Compressor Capacity Questions 2

[BanditKeith]

Hey Everyone,

I just finished a project involving compressors and came across a plethora of abbreviations to describe the capacity of a compressor, namely SCFM, ACFM, ICFM, and FAD to name a few. My current understanding, thus far, is:

SCFM - Standard cubic feet per minute - The capacity at standard conditions (68 F, 14.7 psia)
ACFM - Actual cubic feet per minute - The capacity at the actual operating conditions
ICFM - Inlet cubic feet per minute - The capacity at the suction side of the compressor at ambient conditions
FAD - Free air delivery - The free air delivery at the discharge at standard conditions (68 F, 14.7 psia)

I know there's more, but these are the main ones I keep seeing. My biggest questions are:
1. Why are there so many different ways on specifying capacity? Kinda leads to some confusion, don't you think?
2. Some of these can clearly be quite similar if not the same thing? For example, SCFM and ICFM can theoretically be the same if the ambient conditions are exactly the same as the standard conditions. Another example, aren't SCFM and FAD more or less the same given how they are based at standard conditions, or am I missing something between the two?
3. Why are some compressors (ICFM) rated based at the inlet flow rate vs outlet flow rate? Won't most people be interested in what the compressor is actually delivering at the discharge versus what it is taking in?
4. I see most vendors rate their units at some CFM at some PSI (example: 1600 CFM @ 150 PSI). Is it safe to assume that this always refers to FAD, or is the best way to avoid confusion in the future is to verify from the vendor what the flow rate specifically means (operating conditions, intake capacity, etc.)?

Thanks in advance.

 

[zdas04]

The air compressor industry predates standards. In gas compressors, the suction pressure is rarely anywhere close to anyone's "standard" so that industry adopted pretty rigid classification around SCF (which is typically somewhere around 14.7 psia and 60F, not 68F, but is whatever two people decide it is), not ICF or ACF.

Before standards, each manufacturer used their own designations and they were truly all over the map. Funny thing is that if you pick a pressure and temperature to state your capacity then you are just defining "standard" conditions. If I say "150 ICFm", but I really mean "150 ICFm at my factory in Mt Vernon, OH where the atmospheric pressure is 14.0 psia and the temperature is always a balmy 70F" then I've defined the standard at 14.0 psia and 70F that will be used to state volume for discharge conditions and usage conditions as well. It works fine. "ACF at suction conditions" is the same thing (not really an ACF at all).

The 1600 CFM at 150 psi really doesn't mean what it looks like it should mean. Generally it means "1600 ICFM with a maximum discharge pressure of 150 psig". But not always. I've never seen it to mean "standard pressure for calculations is 150 psig plus local atmospheric pressure".

Air compressor terminology is incredibly imprecise and it behooves you to get definitions early and to verify that the vendor is following his definitions.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[rakuday]

One reason for specifying the compressor by inlet CFM, is because, most of the appications are specified by CFM. For instance, air tools are specified by the air consumption, given in terms of CFM (ex: 3cfm @ 90 psi). It becomes easy to draw the final air consumption for all the applications, including losses etc.. and can arrive at the compressor specs which should cater for the whole need.

To aid the end users or customers of compressors, CAGI datasheets are available, wherein the compressor manufacturer will have to specify the data in a common condition. This helps the customer to compare compressor in apple to apple comparison. Further, the flow data can also be converted to Nm3/hr for quick comparison.

 

[rotw]

I came across a very basic question recently. API617 datasheet template point out to MMSCFD flow on dry basis.
Wet or dry is referring to water content if am correct. Since Water vapor at standard conditions is liquid, I was wondering if it makes sense to indicate MMSCFD on wet basis or is just a matter of convention ?

 

[LittleInch]

If I was you I would ignore most of the others and just use scfm. That way everyone uses the same conditions. You can then modify to suit your actual pressure and temperature if you need to but avoids the confusion you note above.

The only time you really need acfm is to size pipework for in line compressors in a piping system. Air compressors and air users are different to gas compressors where suction could be 500 psig.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

Rotaryworld,
"Water vapor at standard conditions is a liquid" is an incorrect statement. Ever seen the condensation form on the outside of a cold class on a humid day? Dew on your grass in the morning? Gases will be saturated with water vapor (gas) at the surface of every coherent gas liquid interface. "Dry" never really means dry. Typically we use 7 lbm/MMSCF or 112 mg/SCM as the standard for dry. It turns out that this number is the dew point of methane at 1000 psig and the freezing point of water. Low pressure gas can easily have 2-8% (by volume) of water vapor. 8% water vapor has a significant impact on density at a given pressure, so it can impact the performance of a dynamic machine.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[rotw]

Zdas,
Thanks for your insight.
I would like to reformulate my concern. Suppose there is some water in a gas flowing through a compressor, the ACFM flow (or wet) going through the machine should be the sum of a ACFM of water (in vapour form) and ACFM of hydrocarbons (the dry flow portion). Do you agree?So if I would have to state the dry actual flow then I would have to pick up the later value. However If I also have to indicate beside the actual flow, the standard flow MMSCFD, is it technically correct to refer to a wet basis ?

Thanks

 

[zdas04]

All that the conversion to standard conditions does is convert the mass flow rate of a gas (whatever its components) to the volume flow rate (of that set of components) it would exhibit at your standard conditions. That is all. If the gas has 10 ACF/hr at 1450 psia and 60F and your standard is 14.5 psia at 60F then the gas will have 1,000 SCF/hr if you disregard changes in compressibility or 1,245 SCF/hr if you don't ignore compressibility (assuming methane as the gas with no nitrogen, CO2, or H2S). If you saturate the gas with water then you will get a slight change in compressibility and a small change in the SCF/hr.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"