Sciencemadness Discussion Board

Some fumehood trivia

Gon - 10-1-2010 at 15:53

Hello. I have been a lurker here for a very long time, but as I am not currently practicing much chemistry, I have not felt that it would be prudent to join in the forum discussions.

However, having spent one of my previous lives as an industrial plastic fabricator in the US, the kind of job where one builds fumehoods and equipment for corrosive and dangerous environments, I wanted to point out a secondary design objective that has not AFAIK been mentioned in the DIY discussions.

The outlet of a fumehood in industry is usually a smooth tall PVC pipe with a somewhat larger ID pipe affixed to the end of the stack with short ribs that bring the larger pipe a bit higher than the inner pipe, allowing rain to pass down the inside of the larger pipe, while shielding the inner pipe's output from the wind.

It is a type of weathercap that is pretty unique to fume exhaust systems as far as I know. Note this if you do not want to be noticed. ;)

The purpose of this specialized structure is to propel the exhaust high above the building's roof, and is designed to optimize the laminar flow out of the stack. This is done so that the fumes that are being removed does not travel over and corrode the other things that are on the roof (ie air conditioner, etc) and to allow the fumes to be propelled past the wind envelope of the building that it is housed in.

This implies that for a successful fume exhaust system, you need to not only design for the intended hood, but you will want to be sure that you choose your blower and stack materials for features which promote the smooth acceleration of the exhaust stream.

The design of ducting and ventilation systems has a lot written about it, and a Google search will surely bring a lot of info, but one basic principle that is always observed is that turbulence is very expensive to the system's performance. Smooth duct materials, large radius bends (if bends are absolutely necessary), and avoiding things like abrupt changes in duct diameter, etc will promote laminar flow through the system, and the maximum energy to move material / gas / mist out the stack.

This kind of thing should not make an effective fume removal system out of reach for the amateur; in fact, observing industrial practices should permit scaling such systems down to any size if it is designed with laminar flow in mind.

In fact, (I have not tried this) it might be possible to accelerate the exhaust stream in novel ways like using multiple small blowers in the system, things that would be absurd in industry, but might be very practical to an experimenter.

As long as principles of aerodynamics are kept in mind (simple, like designing your own jet plane) :)


Magpie - 10-1-2010 at 16:22

Hello, Gon, thanks for telling us about your industrial experience. You say that the individual can do things that would be absurd in industry. I think that the converse holds true as well. To the individual, space, noise, and cost predominate. Also, as nice as that stack design sounds, a blower on the roof with a 10 ft stack with or without a fancy rain guard would, as you say, attract a lot of unwanted attention. ;)

A question that comes up here often is the necessity or not of having an explosion proof blower when handling flammable solvents, etc. There is a great difference in cost. What is your opinion on this question?

Jor - 10-1-2010 at 16:44

To be honest, i think this is really not necessary, as a good flow dilutes the fumes to a great extent, and even for the most flammable solvents you need at least a concentration of 1% to get an explosion or flash fire. Ofcourse if large amounts of ether (a large spill) are released at once, you better turn off the hood, or put the blower on a higher speed. This is problematic when you work with CS2 though considering toxicity.

I think corrosion due to acidic fumes is a much more serious concern. I wonder when mine will give up, it's been sucking up a lot of HCl, HNO3, NO2, etc. That's wont be a real problem though, I will just replace the blower, one costs about 100 EUR.

Gon - 10-1-2010 at 18:04

Most of the systems that I have worked on have been used in highly corrosive environments such as metal finishing and printed circuit houses, using corrosion resistant blowers (usually fiberglass housings with polypropylene impellers).

Because these fans are induction motor squirrel cage designs, the motor is usually outside of the exhaust stream. Because corrosive vapors and mists are pretty harmful to motors, the usual case is for the motor to be inside the building, and they are usually not affected by corrosion unless something goes wrong with the system.

That said, I agree with Jor about the dilution issue, but would be very concerned about large amounts of carbon disufide or ether being exhausted without an explosion proof motor.

I have a personal philosophy about safety that is a little complicated when it comes to chemistry and other potentially dangerous activities: you cannot prevent accidents 100 per cent except for simply not doing it.

You can do many things to diminish the odds of an accident, however.

I would rather have a fume hood for a lot of the things that one does in a lab than not have one, but there are limitations that have to be kept in mind as well. As an amateur, I would want to build in as many safety factors in a hood that I could.

One thing would be that in case of a fire or explosion that the hood and associated ductwork be fireproof. PVC is used because it is a "self extinguishing" material. Stainless steel would be nice, but would be very impractical for DIY.

How much flammable vapor one works with would have to be a major safety concern. There has to be a common sense point where one makes the decision to take whatever it is outside and / or limit what you can do in a amateur lab.

When you have a well designed hood, it acts like a chimney flue, with a natural draft independant of the blower. Granted this takes a tall stack or other considerations, but it is something that you would want to consider in a DIY project anyway to make sure that those vapors are exhausted and not just blown back into your attic (or blower motor). Or into your face if the wrong wind comes up.

Yes, an explosion proof blower motor is a great idea, it makes a fumehood that much more safe. But with fumehoods, you still have limitations, and as with any tool it is crucial that you work within them.

Washdown system, anyone? :)

Magpie - 10-1-2010 at 18:19

My ducting for my homemade fume hood is 8" PVC. I would have liked to have put in stainless steel but that would have been horrensously expensive and may have required fabricators to come onsite. With PVC and the help of my son I could self-install. I did not realize that PVC is self-extinguishing. That is good to know.

I have toyed with the idea of installing a small port in my ducting, about midway, to which I could connect a fire extinguisher. It seems that being able to flood the whole system rapidly with CO2 would be a good safety feature. What do you think? Did you do this for any of your customers?

watson.fawkes - 10-1-2010 at 20:02

Quote: Originally posted by Gon  
The outlet of a fumehood in industry is usually a smooth tall PVC pipe with a somewhat larger ID pipe affixed to the end of the stack with short ribs that bring the larger pipe a bit higher than the inner pipe, allowing rain to pass down the inside of the larger pipe, while shielding the inner pipe's output from the wind.
Thanks for your participation. I appreciate it.

I have a few questions about construction details. The first set is about drainage. Since one of the purposes of the outer pipe is to conduct liquid, is the drain for this liquid simply the lower end of the outer pipe, letting it fall onto the flashing of the inner exhaust pipe? Is there ever a problem with condensate from the exhaust taking the same drain path?

The second set is about the air flow path between the inner and outer pipes. How far down from the end of inner exhaust pipe does the larger pipe typically extend? Is their significant flow through this channel? It seems possible for this to be too short, allowing wind gust to interfere with exhaust flow. But is it possible for it to be too long? Are there rules of thumb for dimensions?

Finally, is there a rain cap on top?

Gon - 11-1-2010 at 17:59

@Magpie:

I have heard of extinguishers in fume hoods, I don't see that something like that would do any harm, as long as the blower is off. But personally, I would want to have a device like that where it would work in spite of the blower.

I have read lab accident reports involving materials that have deposited on the inside of the duct like peroxides and have ignited from shock.

If you had water streaming down through the ducts, it would likely cool any fire enough to get it under control.

I should make it clear that I have not built laboratory fumehoods or ventilation systems, only dedicated industrial ones.

@watson.fawkes:

First, an apology for a not too good description of that weather cap; included is an image that I snagged off of Google Images. It prevents nearly 100% of rain from entering the exhaust duct, but allows it to drip down from the collar.

Most of my experience has been with hoods for heated tanks, where condensation is always an issue. This is handled by hanging the exhaust ducting in such a way where any condensation would be drained back into the bath.

Thermoplastic ducts are usually hot air welded or glued together to make a watertight seal, as having icky stuff drip out of the exhaust system onto your head is a very unpleasant thing. :(

Since condensation will likely be a issue with a fumehood, as well, drainage should be provided for. Besides why go to the trouble of building a hood without including at least a cup sink?

http://ateam.lbl.gov/Design-Guide/DGHtm/bitmaps/help0052.jpg

BTW, follow this link for more information on the weather cap.

http://ateam.lbl.gov/Design-Guide/DGHtm/stackdischargefittin...

[Edited on 1-12-2010 by Gon]

Magpie - 11-1-2010 at 19:12

Quote: Originally posted by Gon  

http://ateam.lbl.gov/Design-Guide/DGHtm/stackdischargefittin...


That's a very impressive document. Thanks for posting this link.

watson.fawkes - 12-1-2010 at 08:03

Quote: Originally posted by Gon  
included is an image that I snagged off of Google Images. It prevents nearly 100% of rain from entering the exhaust duct, but allows it to drip down from the collar.

Most of my experience has been with hoods for heated tanks, where condensation is always an issue. This is handled by hanging the exhaust ducting in such a way where any condensation would be drained back into the bath. [...] Since condensation will likely be a issue with a fumehood, as well, drainage should be provided for. Besides why go to the trouble of building a hood without including at least a cup sink?
The diagram you posted answers my question. By specifying the dimension of the outer pipe as "D+1" (and not, say, as a percentage of D), it means that the lower gap is used for pressure equalization, at most, in addition to simple clearance. What I had been wondering is how much leeway the construction was. To be surreptitious in constructing one of these exhausts, it seems reasonable to bring the end of the pipe down to an inch or two of the roof and to cut the end of the pipe parallel to the roof slope. You'd have to have a pretty keen eye to notice that little gap.

I had intended the question about condensation as about the outer collar, but I didn't make that particularly clear. What you've said, though, is that condensation is typically within the duct system itself before the exhaust pipe. That means that condensate dripping onto the roof itself isn't such a problem.

You've also inspired an idea for folks who might have condensation problems: a cold finger within the exhaust piping. The temperature of this finger only needs to be as cold as the outside air, since that's the condensation temperature otherwise. This means the cold finger can be a simple, closed recirculation loop with a heat exchanger outside. The only mechanical piece is a circulation pump; there's no need for a refrigeration system. I don't particularly need this, but others might. Two other advantages come to mind. First, it isolates internal condensation to make drainage easier. Second, it reduces visible emissions (mist, fog, condensing water vapor) by condensing the bulk of vapors prior to their leaving the stack.

Gon - 14-1-2010 at 18:18

Interesting using the weather cap in the way that you suggest, it would certainly not interfere with the vertical discharge flume.

I wonder if cladding the stack with thin stainless steel might not make it look like a sauna exhaust or like a new fangled pellet furnace stack. :)

The main reason why I posted some of my understanding of design guidelines for fume exhaust was that I think that looking at it as a streamlined system might make some interesting things possible. As was mentioned earlier, having a fairly high stack velocity would insure that the exhaust would diffuse rapidly and move it away from the building.

Acheiving that flow is what I think is a little exciting. Even with the most powerful blower and perfect ducts and stack, the exhaust system would still be limited by the available air that could be drawn through the hood.

Like in industry, you would also need to provide for make up air to replace the air that has been drawn out of your lab. Squirrel cage blowers do not work very hard moving air if the input to the blower is constricted; they simply move the available air around in their housing. But when lots of air is available to the impeller, they become very impressive air moving machines.

Suppose that you have a hood that needs a certain amount of airflow through it to function efficently; you don't necessarily have to rely solely on the blower that you have downstream to draw the air into the hood, you could mount a blower in the wall that forced clean outside air into the lab and then into the hood. It simply does not have anywhere else to go (assuming that there is a door that closes off the lab from the rest of the building). The blower pushing air in and the other blower pulling air out gives you a lot more air moving, but only one that works in the exhaust stream.

There is no reason why the lab could not draw tempered air in from the inside of the building (presuming that makeup air would be drawn in though the furnace) and keep the lab comfortable (but breezy).

Magpie - 14-1-2010 at 18:53

Quote: Originally posted by Gon  

There is no reason why the lab could not draw tempered air in from the inside of the building (presuming that makeup air would be drawn in though the furnace) and keep the lab comfortable (but breezy).


My lab is located in my garage. I installed a filtered inlet in the wall to the attic space in the peak of my garage. This let's in plenty of air but it has a faint odor of attic insulation, which I don't like. So I usually just leave the door to my living space partially open to provide most of the inlet air. This has the added benefit of keeping the lab more temperate in both winter and summer. It's just costing more that way.

I don't seem to have an issue with fumes being too close to the house at the outlet in my garage peak. I see no deposits or corrosion at the louvre there. I really don't even have a stack. The eave plus the louvre design keep all the rain out and I get no condensate. I do see a little dust in the hood after a good windstorm, however. It's minor maintenance to wipe it up with a damp paper towel.

watson.fawkes - 15-1-2010 at 04:23

Quote: Originally posted by Gon  
Acheiving that flow is what I think is a little exciting. Even with the most powerful blower and perfect ducts and stack, the exhaust system would still be limited by the available air that could be drawn through the hood.
That's only true if the fume hood is the only air inlet. You can decouple fume hood flow and exhaust stack flow with a second fan with its own independent outside air intake. The second fan has two inlets: the fume hood exhaust and its independent air inlet. If that second fan fails, fume hood exhaust might leave through the outside air inlet, so it should be located not stupidly. If you want to leave the second fan off for nuisance fume hood operation, install a damper on the outside air inlet. The first, fume hood fan will be exhausting against a lower pressure as long as the second fan is running, which will increase its flow rate. Summary: first fan provides correct face velocity at hood; second fan provides adequate stack velocity at exhaust.

Gon - 16-1-2010 at 11:21

@Magpie:

Thank you, you helped make my point. It sounds like you have a very good exhaust system! Are you using a Nutone-type fan in your ceiling?

@Watson.fawkes:

Nice suggestion. Would you say that you are describing what is typically used with state-of-the-art hoods that port makeup air into the hood when the cover is completely closed or are they generally passive vents?

What you describe still relies on acheiving a high face velocity based on pulling air into the hood, but does not mention the possibility of pushing air in because of a slightly positive pressure maintained in the lab supplied by a wall or ceiling mounted fan.

Push-pull exhaust systems are common in plating and metal finishing because of the makeup air needs of big tanks and to decouple the tank's exhaust from the plant's HVAC. This is where a blower will direct air though a slot at one end of the tank sending it across the surface of the bath and into a hood at the other end with a pretty large capture area. These hoods are pretty similar in design to the vent in a fumehood, but without the front shield.

I have noticed that most of the discussion of DIY fume hoods has been centered on building a box that sucks :) and not on the subject of directing air through your lab in a benefical way.

In it's simplest form, I think that a DIY hood requires two fans, one bringing air into the lab, and one directing air out of it. The design of the hood can vary a lot, including "snorkel" type of collectors.

Perhaps I have missed something somewhere.

Magpie - 16-1-2010 at 12:15

Quote: Originally posted by Gon  
@Magpie:
Thank you, you helped make my point. It sounds like you have a very good exhaust system! Are you using a Nutone-type fan in your ceiling?

...The design of the hood can vary a lot, including "snorkel" type of collectors.


Yes, I am quite satisfied with my exhaust system. I am using a Grainger squirrel cage, closed one-side fan with a 9"ID suction. It is rated for 450cfm @ 0.75" H2O pressure drop, which is my operating point.

I have noticed that some of the newly constructed labs have the "snorkel" type collectors, one at each work station. Do you know if these are working out OK?

Gon - 16-1-2010 at 20:58

Nice choice of blower, Magpie.

I can't say that I know if snorkel exhaust work or not, as I have not worked with them.

But, as I work in a manufacturing facility in the US, and have to wear safety glasses in areas where there is no machining or activities that could endanger one's eyes, I have a feeling that the snorkel exhaust systems solve some issue with workplace exposure to hazardous substances.

And I am happy that folks that use them don't have to spend the day wearing respirator masks.

On the other hand, I could find a lot of uses for something like exhaust collectors that could be mounted to supports like glassware. While a proper fumehood is indispensible for some activities that a mad scientist might take on, I would prefer any method of fume control, no matter how humble.

watson.fawkes - 17-1-2010 at 11:22

Quote: Originally posted by Gon  
Would you say that you are describing what is typically used with state-of-the-art hoods that port makeup air into the hood when the cover is completely closed or are they generally passive vents?
It's a passive vent of outside air. There's an upper bound on the face velocity through a fume hood so that you maintain laminar flow across the face. Too high a velocity, this breaks down. So if you've got the sash down but not closed, there's not a lot of air coming through the face, but you cannot just pump lots of air naively into the hood, because you've got to maintain the face flow as both inward and laminar. As a result, you introduce air into the exhaust duct after the fan for the hood itself. The hood fan, therefore, has the responsibility for maintaining the face flow correctly.

Now you have to deal with achieving minimum exhaust velocity. The chimney is of fixed diameter, and that translates into minimum volumetric air flow to the chimney. The air volume coming from the hood is allowed to be less that this flow rate, by assumption, or else you don't need this technique. To make up the extra volume, you have a passive inlet vent from the outside. To ensure adequate exhaust stack velocity, you have a second fan.
Quote:
Push-pull exhaust systems are common in plating and metal finishing [...] I have noticed that most of the discussion of DIY fume hoods has been centered on building a box that sucks :) and not on the subject of directing air through your lab in a benefical way.
This idea is also used in some fume hood installations. The "push" side is an air curtain mounted just forward of and above the sash. You get laminar flow down the outside of the sash and in through the face. These are evidently good enough that in many cases you can use unconditioned air for the curtain without much adverse impact on the lab environment. Even if you don't do this, installing an intake fan gives an assurance of adequate face flow without installing sensors, such as a pitot tube.

As for snorkel collectors, they can improve isolation of contaminants when used inside and in conjunction with a fume hood and its fan.