reducedpressure

Reduced pressure is a dimensionless thermodynamic quantity used to compare the behavior of fluids under different conditions. It is defined as P_r = P / P_c, where P is the system’s actual pressure and P_c is the substance’s critical pressure. The concept is part of the corresponding-states framework, which also uses reduced temperature T_r = T / T_c and reduced volume V_r = V / V_c. When data are expressed in reduced variables, many fluids show similar P–V–T behavior, enabling cross-substance scaling and transfer of information from one substance to another.

Applications include the estimation and comparison of gas nonideality, aiding design and analysis of equipment, and the construction of generalized compressibility charts. In practice, reduced properties are used in conjunction with equations of state such as Peng–Robinson or Soave–Redlich–Kwong, and with charts of the compressibility factor Z as a function of P_r and T_r.

Limitations: The corresponding-states principle is an approximation. Its accuracy declines near the critical point and for substances with strong polarity, hydrogen bonding, or complex molecular structures. Deviations can arise due to molecular complexity or significant intermolecular forces, limiting universal applicability.

In summary, reduced pressure provides a normalized, dimensionless framework for comparing and scaling thermodynamic behavior across substances by reference to their critical properties.