Vapor-compression refrigeration cycle

[zircons]

Homework Statement

A conventional vapor-compression refrigeration cycle uses ammonia as the refrigerant. Evaporation and condensation are at -4°C and 34°C respectively and the refrigeration rate is 5kW. The compressor efficiency is 0.75. Assume that the throttle process is isenthalpic.
Calculate the work done by the system, the ammonia circulation rate, the rate of heat transferred to the fluid in the condenser, the power requirement, and the COP of the cycle.

Homework Equations

diagram:http://i.imgur.com/QAQpxBg.jpg

Work done by the system = work done by compressor = H₃-H₂

The Attempt at a Solution

From the problem statement: H₁=H₄, S₃=S₂
I started by finding the enthalpy of vaporization at -3°C and 34°C for saturated ammonia. Now I have H1, H2, H4. I also found the entropy of vaporization at -3°C, so I have S2 and S3.

 

[SteamKing]

Do you have a question about this problem? Without showing any work, it's hard to figure out why you made this post.

 

[zircons]

Yes, I just posted everything since each one is interrelated.

So to start off with, calculating the work: I don't understand how get H3. I thought I could get it via the entropy (S2) since S2=S3, but I don't think there is a relation there. Now I'm thinking I could get it via the compressor efficiency, but for that I need to calculate the work first. I guess I'm just stuck on where to begin.

 

[zircons]

Nevermind, figured it out!

 

[Nickie]

Now I'm not sure what to do next.

I would approach this problem by using the principles of thermodynamics and the vapor-compression refrigeration cycle to solve for the unknown values.

First, I would use the given information to plot the refrigeration cycle on a pressure-enthalpy (P-H) diagram. This will help visualize the process and determine the values of enthalpy and entropy at each point.

Next, I would use the first law of thermodynamics (energy conservation) to calculate the work done by the system, which is equal to the work done by the compressor. This can be determined by finding the difference between the enthalpies at points 3 and 2 on the P-H diagram.

To calculate the ammonia circulation rate, I would use the mass flow rate equation, which is equal to the refrigeration rate divided by the enthalpy difference between points 3 and 4.

The rate of heat transferred to the fluid in the condenser can be calculated using the energy balance equation, which is equal to the refrigeration rate plus the work done by the compressor.

The power requirement can be calculated by dividing the work done by the compressor by the compressor efficiency.

Lastly, the coefficient of performance (COP) can be calculated by dividing the refrigeration rate by the power requirement.

In summary, as a scientist, I would use the principles of thermodynamics and the vapor-compression refrigeration cycle to solve for the unknown values and ensure that my calculations are accurate and consistent with the given information.