Both gas and liquid flows can be measured in volumetric or mass flow rates. With noncompressible liquids, these measurements are nearly the same, sans the effects of temperature. With compressible gases, though, they are very different.
Gases are composed of a large number of particles that behave like hard, spherical objects in a state of constant, random motion. They have no affinity for one another or the walls of their container. These particles (moles) move in a straight line until they collide with another particle or the walls of the container. Most of the volume of a gas is therefore empty space, which is why a gas is compressible.
When measuring the flow of a compressible gas through a pipe, you are measuring volumetric flow. Unlike with noncompressible liquids, this measurement is of little value unless it is converted to mass flow, which is dictated by the pressure and temperature.
The pressure of a gas results from collisions between the gas particles and the walls of the container. Each time a gas particle hits the wall, it exerts a force on the wall. The force or pressure is dependent on the frequency of collisions against the wall and the kinetic energy of the molecules.
The speed at which the molecules travel is dependent on kinetic energy, which is determined by thermal energy. The warmer the molecules are, the more energy they have, and the faster they will travel. Any object in motion has a kinetic energy that is defined as half the product of its mass times its velocity squared:
KE = 1/2 mv2.
The average kinetic energy of the particles in a gas is proportional to the temperature of the gas. Because the mass of these particles is constant, they must move faster as the gas becomes warmer. If they move faster, the particles will exert a greater force on the container each time they hit the walls, increasing the pressure of the gas.
Thus, in measuring gas flows, pressure and temperature need to be factored in and are calculated by the ideal gas law, stated as
PV = nRT = NkT,
- V = volume in meters cubed
- P = pressure measured in pascals
- n = number of moles
- R = universal gas constant (8.3145 J/mol K)
- N = number of molecules
- K = Boltzmann constant (1.38066 x 10-23 J/K = 8.617385 x 10-5 eV/K)
- k = R/NA
- NA = Avogadro's number (6.0221 x 1023 /mol)
The second way to effect pressure is by compression, which reduces the volume. With a given volume at atmospheric pressure, there will be a definitive number of gas molecules. If the volume is decreased, you still have the same number of moles (mass), but, as described earlier, the frequency of collisions with the walls will increase, thus exhibiting a higher pressure.
So it would be inaccurate to compare flow rates of unequal pressure and temperature. Thus, a common set of conditions is needed for measurements to allow comparisons to be made between different sets of data. ISO 2314, and ISO 3977-2 established standard conditions to be 59F and 14.696psi.
To measure gas flows effectively, the volumetric flow rate has to be converted to standard conditions for temperature and pressure. Simply put, it is the volume the gas would occupy at atmospheric pressure (14.7 psi), and it is defined as standard cubic feet per minute (SCFM).
To convert flow from cubic feet per minute (CFM) to SCFM, we use
Qg = Q x P/14.7,
- Qg = gas flow in SCFM
- Q = volume flow rate in CFM
- P = line pressure absolute (gage pressure +14.7)
For example, let's convert gas flow expressed in CFM (CFM) to units of SCFM. Given:
- Q = 20 CFM
- P = 114.7 (100psi gauge reading +14.7)
- Qg = Q x P/14.7
- Q = 20 CFM x 114.7/14.7
- Q = 156 SCFM
Flow meters used to measure gases usually are calibrated for readings at atmospheric pressure. When the flow is under pressure, the meters provide a chart of factors associated with various pressures to multiply against the visual reading.
Finally, when measuring gas flow, since temperature and pressure need to be factored, consult the owner's manual for the flow meter. Flow meters used to measure gases usually are calibrated at standard temperature and pressure. When the flow is measured at conditions other than ST&P, the meters provide a chart of correction factors.
Joe Panfalone is an applications engineer for Exair. He holds an AE in industrial engineering from the University of Dayton and a BS in business management from Defiance College. He has more than 40 years of experience in the fluid power industry, holding positions in engineering, management, and marketing.