Answers to Frequently Asked Questions
The following information gives answers to some of
the most frequently asked questions concerning the REFPROP software (that is
described in the web page at www.nist.gov/srd/nist23.htm). To download any of the files listed
below, right-click on the file and select Save Target As... and then add
the file to your Refprop or other appropriate directory.
For questions not answered here, please see the user's guide at www.nist.gov/srd/PDFfiles/REFPROP8.PDF
1. For differences in enthalpy and entropy between the Refprop graphical interface and the Refprop Excel sample spreadsheet, or for differences in enthalpy and entropy between Refprop and tables of properties given in handbooks:
The absolute values of enthalpy, entropy, and energy at a single state point are meaningless. It is only the difference between two different state points that matter. Thus, the value for a single state point can be set to any arbitrary value. Many handbooks set the arbitrary state point so that the values of these properties are positive for most liquid or gas states. The user can change the values of the arbitrary state points by going to the Options/Reference State menu.
For mixtures, there are additional options that can be set to affect the manner in which these properties are calculated. The preset values sent out with Refprop 7.0 are different between the graphical interface and the Excel file. In the Options/Reference State menu, there are two choices at the bottom of the menu on the far right. By changing this option, the two programs will then return the same values. This option can be permanently saved in the graphical interface by selecting Options/Save Options, and the saving the options under the file name defaults.prf. To change the default in the Excel file, press Alt-F11 to bring up the VB code. If the code does not appear, make sure the project explorer is visible (View/Project Explorer), and then click on modules and then on module1. Search for the call to SETREF, and change the second input from 2& to 1&. More information on this can be found in your Refprop\Fortran directory in the file SETUP.FOR under the SETREF subroutine.
To permanently change the default setting for a pure fluid, edit the fluid file and look at the 14th line of the file, which will appear similar to this:
IIR !default reference state
Remove this line and add two lines in its place, similar to the following:
OTH !default reference state
300.0 1.0 10000.0 100.0 !tref, Pref, Href, Sref
These lines will set the enthalpy to 10000 J/mol and the entropy to 100 J/mol-K at 300 K and 1 kPa. These values can be changed to any other desired values.
2. The following reference can be used to cite the REFPROP program in publications:
Lemmon, E.W., Huber, M.L., McLinden, M.O. NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP, Version 8.0, National Institute of Standards and Technology, Standard Reference Data Program, Gaithersburg, 2007.
3. The most recent help file from Refprop is available below.
4. There are cases where an input state point can result in two separate valid states. The most common is temperature-enthalpy inputs. Viewing a T-H diagram will help show how there can be two valid states points for a given input. For example, nitrogen at 140 K and 1000 J/mol can exist at 6.85 MPa and at 60.87 MPa. When this situation occurs, Refprop returns the state with the higher density. For these state points , the character '<' or '>' can be added to the number (before or after) to find the lower or upper root, respectively. These same characters can also be used to find metastable fluid states for temperature and pressure inputs. For example, the saturation pressure for nitrogen at 100 K is 0.778 MPa. Inputs of 100 K and 0.777 MPa will return a vapor state, but inputs of '100' for temperature and '0.777>' for pressure will return a metastable liquid state. Item #11 below gives additional information about the use of the '>' and '<' symbols.
5. If the following error message appears when the graphical interface for 7.0 is launched:
run-time error 339
component MSHFLXGD.OCX or one of its dependencies not correctly registered: a file is missing or invalid
then download the file from the link below and save it in your Windows\System32 or Windows\System directory.
The comdlg32.ocx file may also sometimes be needed:
6. The fortran code and Excel sample file in version 7.0 are not able to read the *.mix files, but that has changed in version 8.0. However, the .mix files are ASCII files and thus can be read from any editor and are quite simple to understand. The first two lines are just information and can be ignored.
7. The transport properties for Nitrogen, Argon, and
Oxygen in NIST12 did not include the thermal conductivity enhancement for the
critical region and did not represent the experimental data as well as
possible. The transport equations
for these fluids have been redone recently and now represent the data to within
their experimental uncertainties.
The publication below documents the new equations and shows all of the
comparisons to data.
N2-Ar-O2 Transport equations documentation
8. For
subroutines such as ENTHAL, ENTRO, CVCP, etc., where temperature and density
are the input properties, the output property data will differ from those
returned when using routines such as PHFLSH or TPFLSH when in the two-phase
region. Any
routine (except TDFLSH) that uses temperature and density as inputs will return
what appears to be erroneous results in the two-phase region. These
results actually show calculations directly from the equation of state
without taking into account the fact that the mixture has split into two
phases. The results would be valid for such situations where a substance
is heated beyond its boiling point (a metastable state), but without boiling
(such as water in a glass container being heated in a microwave). The
results eventually end up at the spinodal, beyond which it is no longer
possible to increase the temperature without boiling the liquid. Thus,
for typical results these routines should never be used for two-phase
calculations, rather the FLSH routines should be called. The flash
routines return the properties in the liquid and vapor states, and these can be
used to call routines such as THERM, CVCP, TRNPRP, etc., with the associated
liquid or vapor density at the saturated temperature.
9. There are several fluid files that
Refprop accesses in order to run properly. These are nitrogen.fld, propane.fld, R134a.fld, and C12.fld
(dodecane). These fluids are used
as reference fluids in extended corresponding states methods employed in
Refprop to predict transport properties for some instances. These fluids (and the hmx.bnc file)
should be distributed in addition to those required in a particular application
if the Refprop routines have been incorporated into a software package.
10. Although version 8.0 allows mixtures of
nitrogen, argon, oxygen, and water as a consequence from the addition of the
new natural gas mixture model, calculations for moist air have not been tested
yet. It is likely that calculated
values are reasonable, however, Refprop may not return results because the
saturation routines may fail. The
program calls the saturation routines to determine if the state is vapor,
liquid, or two-phase. If you know
that your state point is in the vapor phase, you can avoid the call to the saturation
routines by using TPRHO instead of TPFLSH. The fortran file FLSH_SUB.FOR gives additional information
concerning the inputs to these routines.
A moist air mixture could be made up starting with the composition of
dry air used in Refprop: 0.7812 nitrogen,
0.0092 argon, and 0.2096 oxygen (on a mole basis). A small amount of water could be added to this composition
and then normalized.
11. When using mixtures, error messages
will sometimes be reported when Refprop fails to converge during an iteration,
usually during the calculation of a VLE state close to the critical point. In some situations you may know that
the point is in the single phase and wish to force the calculation in spite of
the error message. This can be
done with the '>' and '<' symbols.
When one of these is attached to a number in the Specified State Points
table, Refprop will assume that the point is in the liquid phase when '>' is
attached and in the vapor phase when '<' is attached. Care must be taken because metastable
states will be returned in the point is in the two phase. When a state point is above the
critical point but Refprop reports a nonconvergence, you can use either of
these symbols to force the calculation.
For example, a 0.5/0.5 mixture of methane/propane will report a
nonconvergence at 300 K and 10 MPa.
A P-rho diagram shows that the point is above the critical
pressure. Entering '300' for
temperature and '10>' for pressure will return the valid state point at
10.826 mol/l.
12. The error message "No mixture data are available for one or more binary pairs in the specified mixture. The mixing parameters have been estimated." (message #117) occurs when an interaction parameter for two different fluids is not available in the program. The most common reason for this occurrence is a lack of experimental data to describe VLE, densities, and so forth for the binary pair. In other situations, such as many mixtures with water, interaction parameters have not been fitted to the REFPROP model even though data exist. An estimation scheme is available in the program to approximate one of the interaction parameters that helps improve bubble and dew point pressures. This error message indicates that you should be aware that calculated properties are estimates only. For similar fluids, especially among the refrigerants, the scheme works fairly well. It breaks down with dissimilar fluids and eventually the scheme will produce large interaction parameters and will report the error message "No mixture data are available for one or more binary pairs in the specified mixture. The mixture is outside the range of the model and calculations will not be made." The most common occurrence of this message comes from mixing methane with higher hydrocarbons that are not in the GERG-2004 mixture model. For example, mixing natural gas components with propylene will generate this message. With time we will add additional interaction parameters for those binary mixtures which are currently not described. At that time, further information will be made available about the status of the new mixing file.
13. The synonym "ethylene oxide" was incorrectly included in the DME.FLD file for dimethyl ether. The correct fluid file is available below:
14. When using the methods described in the help file to make the Excel routines available globally, the Step 1 Instructions will only work if a fresh worksheet is used without ever opening up the REFPROP.xls file. Since a user may need to open up this worksheet for help with syntax examples, this process is not always recommended. Rather, the Step 2 Instructions work better in this case, where the REFPROP.xls VB module is modified to make the FluidDirectory and MixturesDirectory public and to add the c:\program files\refprop directory to these variables. The Add-On should be made from the modified REFPROP.xls file as instructed.
Please send questions to Eric Lemmon <ericl@boulder.nist.gov>
Last modified: October 5, 2007