Contents - Index



EES provides properties for many pure fluids and common mixtures such as R407C, R410A, and ammonia-water. Click here to view a list of the fluids for which property data are built into EES.  The National Institute of Standards and Technology (NIST) has developed REFPROP (also called NIST Standard Reference Database 23), which provides high accuracy property data for pure refrigerants and refrigerant mixtures. REFPROP is the most authoritative source of pure fluid property data.  REFPROP also provides high accuracy data for refrigerant mixtures, ammonia-water, and most mixtures of organic fluids. 


The EES_REFPROP is an optional interface has been developed for REFPROP (version 10 and newer) that allows the NIST property data base to be used with the equation-solving and other features of EES. 


Starting with EES version 11.477, property data can be obtained from REFPROP with the EES_REFPROP interface using exactly the same property commands as used for built-in EES fluids.  All that is needed is to add _RP to the name of the fluid that REFPROP recognizes.  For example, the following program calculates the properties of propane using REFPROP:


$UnitSystem SI C kPa mass

T=25 [C]

P=100 [kPa]








h=630.4 [kJ/kg]

s=2.847 [kJ/kg-K]

k=0.01185 [W/m-K]

ST=0.007303 [N/m]



Property calls to made in this manner use the unit system specified in EES.  The independent properties in the above program are temperature and pressure, but any valid set of independent properties can be provided, just as for the built-in EES functions.  The fluid name that is provided in the property functions, (e.g., Propane_RP) must be recognized by REFPROP (without the _RP).  The fluid name may be a pure fluid (e.g., Propane_RP) or predefined mixture (e.g, R466A_RP).  It is not possible to specify a mixture with a fluid name containing a + symbol (e.g., propane_rp+butane_rp), which is possible using the Call EES_REFPROP procedure described in the EES_REFPROP manualProperty plots using data from REFPROP can be generated by selecting the EES_REFPROP button at the top of the Property Plot dialog.


The reference state for specific enthalpy and entropy will be as set by REFPROP, but it can be changed using a $Reference directive, e.g.,




An EES_REFPROP interface dated May 27, 2022 is required to use EES format for property functions or to produce plots. You can determine the date of your EES_REFPROP interface using the EES_REFPROP_DATE$ function. If you wish to update to the current version and you purchased it less than one year ago, send an email to to request a free update. If you purchased the interface more than one year ago, you can order the update for 20% of the current cost. The order can be placed at our webstore


The EES_REFPROP manual provides a detailed description of the EES calls needed to access the REFPROP data using the older Call EES_REFPROP format.  The $ConvertEESREFPROPUnits directive provides automatic unit conversion of the inputs to and outputs from the EES_REFPROP interface with the Call EES_REFPROP format.  This directive is not needed when using the EES property function calls.


The EES_REFPROP interface can be ordered from the F-Chart website.  Note that the EES_REFPROP interface is paired to an EES license and it will only operate with that license. The number of users for the EES_REFPROP interface must match the number of users for the EES license. The standard EES_REFPROP interface is a 32-bit dynamic link library (DLL) that interfaces with the 32-bit REFPROP program. A 64-bit license of EES is available and it requires 64-bit DLLs, including the 64-bit EES_REFPROP interface.


Note that EES can use REFPROP .FDL files directly to provide property data for pure fluids without use of the EES_REFPROP interface. However, this method can not be used for mixtures.


The EES_REFPROP inteface is updated occasionally.  You can see the date of the interface using the EES_REFPROP_Date$ function.



See also: EES_REFPROP manual

 Using REFPROP .FDL files


 EES_REFPROP Property Plots