Thermodynamic Steam Tables 1

[Anthonycrf450r]

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

 

[racookpe1978]

What pressure do you "think" you have at the "exit" of the turbine?
What temperature?

 

[Anthonycrf450r]

given a pressure of 9kpa and an enthalpy at this point of 2370kj/kg. The temp is unknown. So from the theory stated, the pressure will give you temperature????.....but.....on this same line in the table the enthalpy does not equal the given value....hence wondering if the data is wrong? or my interpretation of theory is wrong???

 

[Engdoitbetter]

Dear Anthony,

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

 

[25362]

[•] 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 43.76[sup]o[/sup]C.

 

[Latexman]

I believe Anthonycrf450r meant 9 kPa guage, not absolute.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Latexman]

On second read, he never really said whether the turbine was condensing or non-condensing and the enthalpy is out of line with my references. Confusing.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[25362]

Anthonycrf450r,

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%.

 

[Anthonycrf450r]

ahhhhhhh, thanks to all. I now understand.

In addition to the above theory, quality is the next determining factor to get the enthalpy of steam!!!!!!!!!!

 

[racookpe1978]

Condensing or non-condensing turbine?

From
• 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.

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.

 

[Latexman]

In the big chemical plants I've worked in, turbines without condenser are usually used in the process areas and turbines with condenser are usually used in the "powerhouse". It's not always this way, byt usually.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[25362]

See please, thread666-143727