compressibility factor (z)

In general, deviation from ideal gas behavior becomes more significant the closer a gas is to a phase change, the lower the temperature, or the higher the pressure. The compressibility factor (Z), also known as the compression factor, is the ratio of the expected volume of a gas to the volume of an ideal gas at the same temperature and pressure. The compressibility factor is a useful thermodynamic property for correcting the ideal gas law to account for the gas applications encountered in the real world.

If we insert the compressibility term into equation 1, we get:

After a little manipulation of above equation, we arrive at the ACFM equation with the compressibility factor z:

Let’s look briefly at how the compression factor changes as a function of pressure for natural gas with a specific gravity of 0.6. (Specific gravity for a gas is defined as the ratio of the density of the gas to the density of dry air both measured at the same specific conditions of temperature and pressure.) Notice that for z to significantly reduce the idealized flow, the pressure must exceed 1000 psia for natural gas at 100 oF. As a rule of thumb for natural gas, we can say that z will vary from 1.00 only a few percent if your suction pressure is only a few hundred pisa. However, if your suction pressure is much greater than a few hundred psi, you need to include z in your calculation of the ACFM.

Always remember that the compression factor is a function of pressure, temperature, and the actual mixture of gas analyzed.

The compression factor should not be confused with the compressibility (also known as coefficient of compressibility or isothermal compressibility) of a material, which is the measure of the relative volume change of a fluid or solid in response to a pressure change.

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