A typical thermodynamics question

In summary, the conversation is about a question regarding the first law for control volume and the appropriate method to solve it. The person initially thought to apply the first law for the evaporator system and then for the air, but was unsure if this was the correct approach. Another person reassures them that their method is good and that it doesn't matter if part B is solved before part A. They also suggest checking the work and using the table. The conversation ends with a suggestion to post the question in a homework help forum. The following comments point out potential errors in the initial solution, such as not considering the 30% quality of the fluid and using the ideal gas equation to calculate mass flow rate instead of assuming standard air pressure.
  • #1
tony_engin
45
0
Hi all!
For this question's part (a), I initially thought that I should first apply the first law for control volume for the system of the evaporator so that the rate of heat transfer from the evaporator to the air can be found. Then apply first law again on the air with using the specific heat to find out the exit temperature. Does this method sound?
It seems that I'm doing part(b) before part (a)...
What should be the more appropraite method?
 

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  • #2
Please kindly help...
 
  • #3
Yes, your method sounds good and yes, it'll yield the answer to B before A. I wouldn't worry about that. If you post some of your work, we can check it. Do you know how to use the table?

Btw, homework questions belong in homework help (and you'll probably get more responses that way too).
 
Last edited:
  • #4
yes..attached is my "solution" to this problem..
please point out my faults..
 

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  • #5
Procedure is ok. I think you didn't consider the 30% quality. 30% fluid is already in gaseous state before the evaporator, so heat extracted from air is equal to latent heat of 70% of R12.

Secondly, it is better to calculate the mass flowrate based on ideal gas equation. 100kPa is a bit lower pressure than that of standard air.
 

What is thermodynamics?

Thermodynamics is the branch of science that deals with the study of heat and its relation to energy and work. It explores the principles and laws that govern the behavior of thermodynamic systems, including the transfer of energy as heat and work.

What are the laws of thermodynamics?

The laws of thermodynamics are fundamental principles that govern the behavior of energy in a thermodynamic system. There are four laws: the zeroth law, which states that if two systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other; the first law, which is the law of conservation of energy; the second law, which states that the total entropy of an isolated system will always increase over time; and the third law, which states that the entropy of a perfect crystal at absolute zero temperature is zero.

What is the difference between heat and temperature?

Heat is a form of energy that is transferred from one system to another due to a temperature difference. Temperature, on the other hand, is a measure of the average kinetic energy of the particles in a system. It is a measure of how hot or cold a system is.

What is the Carnot cycle?

The Carnot cycle is a theoretical thermodynamic cycle that describes the most efficient way to convert heat into work. It consists of four processes: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. The Carnot cycle is important in understanding the limitations of real-life engines and helps to establish the maximum possible efficiency for a heat engine.

How is thermodynamics applied in real-life situations?

Thermodynamics has various real-life applications, including power generation, refrigeration, air conditioning, and chemical reactions. It is also used in the design of engines, turbines, and other energy conversion devices. Additionally, thermodynamics is applied in environmental studies, such as climate change and global warming, as it helps to understand the transfer and transformation of energy in natural systems.

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