Vapor-compression refrigeration system - boiling & critical point

[yecko]

Homework Statement

In vapor-compression refrigeration system, should boiling & critical points be the higher the better or the lower the better?

Relevant Equations

T-s diagram

Is it the lower the boiling point the better? As room pressure and temperature already can change the phase without working at vacuum?
And the higher critical point means it is hard to reach super critical phase?

But there is transcritical CO2 cycle, which the critical point is so low, and people are using it... is it an exceptional case? or is my concept correct?
thank you in advanced.

 

[Twigg]

Honestly, this homework question is pretty vague. I'd ask your instructor to specify what his criteria for "better" are. My interpretation is based on coefficient of performance.

I'm assuming "boiling point" here means the vapor's state at the evaporator. And I think you have the right idea that the critical point limits the condenser temperature. Think about a Carnot refrigeration cycle. What do you want from the condenser and evaporator temperatures to get a higher coefficient of performance?

I would not worry about transcritical cycles because the critical point is not a relevant parameter for them. I don't think this problem expects you to think about them.

 

[yecko]

This is the problem, which I am quite confused with the concepts.

What do you want from the condenser and evaporator temperatures to get a higher coefficient of performance?

A higher TL and a lower TH?

I would not worry about transcritical cycles because the critical point is not a relevant parameter for them. I don't think this problem expects you to think about them.

We also learned about transcritical CO2 cycle in the same chapter. Which without change of state, it is correct to say " heat be absorb and release only by heat capacity"?

Thank you

 

[Twigg]

A higher TL and a lower TH?

This is correct, but somehow when I posted my reply I had it backwards in my head. Please disregard that suggestion about using the Carnot cycle coefficient of performance. Really sorry about that.

Why don't you draw a TS diagram for a sub-critical vapor-compression cycle, labeling the critical point and "normal boiling point" (as the evaporator temperature). Is a refrigerant with a higher/lower critical point more or less versatile? Do the same exercise for the boiling point. Next, draw a transcritical cycle. Do the same limitations apply?