Thermodynamic Steam Tables
Thermodynamic Steam Tables
(OP)
Hi All, first post here....
at the exit of a turbine in a power plant you have saturated steam at low pressure, eg 10kPa. It is stated that at pressure below 10kPa steam can be considered an ideal gas as the % error based on this assumption is minimal. Based on the pressure the temperature of the steam can be found directly from the steam tables. It is also said that enthalpy of an ideal gas is directly dependent on temperature.
So for an ideal gas, the pressure determines the temperature which in turn gives you your enthalpy? is this correct?
Is there any case where enthalpy will be different? Currently I am given some data where the pressure and enthalpy values at a specific point in a system, do not correspond to the same temperature in the steam tables....so I am wondering if the data is wrong or if I have the theory wrong? I am by no means an experienced 'thermodynamacist', so I am expecting it is the later.
Thanks
at the exit of a turbine in a power plant you have saturated steam at low pressure, eg 10kPa. It is stated that at pressure below 10kPa steam can be considered an ideal gas as the % error based on this assumption is minimal. Based on the pressure the temperature of the steam can be found directly from the steam tables. It is also said that enthalpy of an ideal gas is directly dependent on temperature.
So for an ideal gas, the pressure determines the temperature which in turn gives you your enthalpy? is this correct?
Is there any case where enthalpy will be different? Currently I am given some data where the pressure and enthalpy values at a specific point in a system, do not correspond to the same temperature in the steam tables....so I am wondering if the data is wrong or if I have the theory wrong? I am by no means an experienced 'thermodynamacist', so I am expecting it is the later.
Thanks





RE: Thermodynamic Steam Tables
What temperature?
RE: Thermodynamic Steam Tables
RE: Thermodynamic Steam Tables
It is true that for saturated steam you do need only one variable to define the state. So you can obtain temperature simply from pressure.
But when you try to calculate enthalpy from temperature, you must take into account that:
a) specific heat of steam may not be constant with temperature;
b) the enthalpy you're calculating is for steam only and does not consider the presence of water.
Hope it helps.
Stefano
RE: Thermodynamic Steam Tables
• There are no ideal gases, only asymptotic convergence of real gases to ideality.
• NIST tabulated enthalpy of saturated steam at 9 kPa is 2580.2 kJ/kg.
• NIST gives a corresponding temperature of 43.76oC.
RE: Thermodynamic Steam Tables
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Thermodynamic Steam Tables
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Thermodynamic Steam Tables
Please note that the turbine exhaust is of a quality of around 90%.
In the given example the enthalpy of water at the given pressure is 183,25 kJ/kg. Therefore, to get an enthalpy of 2370 kJ/kg, as reported by you, one must have a steam quality of 91.23%.
RE: Thermodynamic Steam Tables
In addition to the above theory, quality is the next determining factor to get the enthalpy of steam!!!!!!!!!!
RE: Thermodynamic Steam Tables
That 43.7 degrees C is much higher temperature than should be expected for a modern steam turbine. A condensing turbine should be exiting into a vacuum (absolute pressure less than atmospheric) at less than 30 C.
RE: Thermodynamic Steam Tables
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Thermodynamic Steam Tables