Flow Measurements

Flow through a compressor may be stated in a number of different ways:
• MMSCFD
• Moles/Hour (MPH)
• SCFM
• ACFM
• Weight Flow

MMSCFD
MMSCFD denotes millions of standard cubic feet per day, where “standard” means 14.7 psia and 60°F. This notation is often used in gas plant, gas transmission, and refinery applications.
Moles/Hour (MPH)

Process engineers often use MPH in material balance computations. (A “mole” is a fixed quantity of molecules. This concept greatly simplifies process calculations.) A mole of any gas occupies approximately 379.4 cubic feet at standard conditions (14.7 psia, 60°F), and it has a weight in pounds equal to the molecular weight of the gas. For example, a mole of methane (CH4) would have a volume of 379.4 cubic feet at standard conditions, and that volume would weigh 16.04 pounds. Knowing the moles per hour, the MMSCFD may be determined from:

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SCFM
SCFM denotes standard cubic feet per minute, and is frequently used in compression work.

ACFM
Actual cubic feet per minute (ACFM) at the inlet, often called Q, is related to the physical size of the compressor. Several design parameters are based on Q. ACFM at inlet is also abbreviated ICFM. ACFM at the compressor discharge is sometimes of interest, and in this manual it will be abbreviated DCFM (discharge cubic feet per minute).

However, note that ICFM is the more appropriate term to use when referring to inlet conditions. In many cases, ACFM is often used interchangeably with ICFM. If there is any doubt, be sure to get clarification.

SCFM may be converted to ACFM, or Q, by:

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where:
P1, T1, Z1 = Absolute pressure (psia), absolute temperature (°R), and compressibility at the condition of interest.  Zo = Compressibility at standard conditions.


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