Construction of high stage (cascade)

[floshey]

This is not a how-to thread, more a NEED HELP thread :D

Okay guys, about 6months ago i bought a used Vapo PE, and ever since i've been very fascinated by phase-change. I got a friend of mine to refill my PE with R404a, and it works perfectly.
Now i am ready to take my work a step forward, i will start on buliding a cascade, easy now, i won't build it all in one. Im starting off designing and building the high stage. I have some questions which i hope you can answer before i order the parts neccesary for this project.
Ive been reading allmost every thread here at xtremesystems for the last 6months, ive read phase change basics and phase-change.com forums. I have even taken crash courses in organic chemistry. My point beeing, i really look op to some of you guys, your talent are just amazing. Ok, now to the thereticly plan.
I have modified chilly1's cascade drawing a bit, just to create a easy overview.
http://www.upit.dk/u/mini60.JPG
Compressor:
I need a BIG strong compressor, the most important is that it can hold a big load at a decent temperature, the gasses i can obtain are R290 and R1150. I need a little advice on which compressor to use, i would be very glad if you could throw some specs or url's with your suggestions.
I'm thinking 1HP and maybe even bigger, but i could be wrong. I want to use the high stage compressor for a dual evap project later on, so better safe than sorry.

Condensor:
How big a condensor do i need? My plan was to pm chilly1 when i had found the correct size and type, i have looked at some condensers from rparts. I dont think the condensor will be a problem, just get a big one.

Filter:
Not much to say here, just need a std filter/dryer.

HX:
As for the HX i've been thinking alot of different designs, but as most of you guys keep saying, the coaxial design is the best, or what? I need some thoughts regarding this aswell.

Subcooler:
I will just use a normal tube-in-tube design for this part of the system, not much to say here really.

So, i'll start ordering parts for the system as soon as my idea gets approved by you guys.
So building the high stage will be my hands on experience, i will fine tune it and play with it till i feel safe enough continuing to the low stage construction. I wont do anything impulsive before asking you guys.

As for the refrigerants i only have access to R290 and R1150, i have pro help to fill the ethylene onto the system, i would never come near that stuff alone ;)
Im considering a 75:25 ratio (R290/R1150) to lower Propane's boiling point a bit, i dont think -42c will be enough to keep the HX at a proper temperature when kept under load, i could be very wrong though.
As for ordering the tools for the job i need a good vacuum pump, i don't quite understand how you measure a pump's effiency. The specs i have seen on different pumps is micron, the larger number, the better vacuum? could someone explain that?

And sorry for my bad english, if you can't understand me, please let me know, and i will try to explain it better.

Okay guys, i'm waiting for your assistance! :toast:

[oc_nightmare]

ethylene wont condense in the first stage condenser at the ambient temp... u will gain nothing in putting it with r290 in the first stage.
about vacuum, the lower is better.... first vacuume to 250micron then break the vacuum with r290 then vacuume again to 50micron and charge the system.

[floshey]

Is it possible to build the high stage of R290, or do i need to find some R507 / R410?
I still need some answers on all my other questions, but thanks for the help so far oc_nightmare.
I kinda had in mind that putting in the ethylene wouldnt help me, it's just, ive seen many people put in a HP gas to lower the boiling point, i guess putting in R1150 in first stage is just _TOO_ extreme.

[blinky]

yah u can do a high stage of r290 and low stage of r1150, but it wouldnt get that cold because the r290 cant get as cold as r507/r404a. so i would recommend using r22/r404a/r507 for the high stage

[froudeg]

You need to be able to hold -40c under full load on the 1st stage interstage heat exchanger for ethylene to condense at about 220psi on 2nd....even if you can only hold -30c, the pressure will increase to 350psi - which is starting to get somewhat dangerous.

Load capacity of 1st stage should be designed to do 500+ watts, again at -40c sorta temps...It is possible to get this with r290, but its difficult - you will need a far beefier compressor to get 500 watt capacity at -40c evap temps with r290 - its far easier to do it with r404a/507.

[floshey]

Yeah i will go for R404a / R507 on first stage, lets just hope i can find it somewhere, now enough about refrigerants.
As for the compressor i was thinking going for a rotary 1HP / 1½HP in first stage. Could someone please explain to me what the difference is between a rotary and a normal compressor, cause the way i see it, the rotary are able to pull a deeper vacuum?

[froudeg]

As indicated, a propane/ethylene mixture in the 1st stage isn't going to work....the critical point of ethylene is ~10 celcius (critical point is the maximum temperature that a gas will condense, beyond this temperature no matter what pressure is applied, it just wont condense)

Now a ethane/propane mixture should work well - and in fact there is a refrigerant made by care (care 50) that is just that with a boiling point of ~ -50c celcius.