EES provides properties for many pure fluids and some refrigerant mixtures such as R407C, R410A, R449A, R450A. R452A, R452B, R454C, R507A, R508B, R513A, R514A and ammonia-water. Click here to view a list of the fluids for which property data are available in 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. Included in the database are the "natural" refrigerants, carbon dioxide, ammonia, propane, isobutane and other hydrocarbons. REFPROP also provides high accuracy data for mixtures such as ammonia-water and most organic refrigeration mixtures. REFPROP uses the most advanced methods for estimating property data. Transport properties as well as thermodynamic properties are provided in all fluid regimes, including compressed liquid and near critical states.

An interface has been developed for REFPROP that allows the NIST property data base to be used with the equation-solving and other features of EES. If you are involved in modeling refrigeration systems that use property information not included in EES, you will find the NIST REFPROP program and the EES_REFPROP interface to be of considerable interest. Contact NIST to order REFPROP. Single and multiple-user licenses for the EES_REFPROP interface can be ordered. 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. If, for example, an EES 5-user license is in use, a 5-user EES_REFPROP interface is required. Click here to view the EES_REFPROP user instructions.

The $ConvertEESREFPROPUnits directive in EES (version 10.119 and newer) provides automatic unit conversion of the inputs to and outputs from the EES_REFPROP interface to the unit system set in the EES program.

The $REFERENCE directive in EES (version 10.364 and newer) can be used to set the reference state for specific enthalpy and entropy values provided by the REFROP program.

Starting with EES version 11.366, the Property Plot command can be used to prepare property plots using REFPROP data. An EES_REFPROP interface dated May 27, 2022 is required. 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 standard EES_REFPROP interface is a 32-bit dynamic link library (DLL) that interfaces with the 32-bit REFPROP program. A 64-bit version of EES is available and it requires 64-bit DLLs. If you have previously purchased the EES_REFPROP interface and are using the 64-bit version of EES, you may wish to upgrade to the 64-bit EES_REFPROP interface. Calculation speeds can be significantly improved with the 64-bit version.

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