$Reference

$Reference

The $REFERENCE directive allows the default reference state for a specified fluid to be changed. The format for this directive is

$REFERENCE Fluid ReferenceID

where

Fluid is the name of the fluid, e.g., R134a. The fluid name can also be provided with a previously-defined string variable, e.g., R$.

ReferenceID is one of the following:

DFT - the reference is reset to the default value. See the fluid property information for the specified fluid to determine the default reference state

NBP - the values of specific enthalpy and specific entropy are each set to 0 for saturated liquid at the normal boiling point. Note that this option is not applicable to fluids for which the critical pressure is less than one atmosphere. In this case, the reference will be reset to the default value.

ASH - the values of specific enthalpy and specific entropy are each set to 0 for saturated liquid at -40 C (-40 F). This is the ASHRAE standard reference. Note that this option is not applicable to fluids for which the critical temperature is less than -40 C. In this case, the reference will be reset to the default value.

IIR - the value of specific enthalpy is set to 200 kJ/kg and the value of specific entropy is set to 1.0 kJ/kg-K for saturated liquid at 0 C (273.15 K). This is the standard reference state for the International Institute of Refrigeration. Note that this option is not applicable to fluids for which the critical temperature is less than 0 C. In this case, the reference will be reset to the default value.

RXN - the value of specific enthalpy at 25 C, 101.3 kPa is set to the enthalpy of formation and the value of specific entropy is set to a value based on the Third Law of thermoydnamics. These are the reference states used for ideal gases. This reference state allows fluids such as steam, carbondioxide, methane, etc. to be used in combustion calculations, even at conditions in which they do not exactly obey the ideal gas law. Note that the unit system should be set for molar basis with this reference state.

The $Reference directive can not be placed in a Function, Procedure, Subprogram or Module.

Examples:

$REFERENCE R134a IIR

{Determine the Lower and Higher Heating Values of Methane}

$UnitSystem SI C kPa molar

$Reference Methane RXN

$Reference Steam RXN

$Reference CarbonDioxide RXN

P=Po#

T=25 [C]

h_CH4=enthalpy(Methane,T=T,P=P)

h_H2O_vap=enthalpy(Steam_IAPWS,T=T,x=1) {enthalpy of water vapor}

h_H2O_liq=enthalpy(Steam_IAPWS,T=T,x=0) {enthalpy of liquid}

h_CO2=enthalpy(CarbonDioxide,T=T,P=Po#)

HHV=h_CO2+2*h_H2O_liq-h_CH4 {higher heating value}

LHV=h_CO2+2*h_H2O_vap-h_CH4 {lower heating value}

Use with the EES_REFPROP Interface

The $REFERENCE directive can be used for properties determined using the EES_REFPROP interface. In this case, either _RP must be appended to the fluid name or the /EESREFPROP option must follow the $REFERENCE keyword, as shown in the following program. The fluid name must be included in the REFPROP data base. Mixtures created by adding components, e.g., Propane+Butane, cannot be used with this directive.

$REFERENCE R452A /EESREFPROP NBP

$UnitSystem SI K kPa Mass

P1=101.315 [kPa]

h1=enthalpy(R452A_RP,P=P1,x=0)

$REFERENCE R457A_RP ASH

T2=converttemp(C,K,-40 [C])

h2=enthalpy(R457A_RP,T=T2,x=0)

{Solution:

h1=0 [kJ/kg]

h2=0 [kJ/kg]

}