A typical thermodynamics question

  • Thread starter Thread starter tony_engin
  • Start date Start date
  • Tags Tags
    Thermodynamics
AI Thread Summary
The discussion revolves around solving a thermodynamics problem involving an evaporator and heat transfer. The initial approach suggested applying the first law of thermodynamics to the control volume of the evaporator to find the heat transfer rate to the air, followed by calculating the exit temperature using specific heat. Feedback indicates that this method is valid but may lead to solving part (b) before part (a). It is also noted that the solution must account for the 30% quality of the fluid, as 30% of the R12 is already in a gaseous state before entering the evaporator. Additionally, calculating the mass flow rate using the ideal gas equation is recommended due to the lower pressure of 100 kPa.
tony_engin
Messages
45
Reaction score
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?
 

Attachments

  • thermo1.JPG
    thermo1.JPG
    33 KB · Views: 419
Physics news on Phys.org
Please kindly help...
 
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:
yes..attached is my "solution" to this problem..
please point out my faults..
 

Attachments

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.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
View full post »

Similar threads

Entropy and the Laws of Thermodynamics
Replies
3
Views
1K
Thermodynamics: Internal Energy, Heat and Work Problem
Replies
1
Views
2K
Trouble solving for end state of two control volumes in a rigid tank
Replies
12
Views
2K
Significant figures -- wrong answer to Thermodynamics question?
Replies
4
Views
1K
Mixing two ideal gases with different V, T at constant pressure
Replies
16
Views
3K
B Cooling effect of evaporation occurs even if the surroundings are colder?
Replies
10
Views
2K
How Do You Calculate Heat Transfer in a Thermodynamics Piston Problem?
Replies
5
Views
3K
Thermodynamics of an Insulated Box: Water-Steam Question
Replies
1
Views
2K
Adiabatic Expansion of Pressurized Air in a Piston-Cylinder Setup
Replies
8
Views
2K
Dryness fraction of saturated steam
Replies
2
Views
1K

Hot Threads

Recent Insights

Back
Top