CFM vs. PSI when using venturi

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GouletPens

GouletPens

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I was too busy checking out the ladies during my physics class in high school, so I'm not that up on venturis. I have the typical venturi vacuum setup on my PV pot that everyone's familiar with, and I just got a new air compressor. I've been using a 6-gallon pancake compressor with 150 PSI and 1.6 CFM and I've had to draw a vacuum in stages b/c the pressure would drop in the compressor before I could draw my 28" vacuum. I'd have to do about 4 or 5 cycles to get a full draw. I recently got a 60-gallon 2-stage compressor with 135 PSI and 11.3 CFM. I haven't used it for drawing vacuum yet, but I imagine it will blow the old one away. It just got me thinking, when using a venturi valve, is it the PSI or the CFM that makes a bigger difference in the ability to draw a vacuum? My father and I have a power washing business and I know from experience that VOLUME of water is much more important than PRESSURE. I imagine it's similar for air, but I'd love to hear everyone's input on the matter!
 
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I believe it's the pressure. Your compressor's ability to maintain that pressure at a given flow rate is the limiting factor.
 
Hmmm...conflicting info.....I imagine that pressure AND volume are both factors, much like flowing water. But what I'm really curious to know is which one is more important.
 
pipecrafter said:
My vacuum pump requires just shy of 5 CFM at 90PSI. I've never tried to see if more PSI would make it go faster, since it pulls a vacuum so darn fast anyways.
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See, b/c my pancake before was losing pressure (and volume, for that matter) so quickly when using a venturi, I didn't have any controlled environment in which to test changing pressure or volume.

I know I'll be able to experiment a bit with my new compressor but I have yet to wire the 220V line and run the air hoses in the shop b/c my wife has me working on a bathroom project, but that's a whole other can of worms!:eek:
 
Here's a little app note on eductors. If you are evacuating a vessel, you probably want higher suction and so higher inlet pressure. PS: I am not a fluid systems engineer, I just have a little experience with controls for vacuum systems using eductors.
 
GouletPens said:
Hmmm...conflicting info.....I imagine that pressure AND volume are both factors, much like flowing water. But what I'm really curious to know is which one is more important.
Click to expand...

I think this is like an algebra equation, where the two unknowns are both on the same side of the equation, locked in constant relationship:
vacuum = pressure * volume

Change either pressure or volume, and the vacuum is reduced.

So, is one more important? I'd say that pressure might be, since without it, a venturi pump won't work. If your CFM is low, it will still work, just not consistently or reliably. However, if supply-side pressure drops, the venturi won't work at all.
 
pipecrafter said:
I think this is like an algebra equation, where the two unknowns are both on the same side of the equation, locked in constant relationship:
vacuum = pressure * volume

Change either pressure or volume, and the vacuum is reduced.

So, is one more important? I'd say that pressure might be, since without it, a venturi pump won't work. If your CFM is low, it will still work, just not consistently or reliably. However, if supply-side pressure drops, the venturi won't work at all.
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That makes sense, but I wonder if the vacuum and pressure are equally constant, or is changing one will have more of an impact than changing the other....

I'm just thinking say I'm moving 1.6 CFM at 90 PSI now, then with my new compressor I'll be moving 11.3 CFM (max) at 90 PSI, both through the same venturi, the pressure going through the venturi would have to be greater b/c the amount of air going through the same constriction would cause a greater buildup of pressure, then when going through the valve would create a greater vacuum??? I don't know, it's this kind of stuff that always went in one ear and out the other in school. I'm MUCH more of a 'practical application' kind of learner!!!:beat-up:
 
GouletPens said:
That makes sense, but I wonder if the vacuum and pressure are equally constant, or is changing one will have more of an impact than changing the other....

I'm just thinking say I'm moving 1.6 CFM at 90 PSI now, then with my new compressor I'll be moving 11.3 CFM (max) at 90 PSI, both through the same venturi, the pressure going through the venturi would have to be greater b/c the amount of air going through the same constriction would cause a greater buildup of pressure, then when going through the valve would create a greater vacuum??? I don't know, it's this kind of stuff that always went in one ear and out the other in school. I'm MUCH more of a 'practical application' kind of learner!!!:beat-up:
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You are never going to pull more than 29.925" of vacuum. You could have a 150 hp 1000 gallon compressor that put out 2000 cfm and you still would never get past 29.925" of vacuum. The max vacuum that anyone could theoretically pull would be 29.925 inches of mercury vacuum or 1 atmosphere. Of course that is exactly at sea level, so if you are 1000 ft above sea level you will barely register 28". The higher you go up in elevation, the less vacuum you will be able to register. That is basic physics 101

The only difference you will see will be that your new compressor will pull vacuum much faster.
 
jleiwig said:
You are never going to pull more than 29.925" of vacuum. You could have a 150 hp 1000 gallon compressor that put out 2000 cfm and you still would never get past 29.925" of vacuum. The max vacuum that anyone could theoretically pull would be 29.925 inches of mercury vacuum or 1 atmosphere. Of course that is exactly at sea level, so if you are 1000 ft above sea level you will barely register 28". The higher you go up in elevation, the less vacuum you will be able to register. That is basic physics 101

The only difference you will see will be that your new compressor will pull vacuum much faster.
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I'm not trying to pull more vacuum, I just want to pull it without having to wait for the compressor to refill 4 or 5 times while my alumilite is busy setting up in the pot while I'm casting my worthless wood. Time is of the essence there, so speed of vacuum is important. Either way it's irrelevant talking about my equipment cuz I have what I have and I'm positive that have 10 times the CFM and 10 times the tank size is going to make a difference in the way the vacuum draws...I'm still curious to know exactly the relationship between pressure and volume with the bernoulli principle.
 
GouletPens said:
I'm just thinking say I'm moving 1.6 CFM at 90 PSI now, then with my new compressor I'll be moving 11.3 CFM (max) at 90 PSI, both through the same venturi, the pressure going through the venturi would have to be greater b/c the amount of air going through the same constriction would cause a greater buildup of pressure, then when going through the valve would create a greater vacuum??? I don't know, it's this kind of stuff that always went in one ear and out the other in school. I'm MUCH more of a 'practical application' kind of learner!!!:beat-up:
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I think you almost answered your own question but got a little off on your last assumption. The statement "the pressure going through the venturi would have to be greater b/c the amount of air going through the same constriction would cause a greater buildup of pressure" is where you went astray. It will not cause a greater buildup of pressure, it will, I believe, cause the pressure difference to occur faster due to the faster moving air (higher CFM). Make sense?
 
It's not actually an increase in pressure - 90PSI is 90PSI. If your compressor's regulator is set to 90PSI it will maintain that pressure until the air exits the venturi valve.

CFM will also remain constant as long as the PSI remains constant. CFM will be a fixed value based on orifice diameter and PSI - you can only push so much air through a given hole at a given PSI, and you can't change that volume without altering either PSI or orifice size.

Given that, you may *think* you're pushing 90PSI at 1.6CFM because that's what your other compressor said it was doing - but that's a false measurement, That 1.6CFM is the *sustained* volume of air that the small compressor's pump can maintain. Like it or not, the venturi is going to consume it's required 5CFM, though it may exhaust the air storage tank in 10 seconds. The CFM doesn't change - Cubic Feet Per Minute is a constant. If the venturi will consume 5 CFM at 90PSI, it will consume 5CFM for the 10 seconds the small compressor's tank can keep up with the flow - which is why you found you had to evacuate the pressure pot in stages.

The main thing you're gaining by getting a bigger compressor with bigger holding tank is the ability to maintain the *flow rate* (CFM) over a much longer period, if not indefinitely. The CFM rating on compressors is a measurement of the actual compressor pump's ability to deliver the needed volume of air. All things being equal, given a large enough storage tank, you could maintain 90PSI at 5 CFM for a very long time - I just wouldn't want to have to fill that tank with a compressor pump from a pancake-style compressor. :biggrin:
 
pipecrafter said:
It's not actually an increase in pressure - 90PSI is 90PSI. If your compressor's regulator is set to 90PSI it will maintain that pressure until the air exits the venturi valve.

CFM will also remain constant as long as the PSI remains constant. CFM will be a fixed value based on orifice diameter and PSI - you can only push so much air through a given hole at a given PSI, and you can't change that volume without altering either PSI or orifice size.

Given that, you may *think* you're pushing 90PSI at 1.6CFM because that's what your other compressor said it was doing - but that's a false measurement, That 1.6CFM is the *sustained* volume of air that the small compressor's pump can maintain. Like it or not, the venturi is going to consume it's required 5CFM, though it may exhaust the air storage tank in 10 seconds. The CFM doesn't change - Cubic Feet Per Minute is a constant. If the venturi will consume 5 CFM at 90PSI, it will consume 5CFM for the 10 seconds the small compressor's tank can keep up with the flow - which is why you found you had to evacuate the pressure pot in stages.

The main thing you're gaining by getting a bigger compressor with bigger holding tank is the ability to maintain the *flow rate* (CFM) over a much longer period, if not indefinitely. The CFM rating on compressors is a measurement of the actual compressor pump's ability to deliver the needed volume of air. All things being equal, given a large enough storage tank, you could maintain 90PSI at 5 CFM for a very long time - I just wouldn't want to have to fill that tank with a compressor pump from a pancake-style compressor. :biggrin:
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Thank you, this puts a lot of pieces together for me. Thanks everyone for being so patient with me on this and giving me such good information.

I'm still a little miffed about the relationship between pressure and volume in a venturi setup....if you change the pressure but keep the volume constant, does that have a different impact than changing the volume and leaving the pressure constant? I tried to research the bernoulli principle and venturi system and I got a bunch of formulas that I don't really understand. Can anyone just spell it out?
 
pipecrafter said:
It's not actually an increase in pressure - 90PSI is 90PSI. If your compressor's regulator is set to 90PSI it will maintain that pressure until the air exits the venturi valve.

CFM will also remain constant as long as the PSI remains constant. CFM will be a fixed value based on orifice diameter and PSI - you can only push so much air through a given hole at a given PSI, and you can't change that volume without altering either PSI or orifice size.

Given that, you may *think* you're pushing 90PSI at 1.6CFM because that's what your other compressor said it was doing - but that's a false measurement, That 1.6CFM is the *sustained* volume of air that the small compressor's pump can maintain. Like it or not, the venturi is going to consume it's required 5CFM, though it may exhaust the air storage tank in 10 seconds. The CFM doesn't change - Cubic Feet Per Minute is a constant. If the venturi will consume 5 CFM at 90PSI, it will consume 5CFM for the 10 seconds the small compressor's tank can keep up with the flow - which is why you found you had to evacuate the pressure pot in stages.

The main thing you're gaining by getting a bigger compressor with bigger holding tank is the ability to maintain the *flow rate* (CFM) over a much longer period, if not indefinitely. The CFM rating on compressors is a measurement of the actual compressor pump's ability to deliver the needed volume of air. All things being equal, given a large enough storage tank, you could maintain 90PSI at 5 CFM for a very long time - I just wouldn't want to have to fill that tank with a compressor pump from a pancake-style compressor. :biggrin:
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Very well defined response. I agree completely. I just couldn't come up with the right words.
 
I'm trying to understand why you would rather get a vacuum than use pressure. :confused:
With a vacuum the best you could get would be 1 bar lower than normal atmospheric pressure, but your tank can cope with 6 or 7 bar overpressure.

So the choice would be
1. Use vacuum ( 1 bar difference)and hope that the trapped gas ( air ) expands enough to escape and float to the surface of the quick setting resin.

or...

2. Crush the little blighters into microscopic size with a good wollop of 6 or 7 bar instantly from that nice big storage tank:biggrin:


I always figured that vacuum was cool for de-gassing unmixed resin or stabilizing and pressure was best for casting:wink:
 
skiprat said:
I'm trying to understand why you would rather get a vacuum than use pressure. :confused:
With a vacuum the best you could get would be 1 bar lower than normal atmospheric pressure, but your tank can cope with 6 or 7 bar overpressure.

So the choice would be
1. Use vacuum ( 1 bar difference)and hope that the trapped gas ( air ) expands enough to escape and float to the surface of the quick setting resin.

or...

2. Crush the little blighters into microscopic size with a good wollop of 6 or 7 bar instantly from that nice big storage tank:biggrin:


I always figured that vacuum was cool for de-gassing unmixed resin or stabilizing and pressure was best for casting:wink:
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You're right, I should have clarified. I do cast under pressure with WW, but I degas under vacuum and I also stabilize with Ultraseal. I use my pot for both. The little pancake did okay for pressurizing the tank, but the vacuum was a nuissance. The new compressor will be overkill for both!!
 
Brian, unless you are degassing mixed resin then surely vacuum time is not an issue.? Your compressor could run all day without any bad effects.
I've only used Alumilite once and it started to set long before I would have been able to get the lid on my pot, let alone draw a vac. I think if I had attempted it, then I would have ended up with huge trapped 'expanded' bubbles. :eek:
I'm not sure if I even caught it in time for the overpressure to make a difference.:confused: But luckily the result was bubble free anyway.

I'm not familiar with Ultraseal. Does it set hard quickly? I thought that you could 'sink' your wood in the stuff and still have plenty of time to do the vac-pressure-vac-pressure sequence several times to make the stuff penetrate as much as possible to stabilize?. But I've never tried to stabilize anything anyway:biggrin:

As for the PR.......it stinks too much for me and I believe it rots the brain, so I now get all my stuff from the Princess, Charlie or Jeff:biggrin: We can tell how it's effected them!!!!:biggrin: My kit is pretty much collecting dust now:redface:
 
GouletPens said:
I'm still a little miffed about the relationship between pressure and volume in a venturi setup....if you change the pressure but keep the volume constant, does that have a different impact than changing the volume and leaving the pressure constant? I tried to research the bernoulli principle and venturi system and I got a bunch of formulas that I don't really understand. Can anyone just spell it out?
Click to expand...

Changing pressure, but keeping volume constant, would require also changing the orifice size in the venturi. For instance, if you want to keep 5CFM, but also want to reduce the pressure to 60PSI, you would have to enlarge the orifice. On the same hand, if you want to keep 5CFM, but increase pressure to 150PSI, you would need to reduce the orifice.

I'm not all that up on my fluid dynamics, so I'm not sure what the ideal configuration is for venturi pumps like this, but I can only assume that changing the Harbor Freight pumps most of us use would be beyond the scope of "worthwhile past time" for almost anyone. :wink:
 
pipecrafter said:
Changing pressure, but keeping volume constant, would require also changing the orifice size in the venturi. For instance, if you want to keep 5CFM, but also want to reduce the pressure to 60PSI, you would have to enlarge the orifice. On the same hand, if you want to keep 5CFM, but increase pressure to 150PSI, you would need to reduce the orifice.

I'm not all that up on my fluid dynamics, so I'm not sure what the ideal configuration is for venturi pumps like this, but I can only assume that changing the Harbor Freight pumps most of us use would be beyond the scope of "worthwhile past time" for almost anyone. :wink:
Click to expand...
Okay, I understand that part. But say you were to do either of these...increase the pressure or increase the cfm.....which one would draw a vacuum faster? That's what I'm really trying to understand, what is the end result of the drawn vacuum in relation to the pressure and volume of air moving through the venturi?
 
GouletPens said:
Okay, I understand that part. But say you were to do either of these...increase the pressure or increase the cfm.....which one would draw a vacuum faster? That's what I'm really trying to understand, what is the end result of the drawn vacuum in relation to the pressure and volume of air moving through the venturi?
Click to expand...

I am by no means the expert on this but from what I understand is that if you put more CFM past the suction, it will pull out the air and non-condensable gasses faster. So, I think greater CFM = faster vacuum draw.

BTW, if you increase pressure, it will slightly increase the CFM flow which will draw a vacuum faster also. But not as fast as just changing CFM.

Take this for what it is worth and what you paid for it.......
 
GouletPens said:
Okay, I understand that part. But say you were to do either of these...increase the pressure or increase the cfm.....which one would draw a vacuum faster? That's what I'm really trying to understand, what is the end result of the drawn vacuum in relation to the pressure and volume of air moving through the venturi?
Click to expand...

What I'm trying to say is that, in order to change the CFM, you need to modify the venturi or pressure - you can't just say "change the CFM" and expect it to work. It's all immutably related, and you can't change one thing without changing another.

If you change the pressure, the CFM will change - that's the physics of the situation. If you want to change the CFM, you *must* change the pressure - unless you're going to modify the orifice on the vacuum pump. CFM is the flow rate directly and inexcricably dependent upon pressure and orifice size. If you want to change CFM, you must change one of those other items.

So, want to draw a vacumm very, very quickly? Increase pressure. HOWEVER, the venturi will probably need to be modified to accomodate the increased pressure, since you can easily overpower the valve because most are designed to operate within a set of specific pressures up to a maximum. Lower pressure will usually work without problem, to slow down evacuation, but trying to speed up evacuation by increasing pressure may simply overwhelm the venturi, and end up blowing motive gas (from your air compressor) into the chamber you're attempting to evacuate.

The actual application varies by what you're trying to evacuate - liquid, gas, or particulate matter. The document that Jeff linked to will contain a lot of information about all of that.

What's the ideal pressure? That depends on the specific venturi valve design. My suggestion would be to stick with the pressure requirements of whatever venturi pump you've bought. Increasing pressure slightly may show a faster evacuation of the chamber, but I wouldn't expect it to be able to cope with much beyond about 10% increase in motive pressure.
 
Volume, volume, volume. They way the venturi effect works is that the air moving causes the vacuum to be created. The air is moving from a higher pressure to a lower pressure area and due to the movement of air across a orifice it creates a vacuum in the container that the air is drawn from. The reason small compressors do not work well is that they can not move enough volume of air across the device to create enough vacuum before the volume of air drops as the pressure drops. If you move 50 cf of air across at 10 psi or 100 psi it does not matter. It is the rate of air movement over time not how much pressure is behind it.

Think about the system as an Y. The air flows down the right part of the Y and down the base. The left side of the Y is hooked up to your container that you want to create a vacuum in. As sir moves across the left opening where the right and left sides join and down the Y it pulls air out of the container,out of the left side of the Y. It is the movement of the air, not the pressure behind it that creates the vacuum. Double the pressure and keep the air volume flow the same and you get the exact same vacuum. Double the volume of air flow but decrease the pressure and you get increased vacuum. The reason a larger air compressor makes the vacuum faster is the it can maintain the high volume of air flow longer. It does not need to stop and catch up like your old small compressor did.
 
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