# Calculating CFM

1. Oct 13, 2007

### CSI GIL

I am currently getting setting up a project where we will be abrasive blasting 48" dia steel pipe in 80' lenghts utilizing robotic equipment producing 1600 CFM @ 160 PSI. We are endevouring to identify the CFM requirements for our dust collection system which will be required at one end of the pipe.

So my question is; what impact does the PSI have on CFM and as we will be creating negative air inside the pipe, what impact will the increased velocity have on the CFM requirements?

2. Oct 13, 2007

### FredGarvin

Basically what you'll need to do is convert your flow conditions of 1600 cfm @ 160 psi to standard conditions, i.e. SCFM. That is the same flow if you were to expand it to ambient environmental conditions. Once you do that, then you can go to your dust collection system vendor and give them your flow in SCFM. They should also rate their equipment in SCFM. That way you are able to compare apples to apples and get the proper ventilation system.

It gets a bit tricky when dealing with flows like this because each machine gives a spec based on certain conditions. The robotic equipment make the assumption that your overall delta P for the flow will be 160 psi to atmosphere. Well, you're now not going to atmosphere. You're going a bit less. That will actually drive the CFM number up a bit because the flow will be allowed to expand a bit more. You'll still have the same mass flow, but volumetric flow will go up a bit. That is why using a reference state is so important.

3. Oct 25, 2007

### precog

Calculating Airflow in a Fan

I am trying to calculate the airflow in my current system and cross reference that with the airflow that I should be producing from the information from the manufacturer. The problem is the only information I have to go off on is the amps given off by the equipment and the HP of the motor.

Any assistance would be appreciated

4. Oct 27, 2007

### Tagline

Thing is I've never been able to find a definition of SCFM that everyone agrees with. Some claim, as I remember, the air should be like 68° F, 36% Relative Humidity measured at sea level (Ah! a trip to the ocean). Others say the RH should be zero and still others 50%, on and on. Hmm, makes me wonder what 'Standard' really means. Do you happen to know what the actual standard is?

5. Oct 27, 2007

### FredGarvin

You are correct that there are many different versions of what a standard day is. The one that I have seen most often is 14.696 psi, 60°F and 0% RH. The thing is is that it is an easy calculation to do and that you simply need to coordinate with any supplier as to what standard they are using.

6. Oct 29, 2007

### Tagline

Agree ... obviously you have to use the same calculation the vendor uses but I was wondering if there is a standard writen in stone anywhere that is appropriate for designing blowers and such. I have a lot of military standards from when I was designing Apache's but not one for that and I don't have access to IHS any more.

7. Oct 29, 2007

### FredGarvin

Nope. Unfortunately there is no accepted, set-in-stone standard. It is silly. The one thing I have never done is to see what the deviation in results is by going to all the different versions.

8. Oct 29, 2007

### stewartcs

As long as you define what pressure and temperature you are using in your calculation, then it doesn't matter (assuming they're reasonable values that simulate the specific enviorment you are in). Different mathematical models of the same natural phenomenon will naturally produce different results (similar, but not the exact same). So long as you state your assumptions, in this case, which STP you are using, I think you'll be ok.

Most people use whatever STP they are governed by. I use either ISO or NIST. However, I've seen different STP's within the NIST numbers. So, like I said, just make sure you state your assumptions in your results clearly.

9. Oct 30, 2007

### Tagline

I can see why you've never tested different basic assumptions. Guess there's not many manufacturing companies or engineering firms located at sea-level ... well ... less you are from Louisiana maybe.

Could be the reason for no 'written-in-stone' basic assumptions is because they would, I think, depends on the application and the operating environment. For example: a blower to be used in an SR71 Blackbird would be based on much different assumptions than, say, a blower in a steel mill. Hard to get around actual test data being the best solution to nagging uncertainty I guess.

10. Oct 30, 2007

### Tagline

Yep, like I always say: Good engineers make good guesses.