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Tech
Tips Index
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SCFM vs ACFM When manufacturers list performance for blowers and compressors it is stated as "Capacity" in CFM (Cubic Feet per Minute). This refers to the volume of air at the inlet to the unit, therefore this is often referred to as Inlet CFM (ICFM) or Actual CFM (ACFM). These terms are used interchangeably and mean basically the same thing (CFM, ICFM, ACFM). In many process applications there is a critical demand for a specific minimum flow in terms of some base or reference pressure, temperature and relative humidity. Many standards are used, the most common being the Compressed Air & Gas Institute (CAGI) and the American Society of Mechanical Engineers (ASME) standards, which are 14.7 PSIA, 68Deg. F and 36% relative humidity. When you express your "Demand" in Standard CFM (SCFM), you are saying that you want this compressor or blower to deliver this CFM even at your worst case conditions. If you have a "DEMAND" of 500 SCFM and you pick a unit from the manufacturers literature that indicates a "CAPACITY" of 500 ACFM you will not get the amount of air that you require during times when your inlet conditions vary from the standard conditions. Corrections must be made to assure that the unit furnished will provide the proper amount of air for the process to function properly. To convert your "DEMAND" SCFM to ACFM that you can use to select a unit from the manufacturers literature, you plug your standard conditions and the "worst case" inlet conditions (that will exist at the unit) into the formula shown below:
Specifying "DEMAND" in SCFM is generally limited to process applications and those who write specifications that require SCFM should always provide the following information to the vendor with the request for quotation: - what standard you wish to use (PSIA, temp, %RH) - worst case inlet conditions (PSIA, temp, %RH) SCFM is not normally used when specifying "Plant Air" applications. |
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