GAS COMPRESSION — BASIC THEORY

INTRODUCTION

We all deal with compressed gas on a regular basis, yet there may only be a few of us who actually know enough of the principles necessary to be able to determine a customer's requirements without help from our applications people.

While our discussion here will be too brief to provide more than some basic principles, hopefully it, along with the documentation presented, will take us one more step toward being self-sufficient in these situations.

BASIC CONCEPTS

The First Law of Thermodynamics

This law states that energy cannot be created or destroyed during a process (such as compression or transfer of a gas), although it may change from one form of energy to another. In other words, whenever a quantity of one kind of energy disappears, an exactly equivalent total of other kinds of energy must be produced.

The Second Law of Thermodynamics

This law is more abstract and can be stated several ways.

Heat cannot, of its own accord, pass from a colder to a hotter body.

Heat can be made to go from a body at lower temperature to one at higher temperature only if external work is done.

The available energy of the isolated system decreases in all real processes.

Heat or energy (like water), of itself, will flow only downhill.

Basically, these statements say that energy exists at various levels and is available for use only if it can move from a higher to a lower level. In thermodynamics, a measure of the unavailability of energy has been devised and is known as entropy.

Entropy (as a measure of unavailability) increases as a system looses heat, but remains constant when there is no gain or loss of heat (as in the adiabatic process).

Ideal or Perfect Gas Laws

An ideal or a perfect gas is one to which the laws of Boyle, Charles, and Amonton apply. While there is no such gas, these laws are used for convenience and consistency, and they are corrected by compressibility factors based on experimental data.

Boyle's Law

At constant temperature, the volume of an ideal gas varies inversely with the absolute pressure.

Charles' Law

At constant pressure, the volume of an ideal gas varies directly as the absolute temperature changes.

Amonton's Law

At constant volume, the absolute pressure of an ideal gas varies directly as the absolute temperature changes.

Dalton's Law

This law states that the total pressure of a mixture of ideal gases is equal to the sum of the partial pressures of the constituent gases.

The partial pressure is defined as the pressure each gas would exert if it alone occupied the volume of the mixture at the mixture temperature.

Dalton's Law has been proved experimentally to be somewhat inaccurate, with the total pressure often being higher than the sum of the partial pressures, particularly as pressures increase. However, for engineering purposes, it is the best rule available and the error is relatively minor.

Amagat's Law

This is similar to Dalton's Law, but states that the volume of a mixture of ideal gases is equal to the sum of the partial volumes that the constituent gases would occupy if each existed alone at the total pressure and temperature of the mixture.

Avogadro's Law

Avogadro stated that equal volumes of all gases, under the same conditions of pressure and temperature, contain the same number of molecules.

This law is very important and is applied in determining compressor or booster size. If, for instance, a requirement exists to boost the pressure of a gas mixture, this law allows the calculation to assume that the molecular volume of each constituent gas is proportionally equal. In other words, in a 65-25-10 gas mixture, molecular volumes do not vary and can therefore be equally compressed.

Gas and Vapor

By definition, a gas is that fluid form of a substance in which it can expand indefinitely and completely fill its container. A vapor is a gasified liquid or solid, in other words, a substance in gaseous form.

All gases can be liquefied under suitable pressure and temperature conditions and therefore can also be considered vapors. The term "gas" is generally used when conditions are such that a return to the liquid state (condensation) would be difficult within the scope of the operations being considered. However, a gas under such conditions is actually a superheated vapor.

Changes of State

Any pure substance may exist in three states; as a solid, as a liquid, or as a vapor. Under certain conditions it may exist as a combination of any two phases and changes in conditions may alter the proportions of the two phases. There is also a condition where all three phases may exist at the same time. This is known as the triple point. Water has a triple point at near 32 degrees F and 14.696 psia (at sea level). Carbon dioxide may exist as a vapor, liquid and solid simultaneously at about minus 69.6 degrees F and 75 psia.

Substances, under proper conditions, may pass directly from a solid to a vapor phase. This is known as sublimation.

Changes of state are involved in many processes requiring compression. Two examples are refrigeration and the manufacture of dry ice.

Critical Conditions

There is one temperature above which a gas will not liquefy due to pressure increase, no matter how great. This point is called the critical temperature. It is determined experimentally.

The pressure required to compress and condense a gas at this critical temperature is called the critical pressure.

Specific Gravity

Specific gravity is the ratio of the density of a given gas to the density of dry air, where both are measured at the same specified conditions of temperature and pressure, usually 14.696 psia and 60 degrees F. It should also take into account any compressibility deviation from a perfect gas.

Compressibility

All gases deviate from the perfect or ideal gas laws to some degree. In some cases the deviation is rather extreme. It is necessary that these deviations be taken into account in gas compression calculations to prevent boosters being oversized or undersized, especially in high pressure applications.

Compressibility is experimentally derived from data about the actual behavior of an individual gas or gas mixture. The result of this derivation is known as the compressibility factor "Z", or the Z factor. The Z factor is used as a multiplier in the basic compression formula. The chart on the following page lists Z factors for some commonly used gases.

NOTE: Figures are based on gas at 70 degrees F.