Finding the state in thermodynamics

[alexdeslaurier]

Hi I am having trouble finding the state of a process in thermodynamics from the question for example I have a question that asks



10-kg of R-134a at 300 kPa fills a rigid container
whose volume is 14 L. Determine the temperature and total
enthalpy in the container. The container is now heated until
the pressure is 600 kPa. Determine the temperature and total
enthalpy when the heating is completed.

I don't want to know how to solve the problem I just need help understanding it. The solution states that it this solution is a mixture and thus the temperature is the saturation temperature at the given pressure.

What I would like to know is how do we know from what is given that this is a mixture.

Thanks a lot.

 

[Chestermiller]

Hi I am having trouble finding the state of a process in thermodynamics from the question for example I have a question that asks



10-kg of R-134a at 300 kPa fills a rigid container
whose volume is 14 L. Determine the temperature and total
enthalpy in the container. The container is now heated until
the pressure is 600 kPa. Determine the temperature and total
enthalpy when the heating is completed.

I don't want to know how to solve the problem I just need help understanding it. The solution states that it this solution is a mixture and thus the temperature is the saturation temperature at the given pressure.

What I would like to know is how do we know from what is given that this is a mixture.

Thanks a lot.

Do you have graphs with temperature, pressure, and enthalpy for 134a? What kind of graph is it? If not, do you have tables, including specific volumes of saturated liquid and vapor? If you look up a density of 10/14 kg/L at 300 kPa, you will find that this density is between that of the saturated vapor and the saturated liquid at this pressure.

 

[alexdeslaurier]

I have access to the tables but the tables doesn't have the information at 300 kPa I had to use interpolation to find all the values but from the solution it seems to be obvious that the solution is a mixture here is what is written in the solution

'' The initial state is determined to be a mixture, and thus the temperature is the saturation temperature at the given pressure. From tables A-12 by interpolation.''

It seems that before any work is done its known that its a mixture, why is that?

 

[Chestermiller]

I have access to the tables but the tables doesn't have the information at 300 kPa I had to use interpolation to find all the values but from the solution it seems to be obvious that the solution is a mixture here is what is written in the solution

'' The initial state is determined to be a mixture, and thus the temperature is the saturation temperature at the given pressure. From tables A-12 by interpolation.''

It seems that before any work is done its known that its a mixture, why is that?

Go back to your tables and interpolate to get the density of the saturated liquid at 300 kPa and the density of the saturated vapor at 300 kPa. You will find that your bulk density of 10/14=0.714 gm/cc will lie between that of the saturated liquid and the saturated vapor. This means that you must have a mixture of saturated liquid and saturated vapor. Superheated vapor will have a density less than that of the saturated vapor, and subheated liquid will have a density greater than 0.714.

Chet

 

[rezeew]

I can understand your confusion about determining the state in thermodynamics. In this particular problem, it is stated that 10-kg of R-134a fills a rigid container. R-134a is a refrigerant that is commonly used in air conditioning systems and heat pumps. In its pure form, R-134a is a single component substance, but it can also exist as a mixture with other substances.

In this case, it is likely that the R-134a is present as a mixture in the container. This is because the problem states that it is at a pressure of 300 kPa, which is below the critical pressure of R-134a (which is 4068 kPa). This means that the R-134a is in the liquid-vapor region, where it exists as a mixture of liquid and vapor.

To determine the state of a substance in thermodynamics, we need to know two independent properties such as temperature, pressure, or specific enthalpy. In this problem, the pressure and volume of the container are given, but not the temperature or specific enthalpy. Therefore, we need to use other information, such as the fact that the substance is a mixture, to determine its state.

The solution states that the temperature is the saturation temperature at the given pressure. This means that at 300 kPa, the R-134a is at its boiling point, where it exists as a mixture of liquid and vapor. At this point, the temperature and pressure are in equilibrium, and any additional heat added to the system will cause the substance to transition from liquid to vapor.

When the pressure is increased to 600 kPa, the temperature will also increase, but the substance will still exist as a mixture of liquid and vapor. This is because the temperature and pressure are still in equilibrium at the new pressure. Once the heating is completed, the substance will reach its saturation temperature at 600 kPa, and any additional heat added will cause it to fully transition to vapor.

I hope this explanation helps you understand how we can determine the state of a substance in thermodynamics. It is important to consider all the given information and use the properties of the substance, such as its critical pressure and boiling point, to determine its state accurately.