# Looking for input of other peoples opinions on the relationship of CFM and PSI?

• Mattsnooze
In summary: Since you're not specifying the flowrate, I'm going to assume you want to know how long the tank will last at a specific flowrate of 10 psi.Assuming you have a 1 gallon tank and it is at 250 psi, then you have 1 gallon x (250 psi x 144 in^2/gal)/(14.7 psi) = 2514 in^3 (it is the same number of molecules of air whether the pressure is high or low). If you then reduce the air pressure to 10 psi, the volume of the air that came out is 2514 in^3 x (10 psi)/(14.7 psi) = 1712 in^3. If you now divide
Mattsnooze
Hey. As the title dipicts, is there any relationship between CFM and PSI, linear or non-linear?

Any mathamatical relationship either?

CFM (cubic feet per minute) is a volume over time, and PSI (lbs per square inch) is a weight essentially over an area. So I take it, there's no real way to linearize it?

For instance, what's the compression ratio of a typical 12V compressor (high end)?

Also, if one compressor can produce 100 psi, and another 300 psi. Does the 300 psi unit have a higher compression ratio? I know the compression is usually pistons that very quickly pump in tinny volumes of air (30mm - 60+mm pistons) so I'm sure the size of the piston's determine how fast and how much max pressure they can compress (I think would be inversly perportional ( bigger pistons faster compression, but lower Max pressure?)would it not)?

Also, if a compressor and tank combo store let's say at 200PSI. If you tap a valve directly off the Tank, the second you open the valve air comes out at a Descending PSI rate would it not? So all that being said, the larger diameter the valve, the higher the CFM, but the lower the PSI? Or is PSI the same, but the tank just drains faster, and the PSI drops faster proportionatly?

(sorry for some of my spelling errors, I'm trying to type fast B4 I go to work)

I'm doing research on compressors and how the sizes of the chambers/ports/wheels all play a part in balancing CFM and PSI, depending on the application. So in order to work with them I would like to understand if there is any relationship between the two or not.
So from a design perspective I would like to learn how to design compressors for the variet of applications (from low cfm (high psi), to High CFM(high PSI) and the few combinations between.

I've read a crap load of formulas, and engineering documentation, so now I would like to understand/re cap with what I've learned and hopefully develop upon that.

The designing is for different methods of forced induction on automobiles. So even if you have automotive experience I would appreciate If you chimmed in. thanks in advance.

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Hey. As the title dipicts, is there any relationship between CFM and PSI, linear or non-linear?

Any mathamatical relationship either?

CFM (cubic feet per minute) is a volume over time, and PSI (lbs per square inch) is a weight essentially over an area. So I take it, there's no real way to linearize it?

There is no unit conversion between the two; however, there is a relationship between Volume and Pressure.

Depending on the process by which you went from the initial state to the final state of the gas, you can use the http://en.wikipedia.org/wiki/Gas_laws" .

Also, if a compressor and tank combo store let's say at 200PSI. If you tap a valve directly off the Tank, the second you open the valve air comes out at a Descending PSI rate would it not? So all that being said, the larger diameter the valve, the higher the CFM, but the lower the PSI? Or is PSI the same, but the tank just drains faster, and the PSI drops faster proportionatly?

If the tank has a fixed amount of air in it, and the compressor doesn't make up any that is lost, then the pressure will drop as the gas escapes. Generally, depending on the initial pressure, the flow rate will be relatively constant up until the very end of the discharge.

CS

Last edited by a moderator:
Mattsnooze said:
Hey. As the title dipicts, is there any relationship between CFM and PSI, linear or non-linear? Any mathamatical relationship either?
As has been mentioned, there is no direct relationship except those in the gas laws.

Mattsnooze said:
CFM (cubic feet per minute) is a volume over time, and PSI (lbs per square inch) is a weight essentially over an area. So I take it, there's no real way to linearize it?
What do you mean by linearize it? I am going to assume that you want a relationship between the two. Again, see the gas laws.

Mattsnooze said:
For instance, what's the compression ratio of a typical 12V compressor (high end)?
It will be different for every compressor.

Mattsnooze said:
Also, if one compressor can produce 100 psi, and another 300 psi. Does the 300 psi unit have a higher compression ratio?
It has a higher, overall pressure ratio.

In the end, if you look at any compressor data, you will see stated by the manufacturer the design flowrate at a specified pressure.

Mattsnooze said:
I know the compression is usually pistons that very quickly pump in tinny volumes of air (30mm - 60+mm pistons) so I'm sure the size of the piston's determine how fast and how much max pressure they can compress (I think would be inversly perportional ( bigger pistons faster compression, but lower Max pressure?)would it not)?
There are a lot of styles of compressors other than piston type. You can not infer information about the output based on just the size of piston. In general, the larger the piston, the greater the flow rate. Whether it can generate a higher pressure depends on a lot of other factors including the entire driving engine.

Mattsnooze said:
Also, if a compressor and tank combo store let's say at 200PSI. If you tap a valve directly off the Tank, the second you open the valve air comes out at a Descending PSI rate would it not?
The outlet condition is dictated by the pressure that the valve opens up to. If it opens to atmosphere then that will be the outlet pressure.

Mattsnooze said:
So all that being said, the larger diameter the valve, the higher the CFM, but the lower the PSI? Or is PSI the same, but the tank just drains faster, and the PSI drops faster proportionatly?
The tank drains faster.

One more Question. If I had a 1 gallon tank with a compressor that can compress at 250 PSI, and released it with a regulator at 10 psi, does that mean it could continue to push out 10PSI for (X) amount of time (the gallon worth) if it wasn't "chargeing/compressing"? So in comparison, if with a regulator I was pushing 50 psi out does that mean it would drain 5 times faster than if I only allowed 10 psi to release? (compare by blowing into open air at atmospheric pressure) In other words, if I regulate the psi down, does that extend the amount of time I could continue running at the selected PSI? ie. 250 psi max, but only letting out 10 psi for a duration?

Sorry for any incoherance, I'm having trouble putting my questions into words.

You are correct.

Cool. You guys have been a huge help. Ill post my design in a couple weeks with pictures so you guys see what I'm doing. I have all the info I need, I just have to finish putting the setup together, and flow bench it.

Take it easy, and have an e-beer on me :P
Peace!

Just make sure you pay attention to the thermodynamics. When a compressor compresses a gas, it gets hot. This is a potential source of energy loss and how a compressor does the compression and how a regulator does the expansion both make a difference in how much energy you lose.

## 1. What is CFM and PSI and how are they related?

CFM stands for cubic feet per minute and is a unit of measurement used to describe the volume of air flow. PSI stands for pounds per square inch and is a measurement of air pressure. These two units are related in that CFM is a measure of the amount of air being moved, while PSI is a measure of the force behind that air flow.

## 2. How does CFM and PSI affect the performance of air tools?

The CFM and PSI of an air tool are important factors in determining its performance. The CFM determines how much air the tool can use to operate, while the PSI determines the strength of the air flow. A higher CFM and PSI will result in better performance and efficiency of the air tool.

## 3. Is there an ideal ratio of CFM to PSI for air tools?

There is no specific ratio of CFM to PSI that is considered ideal for all air tools. The required CFM and PSI will vary depending on the type and size of the tool. It is important to match the CFM and PSI requirements of the air tool to the capabilities of the air compressor to ensure optimal performance.

## 4. How does the size of the air compressor affect the relationship between CFM and PSI?

The size of the air compressor does have an impact on the relationship between CFM and PSI. A larger air compressor will typically have a higher CFM and PSI, allowing it to power larger and more demanding air tools. However, it is important to note that the CFM and PSI ratings of an air compressor can also be affected by factors such as tank size and motor power.

## 5. What are some common mistakes to avoid when considering the relationship between CFM and PSI?

One common mistake to avoid is using an air tool with a higher CFM and PSI requirement than what the air compressor can provide. This can result in poor performance and potentially damage to the tool. It is also important to regularly maintain and clean the air compressor to ensure it is operating at its optimal CFM and PSI levels.

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