Autocascade refrigeration system: design verification needed

I am currently investigating the possibility of using an autocascade refrigeration system to chill my CPU. I am not sure this post belongs here in 'Technical Speculations', but since I am just speculating at the moment.....

OK, here we go. I plan to use autocascading of R404a and ethane (R170) or ethylene (R1150). As Gary likes to mention: these systems are not exactly little toys anymore, so I would like you guys to pick out any design errors before I even try to build such a device.

A drawing of the system:

http://www.icecoldcomputing.com/imag...tocascade1.png

The system is basic dual-refrigerant autocascade. A mixture of R404a and R170/1150 is compressed up to 15-20 bar (225-300 psi). By controlling the water temperature, the watercooled condenser helps maintaining sufficient high side pressure, which is needed to condense the more volatile refrigerant component later on. It also serves as a heat accumulator to help during startup; the amount of energy (heat) dumped into the condenser is a fewtimes higer during startup than during operation. In my current R507 water chiller this works perfect.

The liquid R404a and gaseous R170/1150 passes through a filter/dryer and suction gas <-> liquid line heat exchanger. This heat exchanger should rise suction superheat enough to ensure that no liquid R404a from the downstream evaporator arrives at the compressor suction header. After the SG<->LL HX, the oil-rich liquid R404a is separated from the R170/1150. Since temperature is above the R170/1150 critical pressure, the R170/1150 remains gaseous and continues to the R170/R1150 condenser.

In the R170/1150 condenser, the earlier separated R404a is evaporated to bring temperature below the SCT associated with the high side pressure. The evaporated R404a and oil is returned to the compressor.

At this point, we have condensed and subcooled R170/R1150, which is processed as usual: it is passed through another SG<->LL HX which improves hydrocarbon system efficiency, and finally it is expanded through another (adjustable) orifice into the evaporator.

When the R170/1150 condenser is not yet cold enough to condense the refrigerant, high side pressure will rise. To limit the maximum high side pressure I intend to use a valve which should release the overpressure to the low pressure side of the system.

The expansion orifices will be very small needle valves which I will probably make myself.
A description of the half-finished CPU evaporator can be found here.

The choice ethane (R170) or ethylene (R1150) will be based on availability and price. Ethylene is also used by farmers growing bananas to accelerate riping, so it might be easier to obtain.

Personally I prefer ethane since that allows a wider range of working pressures. At 15 bar (~225 psia) high side pressure, ethane condenses already at -18C/0F. Assuming a TD of 8K/14F over the R170/1150 condenser, a low side pressure up to 2.4 bar (~36 psia) is allowed.

With ethylene @ 15 bar, a temperature of -39C/-38F is needed to condense. Assuming the same 8K TD over the R170/1150 condenser, this limits low side pressure to 0.96 bar (~14 psia) max. To maintain the required low interstage temperature, a huge portion of compressor capacity must be devoted to compressing R404a.

A refrigerant like R508b would be ideal for this purpose, but I simply cannot obtain it.

Any comments/remarks/improvements?

Hello re the questions on auto-cascade design.
I am not a refrigeration engineer but I have worked on auto-cascade systems in the past. Systems operated at -90 -100 and -135 C
The -135c was a large system which worked with a three phase compressor. The -90 (upright) and -100 chest used a compressor about 2hp from memory. The original systems that I worked on were made by a company now defunct!. Their systems had problems which were never resolved as far as I know. The company was Queue Systems in Parkersburg West Virginia. Owned and run by Jeff or Geof and John Bergen. I think Jeff went back to his former employer Former. Yes pun intended. They were I think just over the river in Ohio.

Later I had contact with systems made by Sanyo. These achieved similar performance but with very much smaller compressors. less noise and less heat. They continued to work as long as I knew of then. The Queue units had a habit of warming up to -30 or so. If turned off and warmed up they would recover but would fail again later.
If I remember right the original patent was owned by Gulf Western so you may be able to get some info through that.

I would be interested to hear how you get on as I was thrown in at the deep end and got to do things like Change compressors, type of oil, add oil separator,size of condensor, type of phase separator plus many changes to the gas mixture which was always supplied in a pre mixed charge. Years later I still think of these systems and wonder if I could make one that worked properly. I would be very interested in trying one with Hydrocarbon refrigerants. Hope some of this might be of use to you.

where can i find a PT chart for ethane(R-170)

In Coolpack you can find a calculator with PT data for various refrigerants.