Vapor-compression refrigeration cycle

In summary, a conventional vapor-compression refrigeration cycle using ammonia as the refrigerant operates at evaporation and condensation temperatures of -4°C and 34°C respectively, with a refrigeration rate of 5kW. The compressor efficiency is 0.75 and the throttle process is assumed to be isenthalpic. The following parameters were calculated: work done by the system (H3-H2), ammonia circulation rate, heat transferred to the fluid in the condenser, power requirement, and COP of the cycle. The enthalpy of vaporization at -3°C and 34°C for saturated ammonia was used to calculate H1, H2, and H4, and the entropy of vaporization at -3°C was
  • #1
zircons
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0

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.
 
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  • #2
Do you have a question about this problem? Without showing any work, it's hard to figure out why you made this post.
 
  • #3
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.
 
  • #4
Nevermind, figured it out!
 
  • #5
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.
 

Related to Vapor-compression refrigeration cycle

1. What is a vapor-compression refrigeration cycle?

A vapor-compression refrigeration cycle is a thermodynamic process that uses a refrigerant to absorb heat from a low-temperature area and transfer it to a high-temperature area, resulting in the cooling of the low-temperature area. It is the most commonly used method for refrigeration in household and industrial applications.

2. How does a vapor-compression refrigeration cycle work?

The cycle begins with the refrigerant in a low-pressure, low-temperature liquid state. It is then compressed by a compressor, which increases its pressure and temperature. The hot, high-pressure vapor then travels through a condenser, where it releases heat and condenses back into a liquid. The liquid refrigerant then passes through an expansion valve, which reduces its pressure and temperature, causing it to evaporate and absorb heat from the low-temperature area. The cycle repeats as the refrigerant continuously circulates through the system.

3. What are the components of a vapor-compression refrigeration cycle?

The main components of a vapor-compression refrigeration cycle include a compressor, condenser, expansion valve, and evaporator. Additional components may include a receiver, accumulator, and various control devices.

4. What are the advantages of a vapor-compression refrigeration cycle?

Some advantages of a vapor-compression refrigeration cycle include its efficiency, reliability, and versatility. It can be used for both cooling and heating purposes, and it can be easily controlled and adjusted to meet different temperature requirements.

5. What are the limitations of a vapor-compression refrigeration cycle?

One limitation of a vapor-compression refrigeration cycle is its reliance on a refrigerant, which can be harmful to the environment if leaked. Another limitation is its performance in extreme temperatures, as it may struggle to maintain desired temperatures in very hot or very cold environments. Additionally, the cycle requires regular maintenance and can be expensive to repair if the components fail.

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