Yet another question about FAD vs SCFM and effects of ambient conditions 1

[MidnightRover]

It’s not like there isn’t a lot of information out there on this, but I just don’t seem to get it.

I have a compressor I’m looking to use that is rated at 30 SCFM at 110 psi. What I want to know is my air flow at ambient conditions (40°C, 300m). Sounds simple enough doesn’t it. However there’s enough talk of the importance of derating you air compressor for ambient temperature and pressure that I'm worried that I'm missing something and I haven't been able find definite answers that I'm satisfied with.

1st, as far as I know, your volumetric air flow doesn't change significantly with conditions. For clarity what I mean is the volumetric air flow when the air has gone from ambient (non-standard) conditions, to being compressed, and then returned to ambient and is measured at this point. To differentiate from SCFM I’ll refer to this as the Free Air Delivery (FAD) (which I think is the accepted definition for FAD, please correct me if I’m wrong). But if this is true, I don't see why you need to know the inlet conditions of a compressor when giving a rating.

However your mass flow rate does change and this equates to the power delivered by a flow of air which is what most people want to equate, and this is important to know for pneumatic tools.

So is this is what they mean by derate? That when they are saying that an air compressor has to be derated to 83% in Denver, is what they are really says is “For a tool that requires 83 SCFM of air, you will need compressor rated for 100 SCFM in order to deliver the mass air flow needed by the tool to deliver it's rated output.”

To be more specific on why I want to know, I am interested in modeling a system where I need to deliver a volume of air from a pressurized reservoir for the purposes of cleaning some dust filters. I think I only need to be concerned with volume and not so much mass in this case. As far as I know (and I may post this as a separate question) the volume of air used will be the same regardless of changes in ambient conditions. If that is the case, can I equate the SCFM rating of the compressor to the inlet cfm?

Thank you for any clarity you can give.

 

[MidnightRover]

Just to give some more background of what I do and don't understand, below is some more of what I had written, but then cut from the post as I thought it was getting to long:

I understand that the SCFM rating means is that at standard conditions (14.7 psi, 60 °F) that in one minute, the volume of air that was compressed to 115 psi is 32 ft³ after it’s been allowed to uncompress back to standard conditions.

Now I often read that you need to derate an air compressor to account for temperature and altitude (one example stated that you only get 83% of your capacity when in Denver) and this is where I get confused.

I think I am unsure of what they mean by derate.

I understand that air gets less dense as temperature increases and ambient pressure decreases, and this has the effect of decreasing the mass flow rate for a given volumetric flow rate of air.

But I don’t see how this decrease in density effects the volumetric air flow from the compressor. For clarity what I mean is the air flow when the air has gone from ambient (non-standard) conditions, to being compressed, and then returned to ambient. To differentiate from SCFM I’ll refer to this as the Free Air Delivery (FAD) (which I think is the accepted definition for FAD, please correct me if I’m wrong).

Regardless of ambient conditions, the displacement of a positive displacement compressor is the same. I know that the displacement of a PD compressor doesn’t exactly equate to the FAD. That there is the dead area that doesn’t get evacuated. I have my head around the fact that at higher altitudes the air that is left in this dead area will re-expand more for a given compressed gauge pressure than when at standard conditions. That this has the effect of decreasing the intake air flow because there isn’t as much room for new air to enter the chamber. But one example I found gave the difference in intake flow as you go from sea level to 2000 ft to be only a difference of 0.5%.

So except for this minor difference, I don’t see why my FAD from my compressor would be any different from one place to the next.

 

[zdas04]

You are over thinking this. The space that is filled and emptied by the compressor is 30 cubic ft per minute. If there were no air coming in at all then the compressor is still trying to fill and empty 30 cubic ft per minute. This means that the mass flow rate is 30 CFM times the suction density (you can the divide the mass flow rate by the imaginary density at standard conditions to get SCFM).

If you are in Denver, the atmospheric pressure is 12.22 psia. If the suction temperature is 60 F, then everything except pressure cancels. That means that since 12.22/14.73=0.8295. Multiply that times 30 CFM and you get 24.9 SCFM.

You really do have to think in mass flow terms to get your head around this concept.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[MidnightRover]

David, thank you for your reply.

So SCFM is used as sort of a laymans way of equating mass flow rates? I do understand that the mass flow rate changes with pressure and temperature, but the way it's talked about, it's like the compressor's volumetric flowrate is somehow changing. I also understand that the rate at which a compressor fills a reservoir will be slower with lower pressure ambient since there is less mass per stroke and that with a constant temperature, there is a direct relation between pressure and mass.

Thus for clarity, is this a correct statement to make:
A 30 SCFM compressor will have a flow rate of 30 cfm as measured in terms of the ambient air conditions, regardless of conditions. However what changes with the ambient conditions is the mass flow rate which manifests as slower flow rate at the reservoir pressure. 1 cfm as measured at 100 psig equates to 1*114.7/14.7 = 7.80 cfm of ambient air at standard conditions. However in Denver it equates to 1*112.22/12.22 = 9.18 cfm of Denver's ambient air. (This is of course assuming that all of these flow rates are at the same temperature of 60°F) Thus the compressor has to run 9.18/7.8 = 1.17 times as long in Denver to produce an equivalent amount of high pressure air flow. But note that even thought the high pressure volumetric flow rate is the same, the mass flow rate is still different due to the difference in absolute pressures (112.22 psia vs 114.7 psia).

Thanks,
Barry

 

[zdas04]

Your clarifying statement is correct (I tend to us ACFM to indicate "at actual conditions" to differentiate it from physical volume displaced, but most people don't). The physical volume displaced by the cylinder (at a certain rpm) is 30 CFM, so it will always move 30 ACFM. If the suction conditions are 14.73 psia and 60F then it moves 30 SCFM. If the suction pressure is 12.23 psia then the compressor will move 24.9 SCFM at that rpm (but still 30 ACFM). If the suction pressure is 15.025 psia it will move 30.6 SCFM at that rpm. Standard conditions are a surrogate for mass flow rate, so if mass flow rate changes because of suction pressure changes then volume flow rate at standard conditions will change in the same direction.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

ZDAS04,

I bow to your greater knowledge of air volumes, but in the list above you seem to have ignored pressure. in the example above, I would agree that the compressor is oving 30 ACFM based on the inlet pressure, but in the two instances, if the outlet pressure stayed the same then ACFM of the pressurized air at the same pressure ( lets say 100 psig) would be lower for the lower inlet pressure compared to the compressor at sea level??

Hence the reason why SCFM is so good because everyone is working on the same basis. There's not much point in an air compressor which doesn't compress air to a set pressure.

Bottom line - If you want a set volume of air at a set pressure, then if you lift the compressor up several thousand feet, it will have to work much harder to achieve the same thing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zdas04]

LittleInch,
I was just talking about suction conditions. We know that mass flow rate is constant across the compressor, therefore volume flow rate at standard conditions is constant between the suction and discharge, BUT the volume flow rate at actual conditions is no where close to constant between suction and discharge. If we assume that suction pressure and temperature are 14.7 psia and 60F and discharge pressure and temperature is 114.7 psia and 120F, then the ACFM goes from 30 ACFM on the suction to 4.3 ACFM on the discharge. If the suction and discharge piping is the same size (uncommon for a compressor this big, but lets pretend) then the velocity on the suction would be 7 times faster than velocity on the discharge.



David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

My point was aimed at this " But note that even thought the high pressure volumetric flow rate is the same, the mass flow rate is still different due to the difference in absolute pressures (112.22 psia vs 114.7 psia). " Using the same compressor but at a different elevation this is not correct. The "high pressure" acfm flowrate assuming the same "high pressure" will be lower as the mass flowrate is lower.

Like you always say, a gas flowrate is utterly meaningless with the statement "at xx pressure"

That's were the confusion comes.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MidnightRover]

David, LittleInch;

Thank you both for your input.

@LittleInch: I'm not sure I follow your point, but I think that may because of a misunderstanding. My note about the high pressure volumetric flow rate being the same was referring to the 1 cfm @ 100 psi g, 60°F. If I understand correctly, though in all inlet conditions I will have 1 cfm @ 100 psi g, the mass flow rate and equivalent ambient air flow rate will differ with the inlet conditions. As my pressure decreases and my temperature increases, the ambient air flow rate will have to increase and my mass flow rate will decrease. For this flow rate I was not meaning to indicate that it was coming from a 1 cfm compressor, but some theoretical steady state source of 100 psi g air. I understand that a compressor that could produce 1 cfm @ 100 psi g steady state at standard conditions would not be able to produce 1 cfm @ 100 psi g in Denver. It would only be able to produce (114.7*12.22)/(14.7*112.22) = .85 cfm @ 100 psi g. And even though at standard conditions and in Denver I have 1 cfm @ 100 psi g, the mass flow rate is not equivalent because the absolute pressure of that 100 psi g is different and hence so is it's density.

And I admit that I am confused by the statement that "a gas flow rate is utterly meaningless with the statement 'at xx pressure'". Assuming the temperature is somehow already known, don't we need the pressure to fully define the air flow? Are you referring to the fact that the statement by itself is meaningless without clarifying whether we are referring to the flow rate at standard conditions or ambient inlet conditions or at the specified pressure conditions?

And actually, maybe this is a good time to clarify terminology since I now think I have been misusing it. Please correct me if the following is incorrect:
1. 100 SCFM @ 100 psi = air flow is equivalent to 100 cfm of air at standard conditions when the compressor is outputting at 100 psi g
2. 100 ACFM @ 100 psi = air flow is equivalent to 100 cfm of air at ambient conditions when the compressor is outputting at 100 psi g (this is the one I think I miss-understood to mean #3 until I read David's comments above)
3. 100 CFM @ 100 psi = air flow is 100 cfm of air at 100 psi g (and some already indicated temperature).

Of the three the first is the only one that can be meaningfully used to spec a compressor since the others are dependant upon the ambient conditions. If you know the SCFM rating, you can determined the other two if you know your local ambient conditions.

Thank you both for your help in clarifying this.

 

[zdas04]

MighnightRover,
Your clarification is a really good example of why I try to never use "psi" (no this is not a "marvels of SI" rant). I use "psig", "psia", "psid", "bar(a)", "bar(g)", kPaa, etc. The way I would say your list is:

[ol 1]
[li]100 SCFM at any pressure = 100 SCFM[/li]
[li]100 SCFM at 100 psia and 60F = 14.7 ACFM[/li]
[li]100 ACFM at 100 psia at 60 F = 678 SCFM (assuming compressibility of air is 1.0)[/li]
[li]100 CFM can't be at 100 psia since (by convention in the air compressor industry) "CFM" is reserved for "ACFM at suction conditions"[/li]
[/ol]

LittleInch's comment is exactly right. I can know a mass flow rate of a gas without knowing the local pressure (because of the Continuity Equation which says that "absent additions or removals of mass, the mass flow rate everywhere within a piping system will be equal"). I can know the volume flow rate at standard conditions for the same reason. But, I cannot know the volume flow rate at actual conditions without knowing the local pressure and temperature.

I hope I didn't confuse things more.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

My bad. I meant "is meangless with out..."

Yes that's what I mean.

Your defintions,

1) No. 100 scfm is 100 cubic feet per minute at "Standard" conditions. "Standard unfortunately varies, but is often 1.0325bara at 15C. 100SCFM at 100 psi doesn't work as there are two pressures involved. Standard is standard, not standard. I would go for 100scfm delivered at 100 psig.

2) This is where we might vary. To me 100ACFM at 100 psi means 100 cubic feet of compressed air at 100 psig. If you converted this to scfm you would get about 700 scfm ( or there abouts). Some people though might take your view that it is 100 cubic feet of air at whatever pressure the inlet is , but now compressed to 100 psig

3) I think 2 and 3 are the same thing, but if you think ACFM is actual at the inlet point, then yes CFM @ 100 psig means as you describe.

I agree scfm is the best to work with and then adjust to your local conditions.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MidnightRover]

Thanks again you two for you responses on this, it's really helping solidify my understanding.

The example I gave of "30 SCFM @ 100 psi" was a spec I pulled off of the spec sheet of a compressor I am looking at and I was looking to make sure I was interpreting it correctly. I guess I neglected to mention that it was a compressor spec as it would seem the context goes into correctly interpreting it.

The ACFM was the one that I was least sure of. David seemed to have used ACFM in his post of 11 Apr 16 15:02 to describe the flow of air at the inlet ambient conditions, however then in his most recent post it seems to have been used to state the flow at the compressed conditions. But based on you two's most recent posts, my previous understanding, what I've seen elsewhere, I'm going to go with the interpretation that ACFM means the volume flow rate of air at the conditions stated/implied.

That CFM is typically interpreted as the inlet air flow was one I was not familiar with but know now.

But at this point I feel I have a pretty good grasp of things so thank you again for both of your help.

Barry

 

[LittleInch]

No problem - Its certainly one area where it can be very easy to be confused and where a lot of people don't really understand it, but then get annoyed when you interpet their understanding differently to yours.

That's why scfm is so useful. acfm often means inlet, but not always, so if in doubt question and check. You can usually work it out from the hp of the motor, but there are many posts on this - search acfm on this site to see what I mean.

Glad we could help.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.