Fluid properties for process calculations
Density, viscosity, heat capacity, thermal conductivity and vapour pressure of process liquids, gases and mixtures — how they are calculated, and calculators that do it.
Why property data decides the answer
Almost every process calculation starts with fluid properties. Pressure drop depends on density and viscosity, heat duty on heat capacity, NPSH on vapour pressure, and gas flow on compressibility. A 20 % error in viscosity can move a line from turbulent to transitional flow; a wrong density changes every head and power calculation. Using properties at the actual temperature and pressure, not at 20 °C from a data sheet, removes a common source of error.
Gas density and the compressibility factor
ρ = p · M / (Z R T)
Example. Air (M = 28.96 kg/kmol) at 7 bar g and 40 °C, taking Z ≈ 1: ρ = 801 325 × 28.96 / (8314.5 × 313.15) = 8.91 kg/m³. At high pressure Z departs from 1 and must be calculated; the PlantCalcKit tools use the Peng–Robinson equation of state, which also gives the heat capacity ratio, the speed of sound and the Joule–Thomson coefficient.
Liquids
Liquid density, heat capacity, viscosity, thermal conductivity, surface tension and vapour pressure are taken from correlations in reduced temperature fitted to reference equations of state (CoolProp; Bell et al., Ind. Eng. Chem. Res. 53, 2014) where those exist, and from established estimation methods (Poling, Prausnitz and O'Connell, The Properties of Gases and Liquids) for other chemicals. Liquid mixtures use ideal-solution mixing rules, and the tools state where that is not accurate (strongly non-ideal mixtures such as methanol and water).
What the PlantCalcKit property calculators cover
- 543 chemicals, plus water and steam, gas mixtures, glycol and brine solutions, heat transfer fluids and petroleum fractions from API gravity and the Watson K factor.
- Density, cp, cv, viscosity, thermal conductivity, surface tension, vapour pressure, boiling and dew points, heating values, Wobbe index and flammability limits.
- Your own measured density or viscosity can be entered to correct the library values.
Each tool's Guide lists the method, the data class of each chemical and the measured accuracy against reference data.
Calculators for this
Free calculators
- Gas density and Z factor — Density and compressibility of 21 gases and mixtures at any pressure, with Nm³/h, kg/h and MMSCFD.
- Steam tables (IAPWS-IF97) — Saturated and superheated steam: enthalpy, entropy, specific volume, density, cp and viscosity.
- Psychrometric calculator — Relative humidity, wet bulb, dew point, humidity ratio and enthalpy of moist air at any altitude (ASHRAE).
- Unit converter — Pressure gauge/absolute, flow, Nm³/h ↔ SCFM ↔ kg/h, cP ↔ cSt, energy, power and more.
Online tools and Excel tools
- Fluid Properties Calculator — Properties of about 165 process fluids at any temperature and pressure — water and steam, refrigerants, solvents, glycol and brine solutions, heat transfer oils and petroleum fractions. Use online · Excel tool, USD 29
- Gas Properties Calculator — 543 chemicals in one alphabetical list, or a mixture of up to 12: Z, density, cp with pressure correction, viscosity, conductivity, heating values and hazard data at any T and P. Use online · Excel tool, USD 49
- Liquid Properties Calculator — 543 chemicals in one alphabetical list, or a liquid mixture: density, cp, viscosity, conductivity, surface tension, vapour pressure, latent heat, heating values and hazard data at any T and P. Use online · Excel tool, USD 49
Questions
How do I calculate gas density?
ρ = pM/(ZRT) with absolute pressure p, molecular weight M, compressibility factor Z, gas constant R = 8314.5 J/(kmol·K) and absolute temperature T. Z is close to 1 at low pressure; at high pressure calculate it from an equation of state such as Peng–Robinson.
Which chemicals are in the library?
543 chemicals, plus water and steam, gas mixtures, glycol and brine solutions, heat transfer fluids and petroleum fractions.
Can I mix chemicals?
Yes. Gas mixtures use the Peng–Robinson equation of state; liquid mixtures use ideal-solution rules.
More calculation guides
- Process engineering calculations
- Pump calculations
- Heat exchanger calculations
- Pipe pressure drop and orifice sizing
- Cooling tower calculations
These guides and calculators give engineering estimates for screening, troubleshooting and training. Check important decisions against site data, your equipment maker and your plant's approved procedures.