gas laws

Gas laws; By definition, compressors are intended to compress a substance in a gaseous state. In predicting compressor performance and calculating the loads on the various components, we need methods to predict the properties of the gas. Process compressors are used to compress a wide range of gases over a wide range of conditions. There is no single equation of state (an equation that allows the density of a gas to be calculated if the pressure and temperature are known) that will be accurate for all gases under all conditions. Some of the commonly used ones,
starting with the most simple, are discussed below.

The simplest equation of state is the perfect gas law:

This equation applies accurately only to gases when the temperature is much higher than the critical temperature or the pressure much lower than the critical pressure. Air at atmospheric conditions obeys this law well. gas laws

To predict the properties of real gases more accurately, the perfect gas law is often modified by the addition of an empirical value ‘‘Z’’, called the compressibility, or sometimes the supercompressibility, of the gas. The value of Z is a
function of the gas composition and the pressure and temperature of the gas. The modified equation is:

This equation is accurate if, and only if, Z is known accurately. Z can be estimated with reasonable accuracy in many cases using the Law of Corresponding States which states that the value of Z as a function of the reduced pressure and temperature is approximately the same for all gases. That is:

A curve of Z as a function of reduced pressure and temperature is shown below figure. This gives reasonable results for most gases when the gas state is not close to the critical point or the two phase region. gas laws

It is frequently useful to have an equation to predict Z. This allows calculation of other properties such as entropy, enthalpy and isentropic exponents that are needed to predict compressor performance. The use of an equation rather than charts is also convenient when a computer is used to perform the calculations. Many equations are available: one of the most simple, the Redlich-Kwong Equation of State is given below. Other equations are more accurate over a wider range of gases and conditions, but are more complex. The Redlich-Kwong Equation of State is:

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