Need Help: Formula Needed

[Wilson]

What up everyone, seems like a very intelligent bunch of folks on the site. Hoping that someone may be able to help me out with a problem we are currently having with a project.

How or what formula can I use to get the CFM rating of a motor from a PSI rating?

Thanks for any help and or info in the matter.

Note: The CFM rating is not posted on the motor/tubing/Hp/etc: Basically with this question answered we would know how much air we need to suck out of a container, and which vacuum/purge system to purchase. This is and would be of great help to me and my group on this project.
Thanks again

-Wilson

[evolution]

With just a PSI rating, you can't figure out the CFM; PSI is a pressure measurement, whereas CFM is a volume/time measurement. That's why when you buy a compressor, you'll see two ratings, both the CFM and the PSI.

Do you have any additional information that you can give us, such as the HP? Also, is the container you are working with at atomospheric pressure? Knowing this information may help lead to an answer.

[Jay H 237]

CFM will also be dependant on the ID of the inlet/outlet of the pump.

For instance, you could use the exact same motor but hook it to a different pump or use different ID tubing/piping and have different CFM.

Do you have any futher info you can give us on this project?

[Wilson]

Thanks for such a quick reply.

I understand its not going to be a simple PSI to CFM conversion. However there must be a point where these PSI and CFM meet. Example: PSI to HP then convert HP to CFM.

No we don't know the HP we need, nor the PSI. That’s my dilemma.
By estimated figures we need to be able to suck about 6-11Ft³.
This is not a set in stone figure. The container is in the open environment (pollen, humidity, etc.)(Size of container: 40ftx8.5ftx8ft), and need the formula to figure out the PSI we need within the tubing and that would help us to gain the Hp needed to acquire that PSI or vise-versa.

I'm not the air flow expert in this matter; I came across this site by chance and in hopes that someone may be able to answer this question for me. But I will get one of my other guys to get on this site first thing in the morning and have him take a look and reply back with his knowledge of what is needed. He maybe able to word this better then me, so that you would be able to gain a better understanding of what it is I am asking for.

Thanks
-Wilson

Edit: The inlet/outlet of the motor is which ever works, the size so long it’s not a huge motor. We have been working with 1/4in, 1/2in, 3/4in, and 1'in. Pressure in all cases seems to hold from start to finish in about 30 ft worth of tubing. We are also looking for just one consistent CFM rating all the way threw; we are not trying to change it at any given point. Valves are a problem we are also dealing with losing about 2 PSI per valve.

[Rickertsen2]

In the case of a vacuum system like this there are two factors you need to determine before choosing a system. They are how strong of a vacuum do you need, and in what time do you need to develope this vacuum. Once you figure out how strong the vacuum you need is, look at the datasheet for the pump system. They will usually have a table that will say "it will take X long to pump Y volume of air down to Z pressure".

If you want to determine CFM from PSI, you will need some additional piece of information. You could always buy a flow meter.

[Swampdude]

It sounds like you're trying to pull a negative pressure on a trailer? One of the questions would be how fast do you need to do it, and what is the desired negative pressure? When you say you want to suck the air out, you really need a goal as to what condition you want to leave the space in. Are you also trying to remove oxygen, or get an inert environment etc?
Any pump/hp will eventually get you to some level of negative pressure, given its required length of time. You just need to decide the time, and the desired pressure - or am I missing something?

[Wilson]

Originally Posted by Rickertsen2

Could you please tell us the end application for this and i think we could be alot more helpful.

Just need to take air out of a container.

Thanks

-Wilson

[Wilson]

Rickertsen2
In the case of a vacuum system like this there are two factors you need to determine before choosing a system. They are how strong of a vacuum do you need, and in what time do you need to develope this vacuum. Once you figure out how strong the vacuum you need is, look at the datasheet for the pump system. They will usually have a table that will say "it will take X long to pump Y volume of air down to Z pressure".

Swampdude
It sounds like you're trying to pull a negative pressure on a trailer? One of the questions would be how fast do you need to do it, and what is the desired negative pressure? When you say you want to suck the air out, you really need a goal as to what condition you want to leave the space in. Are you also trying to remove oxygen, or get an inert environment etc?
Any pump/hp will eventually get you to some level of negative pressure, given its required length of time. You just need to decide the time, and the desired pressure - or am I missing something?

Rick:
Without, the PSI, I can’t answer the HP we need. And without the HP we can’t figure out what amount of PSI is needed to get the results we are looking for. The time, we want a large amount of air, in seconds. 680cubic ft was my estimate of the amount of air sucked in a min. Again I’m no expert, just estimation. We have plenty of datasheets on pumps as you are suggesting, however without a formula to guide us in the proper direction to put factual numbers, this is all guess work.

Swamp:
Your right swamp, negative pressure on a trailer. The faster the better, as far as speed is concerned. We want everything completed in a full min. Which means we need a large volume of air, coming in rapidly in seconds to do so. We don’t know the desired amount of negative pressure, as that’s why I’m requesting for the formulas to be able to get that bit of information and/or also from the HP rating aspect of this dilemma. I wish to leave the space intact and not cause the contents inside the trailer damage. We are trying to get the inert environment. Your right on the time aspect, I’m sorry I left that bit of information out. But this post should cover that aspect. As far as the desired pressure/horsepower is the problem.

Thanks again everyone, for taking the time to read thus far and help with this matter.

-Wilson

[Swampdude]

Ok, with that info, it still boils down to the desired pressure. For instance, you could put a fan pointing out a hole and as soon as you turn it on, voila - negative pressure in the room. Not much, but less than ambient. So if your next question is whats a good pressure before the trailer crushes like a beer can? It sounded like you may be contemplating letting this pump run regularly to maintain this pressure over time? Also depending on the contents you may want to remove oxygen, and not necessarily draw a negative pressure. Because this isn't going to have the effect of a ziploc bag since the volume of space will not reduce hence displacing atmosphere. You may be better suited to "fill" the room with nitrogen, a great purging resource that displaces oxygen.
Besides all that, lets say you wanted to get the trailer to -20psi in 1 minute, and you already gave the volume of 40'x8'x8.5'.
2 options from here are:
1. you oversize the pump and use a pressure regulator to regulate with a +/- of acceptable range - which would have a big motor and little pump
2. you size the pump to run constant -20psi - which would have a big pump little motor

I assume you'd want option 1 - for efficiency?

[Wilson]

Originally Posted by Swampdude

Ok, with that info, it still boils down to the desired pressure. For instance, you could put a fan pointing out a hole and as soon as you turn it on, voila - negative pressure in the room. Not much, but less than ambient. So if your next question is whats a good pressure before the trailer crushes like a beer can? It sounded like you may be contemplating letting this pump run regularly to maintain this pressure over time? Also depending on the contents you may want to remove oxygen, and not necessarily draw a negative pressure. Because this isn't going to have the effect of a ziploc bag since the volume of space will not reduce hence displacing atmosphere. You may be better suited to "fill" the room with nitrogen, a great purging resource that displaces oxygen.
Besides all that, lets say you wanted to get the trailer to -20psi in 1 minute, and you already gave the volume of 40'x8'x8.5'.
2 options from here are:
1. you oversize the pump and use a pressure regulator to regulate with a +/- of acceptable range - which would have a big motor and little pump
2. you size the pump to run constant -20psi - which would have a big pump little motor

I assume you'd want option 1 - for efficiency?

If it can suck 680cubic ft, I believe we will be ok. Yes we want this pressure to be consistent, weather it means on and off toggling or consistent pressure. Oxygen will always remain. Great suggestion on the nitrogen, We have already come up with other ways of this as well. I will leave that for another thread. From all the calls placed to different manufactures of vacuum motors seems that the largest tubing usable, would be ¾’in.

I will throw these 2 options to the guys in the morning and reply back a.s.a.p. I guess its as you said, we are going to have to determine a PSI to aim at to achieve and look at the results it provides to find out weather its adequate.

Yes efficiency would be most important to me, however the others may think differently. We all have different aspects of this project and tend to think in different directions, at times.

[Swampdude]

If you're setting the pump outside and sucking i can see how the hose collapsing would be a problem, but could you put the pump inside the trailer and blow out? Then the hose stays open And nobody steals the pump!

Or run pipe instead of hose.

[Wilson]

Originally Posted by Swampdude

If you're setting the pump outside and sucking i can see how the hose collapsing would be a problem, but could you put the pump inside the trailer and blow out? Then the hose stays open And nobody steals the pump!

Or run pipe instead of hose.

LOL, actually no it must be outside. Actually the hose at 1'in ID is said to have problems, and the pump itself would be too huge for this project in mind. The 3/4 ID limits us to the amount of power we are able to achieve. We have goals in mind, how much we will meet them will be dependant on what gets the job done. The goals are such as: speed, noise level, power, compact, weight, etc.

The tubing collapsing has not been an issue, knock on wood. We have been using Teflon with an assortment of ID and OD for testing, so far they have all held up to par.

In the final product of this design it will have to suck/purge which still puts us in another boat of its self. So we aim at one goal before the end picture.

BTW: As a side note nice bike. I have a sports bike; in the future I should have me a cruiser. More of a fatboy / pitbull guy.

[sanddrag]

negative pressure? Is there such thing? I thought zero pressure like the true vacuum of space is the least pressure you can have.

[Wilson]

Originally Posted by sanddrag

negative pressure? Is there such thing? I thought zero pressure like the true vacuum of space is the least pressure you can have.

Yes there is such a thing, there is +/- and 0 pressure. If you deal with a few or even call manufactures that deal with items such as these. They will tell you they will work at +/-/0 in X amount of elevation, Y amount of humidity, Z amount of temperature.

Positive pressure is a pressure within a system that is greater than the environment that surrounds that system. Consequently if there is any leak from the positively pressured system it will ingress into the surrounding environment.

However, use is made of positive pressure to ensure there is no ingress of the environment into a supposed closed system. A typical example of the use of positive pressure is the location of a habitat in an area where there may exist flammable gasses such as found on an oil platform.

Hospitals may have positive pressure rooms for patients with compromised immune systems. Air will flow out of the room instead of in, so that any airborne germs that may infect the patient are kept away.

Negative pressure is a term used to describe a pressure less than that of a surrounding fluid (such as the air). The term comes from Gauge pressure pressure gauges, which measured a pressure against air pressure. These were extensively used in steam engines because the pressure was relative to air pressure, which was the final pressure of the exhaust steam. The standard pressure gauge design (Bourdon tube) measures gauge pressure by default, making this an easy design choice. Negative pressure meant that the pipe or vessel being measured was below atmospheric pressure.

Except in special circumstances, the lowest possible pressure is a vacuum, which corresponds to -1 atm pressure. Actually what is meant is that there is a pressure difference between a region of high pressure and another region of low pressure. A room with a lower pressure than its surroundings will cause air to flow into the room from the outside whenever a door or window is opened. It is often used in a hospital setting to quarantine people afflicted with highly contagious, deadly, airborne diseases. Some player pianos used vacuum to drive the mechanism and measured things in negative inches of water pressure.

[sciguy125]

Originally Posted by sanddrag

negative pressure? Is there such thing? I thought zero pressure like the true vacuum of space is the least pressure you can have.

It's like energy: it depends on your reference. I could have 0 potential energy with respect to the ground. But, if someone digs a hole next to me, I have some potential energy with respect to the bottom of the hole.

Negative pressure usually refers to negative with respect to atmospheric.