Region 1 Pressure with Temperature and Enthalpy Given
When looking up pressure with temperature and enthalpy given in Region 1, actually there are two pressures that satisfying the temperature and enthalpy given. One is the pressure in Region 1 as we expect and the other is saturation pressure at the temperature given at Region 4. The two pressures are different. This is caused by .the fact that isothermal line crosses isenthalpic line twice in Region 1 of Mollier chart..
In this case, when looking up pressure with temperature and enthalpy given, ES_StableIF97 looks up Region 1 first before checking whether the given condition is located in saturated condition. If the given condition is not in Region 1, then ES_StableIF97 checks whether the given condition is located in saturated condition. If the given condition is located in saturated condition, ES_StableIF97 returns saturation pressure at the temperature given.
The sample of this can be found among the test data for Region 1 in the Table 5 of IAPWS-IF97 shown below.
The first test data of the Table 5 is 3 MPa pressure by 300 K temperature and 115.331273 kJ/kg enthalpy input, if looking up pressure with temperature and enthalpy given. This state must be located in Region 1, because the Table 5 is for Region 1.
However, at 300 K temperature the saturation pressure is 0.0035365894 MPa, and the saturation liquid and vapor enthalpies are 112.574991 kJ/kg and 2549.89301 kJ/kg respectively. Therefore, when user inputs 300 K temperature and 115.331273 kJ/kg enthalpy to look up pressure, two pressures are eligible for the condition. One is 3 MPa in Region 1 and the other is 0.00353658941 Mpa in Region 4, saturated condition.
In this case ES_StableIF97 looks up 3 MPa.
Therefore, when the user looks up pressure by temperature and enthalpy given, he has to decide first whether he want to look up saturated condition, or not. If yes, the user must use the saturation function "ESStb_PsatbyT" or "ESStb_XbyTH" instead of "ESStbe_PbyTH".
The picture below shows various ES_StableIF97 function outputs for the sample above. In case that the user looks up quality with the temperature and enthalpy given, the quality(X) is calculated as 0.0011308668 as shown in the picture below.
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