Fume hood venturi.

[eeTHr]

(Note: This post pertains to my previous post, on the preceding page.)

Here are some blowers I found---

Harbor Freight, $49.99
This one puts out 300 CFM. Maybe not quite enough air volume.

Home Depot, $49.00
3 speeds, up to 310 CFM. Same comment as above.

Lowe's, $46.49
480 CFM, but only has one review, and it sounds like it had a factory defect. If the venturi can suck at only 41.7% of the blowers direct CFM, this would produce 200 CFM of suction, which would be enough for a fume hood door opening of 12" x 24" or 17" x 17" or 15" x 19" (at 100 CFM per square foot of fume hood door opening). The Ammen venturi style shown in the diagram on my previous post, tests-out to be the most efficient so far---probably right around the 40-something percent required for this blower.

Harbor Freight, $74.99
Switchable, either 1400 or 1575 CFM. I wonder what would happen if you mounted this on a skateboard with a really long extension cord?

 

[goldenchild]

Harbor Freight, $74.99
Switchable, either 1400 or 1575 CFM. I wonder what would happen if you mounted this on a skateboard with a really long extension cord?

You got me excited for a sec. I thought it was a squirrel cage type. With the way mine is holding up so well I could build a much bigger hood with 1400 cfm. These are the cheapest acid resistant blowers I could find online.

http://www.drillspot.com/products/587555/plastec_pla15st2p_direct_drive_forward_curve_blower

I see a cfm of 550 but don't know what this means. "CFM @ 0.500-In. SP"

 

[eeTHr]

goldenchild---

No, it's not acid resistant, because it relates to the venturi setup on the previous page.

That blower you linked to looks like a turbocharger! Too bad it's soo expensive. That's why I took such an interest in the venturi system configurations.

CFM is Cubic Feet per Minute of air exchange, or air flow volume.

SP is Static Pressure. Basically that's loss from the air intake and exhaust ports' limitations.

 

[goldenchild]

I don't plan on buying one of the expensive blowers as long as the kind I have now holds up for a decent amount of time.

I knew cfm was cubic feet per minute but didn't know what the part in quotes meant "CFM @ 0.500-In. SP". Do you or anyone else know how to translate that into a sentence?

 

[eeTHr]

goldenchild---

Here's a post by samuel-a that shows a diagram of it.

That post is from this thread.

I was searching for other fume hood threads, and found that if you put "fume hood" as the search term, and then down in the lower right corner click the radio button "Topic titles only," you get just the first post from each thread, instead of every post which contains those words. that method works a lot better for certain searches.

 

[eeTHr]

Here some actual photos of the PVC assembly for the Ammen type Suction Venturi---

Overview with an exhaust section

Suction port side

Other side

Adapters view

Inside venturi from output end, without exhaust extension on

The 45 degree fitting is because my shop vac outflow port is at an angle. That goes into a reducer which is made of black ABS type of plastic, because that's all they had at my local ACE Hardware (it's kind of a small town).

The black fitting which connects the 3" "Y" to the 3" thinwall PVC is actually Gorilla duct tape, because that's what I had on hand. The thinwall, I believe it's called schedule 20 (whereas the "Y" fitting is schedule 40), is much less expensive than the thicker pipe, especially in the larger diameters. So the Gorilla tape is merely to increase the outer diameter of the thinwall exhaust section, to fit into the sched 40 "Y" fitting.

The grey 2" PVC is for electrical runs, whereas the white is for water. You can see where I tapered the end of the 2" pipe which comes out of the 45 degree fitting, in order to fit into my shop vac output.

Although this vac has 2" ports, it's hose is only 1 1/4" ID, so it's not the strongest air flow shop vac on the market, by far. That's why, even though this test setup resulted in suction which was too weak for fume hood use, I think the venturi system has a chance of working satisfactorily in a home refinery scenario, if one had a larger blower and used larger diameter PVC and fittings.

P.S. Although the end view of the inside of the venturi looks like the cross sectional area around the outside of the 2" PVC pipe is smaller than that of the inside of the 2" pipe, it calculates to be 3.93 square inches around that outer circle, compared to the 2" pipe which has 3.14 square inches.

Also note that the white "Y" fitting has a larger diameter at the very ends, to accomodate the outside diameter of the thicker schedule 40 PVC. Also, the PVC pipe walls get thicker as their diameter increases, for both sched 20 and 40.

P.S.S. To get the 2" air input pipe, from the vac output, to pass through that black reducer, I removed the stop-lip inside it, with a Dremel barrel sanding bit (it has a replacable sandpaper tube). This lip is inside the reducer, to provide a stop for the smaller PVC pipe to bottom-out on when assembled. This, and attaching to whatever kind of blower one uses, are the only modifications which are required beyond parts which can be purchased. And I am assuming that where I used the Gorilla tape, an adapter can be bought to be used instead.


Edit: Corrected "especiall" to "especially," in paragraph 3.

 

[seawolf]

At Harbor Freight they have a dust collector that might be used for a fume hood.
$129.99
120 vac
1 HP motor
4" intake
4" exhaust
660 CFM
The housing is metal and I think the fan is a hard plastic.
From some of the posts I have read the housing could be disassembled sanded and repainted with a good epoxy paint to prevent corrosion. Then vented into a scrubber before gasses are released into the outside air.

 

[eeTHr]

I like it. The price isn't bad, the CFM seems good, and it looks like it would be easy to connect up-to.

I wonder what kind of plastic, and whether the acids would damage that?

Actually, some of the blowers that I linked to, could have plastic blades, too. Certain kinds of plastics can be a problem, I think.

But 4" blower pipe going into a 6" "Y" fitting would work for the venturi effect, cross sectional wise. And in that case, the plastic wouldn't matter, and it wouldn't require painting. And if it gave only 40% of the rated CFM in suction, then it would provide enough suction for a 2.65 square foot door opening.

 

[qst42know]

Harbor Freight, $74.99
Switchable, either 1400 or 1575 CFM. I wonder what would happen if you mounted this on a skateboard with a really long extension cord?

You got me excited for a sec. I thought it was a squirrel cage type. With the way mine is holding up so well I could build a much bigger hood with 1400 cfm. These are the cheapest acid resistant blowers I could find online.

http://www.drillspot.com/products/587555/plastec_pla15st2p_direct_drive_forward_curve_blower

I see a cfm of 550 but don't know what this means. "CFM @ 0.500-In. SP"

Be careful of that motor voltage (230/460 V) if you don't have access to three phase power. If the power company will provide it where you intend to work it can be extremely expensive to have it installed.

 

[Geo]

something no one has mentioned yet. i know the dynamics of how this works because ive dealt with it for many years at my old job only it was water but still the same effect. the pressure going in works better at the exhaust end, in other words the farther you put the pressure inlet from the fume hood the stronger the vaccum will be. it would be better if it were at the last couple of feet at the end of the exhaust. think of it as a sand blaster and the feed tube in the sand as your fume hood. the air pressure comes in just behind the nozzle where the sand comes out. same difference with this the vaccum is greater this way.

 

[eeTHr]

something no one has mentioned yet. i know the dynamics of how this works because ive dealt with it for many years at my old job only it was water but still the same effect. the pressure going in works better at the exhaust end, in other words the farther you put the pressure inlet from the fume hood the stronger the vaccum will be. it would be better if it were at the last couple of feet at the end of the exhaust. think of it as a sand blaster and the feed tube in the sand as your fume hood. the air pressure comes in just behind the nozzle where the sand comes out. same difference with this the vaccum is greater this way.

That makes sense, because that way there is almost no back-pressure reducing the venturi effect.

So the previous ideas of putting the blower in front of a string of venturis would probably be very difficult, if not impossible, to optimize.

 

[eeTHr]

Harbor Freight, $74.99
Switchable, either 1400 or 1575 CFM. I wonder what would happen if you mounted this on a skateboard with a really long extension cord?

You got me excited for a sec. I thought it was a squirrel cage type. With the way mine is holding up so well I could build a much bigger hood with 1400 cfm. These are the cheapest acid resistant blowers I could find online.

http://www.drillspot.com/products/587555/plastec_pla15st2p_direct_drive_forward_curve_blower

I see a cfm of 550 but don't know what this means. "CFM @ 0.500-In. SP"

Be careful of that motor voltage (230/460 V) if you don't have access to three phase power. If the power company will provide it where you intend to work it can be extremely expensive to have it installed.

Wow, I didn't notice that. 3 phase, too. And only 560 CFM max. That seems strange, because 1/2 HP motors run fine on 110 VAC, and are common. :?:

Makes the Harbor Freight unit look a lot better, and the venturi method.

 

[Barren Realms 007]

Harbor Freight, $74.99
Switchable, either 1400 or 1575 CFM. I wonder what would happen if you mounted this on a skateboard with a really long extension cord?

You got me excited for a sec. I thought it was a squirrel cage type. With the way mine is holding up so well I could build a much bigger hood with 1400 cfm. These are the cheapest acid resistant blowers I could find online.

http://www.drillspot.com/products/587555/plastec_pla15st2p_direct_drive_forward_curve_blower

I see a cfm of 550 but don't know what this means. "CFM @ 0.500-In. SP"

Be careful of that motor voltage (230/460 V) if you don't have access to three phase power. If the power company will provide it where you intend to work it can be extremely expensive to have it installed.

Wow, I didn't notice that. 3 phase, too. And only 560 CFM max. That seems strange, because 1/2 HP motors run fine on 110 VAC, and are common. :?:

Makes the Harbor Freight unit look a lot better, and the venturi method.

Commercial use. We put a number of 3PH blowers for fume hoods in a science lab for fume hoods.

 

[eeTHr]

Yeah, 3 phase is common in commercial and industrial buildings. It's more efficient. But I've usually only seen it in the higher horsepower motors, where electricity savings can be significant. But I guess if there are several of the 1/2 horse, it does make a difference.

 

[Harold_V]

Commercial use. We put a number of 3PH blowers for fume hoods in a science lab for fume hoods.

For VERY good reason, too. Single phase motors are noisy. Very noisy. They hunt contantly, introducing vibrations and other annoying sounds that are absent from three phase motors.

Harold

 

[eeTHr]

Harold---

I've delt with many, many electric motors, in the range of 1/3 HP to 2 HP, used in automated machinery. The smaller ones in single phase, and larger ones 3 phase, with the middle range in both versions. I knew the 3 phase were more efficient, and I know the differences in how they are built and how they work, but I never, ever, considered the fact that the 3 phase were also smoother running!

Now that you mention it, there was a 5 HP electric motor/generator unit, which converted 3 phase to single phase, in one place I worked. I was always amazed at how smooth and quiet it ran, for it's size. You just put 2 + 2 together for me. Thanks (again).

 

[eeTHr]

I thought that my previous photos of the test venturi might be a little confusing, because of the exhaust extension and angled input, so I made some with just the venturi section to show how simple it, itself, actually is---


Venturi Section

Suction Port View (please disregard the tan leaf on the left end!)

Blower Input and Reducer

Output End

Overall View, Including an Exhaust Section and Angled Blower Air Input

I angled the blower air input, only because my shop vac output is at an angle.

The venturi section, itself, consists of only three pieces: The grey PVC blower air input pipe, the black ABS reducer (which holds the blower air input pipe), and the white PVC "Y" fitting. (The black reducer between the "Y" fitting and the exhause section is actually Gorilla tape, because that's all I had on hand. It was necessary only because the exhaust section was thinwall PVC, so it has a smaller outer diameter, and the "Y" fitting is built to accept schedule 40 PVC into it, instead. But the sched 40 is more expensive!) Also note that the ABS reducer requires the inside retaining rim to be removed, to allow the blower air input pipe to slide all the way through it, and past it, and partially into the exhaust ducting.

This is a test version, using smaller ducting and venturi section than may be required, because it fit my shop vac used for the tests.

The reducer plugged into the venturi section is black because it is ABS plastic, used for sewer pipes, because that's all they had for that at my local ACE Hardware, that day.

The grey blower air input pipe might create a better suction, if it extended somewhat farther into the exhause section ducting, getting it a little farther out of whatever turbulance is created by the suction port takeoff on the "Y" fitting. That's my hunch. Maybe another six inches or so?

The suction CFM recommended by 4metals is 100 CFM for every 1 square foot of hood door opening. So far, I'm estimating the tests to have shown a venturi suction of about 40% of the blower air CFM. But the cost of a larger blower, so far, seems to be way less than the cost of an acid resistant blower.

That's pretty much it, in a nutshell.

Any other tests and results posted by others, would be greatly appreciated.

 

[eeTHr]

Correction!

When I made the calculations for the area of the gap between the 2" blower air input pipe, and the 3" inside diameter of the "Y" fitting, I erroneously used the inside area of the smaller pipe (2") to subtract from the inside area of the larger pipe.

I recalculated it, using the outer diameter of the smaller pipe to figure the gap area, and the cross sectional area of that gap is really only 2.635 square inches. That's less than the inside area of the smaller pipe, which is 3.1415 square inches.

I think that for the total air volume in the exhaust duct to be twice as much as the blower (by combining both the air source from the blower, and the air source from the suction port), and thus making the total air volume in the exhause ducting a balanced 50-50 ratio between the blower air and the suction port air, the gap area would need to be equal or greater than the area of the smaller blower air input pipe.

A 50-50 ratio of the gap and blower pipe inside areas seems technically ideal, but to compensate for air resistance, I think the gap area should actually be somewhat larger (or at least the exhaust ducting following it should be). I haven't tested this yet, and I am basing this only on other tests I made with the other type of venturi configuration, where too much back pressure in the exhaust duct would either reduce the suction, or stop it altogether and cause air to blow out of the suction port instead of sucking in. I searched the Web, but couldn't find any formulas for calculating optimum ratios for the suction effect.

If anybody has any information about this, I would appreciate it if you would please post it.

 

[eeTHr]

I decided to try calculating the gap area using a thinwall schedual 20 pipe for the 2" blower air input. But I don't have a piece of it handy, to measure the wall thickness. So I measured the thickness of the 3" exhaust section thinwall, and used that. The actual thickness of the 2" pipe will be slightly less.

Now, the gap area is 3.091 square inches---less than the 3.1415 gap area with the schedule 40 2" pipe. So that should work for the venturi section, especially since the actual wall thickness of the 2" thinwall is even slightly less than what I used in the computation.

Also, using a schedual 20 "Y" fitting and 3" to 2" reducer, will cost less.

I think that using 4" ducting, after a couple feet of the 3", would be best, especially if there are any turns in the exhaust. I think that the 3" section is going to be necessary, however, to keep from reducing the air speed around the tip of the blower air pipe, as the actual venturi suction will be created there, which will be a few inches into that exhaust section, as mentioned before.

For larger blower pipes, the cross sectional areas will need to be figured and applied accordingly (Area of a circle = pie times radius squared), remembering to use the outer diameter of the smaller blower pipe when calculating the gap between the air input pipe and the "Y" fitting.

Edit: Corrected spelling of "decided." Changed wording in paragraph 2 to read "less than the 3.1415 gap area with the schedule 40 2" pipe," for clarity.

 

[Geo]

you may be over thinking it a bit, remember K.I.S.S. keep it simple stupid. if you trade your wet/dry vac for an electric leaf blower you will get more psi on your pressure side. hook it up and if it sucks.