Venturi effect vacuum bag compressor sizes?

[dbp1]

I'm trying to understand how venturi effect vacuum pumps work.

I've seen people say that the compressor uses a surprisingly small amount of air, will only go on over so often, etc (once it actually pulls the vacuum).

But the description of, e.g., the one that WEST has in a vacuum bagging kit (https://www.westsystem.com/tools-sup...m-bagging-kit/) says you need a compressor that can do 3.5 SCFM at 65 PSI (which many compressors can, but smaller, quieter ones; e.g., https://www.makitatools.com/products/details/MAC320Q, cannot. That one can do 3.5 at 40PSI, or 2.6 at 90PSI, so 65PSI should be somewhere in the middle!).

What I don't quite understand is that if the compressor isn't going _continuously_ (which hopefully it's not!), why is there a _continuous_ requirement? If it needs 65PSI, fine, but that compressor (for example) can go well above that, just at a lower continuous rate. Or is the compressor sending out that much air all the time (seems like a lot), and the "on and off" is just the compressor cycling?

[sp_clark]

Never used one by my guess is that much depends upon the capacity of the storage tank taking the compressor output then sending it downstream to the venturi vacuum device.

Minimum CFM is supplied by the storage tank while the compressor cycles on & off to maintain minimum venturi supply pressure. Kinda like when you're running an air-powered DA sander, the volume of air supplied (CFM) is more important than the pressure supplied to the sander. Bigger tank helps over smaller where volume is more pertinent for continuous operation.

With vac-bagging, once the vacuum's pulled the only real demand is to overcome leakage to maintain the vacuum. So all that's needed from a venturi is a very small amount of pressurized air over time to maintain enough of a vacuum effect to prevent atmospheric pressure from back-feeding into the bag... I think.

I'm sure others having more real-time experience with this process will add to the knowledge I lack at present.

https://www.epoxyworks.com/index.php...agging-basics/

https://www.fibreglast.com/product/v...earning_Center

https://www.fibreglast.com/product/1..._Bagging_Pumps

https://www.carbonfiberglass.com/Vac...Generator.html

https://www.instructables.com/Vacuum-bagging-basics./

https://www.sciencedirect.com/topics...ing/vacuum-bag

https://explorecomposites.com/articl...acuum-bagging/

[jpatrick]

I have used a venturi vacuum pump with my bag system for many years. Like any pump, it can take several minutes to draw a near vacuum with a large bag that has a bulky object in it. But once that is done the whole works shuts off unless the vacuum needs to be "topped up." (Or would that be, topped down?) This shutting on/off is, of course, dependent on the system being fitted with a vacuum switch. Some are not. I use a large compressor in my shop that can easily handle it. But I've also done vacuum bagging work on a work site where the only compressor available is a small jobsite type. Something one would use with a trim nail gun.

You can get a lot of good information regarding vacuum pressing from Daryl Kiel at Vacuum Pressing Systems. www.vacupress.com
I am only affiliated with them as a customer.

Jeff

[Spot]

We use them at work. I borrowed one once and found it annoying to listen to it and the compressor for 5 hours while the epoxy set. It was winter and the project had to be brought into the house so there was no escaping the noise.
I would rather use and now have a small Gast vacuum pump than a venturi generator.

[dbp1]

You can get a lot of good information regarding vacuum pressing from Daryl Kiel at Vacuum Pressing Systems. www.vacupress.com
I am only affiliated with them as a customer.

Thanks. The smallest vacuum press on that site actually doesn't seem unreasonably priced (I have battery trim nailers, so have avoided a compressor thus far in my small shop): the CFM is lower than the bigger ones, but that shouldn't actually matter (nothing I'd ever be doing would be big, so shouldn't be much air).

[Nicholas Carey]

"Joe Woodworker" is sort of the canonical source for DIY vacuum press plans. And parts.

He has several variants of plans for vacuum pump/presses, both venturi-bases and utilizing a proper vacuum pump.

[he would, naturally, prefer you bought the various bits and bobs from him.]

https://www.joewoodworker.com/veneering/welcome.htm

Here's a venturi setup that somebody built using surplus oxygen tanks as a vacuum reservoir.

[Nicholas Carey]

I'm trying to understand how venturi effect vacuum pumps work.

I've seen people say that the compressor uses a surprisingly small amount of air, will only go on over so often, etc (once it actually pulls the vacuum).

But the description of, e.g., the one that WEST has in a vacuum bagging kit (https://www.westsystem.com/tools-sup...m-bagging-kit/) says you need a compressor that can do 3.5 SCFM at 65 PSI (which many compressors can, but smaller, quieter ones; e.g., https://www.makitatools.com/products/details/MAC320Q, cannot. That one can do 3.5 at 40PSI, or 2.6 at 90PSI, so 65PSI should be somewhere in the middle!).

What I don't quite understand is that if the compressor isn't going _continuously_ (which hopefully it's not!), why is there a _continuous_ requirement? If it needs 65PSI, fine, but that compressor (for example) can go well above that, just at a lower continuous rate. Or is the compressor sending out that much air all the time (seems like a lot), and the "on and off" is just the compressor cycling?

With any vacuum setup, you have a vacuum source -- be that a vacuum pump or a venturi vacuum generator. That sits behind a vacuum reservoir -- think a compressed air tank that holds vacuum instead of highly compressed air. And in front of that you have a vacuum controller -- pretty much a relay with a vacuum w sensor that switches the vacuum source (be that vacuum pump or air compressor driving a venturi) on and off according to its limit settings. When the air pressure (or lack thereof) drops sufficiently low the circuit powering the vacuum source opens, and when the air pressure (or lack thereof) rises sufficiently high, the circuit closes and the vacuum source powers up to suck down some more vacuum.

Without that vacuum reservoir, the vacuum controller, and the circuitry, the compressor would be running nonstop.

[jpatrick]

My system has no vacuum reservoir and it works perfectly. There is a venturi pump, an air compressor with an air line that leads to the pump, and a vacuum sensor/switch that reads the pressure inside the press. When a sufficient vacuum is achieved in the press (a flexible bag in my set-up) the sensor/switch closes the valve that stops air flow from the compressor... then all goes quiet. The air compressor works like any other. It has its own pressure switch that starts and stops the motor. It keeps a tank of compressed air between set levels.

Vacuum reservoirs are often used to allow a very rapid evacuation of a volume of air. A feature that I've never needed.

I will agree that having to use an air compressor can be a royal pain in the neck. Having an electric vacuum pump is far more convenient... and portable. I have what I have for the simple reason of economics. When I purchased the system it was what I could afford. The venturi was less money by several hundred dollars, as I recall. Enough to give me pause. I had the compressor already and knew that I'd be using the thing in my shop, not on the road. Recently I've considered upgrading. But the truth is that nowadays I use it so infrequently that I'd rather spend the money on something else.

Jeff

[Gordon Bartlett]

I have never seen a venturi vacuum setup. After the compressed air stops moving across the venturi, what keeps air from being sucked back through the venturi and repressurizing the space? Is there a check valve after the venturi?

[nedL]

I have never seen a venturi vacuum setup. After the compressed air stops moving across the venturi, what keeps air from being sucked back through the venturi and repressurizing the space? Is there a check valve after the venturi?

As mentioned, it is as much about the compressed air storage tank as it is about the compressor.

The compressor builds up a supply of compressed air and shuts off. The venturi runs off that stored up compressed air until the storage tank pressure drops to a preset level, then the compressor comes back on and recharges the storage tank. There is no reason for the compressor to run constantly in order for the storage tank to provide a constant supply of compressed air to the venturi.

[Gordon Bartlett]

Ah. Got it.
I misunderstood, and thought it was implied that once the vacuum was achieved there was no further need for compressed air other than to make up for leaks.
So what is the continuous cfm of air required to maintain the vacuum?
And is there any kind of check valve that could be placed after the venturi that would obviate the need for continued air flow?

[jpatrick]

I have never seen a venturi vacuum setup. After the compressed air stops moving across the venturi, what keeps air from being sucked back through the venturi and repressurizing the space? Is there a check valve after the venturi?

The short answer is, I don't know. There's a check valve or, more likely, the vacuum sensor operates a mechanical valve that has positive open/shut stops. I can hear the thing click on and off. Once vacuum is achieved, there is nothing happening.... no air in or out, no compressor noise, just blessed silence. Of course, the bag leaks a bit. This is inevitable for such a well used bag. I used to make furniture professionally and I did a lot of veneering work. Many varied projects.

Jeff

[John Meachen]

Ah. Got it.
I misunderstood, and thought it was implied that once the vacuum was achieved there was no further need for compressed air other than to make up for leaks.
So what is the continuous cfm of air required to maintain the vacuum?
And is there any kind of check valve that could be placed after the venturi that would obviate the need for continued air flow?

The short answer is no,air has to flow through the venturi all the time.With the better units,very little compressed air is sufficient and the last time I looked closely,the figure of 0.1 cfm was quoted.With a leaky bag it would probably be more.Which isn't to say that a non-return valve couldn't be inline but I've never seen one.

[sp_clark]

...think a compressed air tank that holds vacuum instead of highly compressed air.

Any reservations over using a compressed air tank as a vacuum reservoir?

That thread here awhile back illustrating the aftermath of a tank failure while pressurized gave me the heebiejeebies.

Those recycled oxygen tanks depicted in your previous thread to my thinking are engineered and constructed differently than the welded steel tanks I've known and used for compressed air reservoirs.

[jpatrick]

The short answer is no,air has to flow through the venturi all the time.With the better units,very little compressed air is sufficient and the last time I looked closely,the figure of 0.1 cfm was quoted.With a leaky bag it would probably be more.Which isn't to say that a non-return valve couldn't be inline but I've never seen one.

Perhaps my answer in #12 was not clear. With my system, once vacuum has been achieved there is no air flow anywhere in the system. Air does not have to flow through the venturi all the time.

Jeff

[Gordon Bartlett]

Perhaps my answer in #12 was not clear. With my system, once vacuum has been achieved there is no air flow anywhere in the system. Air does not have to flow through the venturi all the time.

Jeff

Good answer. I like that.
A lot.
So that leaves one more question:
Regarding this valve that you can hear clicking on and off, does the vacuum controller need a separate source of power to operate in addition to the power to the compressor?

[Spot]

Any reservations over using a compressed air tank as a vacuum reservoir?

That thread here awhile back illustrating the aftermath of a tank failure while pressurized gave me the heebiejeebies.

Those recycled oxygen tanks depicted in your previous thread to my thinking are engineered and constructed differently than the welded steel tanks I've known and used for compressed air reservoirs.

My non-expert understanding is that SCUBA, welding, hospital tanks are under much more internal pressure 500-5000 PSI versus an air compressor tank 100-200 PSI. Vacuum accumulators would be somewhere between normal air pressure and almost a complete vacuum. When you have a near vacuum, then there is approx. 15 PSI sea level air pressure pushing on the system containing the vacuum, trying to 'smoosh' it.

The O2 tanks would be overkill in a new condition. They might be a good repurpose for use as accumulator tanks, if they still hold pressure but perhaps failed some other element of a mandated inspection for high-pressure use.

[sp_clark]

When you have a near vacuum, then there is approx. 15 PSI sea level air pressure pushing on the system containing the vacuum, trying to 'smoosh' it.

What'all you've written fits with my understanding also yet I'm still left with a nagging uncertainty that what works for vacuum-bagging – the total atmospheric pressure on surface area – is also working to collapse an air tank being used as a vacuum reservoir, just as its pressure is used to keep stuff being glued in intimate contact during a 'bagged cure; 15 psi = a ton per square foot after all.

It's my lack of a grasp of whether 10 tons of pressure per square foot inside a tank (mine operates @ 140 psi max) designed to hold it in acts any different on the tank's structure compared to just 1 ton per square foot outside poses a risk. An exploding tank to my mind would present a greater hazard than a collapsing one.... Perhaps smaller is better here? A small air tank, having less surface area, would be better up to the task despite not having as much capacity for vacuum storage. Were I to add an accumulator into my vacuum system I'd feel more comfortable with the heavier-walled pressure tank's ability to sustain external pressures.

Anyone having a deeper understanding of what I'm describing's invited to add their knowledge here for our general edification.

[jpatrick]

Good answer. I like that.
A lot.
So that leaves one more question:
Regarding this valve that you can hear clicking on and off, does the vacuum controller need a separate source of power to operate in addition to the power to the compressor?

Yes. It plugs into the wall outlet. This is what I have: https://www.vacupress.com/product/to...acupress-pump/

It is no longer sold by Vacuum Pressing Systems. I would have chosen an electric vacuum pump when I bought mine, but back in those days, there was a significant difference in cost. I already had the compressor and an air dryer.

Jeff

[dbp1]

It seems like, summarizing this thread, that there _are_ systems that have some sort of valve so they only need air to counteract leaks (once the vacuum is reached), which may or may not be sold anymore, but the way the simple "vacuum bagging kit" venturi pumps work (e.g., that WEST starter kit), air is flowing the entire time, which is both possibly annoying, but also, requires the compressor can maintain a certain volume of airflow.

Either way, at least for me (who would need to buy a compressor & a venturi), seems like the $400 purpose made vacuum pump (or a used nicer one) is a smarter idea.

[JimConlin]

What are you doing that requires vacuum bagging?
My guess is than only a small minority of WBF participants are doing anything that needs it.
If it’s needed we’d like to learn more about your application.

[John Meachen]

It seems like, summarizing this thread, that there _are_ systems that have some sort of valve so they only need air to counteract leaks (once the vacuum is reached), which may or may not be sold anymore, but the way the simple "vacuum bagging kit" venturi pumps work (e.g., that WEST starter kit), air is flowing the entire time, which is both possibly annoying, but also, requires the compressor can maintain a certain volume of airflow.

Either way, at least for me (who would need to buy a compressor & a venturi), seems like the $400 purpose made vacuum pump (or a used nicer one) is a smarter idea.

For occasional use you might well get away with one of the basic vacuum pumps which are sold for evacuating car a/c systems.One of mine was picked up from a company that vacuum packed meat and which subsequently went out of business.

[dbp1]

What are you doing that requires vacuum bagging?
My guess is than only a small minority of WBF participants are doing anything that needs it.
If it’s needed we’d like to learn more about your application.

That's a good point, and actually, was the other thing I was going to ask:

I'm thinking about building one of these (http://forum.woodenboat.com/showthre...uckworks-Scout) this summer. Lightweight ply (4mm), with glass on either side (sometimes a couple layers), I think glassed flat on the bench first, and then probably another layer on the hull at the end. The goal is to be as light as possible (not only car-toppable, but carryable over reasonable distances myself).

The question is, would I actually expect to see a weight difference from bagging the glass onto the wood (and to the foam parts that exist: the seats, the foils), vs just using peel ply and trying to be careful (or peel ply, plastic sheeting, flat boards, and as much weight as I could find)? I wouldn't be trying to put a bag over the whole hull, just the pieces when they are flat on the bench, but figure at least 13 sq yards of 4oz glass (that's my estimate: the plans call for 25 linear yards of 50" wide cloth, I'm guessing that 10 of those could be done flat, i.e., within reach for my to vacuum bag).

Trying to do the rough math, I suspect the answer is _no_, or at least, the difference would be pretty marginal. The glass weighs ~52oz, at a 50:50 ratio, I'd have 52oz of epoxy, and at a 70:30 ratio, I'd have 22oz of epoxy, which is a difference of less than two pounds.

Assuming my math is right, that seems marginal: appreciated, but probably not worth trying, especially as it assumes I do it well, makes everything more complicated, etc.

[DeanP]

For my project (Beg-Miel), I vacuum-bagged everything I could.

Laminated plywood frames:

DSC01440.jpg

The centerboard well sides:

DSC01448.jpg

And of course, many bags were used to clamp the veneers for the cold-molded hull:

DSC01842.jpg

I used this pump for everything:

DSC01801.jpg

I found that I had to run the pump continuously to maintain a minimum of 15 inches Hg.

Good luck,

[JimConlin]

For my project (Beg-Miel), I vacuum-bagged everything I could.

<SNIP>

Gosh! That's a lot of consumables.

[Nicholas Carey]

[TL;DR — is a pressure vessel suitable for use as a vacuum reservoir?]

It's my lack of a grasp of whether 10 tons of pressure per square foot inside a tank (mine operates @ 140 psi max) designed to hold it in acts any different on the tank's structure compared to just 1 ton per square foot outside poses a risk. An exploding tank to my mind would present a greater hazard than a collapsing one.... Perhaps smaller is better here? A small air tank, having less surface area, would be better up to the task despite not having as much capacity for vacuum storage. Were I to add an accumulator into my vacuum system I'd feel more comfortable with the heavier-walled pressure tank's ability to sustain external pressures.

Anyone having a deeper understanding of what I'm describing's invited to add their knowledge here for our general edification.

Yes. People use all sorts of stuff as a vacuum reservoir: the Joe Woodworker plans (https://www.veneersupplies.com/docs/...Project-V4.pdf) specify to use schedule 40 PVC pipe to construct your vacuum reservoir (I'm a little dubious about the longevity of that, but that's just me).

Other people use all sort of stuff, including schedule 40 aluminum tube, no-longer-usable scuba tanks, fire extinguishers, or medical oxygen tanks, 20-lb propane tanks, air compressor or portable air tanks.

The failure mode is "beer-canning", and especially with steel or aluminum, it's just going to crumple.

[Nicholas Carey]

It seems like, summarizing this thread, that there _are_ systems that have some sort of valve so they only need air to counteract leaks (once the vacuum is reached), which may or may not be sold anymore, but the way the simple "vacuum bagging kit" venturi pumps work (e.g., that WEST starter kit), air is flowing the entire time, which is both possibly annoying, but also, requires the compressor can maintain a certain volume of airflow.

Either way, at least for me (who would need to buy a compressor & a venturi), seems like the $400 purpose made vacuum pump (or a used nicer one) is a smarter idea.

A possible issue with buying a used vacuum pump is that you don't know what they were using it for and what it might be contaminated with (something toxic, perhaps?). Venturi vacuum generators are cheaper (if you already have a compressor), though they can't reach the high vacuums that a good vacuum pump can reach. Also, vacuum pumps need maintenance. They require, for instance, special [expensive] vacuum oil.

And even with a vacuum pump, you generally want the controller and MAC/vacuum valve controlling it.

[sp_clark]

The failure mode is "beer-canning", and especially with steel or aluminum, it's just going to crumple.

Interesting, thanks. Sch40 PVC I'd avoid myself, SCH80's out there but less easily sourced.

The propane bottles are an interesting re-purposing, I think they're made to a higher quality level than your average compressor tank.

I've heard of folks using PVC pipe for dust collection, also of a couple who've had their systems explode from a static spark igniting the dust inside. Plastic shrapnel's no fun. I prefer metal ducting and a solid grounding for it.

A possible issue with buying a used vacuum pump is that you don't know what they were using it for and what it might be contaminated with (something toxic, perhaps?).

I bought a used vac pump decades ago from a used photo equipment outfit in Chicago:

8E9E0898-ACA0-41E7-8883-26915F16B8DD.jpeg

(Not mine but an identical item)

When I ran it the first time (1989?) it didn't pull much of a vacuum so it may not be functional but it appears parts for rebuilding are still available.

Always wanted to try my hand at 'bagging, seems a useful 'tool' for a number of applications.

[John Meachen]

That's a good point, and actually, was the other thing I was going to ask:

I'm thinking about building one of these (http://forum.woodenboat.com/showthre...uckworks-Scout) this summer. Lightweight ply (4mm), with glass on either side (sometimes a couple layers), I think glassed flat on the bench first, and then probably another layer on the hull at the end. The goal is to be as light as possible (not only car-toppable, but carryable over reasonable distances myself).

The question is, would I actually expect to see a weight difference from bagging the glass onto the wood (and to the foam parts that exist: the seats, the foils), vs just using peel ply and trying to be careful (or peel ply, plastic sheeting, flat boards, and as much weight as I could find)? I wouldn't be trying to put a bag over the whole hull, just the pieces when they are flat on the bench, but figure at least 13 sq yards of 4oz glass (that's my estimate: the plans call for 25 linear yards of 50" wide cloth, I'm guessing that 10 of those could be done flat, i.e., within reach for my to vacuum bag).

Trying to do the rough math, I suspect the answer is _no_, or at least, the difference would be pretty marginal. The glass weighs ~52oz, at a 50:50 ratio, I'd have 52oz of epoxy, and at a 70:30 ratio, I'd have 22oz of epoxy, which is a difference of less than two pounds.

Assuming my math is right, that seems marginal: appreciated, but probably not worth trying, especially as it assumes I do it well, makes everything more complicated, etc.

I would be shocked if you could achieve anything better than a 50:50 ratio.I have seen a laminate with a 41% resin content,it was produced in an autoclave with 90psi on the outside of the vacuum bag and was still felt to be a bit dry.On the other hand,you probably won't have air pockets and the laminate will occupy the smallest feasible volume.

[nedL]

I use an old dairy farm vacuum pump, it works like a charm. This past summer I finished up doing a rebuild on a plastic boat of my brothers and vacuum bagged some original glass panels back in place.
With some creativity there isn’t a need for tubes all over the place.

[dbp1]

I would be shocked if you could achieve anything better than a 50:50 ratio.I have seen a laminate with a 41% resin content,it was produced in an autoclave with 90psi on the outside of the vacuum bag and was still felt to be a bit dry.On the other hand,you probably won't have air pockets and the laminate will occupy the smallest feasible volume.

I'm assuming that even if my numbers were wrong, my conclusion was sound? i.e., if the actual number that a careful amateur using peel ply achieves is more like 1.5:1 or 2:1 (not 1:1), and vacuum gets that down to 1:1, the weight saved on a project like this is still going to only be a couple pounds max? Seems like probably my time (& money & materials) are better spent just trying to do the best job possible with peel ply than putting a whole lot of investment (and likely screwing some things up) into vacuum bagging.

[John Howland]

I used Joe Woodwoker to build my setup over 20 years ago. Lots of information, great resources, and kits and parts for the DIY.
https://www.joewoodworker.com/

[John Meachen]

I'm assuming that even if my numbers were wrong, my conclusion was sound? i.e., if the actual number that a careful amateur using peel ply achieves is more like 1.5:1 or 2:1 (not 1:1), and vacuum gets that down to 1:1, the weight saved on a project like this is still going to only be a couple pounds max? Seems like probably my time (& money & materials) are better spent just trying to do the best job possible with peel ply than putting a whole lot of investment (and likely screwing some things up) into vacuum bagging.

I would hope a careful amateur could get to 1.2:1 after a little practice.With peelply,perforated release film and good vacuum it isn't too hard to get to 1:1.How close you can get without all the technology is something best determined by using an offcut of the material and some accurate scales,before and after adding the glass.

[John Howland]

The smallest of pin hole will keep you from achieving the vacuum and can be vexing at first. More things to go wrong than right. Do a dry test at first.
I use polyurethane bags. Your will need a anti creeping glue.
Here is my set up.
Screen Shot 2023-02-04 at 3.26.32 PM.jpg