centrifugal fans

I would use a belt driven furnace blower, they should be readily available at any salvage yard. Gerry :-)} London, Canada

First you assume we're familiar with Gingerys book. Nope. Not this guy in RCF. What does he use to drive the fan, a shaft? If your in an explosive atmosphere I'm not sure you would want to use a belt. May build a static charge.

Second, whatcha doing? Grinding a little anthrax? Cooking a bit of fulminate of mercury? Just curious.

Or have a fan unit that was approved for use. Explosion-proof motors would be essential if the fumes are likely to pass around the motor or if a bearing was likely to allow fumes into the motor area. One of the requirements for fume hoods is that the roof vent usually has to throw the exhaust gases several meters up into the air to obtain adequate dispersion, depending on the local regulations.

One problem with such systems is that the drive mechanism has to be considered and the danger of ignition sources considered. For example, slipping drive belts can result in rapid heat buildup in bearings that could eventually trigger autoignition of flammable solvents. Note that hoods for some materials, such as peroxides, have even more stringent requirements.

You also have to consider issues like local regulations and insurance. They may require any chemical fume hood to comply with a whole suite of performance requirements, certainly they do here in NZ. Homebuilt chemical fumehoods aren't a goer because the cost of compliance testing is so large. Hoods to disperse common fumes, such as from welding, probably don't have to be too clever, and kitchen range hood units may even be acceptable.

I'm not an expert, but I'm not aware of any advantage of downsizing because the usual criteria here include the actual air flow face velocity when the door is fully open ( to protect the person working ), and the need for sufficient volume to avoid dangerous gas concentrations ( or even worse, condensation in the flue ) and also throw the exhaust high above the stack exit. They seem to demand a fairly large volume of air flow.

Microscale experiments probably could be performed in a smaller cupboard, but most people want a fairly large opening and that, along with the need to always ensure that vapours can't reach flammable or toxic concentrations, probably would require quite significant air movement. I suspect that small unit have similar testing requirments to larger units, so there may be minimal price advantage.

Bruce Hamilton

Only if he air is filtered would that be a good idea. Squirrel cage blowers have a way of plugging up, eventually throwing some of the crud, leaving the blower out of balance and rumbling like hell. To get around that problem, one is wise to use a paddle wheel instead of a squirrel cage if there are no plans to filter.

Harold

A fume hood should be explosion proof, capable of moving a high volume of air at low pressure and most of all quiet. Vacuum cleaner blowers are none of those things.

I would suggest a large salvaged furnace blower with a 3:1 belt drive to a 1/2 HP TEFC 1725 RPM motor. Mount the motor away from the ductwork and bond it to the fan bearings and duct work with some wire to drain off any static charge from the belt. Running at 550 to 600 RPM on good isolated bearings the blower will move a lot of air and make an acceptably quiet low rumble. Filters need to be placed before the blower.

Allan Adler wrote:

Normally the blades on a explosion proof fan are made of aluminum or some other material that will not spark if a bearing lets loose and allows the fan to scrub on the venturi ring, and the motors are rated as explosion proof. Can't say about fan driven ones, as I have only seen direct drive units. I would shy away from a squirrel cage type as they are prone to blockage and crud buildup, but if they have a filter beforehand I guess they may ok.

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In addition, the fume hoods at work are designed to eliminate eddies, achieve laminar flow, etc. Velocities at the sash must be in a specified range and are tested. Also, equipment cannot be set too near the sash because of eddies. Ours are now on a central control which automatically raises and lowers the sash based on a detector. The sash raises if you stand in front for any length of time. These fume hoods are designed to handle really toxic stuff.

On the other hand, engineering type controls for nuisance fumes are not nearly as rigorous. Niederman arms and suspended flex duct are used commonly around extruder vents, etc. We also use "ventilated enclosures" for flammables control where a hood is not required. These require much less stringent checks and records.

Pete Keillor

All this talk about explosion proof motors and blowers makes me chuckle.

While "explosion proof" equipment is more resistant to causing an explosion, as I understand it, the term more appropriately refers to the fact that WHEN an explosion occurs, the device is more likely to contain the explosion, and prevent sending shrapnel in every direction.

I have been making "explosion proof" mine elevator doors for 15 years.

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The fume hoods at the lab I work for are not explosion proof. They use a squirrel cage fan attached directly to a motors shaft.

Use of solvents in the hood are such that the vapor concentration doesn't reach flammable levels, let alone explosive.

Dave

Why would vapor have to pass "through the motor" anyway? Just hook up the blower end to a pipe that intersects with the hood outlet. The fumes will be sucked out, but still blown *away* from the motor.

Explosionproof apparatus: "Apparatus enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby." Refer NFPA 70

Note that important point about not causing ignition. Units claiming to be explosionproof, or even rated for working with flammable vapours, usually have to be certified by an independent laboratory, eg Underwriters in the USA.

Most of the cheaper fume hoods here use hermetic sealing of motors into non-flammable plastic or metal containers with PVC fan blades, along with current sensing and winding and surface overtemperature cutouts, rather than expensive motors rated for use in flammable and corrosive environments. Not sure if they would be approved in the USA.

It's also worth commenting on a couple of points raised by other posters.

At least in NZ, adding a filter to a fumehood usually changes it into a "cabinet" ( eg cytocabinet ) or "isolation" unit, and different rules apply. They have to be able to detect when the crud on the filter is affecting flow, and that the filter doesn't create a new hazard ( by collecting nasties that could react ). They usually are far more expensive. Fume hoods follow the simple principle of the solution to pollution is dilution.

Using the venturi concept to avoid passing fumes across the fan is hugely wasteful of energy, as the flows through the hood determine the minimum volume of air. There would have to be additional flow-pressure monitoring to ensure that all heavier-than air molecules ( eg chloroform ) were being drawn from the work area. There are many fumehoods that adjust the fan to maintain critical face and cabinet velocities across the opening and workplace, but they still move large amounts of air.

Bruce Hamilton

You can move air by entrainment, like a jet pump, so the contaminated air never gets near the blower or motor. A higher-velocity lower- volume jet (as from a vacuum cleaner in blower mode, or a leaf blower) entrains the contaminated air in a duct and propels it. The higher the jet velocity, the lower the jet volume required to move a given (larger) volume flow of exhaust air.

Sash velocities on fume hoods are typically 100 fpm IIRC. (That's about 1.1 mph) Multiply that by sash opening area to get required exhaust volume in CFM.

The classic "wet vac" has what is called a bypass motor. The air stream being vacuumed in that case does not pass through the motor. Ordinary vacuum cleaners which have the airstream pass through the motor for cooling purposes are generally of the universal motor style which use brushes to pass current through the armature. The sparking there is not something you want interacting with many of the products one finds under a fume hood.

If MUST do a quick and dirty fume hood using a vacuum cleaner, at the very least use a wet vac. Remove the filter in it and get rid of any dirt/dust before you start in order to keep those things from absorbing whatever toxins / dangerous_stuff you're exhausting.

The one I salvaged 18 years ago had about five pounds of drywall mud that had been going round and round for 16 years at that time, and even though the bottom four inches of furnace were gone, the fan still seemed happy to keep on spinning in all its inefficient glory. Gerry :-)} London, Canada

Wow! That's pretty good performance. I used a new squirrel cage blower for my first fume hood and regretted every minute of it. I was constantly fighting balance and rumble. Maybe I should have coated it with drywall mud! :-)

Happy Holidays!!

Harold

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What newsgroup is RCF? I see that this is cross-posted between rec.crafts.metalworking (where many know of Gingery's books, if not necessarily the details about all of them), and sci.chem (which I know nothing about).

If *I* were to build a fume hood, it would be to extract the fumes when developing photoresist (the developer is 1,1,1 trichloroethane IIRC) -- a bit nasty for your liver at the least, but not explosive.

The other possible use would be for carrying away the acidic fumes from pickling metals -- which would suggest that all metallic parts (including the blower) should be stainless steel, not the less expensive galvanized steel.

The ones which I used at work had the blower mounted on the roof, driven by a big three-phase motor (thus no sparks from switching within the motor), mounted external to the ducting, and connected to the squirrel-cage blowers by multiple V-belts in parallel. Part of the energy of the motor was used to run another blower which pushed air into a second duct, to make up for the air sucked out by the main blower. Otherwise, the partial vacuum in there would make it very difficult to slide the glazed door up and down -- and might even develop enough force to break the glass.

Most of our work was with similar substances -- though there were some who worked with *really* nasty stuff.

Enjoy, DoN.

For small scale work with not to nasties, a ductless hood might be just the ticket. They occasionally come up on auction sites, here is one example

You do have to pay attention to keeping the sorbant fresh

josh halpern

Glenn Ashmore wrote:

When I worked with trichlorethylene in a facility testing asphaltic materials back in the early '60's, it was determined that the fumes were not a good product to be breathing, and was even worse after they decomposed in a gas fired oven. Anyhow, a fume hood got installed over the area were the work was carried out. We soon discovered that the wearing of ball caps in this area was not a good idea as very few of them survived the trip outdoors. Gerry :-)} London, Canada

Damn. That sounds like the "chicken sucker" I had on the end of my extruder. 12" elephant trunk with mega velocity. We said that any chicken dumb enough to walk by would be chicken salad.

Pete Keillor