Sciencemadness Discussion Board

homebuilt fume hood

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DNA - 1-9-2008 at 22:39


Sideview of fumehood

Like this?

the 5cm from the back of the hood you were mentioning.
Do you mean 5cm space between the baffle and the backwall or do you mean around the edges?


Frontview

I was bored and I made another one just for clarity and practising my paint skills.


[Edited on 2-9-2008 by DNA]

[Edited on 2-9-2008 by DNA]

[Edited on 2-9-2008 by DNA]

[Edited on 9-9-2008 by Polverone]

Magpie - 2-9-2008 at 07:44

Quote:

Do you mean 5cm space between the baffle and the backwall or do you mean around the edges?


The backwall. The baffles are tight up to the edges.

I will prepare a couple sketches to show you what I mean. I need to learn how to use "paint" I guess. I'm a troglodyte and don't even know if I have that capability on my computer.

Magpie - 2-9-2008 at 10:07

[Edited on 2-9-2008 by Magpie]

Oops! Somehow during my edit I lost the text to my message. I reconstruct as:

Here is the sketch of the hood baffles. Keep in mind that the baffles do not in themselves create suction. They are there to direct air flow. Adequate suction requires a sufficiently large blower and duct size to provide sufficient airflow. Sufficient airflow in turn gives sufficient "face velocity," previously defined.

If you have any further questions let me know.

[Edited on 2-9-2008 by Magpie]

[Edited on 2-9-2008 by Magpie]

[Edited on 2-9-2008 by Magpie]

hood.bmp - 1011kB

Picric-A - 2-9-2008 at 12:22

ozone labs, what do you use your nitrogen for? a inert gas?
On my home built fume hood it doesnt extract the gas well, i often find it pushes it back at me,
will those air foils stop that?

Magpie - 2-9-2008 at 15:31

Quote:

On my home built fume hood it doesnt extract the gas well, i often find it pushes it back at me, will those air foils stop that?


A fume hood shouldn't do that - that defeats the purpose of the hood.

Whether baffles will solve this problem or not is hard to say without knowing your configuration in detail. They might.

I would take a look at the configurations shown on the Kewaunee Scientific Corporation website (and others). See if you can modify to their designs. At least you might get some ideas for improving your air flow pattern.

[Edited on 2-9-2008 by Magpie]

DNA - 3-9-2008 at 06:07

I found a table for airsuction in fumehoods after searching for a while.
It might be useful for the people that already made a fumehood and ofcourse the people planning on making one. The information below is for the industry and university's in The Netherlands as defined by the law.

Standard fume hood:
Cabinet width 100 cm: 450 - 630 m3/h

Cabinet width 120 cm: 540 - 756 m3/h

Cabinet width 150 cm: 675 - 945 m3/h

For all above mentioned sizes with the sash opened for 50cm there should be a flow of 0,25 - 0,45 m/sec.

If one has cupboards below the fumehood then that room should be refreshed 20 times (8,5 m3/h) an hour by
the suction in the fume hood.

The fan should not make more noice then 45 dB(A).

I looked back to what my fan does...*shame* 88.3 m3/h
WAY too little so I'll be getting myself an :( expensive squirrel-cage fan but hey look at the positive side of it, I'll probably keep living :D

[Edited on 3-9-2008 by DNA]

Magpie - 3-9-2008 at 07:50

Quote:

Standard fume hood:

Cabinet width 100 cm: 450 - 630 m3/h

Cabinet width 120 cm: 540 - 756 m3/h

Cabinet width 150 cm: 675 - 945 m3/h

For all above mentioned sizes with the sash opened for 50cm there should be a flow of 0,25 - 0,45 m/sec.


That's some good information DNA. For US usage I converted the figures for the 120cm (47.2 inches) width hood:

flow: 317 - 441 ft3/min
face velocity: 0.82 - 1.46 ft/sec

There really is no substitute for an adequate flow.

DNA - 3-9-2008 at 23:13

Just got myself a 1000 m3/h "squirrel-cage" fan 8" (20cm diameter) for 80 euro's (new price 208 euro).
Did you attach the fan at the beginning (above the hood) or at the end of the ducting (exhaust pipe)?

Has anyone got an idea to measure the airflow in m/sec. at the sash opening?

Also leave enough space between the hood and yourself so that you don't obstruct the airflow going inwords.

Do mind that little bobble at around your stomach hight, this is quite important for the airflow.

In understanding url you can see a animated picture of airflow of a new and a old fumehood.
The text says that with the new fumehood when opening/closing the window the vortex is not disrupted, while as you click the button you see the vortex being disrupted of the "old" fumehood.
I tried to make a drawing of the baffles of the new one:


http://www.splusb.nl/sb2003/ZK_product/zk_werking.asp?link=0...

[Edited on 4-9-2008 by DNA]

Magpie - 4-9-2008 at 08:15

Quote:

Just got myself a 1000 m3/h "squirrel-cage" fan 8" (20cm diameter) for 80 euro's (new price 208 euro).

Did you attach the fan at the beginning (above the hood) or at the end of the ducting (exhaust pipe)?


Wow! That's quite some blower, and for a good price too. ;)
You could vent a 5 ft (150cm) wide hood with that!

I placed my blower at the end of the duct. Commercial applications usually do this by placing the blower on the roof. Mine is just inside the garage attic wall. You can see it in the rafters in one of my pictures early in this thread. For air movement I don't think it matters where the blower is placed. However, having it at the end keeps the ducting under suction. This is better for any small duct joint leaks,etc, I think. Edit: Also, it gets the blower noise as far away from you as possible. Although I have read that most of the noise is just due to the air rushing through the ducting.

Your sketches show a hood with variable opening. By changing this opening you will change the face velocity. This is something you should give some thought: Do you want a varable area opening or a fixed area opening? I have a fixed area opening. My face velocity is constant no matter where I place my sliding window.

Edit:

That seems to be a high capacity blower for a 20cm opening. What is the blower rpm? Also, is the inlet to the blower on one side only, or both sides? If on both sides I suppose you will have to blank off one side somehow. Or construct a huge inlet plenum. I hope you don't have to do that.

[Edited on 4-9-2008 by Magpie]

DNA - 4-9-2008 at 22:47

I have the option on two fans I will show them below and their specs.



Left: 90 euro, brand: FMV "lamel", type is C180x180, 800 m3/h, speed is adjustable.
Right, 80 euro, 1000 m3/h @ 50 Pa, 2550 rpm, speed not adjustable.

What do you mean with a varable area or fixed area opening, when you have a slide window I thought it was varable and when your window cannot slide it is fixed or do you mean something else?
What fan would you prefer?
About the right one it says "can be placed horizontally and vertically in ducts" So I'm afraid this one has two openings where a tube needs to be attached.

Right fan:
http://img175.imageshack.us/img175/4114/afb024ic0.jpg
http://img66.imageshack.us/img66/1393/afb025ev4.jpg
http://img98.imageshack.us/img98/764/afb026aat7.jpg

Left fan:
http://img360.imageshack.us/img360/9241/dsc00477fj9.jpg
http://img352.imageshack.us/img352/1019/dsc00478wj6.jpg
http://img156.imageshack.us/img156/2875/dsc00479km4.jpg

Magpie could you look at what one I need I think I need the left fan as it is adjustable in speed and it is an actual squirel cage fan and the right one well...I don't know it is 1000 m3/h which is a lot I also don't want powders being sucked up from the working floor of the fumehood into the fan...and it needs to be placed inside a tube and the other one more looks like commercial suction of fumehoods.

[Edited on 5-9-2008 by DNA]

Magpie - 5-9-2008 at 10:01

DNA I am concerned that neither of those blowers may be good for a lab fume hood.

1. My first concern is the placement of the motor. In the one on the right it is definitely right in the path of the vapors. I wouldn't recommend this for chemical and solvent fumes. I'm not sure about the one on the left but its motor also looks like it may be exposed to the fumes.

2. I am also concerned about the 50 Pa pressure drop. This equates to 0.20 inches of water static pressure. My blower is rated at 475 ft3/min at 0.75 inches of water static pressure. So your 50 Pa blower may be good for moving a lot of air but only at a low duct resistance.

In the following thread I explain more fully the difference between a fixed hood opening area and a variable hood opening area, and their effect on face velocity. Also I show a picture of my blower. As you can see it is belt driven. This removes the motor from the path of the fumes. Also, what you can't see is that the back side of the blower is blanked off, so it sucks air only on the front face of the squirrel cage. This makes it easy to attach to the end of the duct.

http://sciencemadness.org/talk/viewthread.php?tid=10425&...

Please understand that I am not trying to discourage you from getting a blower, quite the contrary. I just don't want to see you spend a lot of money for one that will not do the job. ;)

watson.fawkes - 5-9-2008 at 16:05

Quote:
Originally posted by Magpie
2. I am also concerned about the 50 Pa pressure drop. This equates to 0.20 inches of water static pressure. My blower is rated at 475 ft3/min at 0.75 inches of water static pressure. So your 50 Pa blower may be good for moving a lot of air but only at a low duct resistance.
The full rating for a motor includes a graph relating volume flow vs. static pressure. Many times a vendor will only quote a single point on this graph (and typically on one of the axes, an unrealistic operating point). It may be that this fan is just fine for you, but you need some idea what the static pressure is. There are tables all over the HVAC world for such estimation. The main parameters are diameter of duct, length of straight runs, and the number and geometry of bends and joins.

There's something of a risk of specifying a fan that's too strong for a hood. When face velocity gets too high, that itself creates eddy currents and diminishes containment (which is, after all, the actual goal). If you fear that might be the case, install a damper in the duct line or a variable-frequency drive on the fan motor.

Finally, allocate a little extra static pressure at the outlet. This prevents outside winds from temporarily blocking or even reversing the air flow through the duct.

Hood

tapira1 - 6-9-2008 at 17:40

In order to be able to have any opening in the window of the hood, I have added a couple of counterweights linked to the window through steel cables and running through a poley (at both sides); the counterweighs weigh as much as the whole window. The window stays at the exact position where you leave it; there is no risk of moving down "accidentally"

DNA - 8-9-2008 at 03:23

I found the datasheet of pressures of the right fan.
The left one I will look for but for so far the right one.

http://rapidshare.com/files/143588026/ck_grafiek.pdf.html
It's the 200 model.

[Edited on 8-9-2008 by DNA]

bfesser - 8-9-2008 at 07:42

Would it be unreasonable to ask that the hood/blower conversation become a new thread so that this one can remain on topic?

homebuilt fume hood

Magpie - 8-9-2008 at 11:18

As suggested I have started this thread for the discussion of design and construction of homebuilt fume hoods.

As indicated by watson.fawkes selection of an appropriate blower (or fan) should be based not only on the characteristics of the blower but on those of the ducting as well. The ducting will provide a certain static pressure resistance to a given flow. The blower will be able to deliver a certain airflow at a given static pressure resistance. This is shown in the diagram below.

The resistance curve is in red, and the fan curve is in black. Where these two curves intersect is the "operating point." This determines the fume hood airflow.

Fan curves can be obtained from the fan manufacturer. Determining the duct resistance curve is trickier. As watson.fawkes has indicated this must be calculated based on duct ID, duct length, number and type of elbows, reducers, increasers, duct smoothness, etc, and airflow.

I calculated my duct resistance to be about 0.75 inches of H2O at 475 ft3/min airflow. This matched a point on my fan curve. On this basis I judged my hood/duct design and blower selection to be acceptable. Ideally all of this design work is done before any procurement of materials is started.

If all these calculations seem overwhelming then you are left with making fan and duct design selections based on the successful experience of others. Using some good judgement you will likely do OK.

Here is some of the text that came with the diagram below:

"The governing principle in fan selection is that any given fan can only deliver one flow at one pressure in a particular system. This "operating point" is determined by the intersection of the fan static pressure curve and the system pressure curve. Figure 3 illustrates the operating points of both high and low resistance systems. It is best to select a fan that will give an operating point being toward the high flow, low pressure end of the performance curve to maintain propeller efficiency and to avoid propeller stall. Each particular electronic packaging system should be analyzed for possible reduction in the overall resistance to airflow. Other considerations, such as available space and power, noise, reliability, and operating environment should also be brought to bear on fan choice."

fan curve.jpg - 17kB

jarynth - 8-9-2008 at 16:35

First of all, congratulations on your beautiful fume hood, perfect construction skills and an overall efficient lab. I believe appropriate apparatus is mandatory for successful amateur science. In chemistry, sometimes even more so than purity of reagents.

Unfortunately most of my lab is still boxed, but I do have a thin long centrifugal fan at hand, bought for the explicit purpose of building a hood but never used. It's very quiet, hence probably too weak (I don't have the data with me) even for a midget-sized hood. It looks almost like this one:

http://www.eol.ucar.edu/lidar/real/ncar_cell/squirrel_cage.J...

The intention was to have a laminar flow window as in professional applications. That would have probably required another exhaust fan. Well, I was just a confused teenager :D

Furthermore, I gleaned from the discussion in the 'Tour My Lab' thread that the fan should be placed as far as possible from the hood, for best suction. Can you still envision a way I can salvage my fan and still build a fume hood for the more benign reactions, or at least a chemical closet?

Best of luck with your projects and keep up the good work!

[Edited on 8-9-2008 by jarynth]

[Edited on 8-9-2008 by jarynth]

Magpie - 8-9-2008 at 17:45

Jarynth thanks for the kind comments.
Quote:

Furthermore, I gleaned from the discussion in the 'Tour My Lab' thread that the fan should be placed as far as possible from the hood, for best suction.


No, this is not quite what I meant. I don't think the suction will necessarily be affected. But by having the fan at the end of the duct you keep the duct under a slight suction. If you place the fan right at the hood the duct would be under a slight pressure. From a containment perspective it seems better to keep the duct under a slight vacuum.

I have never seen a blower (fan) like that in your picture file. I would love to see its fan curve. :o I couldn't comment on its use without seeing the curve. Also I'm prejudiced against any blower that has its motor in the path of the fumes.

497 - 8-9-2008 at 20:41

I am considering building a small fume hood some time in the near future. The biggest hurdle now is finding a good place to put it...

I found a pretty nice squirrel cage fan in an old high powered microwave I found in the dump. IIRC the motor draws at least 250 watts, it seems to move a lot of air, I don't know the actual number of cubic feet though. I doubt it can do a lot of suction so hopefully my duct path will be very short. It has plastic blades and housings (there's one on each side of the motor) and I think with a little modification the motor could be pretty well protected from fumes. I think it would hold up well, except to a few solvents possibly. But I shouldn't be needing to vent large quantities of any solvent. I guess I'll just have to test it and see how well it works. Anyway, just wanted to say that trashed microwaves are probably a good source of fans. They're probably not ideal, but hey, they're free! ;)

DNA - 9-9-2008 at 00:27

I found the datasheet of pressures of the right fan.
The left one I will look for but for so far the right one.

http://rapidshare.com/files/143588026/ck_grafiek.pdf.html
It's the 200 model.

Here a screenshot of the pdf.

This fan is the one that needs to be placed within a duct.
It has classifications IP44, isolationclass B special impregnated against humidty and moisture. And important the motor is outside the tube so fumes will not atack the motor...
Still looking for the left one...



[Edited on 9-9-2008 by DNA]

[Edited on 9-9-2008 by DNA]

[Edited on 9-9-2008 by DNA]

DJF90 - 9-9-2008 at 03:02

Magpie has already made a new thread.

watson.fawkes - 9-9-2008 at 05:41

Quote:
Originally posted by jarynth
Unfortunately most of my lab is still boxed, but I do have a thin long centrifugal fan at hand, bought for the explicit purpose of building a hood but never used. It's very quiet, hence probably too weak (I don't have the data with me) even for a midget-sized hood.
You're right, it doesn't look anywhere near strong enough to provide adequate face velocity. It could, however, make a reasonable fan for an air curtain. This is an air flow blowing gently down over the operator. It's placed in front of the fume hood, about a foot over head height. The flow should be gentle enough not to feel drafty. Air curtains improve the consistency of flow through the face of the fume hood. They also provide better isolation of breathing air.
Quote:
Furthermore, I gleaned from the discussion in the 'Tour My Lab' thread that the fan should be placed as far as possible from the hood, for best suction. Can you still envision a way I can salvage my fan and still build a fume hood for the more benign reactions, or at least a chemical closet?
A weak fan just won't exhaust fumes thoroughly enough. The problem is that in an enclosed room, contaminants will build up over time. There's just no substitute for an adequate rate of air exchange, and that requires an adequate fan.

watson.fawkes - 9-9-2008 at 06:05

Quote:
Originally posted by Magpie
But by having the fan at the end of the duct you keep the duct under a slight suction. If you place the fan right at the hood the duct would be under a slight pressure. From a containment perspective it seems better to keep the duct under a slight vacuum.
What matters is the pressure difference between the face of the hood and the outlet of the hood to its exhaust duct. Any method you can use to create that pressure difference (at its required value) will move air. You can put a fan at the exhaust port to the outside. You can put a fan at the hood outlet. You could even make a well-sealed room and use a fan at the inlet to pressurize the entire room. This technique used, for example, to protect immunosuppressed patients, where you also want the room at positive pressure (with respect to ambient) to ensure that all incoming air is well-filtered. There's plenty of room for creativity here. The situation of a fume hood inside a laboratory, though, implies that you can only control the exhaust air, so that's the side where the fan goes.

The best reason to put the fan at the exhaust port is noise control. Moving the fan away from the hood is the first step in controlling fan noise.

watson.fawkes - 9-9-2008 at 06:19

Quote:
Originally posted by 497
I found a pretty nice squirrel cage fan in an old high powered microwave I found in the dump. IIRC the motor draws at least 250 watts, it seems to move a lot of air, I don't know the actual number of cubic feet though. I doubt it can do a lot of suction so hopefully my duct path will be very short. [...] Anyway, just wanted to say that trashed microwaves are probably a good source of fans.
With a microwave oven fan you might be able to make a hood the size of a microwave oven. If it's strong enough, that is. Which I doubt, since that fan provides cooling air for the magnetron, not exchange air for a volume of space.

DNA - 9-9-2008 at 06:43

Here is the other one the squirrel cage one...

watson.fawkes - 9-9-2008 at 06:50

Quote:
Originally posted by Magpie
Fan curves can be obtained from the fan manufacturer. Determining the duct resistance curve is trickier. As watson.fawkes has indicated this must be calculated based on duct ID, duct length, number and type of elbows, reducers, increasers, duct smoothness, etc, and airflow.

I calculated my duct resistance to be about 0.75 inches of H2O at 475 ft3/min airflow. This matched a point on my fan curve. On this basis I judged my hood/duct design and blower selection to be acceptable. Ideally all of this design work is done before any procurement of materials is started.
The authoritative reference on this subject the ASHRAE Handbook--Fundamentals, in the chapter entitled "Duct Design". It explains all the losses and how to compute them. There are excellent worked examples included. Most reasonable reference libraries have a copy.

@Magpie: Thank you for setting an example of computing an estimate of your duct and fan performance. This is certainly a case where a false sense of protection is actively harmful.

Having said this, a fume hood for a single operator in a garage is a pretty modest system. Use as few bends as possible, since dynamic losses from duct turbulence usually dominate in a small system. Use shallow bends (say, 45°) instead of right angle ones if possible. Use long sweeps rather than tight ones.

Magpie - 9-9-2008 at 10:16

DNA I will comment on the fan curve for the "right fan" first. Now that you have indicated that the motor is outside the flow path I think it is a viable candidate. Below is shown the fan curve as marked up by me to show two hypothetical duct resistance curves, in red. These curves intersect fan curve B at 0.15 and 0.21 m3/s, respectively. From the data you posted earlier these are acceptable flows for a hood of 120cm cabinet width. So, on that basis, a duct system that will produce a resistance curve anywhere on or in between those two red lines would be acceptable.

The duct resistance and likewise the fan static pressure at the two operating points (black squares) I have shown are 250 and 340 Pa, respectively. In my world these convert to 1 and 1.36 inches of H20, respectively. These seem to be fairly high pressures and I am therefore impressed with the capabilities of this fan. IIRC you said this fan had a 20cm opening. I think you should be able to assemble a duct system of 20cm ID that would yield an acceptable duct resistance curve.

Watson.fawkes has told how one can calculate points for a duct resistance curve using the ASHRAE handbook. I recommend you give this a try. If you give me the parameters of your ducting I will be glad to check your calculations.

right fan curve - DNA - marked up.JPG - 25kB

Magpie - 9-9-2008 at 11:43

DNA here's my comments for the "left fan:"

1. The manufacturer says it has a "double flow wheel." I'm interpreting this to mean that air flows in from each end. This would put the motor in the flow path - not good.

2. This is not a fan curve but an envelope that includes all the fan curves for the different mechanical and electrical configurations that this fan could have. You really need just the curve for the particular configuration (wheel size, rpm, etc) that you would have.

3. No matter what configuration you have you would be trying to operate in the far left end of that envelope to meet the flow requirements of 540-756 m3/h for a 120cm cabinet width. That's not the best place to be on a fan curve in my understanding.

Jor - 9-9-2008 at 13:54

Talking about hoods, Magpie, you wanted me to show my hood.
I have an old picture. The sash is ready but not yet put into the hood, and the blower has been ordered. however I will be very busy next weeks, so will take some time.

This is the hood itself, not yet the fan and sash (sash is ready though).

http://img147.imageshack.us/my.php?image=p1090617jc5.jpg
http://img49.imageshack.us/my.php?image=p1090619rv9.jpg

Magpie - 9-9-2008 at 14:27

Looking very nice Jor! At some point you will have to give us all the details. ;)

DNA - 10-9-2008 at 00:06

Just off topic, how the f*ck is everyone getting these acros bottles...I thought you could only order there as a company and not as a private person.
Almost every fumehood picture I've seen it has acros bottles in it...
I e-mailed the company for another graph but they haven't replied since yesterday...

DNA - 10-9-2008 at 01:57

Here are the specs of the squirrel cage the correct ones.

Jor - 10-9-2008 at 03:14

Quote:
Originally posted by DNA
Just off topic, how the f*ck is everyone getting these acros bottles...I thought you could only order there as a company and not as a private person.
Almost every fumehood picture I've seen it has acros bottles in it...
I e-mailed the company for another graph but they haven't replied since yesterday...

I can order from Merck, Acros, Fisher and even Aldrich. But not by myself. impossible. A single person is just like an annoying fly to them.
How can you order? Find a small supplier who can sell to individuals. Ask this company to order from the big ones. I buy all my things through a small supplier, and he adds 1 euro to the price per chemical.

panziandi - 10-9-2008 at 04:38

I made a fume hood. I constructed two H frames (front and back) and joined them with side bars, also placed struts across the bottom for storage of bulky equipment, the work surface inside the hood was a stainless steel sink and draining board unit. The sides were tinted plexiglass as was a stationary front. The roof and back was plyboard and a false back was installed to distribute air flow. My fan was a blower similar to the one on the first page of this thread but the motor is exposed to the fumes. I have not yet linked the ducting to the fan or installed a sash. I am currently looking at these fans

https://www.machinemart.co.uk/shop/range/details/ventilators...

What do people think of these?

DNA - 10-9-2008 at 05:07

You'dd better post specifications of that fan, concerning the pressure drop so to see if the face velocity is okay.
Since a lot of air movement is okay, but then still your fan also needs the power.

Simplified sayd, your fan might move a lot of air with no resistance but as soon as you add a duct to it there will be to much resistants for the fan to still move all that air threw your duct...therefor we need to know the pressure drop. It also depends on the size of your hood.

Please post the measurements.

Magpie - 10-9-2008 at 10:16

DNA I have marked up the fan curve you posted for the "left fan." As I suspected using this fan for a 120cm wide cabinet would place you way to the left on the curve, an unstable region, and not the best place to be. I also placed the resistance curves for the 150cm wide cabinet on the diagram in green. This improves things a little.

I'm also concerned about the fairly low static pressure that this fan is able to develop - only 12mm of H20. To move this much air with only that amount of pressure would require a large duct diameter. I'm guessing at least 25cm, possibly larger.

I want to thank chemoleo for cleaning up the "Tour My Lab Thread" by moving the fume hood posts to the beginning of this thread.

DNA left fan curve - marked up.JPG - 92kB

watson.fawkes - 10-9-2008 at 11:59

The following references contain recommendations on face velocity for fume hoods. I've lifted them from the footnotes of the relevant sections of Industrial Ventilation Design Guidebook.Summarizing the Guidebook, the upshot is that a good target range for face velocities in the hood is 0.4 - 0.5 m/s. 0.3 m/s should be considered a bare minimum for ideal circumstances.

Magpie - 10-9-2008 at 13:00

Yes, watson.fawkes, I think it is good to keep in mind that proper face velocity is the starting point for good hood design.

The Dutch standards posted by DNA above come in pretty close to what you have found, ie, 0.25 - 0.45 m/s.

For my design I chose a face velocity of 1 ft/s, which is 0.3 m/s. But as you say I have seen more modern US recommendations for up to 1.5 ft/sec which would be 0.46 m/s.

Then the designer has to know what is the largest sash opening, including any bypass, with which he will be working. Once that area, A, is known the design air flowrate, Q, can be easily computed from Q =VA, where V= face velocity.

Then you try to find a fan that will give you the airflow Q that you need at a sufficient pressure to balance your duct resistance.

DNA - 11-9-2008 at 00:57

Thank you Magpie, I'll go and buy the "right" fan which has an external motor, protection against moisture and still has a good pressure drop.

In the meanwhile I made pictures of my fumehood (see "tour my lab") and I made a more clear picture of what standards are needed and for what sizes of fumehoods and what kind of fumehood etc.

Magpie - 11-9-2008 at 14:59

from DNA above:
Quote:

I found a table for airsuction in fumehoods after searching for a while.
It might be useful for the people that already made a fumehood and ofcourse the people planning on making one. The information below is for the industry and university's in The Netherlands as defined by the law.

Standard fume hood:
Cabinet width 100 cm: 450 - 630 m3/h

Cabinet width 120 cm: 540 - 756 m3/h

Cabinet width 150 cm: 675 - 945 m3/h

For all above mentioned sizes with the sash opened for 50cm there should be a flow of 0,25 - 0,45 m/sec.


There's something not quite right here. The flows all compute for the face velocity of 0.25m/s, but the flows for the 0.45m/s don't compute, eg:

Q=AV = (120cm)(50cm)(0.45m/s)(3600s/h)(m/100cm)(m/100cm) = 972m3/h. The table gives 756 m3/h. Same discrepancy for the other 2 cabinet widths.

This should be resolved.

[Edited on 11-9-2008 by Magpie]

watson.fawkes - 11-9-2008 at 18:40

Quote:
Originally posted by DNA
Cabinet width 100 cm: 450 - 630 m3/h
Cabinet width 120 cm: 540 - 756 m3/h
Cabinet width 150 cm: 675 - 945 m3/h

For all above mentioned sizes with the sash opened for 50cm there should be a flow of 0,25 - 0,45 m/sec.
Quote:
Originally posted by Magpie
There's something not quite right here. The flows all compute for the face velocity of 0.25m/s, but the flows for the 0.45m/s don't compute, [...]
The upper limit flows are all computed for 0.35 m/s.

Jor - 16-9-2008 at 09:35

I can now get a fan, wich moves 500m3 per hour. It makes almost no sound wich is good. The motor is made from stainless steel, and it's not spark proof. Therefore no ether, acetone, etc. They are not toxic anyway, so that's outisde the hood. However the metal will be rusted in a year I think. But I'm moving away in 1 or 2 years, so that's not a problem.
A vent pipe will have to go up like 3-4 meters, then the fan will be constructed right under the roof, and then a short pipe will go out. I think I will lose some power, because of the 3-4 meters up. But how much? 50-100m3? or more?
The fumehood is 80 cm wide, 1 meter high,55 cm deep. No baffles, no bypass. See picture on post I posted in this topic a few days ago.

So what do you think magpie, buy it or not?

DNA, please tell me where you buy your sparkless, moisture protective motor!!

Here these are sparkless motors, used in ships:
https://www.marinestore.nl/Boordcomfort_en_Accessoires_Venti...

Magpie, doe you think these are suitable for me, taking the 4 meters of pipe going up into consideration? Although its Dutch, I think you can understand quite some of the page. However these are 24V. I'm really not good at electronics, how to convert to 230?

[Edited on 16-9-2008 by Jor]

[Edited on 16-9-2008 by Jor]

Magpie - 16-9-2008 at 13:21

Jor,

Right now all I really know about your fume hood and system is that: 1) it is 80cm wide, and 2) your ducting may be up to 4m long. I will need to know the following before I can comment on the suitability of any given fan:

1. You say you have a sash. What will be the open area with the sash in place when you are working in the hood with the fan on? (When and if your sash is completely closed your hood will draw no air.)

2. What will be the duct inside diameter? Is the duct smooth?
3. How many elbows will the duct have? What type are the elbows, ie, 90 degree, 45 degree, 22.5 degree, short radius, long radius, etc.
4. Will there be an outlet louvre or screen?
5. What is the fan curve for the proposed fan?

So, you really have to see (on paper) the whole system before you know if it will perform as intended.
Quote:

Although its Dutch, I think you can understand quite some of the page. However these are 24V. I'm really not good at electronics, how to convert to 230?


Jor I am prejudiced against motors that are inline with the flow. I don't think it mattters so much whether or not they are sparkless as long as the motor is outside the flow path.

I am also predjudiced against battery operated fans. I think these fans are meant for short term duty likely exhausting a bilge before starting a marine engine, or where the battery is being continuously charged. I just finished a project today where my fan was operating for almost 2 hours straight. I would be concerned that a battery operated fan would have quit on me.

Jor - 16-9-2008 at 13:51

Quote:
Originally posted by Magpie
Jor,

Right now all I really know about your fume hood and system is that: 1) it is 80cm wide, and 2) your ducting may be up to 4m long. I will need to know the following before I can comment on the suitability of any given fan:

1. You say you have a sash. What will be the open area with the sash in place when you are working in the hood with the fan on? (When and if your sash is completely closed your hood will draw no air.)

2. What will be the duct inside diameter? Is the duct smooth?
3. How many elbows will the duct have? What type are the elbows, ie, 90 degree, 45 degree, 22.5 degree, short radius, long radius, etc.
4. Will there be an outlet louvre or screen?
5. What is the fan curve for the proposed fan?

So, you really have to see (on paper) the whole system before you know if it will perform as intended.

Quote:

Although its Dutch, I think you can understand quite some of the page. However these are 24V. I'm really not good at electronics, how to convert to 230?


Jor I am prejudiced against motors that are inline with the flow. I don't think it mattters so much whether or not they are sparkless as long as the motor is outside the flow path.

I am also predjudiced against battery operated fans. I think these fans are meant for short term duty likely exhausting a bilge before starting a marine engine, or where the battery is being continuously charged. I just finished a project today where my fan was operating for almost 2 hours straight. I would be concerned that a battery operated fan would have quit on me.

Ok I know the following for now:

1. When the windown is maximum open, it will be about 45 cm working height.
When it is completely closed, it's about 4 cm. So incase there goes somethign wrong, I can close it to the 4cm for very effective ventilation, with a speed of at least 4 m/s at opening. But I will generally work at 40-45cm height. So that's where i want the 0,35-0,45. A little more is not a problem I think.
2. Probably about 150mm-160mm diameter duct size. Not sure what material I'm going to use. Maybe a relatively flexible duct (Polyvinyl chloride, the ones wich look like the aluminium ones), but I don't think these are resistent to bromine/chlorine. Otherwise standard duct, don't knwo the material.
3. Im not sure yet. Ii will probably buy the following:
http://www.growcenter-noord.nl/box-silent-air-500-m3-p-818.h...
If I can put it upside down (wich I think must be possible), then one 45 degree angle is enough. If not (unlikely), it's going to be 2 90 degrees angles.
4. I don't understand question 4.
5.I don't understand 5 (I do not have great English. If someone can explain me I will answer).

Again, my goal is not a hood wich works like a commercial one. I'm happy with a capture of about 99% of the fumes, not 99,9999% like real hoods. And I can always close the sash in case there is a violent release of lots of toxic gas.

What is teh problem with a motor in the inline of flow? I cannot find a motor with an external motor. Corrosion is not a problem, I accept it can be destroyed by fumes in 2 years.
But I found some fans with external engine:

http://www.airflux.be/details.asp?language=nl&productID=...

Go to the blue link 'technische documentatie' for technical details in English.
For me only the 160MD and 160LD are interesting.

[Edited on 16-9-2008 by Jor]

[Edited on 16-9-2008 by Jor]

[Edited on 16-9-2008 by Jor]

not_important - 16-9-2008 at 15:36

Quote:
Originally posted by Jor...
4. I don't understand question 4.

Is the exhaust opening just a plain open rectangular or circular hole, or is it covered with screening or similar that will add to the flow resistance.
Quote:

5.I don't understand 5 (I do not have great English. If someone can explain me I will answer).


Fan curves have been given earlier, they show the relationship between flow volume and pressure/flow resistance. You need that to calculate the flow rate in your proposed design.
Quote:

What is teh problem with a motor in the inline of flow? I cannot find a motor with an external motor. Corrosion is not a problem, I accept it can be destroyed by fumes in 2 years.


Explosion or fire from flammable fumes, and corrosion. A little HCl in the exhaust air can trash you motor in much less than 2 years, it can happen in weeks.

Magpie - 16-9-2008 at 20:39

Jor,

Here's what I came up with for your hood:

Assumptions:

1. open face width: 80cm
2. open face height: 45cm
3. required face velocity = 0.35 m/s
4. required airflow, Q = AV= (0.80m)(0.45m)(0.35 m/s) = 0.126m3/s=454m3/h
5. duct ID = 150mm (smooth)
6. 2 short radius (r/d=1) 90 degree elbows
7. 1 entrance and 1 exit
7. duct length = 4m
8. fluid = air at standard pressure and temperature

Calculated duct resistance = 9.3mm H2O.

This is not much resistance. But if your duct is a corrugated flexible tube like a dryer duct then it will not be smooth, but very rough. This will increase your resistance - how much I can only guess. Maybe someone else knows.

So now we need to see your fan curves of any proposed fans. These are published by the fan manufacturer. Examples of what they look like are shown in the above posts.

DNA - 17-9-2008 at 00:17

I'll soon be getting my new fan, with the outside motor.
They are being sold at conrad electronics especially for the dutch people.
And what is the difference between having a circulair or rectangular opening magpie?
I'll also get something else then this flexible tube for a dryer.

Jor - 17-9-2008 at 01:39

Can you please give me the link of the fan you will buy at conrad? This is, because I see now (yesterday I was tired! ;) ) that the company I gave with external motor in in Belgium... They use other voltages and also huge shippings.

And is it ok to add you to msn DNA? So we can talk about the subject more easily.

Thank you very much Magpie for the calculations. Becuase I need a 454 m3, I will go for something like 500m3. But not important just said the motor would be trashed in WEEKS, now that's a bad thing. Need to find an alternative.

DNA - 17-9-2008 at 01:57

http://www2.produktinfo.conrad.com/datenblaetter/550000-5749... here are the datasheets of the fans that conrad has.
You can also see these fans are being used for ventilation of bathrooms (waterproof) and kitchens and several places in the house at the same time, so if you would use it just for one cabinet of 1 m3 then it will definately have a nice flow :D
There is a fan of 980 m3/h for 185 euro.
Motor is outside, the fan is placed in line with the duct and it it's metal is coated to be resistant for compounds passing by and it has explosion proof thing and also another iso certificate.

So the fan I'll be getting has 580 Pa

[Edited on 17-9-2008 by DNA]

Jor - 17-9-2008 at 02:30

wow that's very nice! External motor, not expensive and also spark and corrosion proof!
I think I'm getting the AXC 150B, the one with 530m3, and AXC 160A with 440m3. The PDF says the product is suitable for long ducting, and can overcome wuite some presuure. Not sure what is the best one for me. Wich one would you guys choose (Magpie :P ?) First going to eMail them and ask if they have them both in stock, as I cannot find it to be in the online catalogue.

DNA! Your fan is much cheaper here:

http://www.auerhaan-klimaattechniek.nl/klima/documentatie/me...

However you need to login and they request a company name, that might be a problem. I might just ask my mother to order, because she is a dentist. Not sure if you can.

Same model, costs 87!


[Edited on 17-9-2008 by Jor]

DNA - 17-9-2008 at 03:09

Wow that's a huge difference, but that is the same kind of fans as that you wanted to order.
So same goes for you, order them not at conrad but at the website you just gave me.
I will go for the 200B, but I can get it via e-bay and there it is 80 euro's so well yeah it is still brand new and including shipping costs concerning conrad this is really cheap, concerning the website you just gave it is the same.
About the msn thing I will pm you my e-mail adress where you can add me on.
EDIT: it is already in my profile the mail adress.

[Edited on 17-9-2008 by DNA]

watson.fawkes - 17-9-2008 at 08:01

Quote:
Originally posted by DNA
I'll also get something else then this flexible tube for a dryer.
That's a good thing.

The rule of thumb, as I recall, is that flexible tube has internal resistance of about three times that of ordinary galvanized steel. In practice that means it's a "last meter" option, but not good at all for long runs. For a fixed fume hood, there's generally no need for that flexibility.

As for using bends, if all you are trying to do is to shift the run of a pipe, two 90° bends are much worse than two 45°ones, and two 45° bends are worse than two 22-1/2° ones.

Magpie - 17-9-2008 at 14:53

From DNA:
Quote:

And what is the difference between having a circulair or rectangular opening magpie?


Assuming you mean the outlet of a fan, rectangular is pretty common for squirrel cage fans. Not much you can do about it. I had to build a special plenum to adapt my fan rectangular outlet to the louvred and screened outlet in the wall of my garage. I had a sheet metal shop build it out of stainless steel. Cost me big $. :( I can draw a sketch of this plenum if you would like.

For any ducting a rectangular cross-section is going to have more friction loss (pressure requirement) than a circular duct with the same cross-sectional area. However, as you've probably noticed, retangular ducting is normal for home heating and air-conditioning. I suppose it takes up less space. There are always economic tradeoffs involved.

If I didn't answer your question, let me know.

[Edited on 17-9-2008 by Magpie]

[Edited on 17-9-2008 by Magpie]

Magpie - 17-9-2008 at 15:06

From Jor:
Quote:

I think I'm getting the AXC 150B, the one with 530m3, and AXC 160A with 440m3. The PDF says the product is suitable for long ducting, and can overcome wuite some presuure. Not sure what is the best one for me. Wich one would you guys choose (Magpie ?)


I want to see the fan curves first. :P

Jor - 17-9-2008 at 16:03

http://www2.produktinfo.conrad.com/datenblaetter/550000-5749...


;)

watson.fawkes - 17-9-2008 at 17:39

Quote:
Originally posted by Jor
http://www2.produktinfo.conrad.com/datenblaetter/550000-5749...
I'll reinforce what others have said. Inline fans are not appropriate for fume hoods. Period. Even if they're cheaper. Even if they're quieter. Even if they're easier to install. Even if they're smaller. To review the risks:

Magpie - 17-9-2008 at 17:54

The brochure for Jor's candidates say the the fan is in-line but that the motor is "external".

Edit: I don't understand how the motor can not be in the flow path for these fans. They don't really look like they have a squirrel cage, but instead are just axial fans - like a kitchen or bathroom fan.

If this is true I can't recommend them either.

[Edited on 17-9-2008 by Magpie]

Jor - 17-9-2008 at 22:44

:(:(:(
It's really impossible to find one with an external fan...
I thought external rotor motor means that it is outside.

However, if I cannot find a good fan, I will just buy this one. I have seen many people having hoods made themselves with common fans, and they work good as well, for long times. Take garage chemist's hood, it has an ordinary kitchen exhaust fan, and he is passing the most nasty fumes through it (works with things like acid chlorides and stuff).
I will look further, but if I cant find a better one, I will go for the motor from conrad.

[Edited on 17-9-2008 by Jor]

DNA - 18-9-2008 at 01:28

No no people, the fan that jor and me have been showing you actually have the motor outside.
It is explosion proof and has also got several certificates of safety, waterproof and what not.
So although it is an inline fan it is suitable I think, because the motor is placed outside the duct.
It is a sort of axial fan combined with the advantages of a squirrel cage fan as they say on the specification sheet.
this is the same fan as that I posted the specifications for where magpie calculated the pressure drop for which was quite a nice pressure drop.
I'll have a look for the exact certificates and things that it has.

DNA - 18-9-2008 at 01:31

Here I have some pictures for you guys so you can see the motor being outside and more exact pictures of that particular fan ;)

http://img175.imageshack.us/img175/4114/afb024ic0.jpg

http://img66.imageshack.us/img66/1393/afb025ev4.jpg

http://img98.imageshack.us/img98/764/afb026aat7.jpg

EDIT: pictures are way too big to post them on the forum, so I included the links.
[Edited on 18-9-2008 by DNA]

[Edited on 18-9-2008 by DNA]

watson.fawkes - 18-9-2008 at 05:44

Quote:
Originally posted by DNA
Here I have some pictures for you guys so you can see the motor being outside [...]
http://img98.imageshack.us/img98/764/afb026aat7.jpg
That little white box is an electrical junction box, not a motor. A fan with an inline impeller and an external motor must be connected by some sort of belt or chain, which would require two openings, which wouldn't seal. The reason that other motor configurations can work is that the couplings is a shaft and the penetration has a bearing. The bearing seal is the weak link at that point. Viton bearing seals are available, if that really becomes a problem.
Quote:
It is explosion proof and has also got several certificates of safety, waterproof and what not.
It's possible for a motor to be rated explosion-proof and still not be suitable for acid vapor service. Those are two different hazards. "Waterproof" is a property that can change in the face of corrosive vapors; assume it will change unless specifically designed for it. Since it's cheaper to use another configuration, assume that such a fan is a specialty product and even more expensive than you think it ought to be.

I also looked up "conrad.com" (the site of the fan information), which I was unfamiliar with because it's a European concern and I don't live there. The upshot is that it's not an industrial supplier and (assuming it was going to be your source) it's unlikely you'll find anything suitable there. It's possible, but don't count on it. One of the threshold tests of being a serious hobbyist is whether you can be a business at least for the purpose of gaining access to wholesale suppliers. (Hint: business cards work wonders.)

Jor - 18-9-2008 at 08:05

DNA, how did you order? The version I want and the version you have are both not in their online catalogue...

Magpie - 18-9-2008 at 08:28

I thought it might be good to repost the picture of my fan. As you can see it is a centrifugal type with "squirrel cage" and externally mounted motor. This style I can recommend.

As watson.fawkes indicated trying to buy equipment like this can present obstacles. The local wholesale distributor for this fan would not sell to me. So I asked an HVAC contractor to buy it for me from the distributor. Since he was a nice guy he did so without any price markup.

blower_7C651.jpg - 9kB

DNA - 18-9-2008 at 13:48

My fan I didn't order via conrad but I bought it via ebay.

Jor - 16-10-2008 at 08:22

We now are working here on the ductwork of the ventilation-system, and we run into a problem. A local supplier tells us that it absolutely unadvisable to take a 200mm duct out of the roof (we have an old roof). He advises 150mm, standard size. Now from the hood I will first go up 3 meter with PVC pipe, and then motor will be attached. Next we have to put an adapter to convert 200mm to 150mm between the motor and the roof (distance motor/roof is about 1 meter.. I was wondering, how does this affect the volume of air that can be moved. technical details of my motor says it can move 760m3/h. Will it decrease by 50% (surface area of a 150mm circle is about 50% of the surfae area of a 200mm diameter) , or less? And is it advisalble to put the adapter (200/150) right under the roof, or immediately behind the motor?
Sash is now done! :P

Klute - 16-10-2008 at 08:54

Please post more pics when you can Jor, it is a deligth to see a buildind hood..

watson.fawkes - 16-10-2008 at 10:13

Quote:
Originally posted by Jor
We now are working here on the ductwork of the ventilation-system, and we run into a problem. A local supplier tells us that it absolutely unadvisable to take a 200mm duct out of the roof (we have an old roof). He advises 150mm, standard size.
Having done some work on old houses, this answer makes zero sense to me. Frankly, my first suspicion is that your supplier has much lower margins on the larger vent (which may mean he has to go to the effort of special-ordering it) and is trying to sell you something for his benefit. Regardless, worry about flashing. Worry a lot. Bad flashing can ruin a structure over time. I once lived in a house (rented, thankfully) where some failed flashing caused a stud cavity to fill halfway up with water when it rained. Really. I "fixed" the immediate problem by getting up in the attic and drilling a drainage hole in the (closed) soffit board. The only concern I can imagine with an old roof is to get all the layers of flashing necessary (one for each roof layer, in an ideal world) in place correctly, but I can't imagine how that would differ by diameter.
Quote:
Now from the hood I will first go up 3 meter with PVC pipe, and then motor will be attached. Next we have to put an adapter to convert 200mm to 150mm between the motor and the roof (distance motor/roof is about 1 meter.. I was wondering, how does this affect the volume of air that can be moved.
The effect of a constriction in the pipe is to increase the static resistance of your venting system. This will move the operating point of your fan to a point with less flow. If you fan is of marginal capacity (I don't know one way or another), your hood will have more transient releases. That's bad, and worth fixing up front. It's also the reason why slightly over-specifying the blower and putting in a damper is frequently a worthy cost for prudence. (Variable speed control also works, but not always applicable.)
Quote:
And is it advisalble to put the adapter (200/150) right under the roof, or immediately behind the motor?
If you have to use a smaller exit vent, make the extra static resistance as low as possible by keeping the constriction as short as possible. This means putting the adapter right under the roof.

As long as I'm addressing static resistance, you get static resistance on the intake air side as well. To wit, you can't blow much air out of a sealed room. Have you planned a source of make-up air? This supplies the same volume of air that you're going to exhaust out your vent. If you have hard winters where you live, figuring out how you want to balance your personal comfort vs. the heating cost of exhaust.

Magpie - 16-10-2008 at 10:30

Good points by watson.fawkes.

I agree that I would not give up easily on keeping that duct 200mm all the way. No matter how short the 150mm piece is it will act like a restricting orifice. I'll take a crack at seeing what it might cost you in airflow, ie, I'll do some calculations.

I don't see why you can't do this but then again I don't know the construction of your roof. I installed the vent pipe for my gas hot water heater. It was a piece of cake. Then again I only had to penetrate a 1/2" (12mm) piece of plywood and the asphalt shingles. The vent pipe was only 3" (75mm) diameter resulting in virtually no structural weakening of the roof. I installed a galvanized steel flashing. I have had no problems in over 5 years.

Is your installer concerned about structural weakening of the roof? If so can it be reinforced locally at the penetration point at a reasonable cost?

Or is it the flashing that he is concerned with. Perhaps he doesn't have or can't get standard flashing in a 200mm size. In that case you might have to have one custom made by a sheet metal shop.



[Edited on 16-10-2008 by Magpie]

[Edited on 16-10-2008 by Magpie]

Jor - 16-10-2008 at 11:39

Let's say it like this, it is much harder for us to install a 200mm exit, says the supplier. After all, money is limited, and time. I'm getiing help of someone (I could never do it alone).
A reason it si much more difficult, is that a 150mm would fit by removing one 'pan' (don't know the english word, it's a ceramic material on house roofs), and when taking 200mm we have to remove two, where also quite some open area will be leftover, and it's hard to close that again (my Englsh is real bad here, try to udnerstand :P). Another small thing, the neigbours must be wondering, when a huge 200ml pipe comes out a garage. I have nice neighbours dont get me wrong, we even go with them to football games sometimes (Feyenoord!!), but I don't know their responce to a chemistry lab.

Hmm, yes the flashing is really important... Or I would get water in the blower and hood ... Or a bird ;)
But wich one is recommended? I can imagine flashing gives quite some resistance, as it blocks the exit.

I need about 450m3 in for my hood. Maximum capacity of my blower is said to be 760m3. it has 4 speeds, 25%, 50%, 75% and 100%. If put at full power, even with the resistance, 450m3 should be achievable right? Even a little les (350-400m3) is also enough in my opinion (I don't do highly toxic work, I work in small amounts, and I can almost fully close the sash, with no bypass, So that really effectively sucks away all fumes, when having alot.

Ok, i will put the adapter right under the roof then.

When I'm having the hood on, I will open a door, just a little. I know it will be quite cold in winters, but it is anyway, even without hood.

Also I use the hood only when neccesary. Because I have no external motor, especially HCl (very corrosive to metals, more than SO2, NOx, etc) will be used outside a hood. i really don't care about breathign a little HCl, as long as it;s not a cloud (I had this experience when opeing a 5L jug of lab-grade 37% HCl :o).

I have this hood especially for nasties, like chlorine, bromine, NO2 organic solvent vapours (like benzene, DCM and chloroform), and carcinogens. As my rotor blades of the blower are plastic coated, this is good for vapours like HCl, HBr, acetic acid, NO2, SO2, etc. It's NOT good for Br2 and Cl2. Would it help to buy a can of Teflon spray and spray a thin layer on the blades?

Yes Magpie I have realised it will add resistance, no matter hwo short the 150mm piece. But what i wonder is, how much. I have some extra power (760m3), more than I need, as noted before.

the flashign is indeed also a problem with 200mm. In local stores I have not seen it at 200mm size...

Magpie - 16-10-2008 at 12:41

OK, I think I understand your problem now. You have ceramic or stone tiles for roofing. This is much more difficult to work with than my plywood and asphalt shingles.

When I looked back at my spread sheet for a previous calculation I did for you I saw that the diameter was 150mm. So what is the outlet diameter of your fan? If it is 150mm you might as well make the whole duct system out of 150mm duct, ie, all the way from the hood to the stack outlet.

How high above the roof will your vent extend? I have a 75mm vent pipe and a 100mm PVC vent pipe for my gas hotwater heater and my gas furnace, respectively. They only extend about 0.5m so no exterior support is needed against the force of strong winds. But for chemical vapors you might like it a little longer. You will have to balance this need against any requirement for structural support and appearance to the neighbors. You want to blend right in and not be a topic of neighborhood speculation. I think I would make it look as normal as possible. Those ceramic tiles can take the chemical abuse anyway! ;)

Flashing is the seal between the roof and the vent pipe. It keeps water out of your attic, not your vent pipe. If you need to keep water out of your vent pipe, which you probably do, then there are devices for this such as conical "witch hats" or T's. I'd use whatever is customary in your area.

Edit:
My previous calculation for 150mm ID smooth duct, 4m long, 2ea 90deg ells, 1 pipe entrance, 1 pipe exit, gave 9.3mm H2O pressure drop for 450m3/hr. If this fits within your fan curve you will be OK. I don't know which fan curve you have.

[Edited on 16-10-2008 by Magpie]

Jor - 16-10-2008 at 13:31

Yea my diameter was indeed 150mm at that time, but after all i decided to buy for 30 euros more a blower wich has 4 speeds, with a max of 760m3, so i could try all speeds and decide wich one works best. this way I can also put it on lowest speed when working with things like ammonia, wich just smells but is tot really toxic. This conserves energy. having only the possiblity to turn it at full blast, I didnt like that.
This blower had 200mm and I never even realised this would be a problem.
Why would I put the exit of the exhaust to high in the air? To get it as far away from me as possible? It's close to mine and the neighbours garden anyway. Even with this hood, I can only work with small amounts and smelly substances only at night. For example when I make bromine I do this at 0:00-1:00 at night. this will still be the case.
When I use large amounts, I always use a washbottle.
We really don't want to work for a long time on the roof. This will draw the attention. Although people are not as paranoia here as in the US, it might a bit weird, an exhaust pipe on the roof of a garage.

What do you think about the idea to spray TEFLON on my blower blades wich are palstic coated? Good idea to help them survive chlorine and bromine, or will the layer of teflon be to thin?

By the way, although my motor is not external, it's not right in the path or airflow. It is sealed within some kind of case, protected by thich metal and plastic. The wires for electricity (just the big one you plug in not the small colored ones) are inside the path though, just as some kind of battery looking thing where it is connected. But it's right in the middle, and most air will not go through the middle.

There are no 2 90 degrees angles in the duct anymore, just a single 45 degrees angle. There will probably be the 200-150mm adaption in the last half meter. So what do you think, will I get 400m3or more ? I think I can,I can't imagine a single 200-150mm adaption to take about half of the power of my blower.

WOW, I can talk English decently when talking about normal things, but when trying to explain technical details and such , it gets quite hard! I hope you can udnerstand me :P

Magpie - 16-10-2008 at 14:14

Don't worry about your English; you are doing great. I understand you perfectly!

Post your fan curve if at all possible. If it has been posted before let me know which one it is.

Yes, high stacks are just to get the fumes away from structures and people. Industrial specifications in the US are 10 ft (3.5m). If you keep your noxious odor generation to late at night I wouldn't think it would ever be a problem. Or, as evil_lurker says, when he gets a good southwest wind he is "good to go." I use my fan during the day. I'm just careful about not doing it when the neighbors are in their backyard.

I'm thinking that spraying the blades with Teflon might be overkill. Likely it wouldn't hurt anything except possibly cause the fan to be imbalanced, but not likely.

[Edited on 16-10-2008 by Magpie]

sbovisjb1 - 16-10-2008 at 14:41

Also where you place your hands (where the glass slides down, have a vent blowing air straight up over your arms, so the risk of contamination from outside agents is minimalized.

*Not required, but its a favourite feature of mine.

Can't you buy an old one that is being discarded by a local college or university and repair it? I know in Canada, when a department has to keep to budget, they often get new equipment they don't need.

I found this

http://www.labequip.com/itemcatalog/stkno/26322/Kewaunee-Sci...

It does not come with a fan, base cabinet or work surface but it offers a shell and should cut back on your construction time.

Here is a smaller, but more complete hood (only $200)

http://cgi.ebay.com/BAKER-COMPANY-SG-600-FUME-HOOD-6-16822_W...

[Edited on 16-10-2008 by sbovisjb1]

[Edited on 16-10-2008 by sbovisjb1]

watson.fawkes - 16-10-2008 at 15:02

Quote:
Originally posted by Jor
A reason it si much more difficult, is that a 150mm would fit by removing one 'pan' (don't know the english word, it's a ceramic material on house roofs), and when taking 200mm we have to remove two, where also quite some open area will be leftover, and it's hard to close that again
The ordinary English usage is to call each piece of ceramic a "tile". (You already knew that word, I'm sure, but it's used for overlapping roofing as well as for flat, butt-jointed, and grouted pieces.)

OK. This does make sense (I've visited Netherlands; I know exactly the style you're referring to.) A smallish constriction isn't going to be that bad; you just have to pay for it with extra fan (which it seems you have). On the other hand, if you haven't already bought your vent, see if your supplier can obtain a rectangular vent, say, 150mm by 250mm, or whatever is standard. You can orient that along the tile width. You'd also need a different shaped adapter, but you already need an adapter of some kind regardless.
Quote:
Another small thing, the neigbours must be wondering, when a huge 200ml pipe comes out a garage. I have nice neighbours dont get me wrong, we even go with them to football games sometimes (Feyenoord!!), but I don't know their responce to a chemistry lab.
If it looks like a chimney, with a standard chimney cap, it shouldn't attract much notice. Another hint: if it looks old, it won't be as noticeable. Cheap spray paint is your friend here. It doesn't need to last, since when the paint goes your vent will <i>be</i> old.
Quote:
Hmm, yes the flashing is really important... Or I would get water in the blower and hood ... Or a bird ;)
But wich one is recommended? I can imagine flashing gives quite some resistance, as it blocks the exit.
I believe you're thinking of a chimney cap, which cover the top of the vent. Just make sure that the exit area beneath the chimney cap is of adequate size (a little bigger than the pipe cross-section) and you'll be fine.

Flashing is different. It's a membrane, either rigid (metal) or flexible (EPDM, for example), that takes on all the special shapes required to fit obstructions to the basic repeating pattern of your roofing material. The traditional flashing for tile is sheet copper. It's easy to shape in the field and it solders easily. Roofing is a "shifted sandwich" of tiles or shingle. Flashing for a vent slips into this sandwich and acts like just another piece of roofing. This general article and this specific construction detail should get you started.
Quote:
When I'm having the hood on, I will open a door, just a little. I know it will be quite cold in winters, but it is anyway, even without hood.
You might also consider a second fan and an outside (source) vent and damper, if the layout of everything permits. You'd then just pipe some incoming air to near the top of your fume hood.
Quote:
Also I use the hood only when neccesary. Because I have no external motor [...]
I do understand your limitation. But I would recommend putting a bit of forethought into how you might replace the fan, if necessary. This involves access to the fan site itself, the ability to cut into the duct, and where the electricity is already routed. The idea is to make fan replacement less than a full construction project.

kalacrow - 16-10-2008 at 22:10

Not entirely off topic.. and nifty :)

I built a small Tesla Rotor (Google Tesla Turbine) to play with out of CDs, and it worked much better than I expected for airflow. An advantage of this design is that the air exits axially (or enters, depending on direction) to the rotor, so you could make a simple unit out of your preferred non corrodible material and link it to a motor externally with an air bearing and a rare earth magnet. This is similar to how it is used for corrosive and heavy solids bearing liquid transport by oil companies.

If you made the rotor assembly out of cheap enough materials, you could just dispose of it every few weeks and replace it, thus obviating problems with wear.

Lots of work, but elegant. Its on The List.

Jor - 17-10-2008 at 11:06

Mgapie. i'm sorry I don't have a fan curve. But every fan should be able to blast at at least 50% power if there is minimal resistance besides a smaller duct at the end right?

Thank you watson. I remember all those hints!

I Have been searching for a second hand hood, and found one, but it was too big (2 meter wide).

Magpie - 17-10-2008 at 12:35

from Jor:
Quote:

But every fan should be able to blast at at least 50% power if there is minimal resistance besides a smaller duct at the end right?


Let's take an educated guess. Below is the fan curves originally posted by DNA. On this graph I have drawn in an assumed fan curve for your fan based on it's maximum capability of 760m3/h = 0.211m3/s. From this we see that at 450m3/h = 0.125m3/s the expected fan pressure capability is 150 Pa = 15mmH2O. Since I calculate you would need 9.3mmH20 it seems you will be OK.

Remember, this is an educated guess. I really don't know what your fan curve looks like.

assumed curve for Jor.JPG - 24kB

Jor - 31-10-2008 at 14:24

Here some pics on the process.

This is the hood it self. It's so crowded in the garage at the moments, So I can only stand very close to hood, so cannot zoom out. It's really as mess there, with tools all over the place.

http://img135.imageshack.us/my.php?image=cimg1058kk1.jpg

This is upstairs in the garage. The left pipe you see is the exit pipe. The right one is coming from hood. Well have to make the right one a little shorter, and then make a 40 degrees angle, followed by the motor and another 40 degrees angle. This will be a hard part to go, ass I'm not sure if we can make it in such a short horizontal distance. We cannot do this this weekend, as we sended back the 200mm/760mm motor, as 200mm ductwork is very hard to get and very expensive. Besides , it will look like a factory when people look at the roof :cool:
Motor will arrive monday or tuesday. The one whos helping me will be here again in 2 or 3 weeks, and then we'll finish it off, and do some smoke tests. The new motor will be less powerfull (450m3 WITH carbon filter, so probably more without), so if power is not satisfactory, well have to send it back again and buy another one. But I doubt it, there's not much resistance in the entire system.

The sash was ready, but to make work easier, we ripped it apart again :D. The idea is that there is one stationary plate of glass, wich covers the upper half of the opening. The other half will be moveable. So the hood can be maximum open to about 40-45cm, and it can be closed to about 4-5cm. So I do not worry too much if hood has not enough power, in case chlorine/bromine/etc will be realeased, I will close it to those 4-5cm. I have no bypass.

http://img236.imageshack.us/my.php?image=cimg1053qf7.jpg

It will certainly not be the kind of hood Magpie owns, but it will greatly protect me from harmful vapours. I will be limited in qunatities anyway, as I live in an area where people live.

Finally this is the exit of the system on the roof:

http://img406.imageshack.us/my.php?image=cimg1052tu8.jpg

A picture from the side:

http://img100.imageshack.us/my.php?image=cimg1056xy4.jpg

Soon I will also get a safety cabinet for hazardous chemicals. When I have these safety resources, I think my lab will be close to completion!
I miss one most important thing... A lab partner :(

[Edited on 31-10-2008 by Jor]

S.C. Wack - 31-10-2008 at 16:25

Well now I have nothing against engineering, and I know enough HVAC to be dangerous, but I'm not sure that this needs to be so involved. Not trying to be a dick here, I'm just saying.

We have a Very Big Hood at work, powered by a number of Dayton 3C411 belt-driven explosion-proof axial fans. The company spends huge sums on the explosive and toxic chemicals that are reacted in it (outside, there is at all times a line of railcars, in addition to many large tanks) so you'd think that money would be no object in making OSHA happy. But instead of consulting chemical and HVAC engineers and buying special things, the mechanics just made a frame, bolted sheet metal to it, and stuck a bunch of fans on top, which are connected to 24" round ducts. There are screens in the intakes so nothing solid gets sucked in. That is all. Maybe they have more fan power than they need, oh well. But there are several chemical monitors, and everything is OK.

I'm not so sure of the need for explosion-proof fans on the smaller scale. If you turn them on before there is any vapor, it is difficult for explosive vapors to reach explosive concentrations, due to dilution. The hoods of yore were powered by flame.

Can't recommend a corrosion-proof fan - the internally exposed part of my Soler and Palau TD-150 6" inline fan is 99% plastic, but unfortunately (these are over $100) it has a gap between the blade and the housing for the aluminum rotor.

Klute - 31-10-2008 at 17:01

Very neat Jor!

Magpie - 31-10-2008 at 18:51

Looks very good Jor. I think your 150mm duct is going to be just fine with your hood size. If the new fan you are getting has an outlet size of 150mm that will work out perfect. No need for any transition pieces.

Good luck finding 40 degree angle pieces. Wouldn't 45 degree pieces work? They are much more commonly available. I was even able to buy 22.5 degree pieces for my duct. Then again if you have a sheet metal fabricator helping you then you can get whatever angle you want. :D

watson.fawkes - 31-10-2008 at 19:29

Quote:
Originally posted by S.C. Wack
We have a Very Big Hood at work, powered by a number of Dayton 3C411 belt-driven explosion-proof axial fans [...] which are connected to 24" round ducts.
Sounds adequate.

All the engineering is to protect the not-yet-knowledgeable from an inadequate system.

Jor - 1-11-2008 at 11:21

Today we have finished the sash and electricity as well :P. So there only remains one thing, wich is putting the motor in place. This will not be to easy. We will do that next week or the week after.

Then there will be tests to determine how safe the hood is.
The motor can be put at 4 different speeds (25,50,75 and 100%). The sash at 4 different heights (42 cm, 30cm, 17cm, 4cm). I will test all possibilities (4x4=16), with the following:
-Opening a bottle of 25% ammonia.
-Making some smoke (How? Maybe some KNO3+sugar? Or a safer method? I would not like to use HCl, wich will any time soon be unavailable forever)
-A piece of paper in front of sash, at different points.

- But because I work with loads of heavy gasses, like chlorine, SO2, Br2, NO2,etc. I must test if the hood is laso capable of effectively venting heavy gasses. I do not have baffles, wich commercial fume hoods have. The baffle in the back usually sucks out heavy gasses. But I do not have this feature. What would be a safe heavy gas to test, wich would also indicate whether the hood is effective for NO2/Cl2/Br2/SO2/organic vapours? I was thinking about some solvent, but most are detected at too high levels, to give a good indication if things are working well. Any ideas?

Any other tests I must do?

[Edited on 1-11-2008 by Jor]

Magpie - 1-11-2008 at 15:11

A strong solution of aqueous ammonia would be good. You can also use a strip of tissue paper. For smoke it seems like a cigarette would be a handy generator.

If, after a time, you decide that you want a baffle or two, they are very easy to construct and install.

pyro6314 - 14-11-2008 at 13:46

I picked myself up a blower with heater attachment from Princess Auto. It`s rated for MAX 300CFM which equates to about 500m3/h. Somehow I don't feel that this is enough flow for even say a 100cm x 50cm opening. I don't have a fan curve but the fan is small so I would assume it won't build much pressure. After the restriction from ducting, is this blower going to do the job? I hope so because the heater comes with a hefty amount of NiCr wire I would like to turn into a mantle.

On a second note, how wide have most of you made your fume hoods? I would like to fit a 24/40 distillation rig with room to spare but I can't measure because its in the mail.

Jor - 16-11-2008 at 08:49

Well I just finished my hood. It works really great. It has 4 sash opening heights, I cannot move it freely, that;s the only downpart. In this post I will call the height were it is fully open 1, one height down 2, another one down 3, and when sash is almost closed (4-5cm left) 4.

I did several test at the motor speed 3 (4 is most powerful but also quite noisy).

Ammonia conc.: I opened bottle at all sash heights and didn't smell anything.

bromine: Opened sash at all heights and only smelled a SLIGHT smell at sash height 1 (absolutely less than 1ppm, I hardly smelled it). I putted a 0,5 mL of bromine on a petri dish so quite some vapour was released.

Smoke test. I hold a bottle of HCl and a bottle of NH3 in front of the sash. At all sash heights, all smoke is sucked in.

NO2 test. A small coin was added to 3mL of conc. HNO3. The sash height was 2 (working height). All fumes are sucked away nicely, and no smell was observed at all. When I lowered the sash to 4, at once, ALL brown gas in the beaker was gone, and the air was colorless, and even though it still bubbled, no brown vapour could be observed.

Pictures will follow.


Video, quality is not as good as I thought it would be:
http://www.youtube.com/watch?v=bus-QQgbrWk

Im very happy, this hood gives me a good feeling of safety.
My motor has about 600m3 max power at full speed (4), and 75%, 50% and 25% of that for the speeds 3,2 and 1 respectively.

[Edited on 16-11-2008 by Jor]

Magpie - 16-11-2008 at 12:26

Well done Jor!

Please post some pictures of your fan installation in the attic.

Did you stay with 150mm duct all the way or do you have a mix of 150mm & 200mm duct?

How is the noise level to neighbors?

Did you place anything on your stack to keep out rain?

Jor - 16-11-2008 at 14:23

http://i405.photobucket.com/albums/pp133/Joris12345/CIMG1180...

http://i405.photobucket.com/albums/pp133/Joris12345/CIMG1179...

http://i405.photobucket.com/albums/pp133/Joris12345/CIMG1177...

http://i405.photobucket.com/albums/pp133/Joris12345/CIMG1176...

These are some pictures.
When I'm standing outside I hear nothing. Maybe the neighbours can hear a SLIGHT noise when in their garage, but I'm not too worried.

It's an all the way 160mm ducting.

:cool:

[Edited on 16-11-2008 by Jor]

Picric-A - 17-11-2008 at 09:46

It all looks so neat and tidy! well done!
I wish i could say the same for my lab :P

Jor - 17-11-2008 at 10:39

;)

It's not always like that. After we finished the hood we cleaned everything.

At the moment, it doesn't look as tidy anymore as yesterday :P
It still is tidy however. I clean every month or so, including the workbench.

Picric-A - 5-12-2008 at 05:04

I am having a few problems with my Hood.
It is homemade, i only made it a few weeks. I will try get a few pictures of it next week as i am away form home at the moment. It consistes of an extrctor fan (fairly powerfull) against tthe wall and i basically built a wooden box around it painted in a special fireproof, waterproof paint.
Once finished i decided to test it by igniting a small amount of KNO3 + sugar mix to produce lots of visible smoke.
once lit it seemed to manage ok but i cant help noticing quite a bit is blown out at me. When using it for things like NOx ect.. i dont want any gasses comming out so what can i do to stop this probelm?
thanks,

panziandi - 5-12-2008 at 06:05

Do you have a sash on the front? You want a sash so you can pull it down to reduce the inlet area, creating more displacement. If you already have a sash, perhaps baffles at the back to distribute airflow better?

Jor - 5-12-2008 at 06:22

Maybe it's the power of your motor. It might be too strong, or too weak.

My blower has 4 speeds. You would think that speeds 4 is the best, or 3. It's not. Speeds 1 and 2 are the best with mine, they make no turbulence and suck everything away. When I open the sash fully, I feel no wind or anything at all, and I don't smell anything, not even when I put a coin in conc. HNO3. And I have NO baffles.
So maybe your motor moves too much air, and creates turbulence. Or not enough air is sucked out. You should try different speeds with your motor. With some knowledge on electricity components, you should be able to experiment.

It's worth the time, really. The fume hood was the best thing I ever had in my lab, for 2 reasons:
-It makes me feel so much safer, I remember the nights at 1:00 AM making bromine, when I had a small cough after every synthesis. Or opening bottles of ammonia. I must say, that I have not smelled a single chem last 2 weeks, besides some acetone (cleaning) and sulfur dioxide when cleaning MnO2.
-I use it for storage. I have some real nasties, like 250ml benzene, a few mL of bromine, 30mL CCl4, 10mL SO2Cl2 and 250mL aniline. I keep those in the hood in a plastic containment container. I also have my waste containers (Heavy metals and chlorinated hydrocarbons) in the hood. My hood is running all day long, I havent turned it off until now. But I always have some experiment running, and I just put it in my hood overnight.

Now I think of it, I haven't thanked the people who gave me advice yet ! :o
Especially Magpie! Hereby, thank you very much, I have used all the advices as much as possible, and the result is very good.

[Edited on 5-12-2008 by Jor]

Picric-A - 5-12-2008 at 07:11

This is the motor i have:
https://www.machinemart.co.uk/shop/product/details/cam200-po...
I feel i need air foils to direct the suction better to stop the blowing effect.
I will post pictures as soon as i can.

Jor - 5-12-2008 at 07:20

How large is your hood?
This one moves a lot of volume, similar to that of Magpied. And Magpie has a big hood (with baffles).
So if you have a small hood you should really take a less powerfull motor. It's not the power that's that important, it's the airflow.

Picric-A - 5-12-2008 at 12:00

I cant give accurate details as i am away form home at the moment but it is around 1 meter high and 1.5m wide and 80cm deep.
I feel the motor is too powerfull... I will try make baffles but this will be failr hard as my extractor is right agains the wall.

Lambda-Eyde - 9-12-2008 at 13:06

What does the crowd think about this one?




It's used for vacuum systems in woodworking shops, and it pulls 14,15 m3/M which is equal to 849 m3/h, enough for even a 150 cm hood! :o

The problem is the motor placement. Well, actually I can't see it, but I assume it's in harm's way when talking about corrosive and explosive gases (considering it's only made to tackle sawdust after all).

If the motor is in the way, I think I can move it and put it to the top and attach a belt drive, as I'll soon have my own metalworking shop capable of such modifications.:D
Does anyone in here have any experience with modifying fans? Will it be as easy as I hope?

[Edited on 9-12-2008 by Lambda-Eyde]

watson.fawkes - 9-12-2008 at 16:48

Quote:
Originally posted by Lambda-Eyde
The problem is the motor placement. Well, actually I can't see it, but I assume it's in harm's way when talking about corrosive and explosive gases
That's a centrifugal fan. Since it's made for wood dust it likely has a radial blade impeller on it. That impeller style sheds dust rather than collects it. Most fans of that style use a direct drive motor; it's mounted on a plate on the back side of the fan housing.

Lambda-Eyde - 10-12-2008 at 00:57

Yes, I assumed it would be a radial configuration, but that doesn't mean it'd be insufficient for my use?
As for the motor placement, if your statement is correct, would it be safe to use it for my fume hood?

Edit: Forgive my noobish question, but what do I want from the hood? Pressure or flow? I assume pressure, so a fan with a backward-curved configuration would be preferable, right? Forgive my ignorance, but physics never was my strong subject...

[Edited on 10-12-2008 by Lambda-Eyde]

watson.fawkes - 10-12-2008 at 05:47

Quote:
Originally posted by Lambda-Eyde
[...] a radial configuration, but that doesn't mean it'd be insufficient for my use?
[...] motor placement, [...] would it be safe to use it for my fume hood?
[...]Pressure or flow?
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