Compressibility is the measure of the change in volume a substance undergoes when a pressure is exerted on the substance. Liquids are generally considered to be incompressible. For instance, a pressure of 16,400 psig will cause a given volume of water to decrease by only 5% from its volume at atmospheric pressure. Gases on the other hand, are very compressible. The volume of a gas can be readily changed by exerting an external pressure on the gas. Compressibility is related to thermodynamics and fluid mechanics.
All gases deviate from the perfect or ideal gas laws to some degree and in some cases the deviation is rather extreme. It is necessary that these deviations be taken into account in many compressor calculations to prevent cylinder volumes and driver sizes being sadly in error.
Compressibility is derived experimentally from data on the actual behavior of a particular gas under p–V–T changes. The compressibility factor Z is a multiplier in the basic formula. It becomes the ratio of the actual volume at a given p–T condition to the ideal volume at the same p–T condition.
A series of compressibility and temperature–entropy charts has been drafted to cover all gases on which reliable information could be found. These will be found in specialized texts or handbooks. In some cases, they represent consolidation and correlation of data from several sources, usually with a variance of less than 1% from the basic data. These charts may be considered authoritative.
Temperature–entropy charts are useful in the determination of theoretical discharge temperatures that are not always consistent with ideal gas laws. Discharge temperatures are required to obtain the compressibility factor at discharge conditions as involved in some calculations. These specific Z and T–S charts will provide the necessary correction factors for most compression problems involving the gases covered.
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