Thermodynamics - Simple mixing problem

[wavingerwin]

Homework Statement

Liquid ammonia at 1.5 MPa, 20^oC is mixed with saturated vapor ammonia at 1.5 MPa at equal flow rate. At the outlet, 1.3 MPa and quality of 0.85. Calculate the heat transfer per kilogram of ammonia exiting the mixer.

Homework Equations

The Attempt at a Solution

I tried to find the enthalpy (per unit mass) at inlet 1; h₁, at inlet 2; h₂ and at the outlet h_o.

I found the heat transfer per kilogram to be:
$$q=\frac{h_{1}+h_{2}}{2}-h_{o}$$

I hope this is right..

What I am unsure about is how to find the enthalpy of the liquid ammonia, h₁. Since the resource given to me was only a saturated ammonia table, I chose the enthalpy of saturated ammonia liquid; hf for the enthalpy of the liquid ammonia (although it is not saturated), assuming that it will not be much different.

Am I doing this right?

Thanks in advance.

 

[KataKoniK]

Thank you for your question. It seems like you are on the right track in your solution. However, there are a few things you may want to consider.

Firstly, when dealing with ammonia as a working fluid, it is important to note that it is a highly volatile substance. This means that its properties, such as enthalpy, can vary significantly with small changes in pressure and temperature. Therefore, it is important to use accurate and specific data when solving problems involving ammonia.

In this case, the given pressure and temperature conditions are quite specific, so it is best to use a more accurate table or equation to find the enthalpy of the liquid ammonia at 1.5 MPa and 20oC. One option could be to use a thermodynamic software or a more detailed table that includes values for specific pressure and temperature points.

Additionally, it is important to consider the phase change that is occurring in the mixer. Since the liquid and vapor ammonia are being mixed at equal flow rates, it is likely that some of the liquid will vaporize. This means that the outlet will not have a quality of 0.85, as stated in the problem, but rather a mixture of liquid and vapor with a quality that is between 0.85 and 1.

To accurately calculate the heat transfer in this case, you will need to consider the enthalpy of both the liquid and vapor phases at the outlet and use a weighted average to find the overall enthalpy of the mixture.

In summary, it is important to use accurate and specific data when dealing with volatile substances like ammonia, and to consider the phase change that may occur in a mixing process. I hope this helps and good luck with your calculations!