Understanding Gas Flow and Measurements

Both gas and liquid flows can be measured in volumetric or mass flow rates. With non-compressible liquids these two measurements are very nearly the same sans the effects of temperature. With compressible gasses though, they are very different. The same mass under different pressures will occupy dissimilar volumes.

To demonstrate this, take a folded fluffy comforter and weigh it. Then stuff into one of those storage bags that you suck down with a vacuum cleaner. The physical size becomes very much smaller but the weight (mass) stays the same.

When measuring a flow of a compressible gas through a pipe you are measuring volumetric flow. Unlike non-compressible liquids, it is of little value unless it is converted to mass flow which would be dictated by the pressure it is under. For example the utility company charges by the cubic foot of natural gas and gallons for water. With water you actually get a gallon as measured by the meter. With gas though, the mass you receive depends on pressure it is under.

To effectively measure gas flows, their volumetric flow rate has to be converted to standard conditions for temperature and pressure. Simply put, it is the volume it would occupy at atmospheric pressure (14.7 psi) and defined as standard cubic feet per minute (SCFM).

Convert flow from CFM to SCFM

 Qg = Q x P/14.7

Qg=Gas flow in standard cubic feet per minute (SCFM)

Q=Volume flow rate in cubic feet per minute (CFM)

P=Line pressure absolute (gage pressure +14.7).

Example: Convert gas flow expressed in cubic feet per minute (CFM) to units of standard cubic feet per minute (SCFM).

 Given:

Q = 20 CFM

P = 114.7 (100 psi gage reading +14.7)

Qg = Q x P/14.7     = 20 CFM x 114.7/ 14.7      = 156 SCFM

Flow meters used to measure gasses usually are calibrated for readings at atmospheric pressure. When the flow is under pressure, they provide a chart of factors associated with various pressures to multiply against the visual reading.

Joe Panfalone
Application Engineer
Phone (513) 671-3322
Fax   (513) 671-3363
Web: www.exair.com
Twitter: www.twitter.com/exair_jp
Facebook: http://www.facebook.com/exair

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