it covers the intake.

Further back would be little better, but that means sawing more holes and patching the originals.




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[*] posted on 16-4-2011 at 22:39


I finally got started on my fumehood today, I'm using a dust extractor fan and the shop fridge I posted upthread. I still have to make the sash and baffles but even with nothing on the front it works great for sucking smoke out, even smoke from KNO3/sugar didn't make it outside the hood.
The hood:


The blower:


There are still some holes I have to seal on the blower, I burned a bunch of toluene in the hood with the fan running so the soot would collect around the holes, this is much easier than putting something smelly in the hood and trying to smell for the holes.
Soot around hole:
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GreenD
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[*] posted on 17-4-2011 at 10:50


If anyone lives in the upper midwest of the states I know a place where you can buy a professional lab fume hood for <500$
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[*] posted on 4-6-2011 at 02:26
Fans


I know some people are having trouble locating fans with the external motors, maybe this will help.

http://www.axair-fans.co.uk/products/centrifugal-fans/plasti...

Rgds
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[*] posted on 7-6-2011 at 09:43


If you are looking for something compact, a furnace inducer blower could be used. Most fit within 12"x12" amount of space.





Amana Inducer X89-563.jpg - 43kB





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[*] posted on 13-6-2011 at 07:02


My first post, so I decided to share some pictures of my recently completed fume hood.



The construction consists of an outer shell of wood, and an inner shell of PVC. The PVC was chosen mainly because I had a lot left over from a previous project. It is reasonably fire proof, and according to a compatibility table, is corrosion proof to nearly everything except certain solvents (mainly esters). The base has been tiled, to make more durable to spills.



The baffle system is also made from PVC sheet, the design copied from Magpie's informative pictures. I also copied his front sash, by using a commercial double glazed sash window. The hood has an interior space of roughly 1m wide by 1m high and a depth of approx 0.7m.



The fan is unfortunately an in-line centrifugal model (squirrel cage fans don't seem to have the same availability in Europe), capable of moving 600m3 of air per hour. With careful absorption of nasty gases, I expect it to withstand at least for a while. A small fluorescent tube gives interior lighting from a glass panel bonded to the roof.



With the exception of the fan and the window (which was got at a discount price), the whole hood has been constructed from recycled material, so did not cost too much to build, however makes it far from perfect.


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Magpie
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[*] posted on 13-6-2011 at 09:50


Very nice, Thor. I was curious about your face velocity, so assuming a window height of 0.5m I calculated a face velocity of 1.09 ft/s. This, I feel, is quite adequate (I think mine is 1 ft/s).

Just out of curiosity what is the ID of your ducting, and where did you place the outlet of your duct from the building?

Also, if your fan motor is directly exposed to duct flow I would be very wary of handling flammable solvents. Ordinarily it would probably be safe due to the high dilution with air, but if you spilled a 100mL of ether I wouldn't want to be around to see what happens! :o




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Thor
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[*] posted on 13-6-2011 at 10:34


Thank you Magpie! And that sound's about right for the face velocity, I did the calculation very quickly after I sourced the window unit. Building round about the window limited the design somewhat, but the price of the unit was too good to pass by. My allotted lab space in the garage is very small, so I make do.

The ID of the exhaust tubing is 150mm, I had a choice of this or a 200mm outlet on the fan, however I wasn't comfortable knocking a bigger hole through the wall. The tube exits on the outside wall, the layout of the land outside means its quite high up, and i'm fortunate to be quite far away from the neighbours. I did consider directing the exhaust straight up through the ceiling, but it would have been rather obvious looking and a lot more work getting the roof sealed again.

A quick inspection of the fan, showed it to be a magnetic type induction motor. It seemed reasonably sealed from the airflow path. With that type of motor, there should be no sparking, unless its excessively worn. I am however aware of the risk though, and will need to be very careful. Getting a squirrel cage fan seems to be next to impossible here, It would have more than doubled the cost of the project. For the moment, I will be conducting quite simple experiments, as its a totally different ball game experimenting at home, compared to the labs at university.
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thumbup.gif posted on 17-6-2011 at 01:57


Outstanding work, Thor! May I ask what king of locking mechanism you used on the door to keep it open/prevent it from falling freely?
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[*] posted on 17-6-2011 at 04:49


Secret Squirrel: Thank you! The window is a commercially produced double glazed sash unit. Like one you would buy for your house. The mechanism is a counterbalance system, and is fully integrated into the window frame. The window sits at whatever height you set it at. One could build such a sash, but it would increase the complexity of the build quite a bit.

[Edited on 17-6-2011 by Thor]
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[*] posted on 24-9-2011 at 01:12


Dredging up this here old thread, cause I had an idear and wanted to knock it off some of you more hvac minded individuals..

My current workspace has a blower rated for 6000 cfm on the top of it blowing directly into the room. I recently tried running power to it and discovered it works quite well. Well enough to slam doors just by plugging the thing in.

Its not corrosion resistant, and it's inline, but because it is such a burly blower I was thinking I might just seal the place up, run a duct straight out the wall from the back of the fume cupboard, and install a vent with a baffle above the door to control overall flow to the building. Basically just using the fume hood as a window and keeping the place under positive pressure.

I think my biggest challenge is actually going to be making the duct small enough to provide enough back pressure such that my face volume won't be counterproductive. That and baffling. I'm having some trouble figuring out the best way to calculate this. My fist "duct" is basically directly from the blower into an 8000 cubic foot room, my second would be the opening of the hood, and my third the outlet to outside. How do I calculate the necessary amount of back pressure on the outlet to ensure I don't knock over any glassware with this beast?

Any input/resources?
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[*] posted on 24-9-2011 at 04:18


I'd suggest replacing any glazing with polycarbonate or perspex sheet of appropriate thickness.
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watson.fawkes
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[*] posted on 28-9-2011 at 02:42


Quote: Originally posted by Chordate  
I think my biggest challenge is actually going to be making the duct small enough to provide enough back pressure such that my face volume won't be counterproductive. That and baffling. I'm having some trouble figuring out the best way to calculate this. My fist "duct" is basically directly from the blower into an 8000 cubic foot room, my second would be the opening of the hood, and my third the outlet to outside. How do I calculate the necessary amount of back pressure on the outlet to ensure I don't knock over any glassware with this beast?
Trying to calculate this doesn't seem like the short path to success. Rather than calculating the duct resistance to find the right face velocity, just measure it and adjust the back pressure empirically. All you need is an anemometer. There are hand-held units available for less than $50 that should be adequate. You could mount the anemometer permanently into the enclosure to ensure that your operating parameters don't change.

I should mention that the other way to moderate the face velocity is to create an alternate path for air flow. Opening a door will do this (one of the reasons to continuously monitor the velocity), but so will making another exhaust duct with a damper. Opening this damper will lower the face velocity, just as opening the damper on the outlet of the hood will.
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Neil
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[*] posted on 29-9-2011 at 07:03


Throwing this out there;

If you use a lot of organics; do not use polycarbonate. It will crystallize and shatter with time and exposure.
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[*] posted on 29-9-2011 at 16:03
Ranger blower fan


I'm in the process of building a fume hood on a tight budget "cheap" I was wondering if I could use this blower fan I took out of my 89 ranger the squirrel cage is about 3 x 6 inch
I was hoping it would be strong enough because I like the idea of being able to have variable speed so when i dont need a lot of suction I wont be blowing out all the heat of the basement in the winter. and if the power should go out it will still be able to run off the battery which I intend to have powering it as well as a DC transformer


[img]C:\Users\Steve\Pictures\2011-09-29\2.jpg[/img]

2.jpg - 56kB

[Edited on 30-9-2011 by Steve_hi]
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overload
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[*] posted on 29-9-2011 at 19:40


Quote: Originally posted by Steve_hi  
I'm in the process of building a fume hood on a tight budget "cheap" I was wondering if I could use this blower fan I took out of my 98 ranger the squirrel cage is about 3 x 6 inch
I was hoping it would be strong enough because I like the idea of being able to have variable speed so when i dont need a lot of suction I wont be blowing out all the heat of the basement in the winter. and if the power should go out it will still be able to run off the battery which I intend to have powering it as well as a DC transformer


You should just buy a new sparkless motor that way you will only have to worry about the power going out instead of both the power going out and the motor going out. You said this motor has been used quite a bit right?
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Steve_hi
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[*] posted on 30-9-2011 at 01:21


Actually I didnt say but considering it is a 89 and not 98 as I originally posted and the fact that this is canada and the heater is on 9 months of the year I guess we could say it was used a lot. A sparkless dc motor where could i get such a thing that would fit with this fan. but It is the volume of air I'm wondering about. I can't find any data on the volume that a vehicle fan puts out but I connected it to a battery and it seems quite strong
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