Phase change basics

[saratoga]

I've been reading various people's posts (chilly1 and gary mainly) and links others post in this forum. Lets see if I understand this as well as i think i do before I start brazing parts together and break something :)

1) The unloaded temperature is lowest when the difference in pressure between the high side and low side is greatest. The load temperature is lowest when the pressure difference between the two sides is at the highest possible while still flowing enough refrigerant to absorb the entire load + secondary heat losses.

So the best performing system is one that has a long enough cap tube to create a big difference between the two sides, but not so big that the compressor can't force enough refrigant through to cool the evaporator properly. The correct cap tube length is then determined by the heat of vaporization of the refrigerant, the effectiveness of the compressor and whatever the property of the liquid refrigerant that determines how fast it flows through a pipe with a given pressure is called.

2) The amount of gas you should add to the system is the amount that maximizes the difference between the high side and low side. Since this depends mostly on the compressor we just sort of take an educated guess and then tweak things from there.

Is this pretty close? If not, what am I confused about?

Now I have some more questions:

What is the role of entropy (other then determining temperatures at which phase changes happen)?

Do compressors behave differently with different but fairly similar refrigerants (like say R22 and R290)?

Thanks for any help. I am not very familar with phase change but would like to learn.

[HawainPanda]

hey saratoga!(i live in tucson too!, do u go to skool?..where?)
i think u got all those rite..nyways..compressors are rated to be able to use diff gases...so watever gas u wanna use..u gotta look up if the compressor can pump it

[saratoga]

I'm at the U of A now, but graduated from Sabino high school 2 years ago. Where do you go?

[HawainPanda]

ah...maybe we can meet up in a year..im at University high skool now...gonna be a senior next year, but im taking a vector calc at the u of a during the summer, like in a couple weeks, so i don't need to take anymore math for the rest of my life :P
wow, this is cool, i thought i was the only one interested in this stuff in arizona, lol, well, maybe we could do somethings later, u thinking of doing a cascade in the future?

[saratoga]

Definately be up for something. I work 9-5 over the summer and am also taking math (fourier analysis) this summer, but that doesn't start for another week yet. Good luck with the class, I'm still making up for not having worked harder at math in highschool and will be taking math classes through my senior year of college and probably beyond as a result.

I added you to my AIM list. I'm not around too much, but I'll try and get a hold of you. I'd like to at least discuss projects and such. You could probably give me an idea where I can get tools locally.

---

I'm still curious about the entrophy thing. Lots of tools offer entrophy tables, and I can't figure out what they'd be used for. Seems to me that if you know the temperature and pressure curves as well as the enthaply of evaporation, entrophy isn't too useful.

[kayl]

fourier analysis what subject you doing
electronics engineering or something

[saratoga]

Computer engineering actually, so yeah lots of circuits. Fun stuff. I'm looking to get into Biomedical or some other form of applied research though.

[HawainPanda]

ah...biomedical, i mite be going there :P, hmmm, do u have time saratoga?..i don't have a car yet, but my class starts next week i think, i could meet u at the student union...maybe just talk of something, im working at umc rite now : D

[Gary Lloyd]

Originally posted by saratoga
I've been reading various people's posts (chilly1 and gary mainly) and links others post in this forum. Lets see if I understand this as well as i think i do before I start brazing parts together and break something :)

1) The unloaded temperature is lowest when the difference in pressure between the high side and low side is greatest. The load temperature is lowest when the pressure difference between the two sides is at the highest possible while still flowing enough refrigerant to absorb the entire load + secondary heat losses.

So the best performing system is one that has a long enough cap tube to create a big difference between the two sides, but not so big that the compressor can't force enough refrigant through to cool the evaporator properly. The correct cap tube length is then determined by the heat of vaporization of the refrigerant, the effectiveness of the compressor and whatever the property of the liquid refrigerant that determines how fast it flows through a pipe with a given pressure is called.

2) The amount of gas you should add to the system is the amount that maximizes the difference between the high side and low side. Since this depends mostly on the compressor we just sort of take an educated guess and then tweak things from there.

Is this pretty close? If not, what am I confused about?

Now I have some more questions:

What is the role of entropy (other then determining temperatures at which phase changes happen)?

Do compressors behave differently with different but fairly similar refrigerants (like say R22 and R290)?

Thanks for any help. I am not very familar with phase change but would like to learn.

I hardly know where to begin. What have you been reading? Where did you hear the word "entropy"?

[saratoga]

I hardly know where to begin. What have you been reading? Where did you hear the word "entropy"?

I'm not a complete beginner in physics. I've done calculations with entropy in thermochem and in my thermo when we did heat engines. I'm familar with gibbs law and S = integral(dQ/T), but its been a while since I did anything with it. Plus they just taught us the math and not the applications since I'm not in mechanical engineering. So what are tables of entrophy values used for in refrigeration? That would probably be a good place to start since thats what prompted my question.

The way I look at it, entrophy determines where the phase change occurs according to gibb's forumla for free energy. But if you already have a table of pressure vs temperature with the phase change marked, what use are the entrophy values? I'm guess I'm missing something stupid here.


ah...biomedical, i mite be going there :P, hmmm, do u have time saratoga?..i don't have a car yet, but my class starts next week i think, i could meet u at the student union...maybe just talk of something, im working at umc rite now : D



Sounds cool. I'll send you an email to the address in your profile once I check my schedule of classes.

[JimmyDean]

Hey crazy i live tucson too. Going to be a senior at Sahuaro HS... graduate 2005. you guys definitely arent alone, theres quite alot of people interested in things, although maybe not as "extreme" as us perhaps. you guys have any phase systems going? LOL i would finish mine, and start on another if I had more money, but bleh. Ive got like 3-4 compressors, and 1 system that is almost complete just need cap tube and dryer and evap. I probably will start on a waterchiller after this. Whats your guys' aim? im jimmydean012

[HawainPanda]

hey hey. jimmy! actually, i can't get anything going yet...my mum is a clean nut, hehe, i can't even lay a paper on my desk with out her telling me to clean up my room, i dunno, i think i know enough to do a single phasechange, but im thinking of cascades which, to me seems a whole lot more fun, wow, this is great, where u going for college?

[HawainPanda]

er, about entropy, i dunno, i think from ap chem its about the heat absorved in a reaction...dunno if that helps

[JimmyDean]

probably pima for the first 2 years, then uofa. i could have got to UHS, but meh, I got lazy around 9th grade and really messed up. Now im pullina 2.8gpa, and I really screwed myself. If you guys wanna ever do something sometime im game probably. A cascade huh, I can supply the materials probably for it...