Thermodynamics Energy transfer question

In summary: If you assume constant pressure, then the equation you use is correct. If you don't assume constant pressure, then the equation you use is incorrect, and you get an incorrect answer.Well, then I don't see how they get that answer. If you assume constant pressure, then the equation you use is correct. If you don't assume constant pressure, then the equation you use is incorrect, and you get an incorrect answer.
  • #1
Silverhobbiest
8
0

Homework Statement


In a frictionless piston-cylinder system, there are 3 kg of R-134a initially at 280 kPa and 15 °C. Heat is transferred to the system in the amount 120 kJ. What will the final temperature of the refrigerant be (deg C)?

Homework Equations


Q - W = ΔU (internal energy)
Q - Wb (boundary work) = ΔH (enthalpy)
(if pressure is constant) Wb = P(v2-v1)

[where v is specific volume]

The Attempt at a Solution


I assumed that pressure was constant for this problem because the piston is implied to be free to move. Also, I know that there is boundary work because if the piston is free to move then the volume is changing.

So, I used the equation:
Q - Wb = ΔH

I identified the refrigerant to be in the superheated vapor phase and used the property tables to calculate the specific volume to the refrigerant at the initial state and got 0.078215 m3/kg and calculated the enthalpy to be 264.12kJ/kg.

I was then stuck on how to find boundary work because I have no idea how to find v2 with the given information. I planned on calculating boundary work so that I can use the equation to solve for h2 and then use the property tables to locate the temperature of the substance.

I became frustrated and attempted to ignore it entirely and got an incorrect answer of 59.4 degrees Celcius.

How do I find boundary work? Is my approach correct? Please help me solve thi problem so that I can understand what I'm doing wrong.
 
Physics news on Phys.org
  • #2
If the expansion takes place at constant pressure, the equation you used is incorrect. The correct equation to use for constant pressure expansion is $$Q=\Delta H$$Since you know Q, you also know ##\Delta H##. That means that you know the final enthalpy per unit mass. You then use your tables to find the final temperature that gives that value for the final enthalpy.
 
  • Like
Likes Silverhobbiest
  • #3
Chestermiller said:
If the expansion takes place at constant pressure, the equation you used is incorrect. The correct equation to use for constant pressure expansion is $$Q=\Delta H$$Since you know Q, you also know ##\Delta H##. That means that you know the final enthalpy per unit mass. You then use your tables to find the final temperature that gives that value for the final enthalpy.

I initially did it this way and got 59.4 °C, but that was incorrect. That is what led me to believe that there was boundary work involved because the process takes place inside of a piston cylinder system. So, why isn't boundary work supposed to be considered in this process?
 
  • #4
Silverhobbiest said:
I initially did it this way and got 59.4 °C, but that was incorrect. That is what led me to believe that there was boundary work involved because the process takes place inside of a piston cylinder system. So, why isn't boundary work supposed to be considered in this process?
Maybe they expected you to assume constant volume, and maybe you made a mistake in the calculation. If you want, I'll redo the constant pressure calculation.
 
  • Like
Likes Silverhobbiest
  • #5
I pretty much confirm your final temperature for the heating at constant pressure.
 
  • Like
Likes Silverhobbiest
  • #6
Chestermiller said:
I pretty much confirm your final temperature for the heating at constant pressure.
Okay, then I'll just email the instructor to see if it's another error in the system. Thank you very much for helping me!
 
  • #7
Silverhobbiest said:
Okay, then I'll just email the instructor to see if it's another error in the system. Thank you very much for helping me!
For constant volume, I get something around 63 C.
 
  • #8
Chestermiller said:
For constant volume, I get something around 63 C.

According to the system, the correct answer is 34.3 C. Do you know anyway this answer could have been obtained?
 
  • #9
Silverhobbiest said:
According to the system, the correct answer is 34.3 C. Do you know anyway this answer could have been obtained?
No. Have you given the entire problem statement, word for word?
 
  • #10
Chestermiller said:
No. Have you given the entire problem statement, word for word?
Yes I have.
 
  • #11
Silverhobbiest said:
Yes I have.
Well, then I don't see how they get that answer.
 

1. What is thermodynamics energy transfer?

Thermodynamics energy transfer is the movement of energy from one form to another, according to the laws of thermodynamics. It involves the conversion of energy between different states, such as heat energy, mechanical energy, and electrical energy.

2. What are the three main types of energy transfer?

The three main types of energy transfer are conduction, convection, and radiation. Conduction is the transfer of heat energy through direct contact, convection is the transfer of heat energy through fluids (liquids or gases), and radiation is the transfer of heat energy through electromagnetic waves.

3. What is the first law of thermodynamics?

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only transferred or converted from one form to another. This means that the total energy in a closed system remains constant.

4. How is energy efficiency related to thermodynamics energy transfer?

Energy efficiency is a measure of how well a system or process converts energy from one form to another. In terms of thermodynamics energy transfer, the more efficient the transfer, the less energy is lost in the process.

5. Why is thermodynamics energy transfer important?

Thermodynamics energy transfer is important because it governs how energy is converted and utilized in various systems, such as engines, power plants, and the human body. Understanding energy transfer is crucial in designing efficient and sustainable technologies.

Similar threads

Heat transfer between objects by conduction
    • Engineering and Comp Sci Homework Help
Replies
13
Views
2K
Engineering Average heat transfer coefficient (forced convection)
    • Engineering and Comp Sci Homework Help
Replies
6
Views
2K
Heat transfer problem for a long rectangular bar
    • Engineering and Comp Sci Homework Help
Replies
4
Views
1K
From differential equations to transfer functions
    • Engineering and Comp Sci Homework Help
Replies
5
Views
1K
Thermodynamics: help regarding the coefficient of heat transfer
    • Engineering and Comp Sci Homework Help
Replies
7
Views
1K
Engineering Tubular Reactor with catalytic walls
    • Engineering and Comp Sci Homework Help
Replies
6
Views
764
Flow inside pipe, heat transfer
    • Engineering and Comp Sci Homework Help
Replies
15
Views
2K
Steam heating system transfers heat to a room (Thermodynamics)
    • Engineering and Comp Sci Homework Help
Replies
1
Views
2K
Spec'ing the power of heater for an atypical heat exchanger problem
    • Engineering and Comp Sci Homework Help
Replies
22
Views
1K
Thermodynamics Energy Balance: Solving for Work Input in an Isothermal Process
    • Engineering and Comp Sci Homework Help
Replies
23
Views
3K

Hot Threads

Recent Insights

Back
Top