Engineering guide
This module calculates the thermophysical properties of oxygen according to the VDI Heat Atlas (VDI-Wärmeatlas, 12th edition 2019), either for the single-phase state or for boiling oxygen on the vapor pressure curve. The results include density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, Prandtl number, thermal diffusivity, enthalpy, entropy, heat of evaporation, speed of sound and surface tension, as well as molar mass, gas constant and the critical-point data.
Calculating oxygen properties is required when sizing air separation plants, vaporizers and piping for liquid oxygen (LOX), in the medical technology and steel industries, and in oxyfuel processes and rocket propellant systems. The range of validity extends single-phase from −200 °C to 100 °C at 1 bar to 500 bar, and in the boiling state from −218 °C to −120 °C — i.e. from the triple point to close to the critical point. The module thus covers in particular the cryogenic range, where the properties are strongly temperature-dependent and simple approximations fail.
Scope
- Properties of oxygen
Typical workflow
Select the type of state
First it is specified whether a single-phase state (gas or liquid, defined by temperature and pressure) or the boiling state on the saturation line is to be calculated.
Enter the state variables and check validity
Temperature and pressure are specified; the module checks compliance with the range of validity (single-phase −200 °C to 100 °C, 1 bar to 500 bar; boiling −218 °C to −120 °C).
Calculate the thermal and caloric quantities
From the property correlations follow density, real-gas behavior, specific heat capacity, enthalpy, entropy, heat of evaporation, coefficient of thermal expansion and speed of sound.
Determine the transport properties
Thermal conductivity and viscosity are calculated as functions of the state; from these follow the kinematic viscosity, thermal diffusivity and Prandtl number for heat transfer and pressure drop correlations. In the boiling state, the values are reported separately for liquid and vapor, supplemented by the surface tension.
Frequently asked questions
Where are the characteristic fixed points of oxygen?
Oxygen boils at −182.97 °C at 1.013 bar; the triple point lies at −218.79 °C, the critical point at −118.57 °C and 50.4 bar. The critical density is about 436 kg/m³. Above the critical point no phase boundary exists any more — which is why the module's boiling-state calculation ends at −120 °C, just short of the critical point.
Why do I need real-gas properties in the cryogenic range instead of the ideal gas equation?
Near the boiling line and at high pressures, the compressibility factor of oxygen deviates significantly from 1; density, heat capacity and speed of sound can no longer be derived from the ideal gas equation there. The isobaric heat capacity in particular rises sharply as the critical point is approached. For vaporizers, pumps and expansions in air separation plants, the real-gas correlations of the VDI Heat Atlas are therefore required.
Does the module also cover the safety-related aspects of oxygen?
No. The module provides thermophysical property data only. Oxygen's fire-promoting behavior, the selection of materials and lubricants (oil- and grease-free service, burnout resistance in accordance with the relevant BAM/EIGA rules) as well as limits on flow velocities in oxygen piping must be assessed separately according to the applicable safety codes.
What are typical sources of error in application?
Common mistakes are confusing absolute pressure with gauge pressure, evaluating properties at an unsuitable reference temperature (mean fluid temperature vs. wall temperature) and extrapolating beyond the correlation limits. For two-phase states, care must also be taken as to whether the liquid or the vapor value of the respective quantity should enter the subsequent calculation.
Technical content source: LV-Solved-Problems-Content 628d313247e7