Another weird problem to drive you guys nuts

I am currently in the process of building a cascade to chill my CPU. A picture says more than 1000 words, so here is one:



There are some small errors in this drawing, but it gives you an idea of what I am trying to accomplish.

High stage will be running R507, evaporating at -35C .. -40C. Therefore, low stage condensing will be around -30C. Low stage refrigerant choice is not fixed yet due to obtainability problems and cost. However, it will either be R170 (ethane), R508b (DuPont SUVA-95, the most preferred choice) or R23 (not preferred due to high discharge temps)

To get some experience and to get the high stage working correctly, I intend to use R410A in the low stage before making the switch to one of the 'real' low-stage refrigerants. Why R410A? It features the highest volumetric efficiency at < -65C / -85F temperatures of all refrigerants I can find in an average ACR engineer's truck. I can also recover R410A back into a bottle, allowing me to play with the charge without dumping refrigerqant into the atmosphere. R170/R508b/R23 pressures are too high for that.

But since R410A is only an intermediate step used to gain experience, I would like to run the high stage as if I really needed the low condensing temperatures.

OK, that was the introduction. Now the problem: I need a capillary tube for expansion of the R410A, but the conditions are slightly unusual.

Assumed conditions:

- SCT: -30C/-22F (2.75 bara / 41 psia)
- Subcooling: 5K
- SST: -65C/-85F (0.51 bara / 7.6 psia)
- Superheat: 5K
- Qload: 150W

Which translates to:

- Pressure drop over the captube: 2.5 bar / ~35psi
- Refrigerant mass flow: 2.16 kg/hr (4.76 lbs/hr)

Does anyone has any idea about what size/length captube I would need?