Two Stage Cacsade Question

[XC] gomeler · 01-25-2007, 02:40 PM

Is it possible to use a 1/4 and 1/3 HP compressor for the 1st and 2nd stage and couple them with a large condensor on the 2nd stage to pre-cool the highstage gas prior to the HX between the stages? From my single-stage experience I know it won't be capable of holding the loads that the larger compressors would at the same temperatures but I am hoping to build a system capable of holding ~200 watts @ -80 C while maintaining a smaller area profile. I think I may use a CPEV on the 1st stage to maintain a constant HX temperature and then captube on the 2nd stage. Gasses would originally be r290 and r290/CO2 for 1st and 2nd stage till I can get a cylinder of r1150 for the 2nd stage. Asides from the dice issues with CO2 and it's load-bearing properties, does anyone see anything fundamentally wrong with my concept, would the compressors be overwhelmed? My idea on the 2nd stage condensor would be to help take some of the load off the 1st stage by having the hx condense CO2 @ ambient rather than at discharge temperatures. So far this is just a thought, if it is rubbish or already been done with miserable results do let me know.

n00b 0f l337 · 01-25-2007, 02:46 PM

Yes, first off your 1st stage compressor should be larger in most cases, even with a desuperheater on the 2nd. Second off, don't expect those kinds of temperatures with that size compressor. Probably a good bit warmer +10C range I'd imagine. Also CPEV on the first stage won't work well since the heat load that passes onto the HX changes based on the 2nd stage and the change in heat from the CPU, if anything the CPEV would go on the second stage and a TXV on the first if you don't want to use capillary. Also the co2 definitly won't condense in your desuperheater.

runmc · 01-25-2007, 06:59 PM

Quote:

Originally Posted by gomeler

Is it possible to use a 1/4 and 1/3 HP compressor for the 1st and 2nd stage

Nope

Sure you can make one, but I doubt it will handle load.

[XC] gomeler · 01-25-2007, 07:59 PM

Has anyone tried it before? I've used a 1/6 hp compressor for a single stage, just thought it would be possible for baby compressors to hold some sort of low temp loads.

I didn't plan on condensing the CO2 in the super desuperheater, I just planned on bringing it down to ambient to relieve the 1st stage workload. I guess I'll stick to cap tube until I know how to tune 2 stage systems then i'll mess with valves.

Has anyone ever tried such a small system before? Is it just not enough load carrying capacity in the 1st stage, not enough power to compress a 2nd stage gas, or a combination of a bunch of stuff? I am really interested in making a small cascade as I don't want to venture forth with huge compressors for my first start. Hell I might as well make the system and if all else fails I'll have the parts for 2 single stagers. So has anyone done this before, even just to hold smaller loads? I don't enjoy thinking of having to use 2 1/2 hp rotaries, my ears already hurt with 1

n00b 0f l337 · 01-25-2007, 08:34 PM

Yeah my cascades going together with 2x 1/3+hp compressors and thats deemed pretty small.

[XC] gomeler · 01-25-2007, 09:29 PM

Hmm ok, any timeframe on when your unit will be in working condition NoL? Would be nice to see your results and settings to determine how my system would perform. I think I'm goign to go ahead with a 1/3 and 1/4 or maybe a 1/3 and 1/3 and see how it goes unless I can find one of those fabled 1/2 HP scroll compressors for "cheap". Goodluck with your build

bazx · 01-25-2007, 11:01 PM

my first cascade was 2x 1/3 danfoss nl11f

have a look thro might be something there of use

http://www.ocforums.com/showthread.php?t=457923

[XC] gomeler · 01-26-2007, 09:54 AM

Thanks bazx, got it bookmarked and I'll look through it tonight.

marru · 01-26-2007, 10:54 AM

@bazx, is that a filter on the suction line of the second stage? Is it any good?
I thought it only works best with liquid refrigerant.

bazx · 01-26-2007, 11:04 AM

Quote:

Originally Posted by marru

@bazx, is that a filter on the suction line of the second stage? Is it any good?
I thought it only works best with liquid refrigerant.

its an accumulator

helps to stop flooding back to the compressor

common on vapo chill units

[XC] gomeler · 01-26-2007, 12:39 PM

Bazx, just read through that build thread and it was really helpful. Your work is so clear and precise and makes a great learning manual. I have a few questions for you though.
Do you think that the dual-captube on the 1st stage was more effective than a single longer captube?
Was the handvalve on the 2nd stage oilreturn necessary or could the system be plumbed to directly return the oil to the 2nd stage compressor via the suction line? Could a little bit of oil carrying gas be used to help assist with this such as r290? On the oil seperator did you pack it with steel/copper wool or was it an empty copper tube?
Thanks for the great thread, think I'm going to go ahead with 2 1/3 hp compressors and hope for some decent results.

n00b 0f l337 · 01-26-2007, 12:42 PM

The hand valve is there otherwise youd have a high side feeding directly into low side, it just doesnt work. Even floated only release a bit at a time.

bazx · 01-26-2007, 01:51 PM

Quote:

Originally Posted by gomeler

Bazx, just read through that build thread and it was really helpful. Your work is so clear and precise and makes a great learning manual. I have a few questions for you though.
Do you think that the dual-captube on the 1st stage was more effective than a single longer captube?
Was the handvalve on the 2nd stage oilreturn necessary or could the system be plumbed to directly return the oil to the 2nd stage compressor via the suction line? Could a little bit of oil carrying gas be used to help assist with this such as r290? On the oil seperator did you pack it with steel/copper wool or was it an empty copper tube?
Thanks for the great thread, think I'm going to go ahead with 2 1/3 hp compressors and hope for some decent results.

with that cascade my big problem was the first stage holding the load
from the second

but imo a larger hx coil may have helped, the 2x caps did help but i still feel the danfoss tes2 with 00 orifice that i used in all cascades after this one was the way to go, it is a one stop solution

and tbh not much more in cost then a 15m roll of 0.031 cap tube

the co2 was tricky to use but fun and has been used by many here with other refrigerants

i would use copper wool in the oil sep if you can

i made a mistake in the position of the sight glass on the oil return it should have been on the other side of the tap

as nol said the valve needs to be shut when not returning oil as it will shot circuit the loop

*edit*

i have just checked the cap i used there

and it looks like i made a mistake, i should of used 2.6m of 0.031 lol well it was my first
that may have been my problem with load

[XC] gomeler · 01-26-2007, 02:07 PM

You can't treat the oil like an autocascade? From the oil seperator have a metering device (captube) release the oil and have the high-stage gas exit through the top of the oil sep? Pressure from the high stage would push the oil out the bottom along with the suction from the compressor.

n00b 0f l337 · 01-26-2007, 02:25 PM

Well you can use a capillary but you'll get a higher suction pressure I'd imagine or other anti-beneficial effect.

LukeXE · 01-26-2007, 02:32 PM

Quote:

Originally Posted by n00b 0f l337

Well you can use a capillary but you'll get a higher suction pressure I'd imagine or other anti-beneficial effect.

Right, suction pressure will be way to high, so you can only dream then about good temperatures :p

[XC] gomeler · 01-26-2007, 02:33 PM

Hmm I think I'm going to experiment with the idea of a capillary based oil return system as I would hate to forget to release the valve and oil-starve the compressor. One final question, can a cascade be operated for extended periods of time, possibly 24/7? Or would the 2nd stage end up dying due to the extreme pressure and temperature differences between the discharge and suction lines? Guess that question would only apply if I can solve the oil return hand-valve idea.

edit: I don't understand why autocascades can get away with using oil seperators and phase seperators and still maintain very low temperatures (due to low evap pressures?) yet the same concept couldn't transfer over to what is essentially a single stage. I understand that I would risk having the highstage gas bypass the HX and get sucked through the oil-return line but if a liquid seal can be formed around the captube and the flow can be correctly matched with oil output from the compressor shouldn't the system maintain a certain balance between oil return and oil output?

LukeXE · 01-26-2007, 02:52 PM

I think that you don`t understand how autocascade exacly works.

Really, don`t try with captube, hand valve will be great for that, and you can open it really seldom, maybe 1 time per 3-4 hours of benching ?

[XC] gomeler · 01-26-2007, 06:29 PM

Thought I had a pretty good understanding on how autocascade systems work. I'll just experiment with this stuff and if it sucks then no loss as this system is an experiment

Gotta learn somehow and this substiution doesn't sound dangerous, just sounds potentially performance robbing. I'm sure I'll have more questions, gotta start accumulating parts first though.

marru · 01-26-2007, 06:52 PM

With autocascades, you will have liquid present all the time in the captube.

With cascades, if you would use a captube alone to return the oil to the compressor there will be periods of time when you will return just gas. This is called a bypassing to the suction line and you only do this if you have big load variations. If this is not done corectly you will have high suction pressure and possibly compressor burnout.

[XC] gomeler · 01-26-2007, 10:00 PM

So from what you are saying, I need to insure that there is always a liquid seal to keep the gas from bypassing the system. Could a small amount of an ambient condensing refrigerant added to the system help with this to make sure that a seal is always formed? Sorry if it sounds like I am beating this into the ground but rarely is anything impossible, just very hard to do, and this concept intrigues me.

LukeXE · 01-27-2007, 01:10 AM

But why you can`t use hand valve...

marru · 01-27-2007, 01:58 AM

I think its ok to try to find an alternate solution. Your idea is interesting but i don't think its practical.

The best thing, in my opinion, is to use a solenoid valve controlled to open for a small period of time after an amount of time of system usage (2h maybe?). This, of course, if user independence is what you're looking for.

n00b 0f l337 · 01-27-2007, 10:48 AM

If user indepdence is what you want, then you want to buy a floated oil separator.

[XC] gomeler · 01-27-2007, 12:12 PM

Thanks NoL, I'll look into them. I really want to make a 2 stage cascade something that is as simple as a single stage to operate. I want everything automated such as a 2nd stage delay start circuit, automated fan controller, high-pressure cutoffs on the 2nd stage, and automated oil return lines. Maybe I can make this cap-tube oil return work, enough theoretical questions though as it is time to make something happen. Thanks for all the input guys, I appreciate it.