Topic 7 lesson

Topic 7 lesson

by Professor Tomforde

​New vocabulary word!

​Isentropic: The entropy of the system remains constant.

Much like "isothermal" and "isobaric"​

​Entropy is a property.

​You'll find it in your lookup tables similar to how we have found v, u, and h.

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​The same rules apply--you can check the phase by determining how the entropy value 's' compares to the s_g or s_f in the tables.

​Let's get a sampling of what you need for your homework....

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​(Question 1 is very similar to Example 7-2)

Multiple Choice

What's the specific entropy of saturated liquid R-134a at 120 kPa?

• 0.09269 kJ/kg-K

• 0.94789 kJ/kg-K

• I don't want to bother looking at my tables; I'll just suffer through HW 7

• 0.85529 kJ/kg-K

​If you hate tables....

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​If it's air

​Recall that you can find the enthalpy of air from A-17 using the temperature

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​ (If the temperature doesn't change then neither does the enthalpy)

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​You might happen to need this for question 2.

​When do we use what?

Recall that ideal processes are isentropic, meaning s₁=s₂.

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​For some of your homework problems, you won't be given enough properties to determine the phase and find the values you're looking for without using this.

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​Using the entropy gives you the extra puzzle piece.

Multiple Choice

Steam at a pressure of 300 kPa and an entropy of 7.2699 kJ/kg-K is

• a compressed liquid

• a saturated mixture

• a saturated vapor

• a superheated vapor

​That was a superheated vapor!

​You could tell because in A-5 for P = 300 kPa, s_g = 6.9917 kJ/kg-K.

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​If s > s_g then it's a superheated vapor.

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​Let's try a trickier one!

Multiple Choice

The enthalpy of superheated steam at P =300 kPa and s = 7.2699 kJ/kg-K is closest to which of these?

• 175 C

• 2800 kJ/kg

• 2600 kJ/kg

• 0.634 m³/kg

​You needed A-6 for that one

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​Go to the heading P = 0.3 MPa and scroll through the s column to find the closest values to 7.2699 kJ/kg-K, then check the h values.

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To get a more precise answer, you'd use linear interpolation.

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​--yes, you probably have a homework question on this....

​How do we know when to use it?

​Recall that isentropic devices are the best possible. Some examples:

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• ​the minimum velocity exiting a diffuser (the best diffuser generates no entropy!)

• ​the minimum power needed for a compressor (the best compressor generates no entropy)

• ​Anything that explicitly states "isentropic" or "reversible+isothermal."

​Most of your homework

​Draw a box, identify energy inputs and outputs and write an energy balance equation....

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​If you need to hunt down an enthalpy value to plug into this, consider whether you can use the entropy to find it.

​The last question

​is a real doozy!

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​1. draw the control volume for the turbine. Write the energy balance for this control volume. You'll discover you need an enthalpy value!

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​2. You need to use the entropy to solve for the quality, then use the quality to solve for the enthalpy.

​Closing thoughts

If you have a pressure that's not in the superheated vapor table, it may be helpful to check the NIST webbook and use their table instead of ours.

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​Happy Thanksgiving, everyone!