SCFM versis CFM

Here is the formula to convert cubic feet to standard cubic feet:

AT = Actual Temp + 460 = Temp. Rankin
AP = Actual Pressure + Atmospheric (14.7 @ sea level)

520/AT = Temp Correction Factor
AP/14.7 = Pressur Factor

CF x Temp Factor x Pressure Factor = SCF

So, if you have 1 cf of gas/air @ 30psia & 120F you would have:

30/15 = 2
120+460 = 520/580 = .90

1 x 2 x .90 = 1.8 scf

See how much Temp and pressure makes? The hotter something is the less of it you can store withing a given space at a given temp. Increase the pressure and you can fit more into a given space. Raise te temp. of a gas in a given space and you will raise the pressure. It's called Boyles Gas Law.

So, when you ship something down a pipeline to China, there will not be a war, because you both can agree on the amount you sent was correct after it's converted to a standard cubic foot. Grin.

SCFM stands for standard cubic feet per minute. Air is a compressible fluid and its' volume is dependent on the pressure and temperature (and moisture content) at the point in question. The standard conditions that I am familiar with in the compressed air industry are slightly different than T_Bone's and are 60 deg F, sea level atm pressure (14.7 psia) and dry (0% RH). Unfortunately there are different "standards" used for the same thing in different industries and different countries. The differences are not major for most small air compressor analyses though. SCFM is air flow referred to a common, standard condition so comparisons can be made with other conditions. The term CFM (cubic feet per minute) by itself is meaningless for air because it does not fully define the conditions at the point of measurement.

Technically, compressors are NOT rated in terms of SCFM or even the meaningless CFM, despite data that you see from manufacturers. That is what makes the whole thing confusing to many. Conventional reciprocating compressors suck in a certain amount of air through the intake valve. That quantity of air is dependent on the piston displacement, the speed and the volumetric efficiency at the point of operation. Compressors are correctly rated in terms of this air at the conditions of temperature, pressure and moisture content AT THE INLET OPENING. In technical circles this is referred to as ICFM for cfm at inlet conditions. It is also sometimes referred to as "free air", especially if there are no inlet restrictions such as piping or filters. See the link for some definitions.

Whenever you see a compressor rated similarly to "8.2 cfm @90 psig and 9.8 cfm @40 psig" (as you typically will), you should assume that this is actually ICFM. The pressures indicated are only to show the point of operation. The difference in the cfm quantities are due to the decrease in volumetric efficiency that always occurs with increasing discharge pressure and is most noticeable in single stage machines. The actual pressure reference that the cfm is referred to is the pressure existing at the intake opening, not that at the discharge. This pressure may be atmospheric or slightly sub-atmospheric. If the compressor is located at sea level on a mild day with low humidity and no inlet restrictions, this is very close to SCFM. If you take this same compressor to Denver, CO with the same temperature and humidity conditions, this compressor would have to be derated (in terms of SCFM) by about 20%, as I recall, due to the lower atmospheric pressure at that altitude. The ICFM would be the same, however. This is why it is not technically correct to rate compressors in terms of SCFM...the ratings would have to change depending on where the compressor is located.

To summarize, if you are not at extremes in regard to temperature, elevation above sea level, or humidity, you can say the cfm numbers as indicated above are quite close to SCFM. This is provided that you do not restrict the intake in any way. Dirty intake filters can reduce compressor capacity significantly because they lower the intake pressure at the intake opening, the same as locating the compressor at higher altitudes.

You may also see the term ACFM for actual cfm but that term has no particular use with compressors, arguably, IMHO. It is best reserved for a discussion regarding the air consumption of air tools...a different subject altogether.

I hope I haven't confused the matter more for you :o).

Rod

• SCFM Discussion

Standard CFM has to meet cirtian testing reqiurements. Standard air is 70*f, 50%rh and weighs .00075lbs/cft. CFM is just a flow measurement without correcting for standard air. So a compressor rated at SCFM is what your buying at a rating where a compressor just rated CFM will/could/maybe have less of an output CFM and a good indication your not buying a "rated" compressor that had to meet "standards" or a cheap compressor.

As an engineer you can calculate SCFM in a given piping system but just stating CFM is not complete enough info to engineer air piping as tempertaure and humidity play a big roll in caculations as CFM will change as the air temperature/humidity changes.

T_Bone