3-stage cascade

[before]

Hi there,

First of all, let me introduce myself. I'm not a refrigeration professional and certainly not an expert among amateurs. I'm a former phd in psychology; who has taken another occupational way. In fact, I'm working now as an executive for a small computer parts reselling company.

In my job, I'm used to perform CPU testing on a daily basis and of course overclocking has become an hobby. I've already owned a 2-stage cascade (R290 & R170) during the past year but the evaporating temp wasn't enough (about -100C unloaded, -91.5C at about 270w of load)... I wished for less.

So; my project was/is to add a third stage to that 2-stager unit.

The whole unit has already been assembled, but results aren't very good for the moment. Briefly, here's what it actually looks like:

Actual Cascade

1st stage

Condenser + Fan
Rotary cp 10,000Btu/h
Captube .8mm (1.9m)
HX 14 plates

R404a

2nd stage

Rotary cp 8,100Btu/h
Manual Oil separator
Desuperheater (huge condenser split into 2 parts, one for 2nd stage and one for 3rd stage)
Manometer for HP
Expansion tank
Pressostat
Deshydrater
Captube .8mm (2.05m)
Coaxial HX

R1150

3rd stage

Rotary cp 17,000Btu/h
Automatic Oil separator
Desuperheater (huge condenser split into 2 parts, one for 2nd stage and one for 3rd stage)
Expansion tank
Nanometer for HP
Pressostat
Deshydrater
Captube .8mm (2.30m)
Evaporator

Mixture: R50 + R1150 (71.5% + 28.5% at the moment; i've tried lots of amounts; but also only R50 and only R1150. R50 alone requires to much of HP; and R1150 alone isn't really good)

Observed problems

- Unsufficient hold of load of 2nd stage: SL can't stay frozen no matter the amount of R1150 while 3rd stage's running; and HX temperature gets hotter and hotter... HP diminushes accordingly. Supposed solution: substituing the compressor by a more powerful one.
- Redundant missfunctionning of 3rd stage: sometimes it works really well... but it may also fails to produce cold into the evaporator while making strange sounds into the compressor (sounds of liquid or oil; dunno). Also, it can start correctly and produce a satisfactory evaporating temperature (let's say -110C with R1150 only), but (1) the end of suction line doesn't freeze as it usually do (I'm thinking about a schrader gauge which is located at the end of the SL near the compressor), and (2) the load cannot be hold. Supposed solution: The matter is I don't understand what happens Couldn't it be a flash gaz issue? ...evaporation occurs inside the capillary tube and then can sometimes produce some cold into the evaporator but cannot hold the load correctly...

Things to work on / think about:

- Rearranging/replacing compressors relativily to their power: 24000btu/h on 1st stage, 17000btu/h on 2nd and 13000btu/u on 3rd (instead of 10000tbu/h, 8100btu/h and 17000btu/h)
- Thinking about making something like an SLHX into 3rd stage. In fact, it mostly depends on what the actual problem is: is it because of flash gas? If yes an SLHX can be the solution. Additionaly, also about flash gas, an exchange of heat into the suction line might be benefical for the R50/R1150 mixture.
- Thinking of replacing coaxial HX with a plate one.
- Thinking of increasing the lentgh of 3rd stage capillary tube.
- May be other things...

Thx for reading. I'm impatiently looking forward to read your comments and inputs (god knows that I need some... )