nist-refprop

NIST REFPROP Database Skill

Query high-accuracy thermodynamic and transport properties using the NIST REFPROP (Reference Fluid Thermodynamic and Transport Properties) database - the gold standard for thermophysical property calculations.

Overview

NIST REFPROP is a commercial-grade thermophysical property database developed by the National Institute of Standards and Technology (NIST). It provides:

• Pure Fluids: 147+ pure fluids with the highest available accuracy
• Mixtures: Predefined and custom mixtures with advanced mixing rules
• Refrigerant Blends: All common refrigerant blends with temperature glide
• Transport Properties: Viscosity, thermal conductivity, surface tension
• Thermodynamic Properties: Enthalpy, entropy, density, specific heat
• Phase Equilibrium: VLE, LLE, VLLE calculations for mixtures
• Highest Accuracy: Reference-quality data validated against experimental measurements

REFPROP is the industry and research standard for:

• Critical engineering design requiring highest accuracy
• Validation of other property libraries (CoolProp is validated against REFPROP)
• Mixture calculations with complex phase behavior
• Refrigerant blend applications with temperature glide
• Research and publication-quality data

Licensing

COMMERCIAL LICENSE REQUIRED

NIST REFPROP is NOT free or open-source software. You must purchase a license from NIST:

• Standard License: ~$300 USD (one-time purchase)
• Academic Pricing: Available for educational institutions
• Purchase: https://www.nist.gov/srd/refprop

License includes:

• REFPROP software (Windows GUI)
• Fluid database files
• Excel add-in
• FORTRAN/DLL libraries
• Python wrapper compatibility

Important Notes:

• License is per-user, perpetual (one-time fee)
• Updates and new fluid files provided for ~5 years
• Required for commercial applications
• Cannot be redistributed

Installation

Step 1: Purchase and Install REFPROP

1. Purchase REFPROP license from NIST: https://www.nist.gov/srd/refprop
2. Download and install REFPROP (typically to C:\Program Files (x86)\REFPROP\)
3. Note the installation directory path

Step 2: Install Python Wrapper

pip install ctREFPROP

The ctREFPROP package provides a Python interface to REFPROP using ctypes.

Step 3: Set RPPREFIX Environment Variable

The Python wrapper needs to know where REFPROP is installed:

Windows:

# PowerShell
$env:RPPREFIX = "C:\Program Files (x86)\REFPROP\"

# Or set permanently via System Properties > Environment Variables

Linux/Mac:

export RPPREFIX="/path/to/REFPROP/"

Python (alternative):

import os
os.environ['RPPREFIX'] = r'C:\Program Files (x86)\REFPROP'

Step 4: Verify Installation

from ctREFPROP.ctREFPROP import REFPROPFunctionLibrary

# Initialize REFPROP
RP = REFPROPFunctionLibrary(os.environ['RPPREFIX'])
RP.SETPATHdll(os.environ['RPPREFIX'])

# Test query
print(RP.RPVersion())

Available Fluids

REFPROP includes 147+ pure fluids organized by class:

Common Industrial Fluids

• Water (H₂O) - IAPWS-95 formulation
• Air - Dry air mixture
• Carbon Dioxide (CO₂)
• Nitrogen (N₂)
• Oxygen (O₂)
• Argon (Ar)
• Helium (He)
• Hydrogen (H₂)

Hydrocarbons (40+ fluids)

• Alkanes: Methane, Ethane, Propane, Butane, Pentane, Hexane, Heptane, Octane, Nonane, Decane (and isomers)
• Alkenes: Ethylene, Propylene, Butene
• Aromatics: Benzene, Toluene, Xylenes
• Cyclic: Cyclohexane, Cyclopentane

Refrigerants (60+ fluids)

• CFCs: R11, R12, R13, R113, R114, R115
• HCFCs: R22, R123, R124, R141b, R142b
• HFCs: R23, R32, R125, R134a, R143a, R152a, R227ea, R236fa, R245fa, R365mfc
• HFOs: R1234yf, R1234ze(E), R1233zd(E), R1336mzz(Z)
• Natural: R717 (Ammonia), R744 (CO₂), R290 (Propane), R600a (Isobutane)
• Blends: R404A, R407C, R410A, R507A, R407F, R448A, R449A, R450A, R513A

Cryogenic Fluids

• Helium, Neon, Argon, Krypton, Xenon
• Hydrogen, Deuterium, Parahydrogen
• Nitrogen, Oxygen, Fluorine
• Methane, Ethane (LNG components)

Specialty Fluids

• Siloxanes: D4, D5, D6, MD2M, MD3M, MD4M, MDM, MM (ORC applications)
• Alcohols: Methanol, Ethanol, Propanol, Butanol
• Other Organics: Acetone, Benzene, Toluene
• Inorganics: Ammonia, Sulfur dioxide, Hydrogen sulfide

See reference.md for complete fluid list.

Property Queries

Core Functions

REFPROP uses different function calls than CoolProp:

from ctREFPROP.ctREFPROP import REFPROPFunctionLibrary
import os

# Initialize
RP = REFPROPFunctionLibrary(os.environ['RPPREFIX'])
RP.SETPATHdll(os.environ['RPPREFIX'])

# Define fluid
MOLAR_BASE_SI = RP.GETENUMdll(0, "MOLAR BASE SI").iEnum
fluid = "WATER"

# Single component
r = RP.REFPROPdll(fluid, "TP", "D;H;S;CP;CV;W;VIS;TCX", MOLAR_BASE_SI, 0, 0, 298.15, 101.325, [1.0])

# Access results
density = r.Output[0]      # kg/m³
enthalpy = r.Output[1]     # J/kg
entropy = r.Output[2]      # J/kg/K
cp = r.Output[3]           # J/kg/K
cv = r.Output[4]           # J/kg/K
sound_speed = r.Output[5]  # m/s
viscosity = r.Output[6]    # Pa·s
conductivity = r.Output[7] # W/m/K

Input Specifications

Output Properties

Multiple properties can be requested in one call, separated by semicolons: "D;H;S;CP;VIS;TCX"

Mixture Calculations

One of REFPROP's key advantages is sophisticated mixture handling:

# Define mixture by components and mole fractions
fluid = "METHANE;ETHANE;PROPANE"
z = [0.9, 0.07, 0.03]  # Mole fractions (must sum to 1.0)

# Query mixture properties
r = RP.REFPROPdll(fluid, "TP", "D;H;S", MOLAR_BASE_SI, 0, 0, 250.0, 5000.0, z)

Mixture Features:

• Up to 20 components in a mixture
• Advanced mixing rules (GERG-2008, Kunz-Wagner, etc.)
• Temperature glide calculations for zeotropic blends
• Vapor-liquid equilibrium (VLE) calculations
• Liquid-liquid equilibrium (LLE) for certain systems

Advantages Over CoolProp

While CoolProp is excellent and free, REFPROP offers:

1. Higher Accuracy

• REFPROP is the reference standard that other libraries validate against
• More precise equations of state, especially near critical point
• Better transport property correlations
• Typical accuracy: 0.01% for density vs 0.1% for CoolProp

2. More Fluids

• 147+ pure fluids vs ~120 in CoolProp
• All latest refrigerants (HFOs: R1234yf, R1234ze, R1233zd, etc.)
• More industrial and specialty chemicals
• Frequently updated with new fluids

3. Better Mixture Handling

• Superior mixing rules (GERG-2008, advanced models)
• Handles complex non-ideal mixtures
• Temperature glide for zeotropic refrigerant blends
• VLE/LLE calculations
• Up to 20 components vs limited mixture support in CoolProp

4. Extended Property Range

• Valid over wider temperature and pressure ranges
• Better extrapolation behavior
• More reliable in extreme conditions

5. Additional Capabilities

• Humidity calculations (psychrometrics)
• Flash calculations (PT, PH, PS, etc.)
• Saturation properties with Jacobian
• Ancillary equations
• Pseudo-pure fluid approximations

6. Official Support

• Maintained by NIST (National Institute of Standards and Technology)
• Technical support available
• Regular updates with latest research
• Publication-quality results

When to Use REFPROP vs CoolProp

Use REFPROP when:

• Highest accuracy is required (±0.01%)
• Working with complex mixtures
• Need latest refrigerant data (HFOs)
• Publication or critical design work
• Budget allows commercial license
• Working with zeotropic blends (temperature glide)

Use CoolProp when:

• Budget constraints (free/open-source)
• Basic property needs (±0.1% accuracy acceptable)
• Pure fluids only
• Open-source license required
• Quick prototyping

Validation Note: CoolProp is validated against REFPROP, confirming REFPROP as the reference standard.

Common Workflow Example

from ctREFPROP.ctREFPROP import REFPROPFunctionLibrary
import os

# Setup
os.environ['RPPREFIX'] = r'C:\Program Files (x86)\REFPROP'
RP = REFPROPFunctionLibrary(os.environ['RPPREFIX'])
RP.SETPATHdll(os.environ['RPPREFIX'])

MOLAR_BASE_SI = RP.GETENUMdll(0, "MOLAR BASE SI").iEnum

# Query water at 25°C, 1 bar
fluid = "WATER"
T = 298.15  # K
P = 100.0   # kPa

r = RP.REFPROPdll(fluid, "TP", "D;H;S;CP;VIS;TCX", MOLAR_BASE_SI, 0, 0, T, P, [1.0])

print(f"Water at {T} K, {P} kPa:")
print(f"  Density: {r.Output[0]:.4f} kg/m³")
print(f"  Enthalpy: {r.Output[1]:.2f} J/kg")
print(f"  Entropy: {r.Output[2]:.2f} J/kg/K")
print(f"  Cp: {r.Output[3]:.2f} J/kg/K")
print(f"  Viscosity: {r.Output[4]*1000:.4f} mPa·s")
print(f"  Conductivity: {r.Output[5]:.4f} W/m/K")

Error Handling

REFPROP returns error codes and messages:

r = RP.REFPROPdll(fluid, "TP", "D", MOLAR_BASE_SI, 0, 0, T, P, z)

if r.ierr > 0:
    print(f"Error {r.ierr}: {r.herr}")
elif r.ierr < 0:
    print(f"Warning {r.ierr}: {r.herr}")
else:
    print(f"Success: {r.Output[0]}")

Units

REFPROP uses different unit conventions than CoolProp:

Default Units (with MOLAR BASE SI):

• Temperature: K (Kelvin)
• Pressure: kPa (kilopascals) - Note: CoolProp uses Pa
• Density: kg/m³
• Enthalpy: J/kg
• Entropy: J/kg/K
• Viscosity: Pa·s
• Thermal conductivity: W/m/K

Unit Systems Available:

• SI with molar units
• SI with mass units (default shown above)
• English units
• Custom units

Best Practices

1. Always set RPPREFIX before importing ctREFPROP
2. Check ierr for errors after each call
3. Use exact fluid names (case-sensitive, from FLUIDS directory)
4. Pre-calculate compositions for mixtures (must sum to 1.0)
5. Cache RP instance - don't reinitialize repeatedly
6. Use appropriate input pairs for each phase region
7. Validate results against known values initially
8. Read error messages - REFPROP provides detailed diagnostics

References

Official Resources

1. NIST REFPROP Website

• URL: https://www.nist.gov/srd/refprop
• Purchase and download

1. REFPROP Documentation

• Included with installation
• REFPROP.pdf in installation directory

1. ctREFPROP Documentation

• URL: https://github.com/usnistgov/REFPROP-wrappers/tree/master/wrappers/python
• GitHub: https://github.com/usnistgov/REFPROP-wrappers

Key Publications

• Lemmon, E.W., Bell, I.H., Huber, M.L., McLinden, M.O. (2018). "NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP, Version 10.0", National Institute of Standards and Technology.

Support

• NIST Support: refprop@nist.gov
• Issue Tracker: https://github.com/usnistgov/REFPROP-issues
• Forum: https://groups.google.com/g/refprop

Quick Reference

# Initialize
from ctREFPROP.ctREFPROP import REFPROPFunctionLibrary
RP = REFPROPFunctionLibrary(os.environ['RPPREFIX'])
RP.SETPATHdll(os.environ['RPPREFIX'])
MOLAR_BASE_SI = RP.GETENUMdll(0, "MOLAR BASE SI").iEnum

# Pure fluid
r = RP.REFPROPdll("WATER", "TP", "D;H;S", MOLAR_BASE_SI, 0, 0, 300.0, 101.325, [1.0])

# Mixture
r = RP.REFPROPdll("METHANE;ETHANE", "TP", "D;H;S", MOLAR_BASE_SI, 0, 0, 200.0, 5000.0, [0.9, 0.1])

# Check for errors
if r.ierr != 0:
    print(f"Error/Warning: {r.herr}")

NIST REFPROP is the gold standard for thermophysical properties. While it requires a commercial license, the investment is justified for applications requiring the highest accuracy, mixture calculations, or publication-quality results. For open-source alternatives, see the `coolprop-db` skill.