Properties of water at a temperature of 300°C and a pressure of 3000 kPa

Question

I'm using a online calculator, and it says that the boiling point of water at a pressure of 3000 kPa is 233.7°C.

So then I understand that the water in is a superheated state (and thus still a liquid). I see in a table of superheated water (Cengel, Thermal Fluid Science, Table A6) that at a saturation temperature of 300°C and a pressure of 3000 kPa (3 MPa), water has the following properties:

Specific volume: $0.08118\,{\rm \frac{m^3}{kg}}$.

Internal energy: $2750.8\,{\rm \frac{kJ}{kg}}$.

Enthalpy: $2994.3\,{\rm \frac{kJ}{kg}}$.

Converting to liters: 1 kg of water then has a volume of $8.118\cdot 10^{-5}\, {\rm dm^3}$ = 8.118 L. Knowing that 1 kg of water has a volume of 1 L at 1 atm, I find this value rather high.

Do I understand this correctly? One more thing: Is the saturation temperature the same as the boiling point? The seems to be disagreement when I google it.

Answer 1

Do I understand this correctly and one more thing: saturation temp is that the same as boiling point. Seems to be disagreement googling it.

No.

The saturation temperature is the same as the boiling point, but the mistake you are making is thinking that the specific volume, internal energy, and enthalpy for superheated steam at 300 $^0$C and 3000 kPa are for liquid. They are not. They are for the superheated vapor.

To determine the values for the liquid, you need to go to the saturated steam tables and interpolate between 230$^0$C and 235$^0$C. Then you can determine $v_f$, $u_f$ and $h_f$.

I understand I need to use the 2 tables: saturated water -pressure and saturated water temperature. But I also realize now that I need to read more to fully understand this

The following is offered as a primer on the steam tables which may help you to begin to more fully understand them.

First of all, the two tables you refer to are essentially the same table except that the first two columns are reversed. The first table has the saturation (boiling) pressure in the first column. The second table has the saturation (boiling) temperature in the first column. After that, the contents are the same. You can use either, but I prefer the second table if you are given the saturation temperature (boiling temperature).

The tables you were using are the Superheated Water Tables. These you need only to determine the saturation temperature (boiling point) corresponding to 3 MPa. Other than that, they only cover superheated vapor. You are interested in the liquid component of the steam. That you get in the Saturated Water-Temperature Tables.

Just so you can put the tables in perspective, see the diagram below. The overall curve that includes the saturated liquid line and saturated vapor line is sometimes referred to as "steam dome". Lines D, H, L etc. are constant pressure lines.

The area under the dome covers a mixture of liquid and vapor, called saturated steam. This area under, including the dome curve, is covered by the Saturated Water-Temperature Tables.

The area to the right of the dome is superheated steam and is covered by the Superheated Water Tables.

The area to the left of the dome is pure liquid.

The lines, points 1,2,3, and text in red is mine and is for your example. To find out how much liquid can be extracted from the superheated water of 300 C and 3 MPa, go to the Saturated Water-Temperature Tables. Interpolate between 230 C and 235 C to determine the specific volume of the liquid, $v_f$ in the table, where $f$ stands for fluid. This corresponds to point 3 in red (saturated liquid). You will see from the tables it is somewhere between 0.001209 and 0.001219.

Hope this helps.