Hmmm... I think I should go into a little more theory on why I chose the refrigerants that I did for this project.
First of all the spec I was working towards:
Desired Evaporator temperature: -30C (no colder then -40C... O-Ring leaks)
Maximum Charge Pressure: 150 psi
Fast pull-down and ability to handle large mass.
Although R-22 seems like it has the right boiling point (-40C at 1 atmosphere) to do the trick, it would require running the compressor suction close to 0 psi (considering pressure drop, and heat transfer losses). With the small compressor I was proposing to use, this would have required a fairly low mass flow through the system in order to maintain such a low suction pressure. Low mass flow equates to more time required to transfer heat of the evaporator, through the compressor, and out through the air cooled condenser.
I really wanted this thing to be up an fully operational within a few minutes (down to alcohol trapping temperature).
So using another refrigerant of a lower boiling point in an autocascade, would allow me to have a higher mass flow (faster heat transfer). I had R-23 available (-84C at 1 atmosphere). By balancing the proportion of R-23 and R-22, and adjusting the cap tube flows, I could achieve -30C with a very high mass flow.
The R-123 (+28C at 1 atmosphere) that was also used improves the heat transfer in the warmest stages, including the energy put into the compression process, heat due to electrical resistance in the motor windings, and rejection of heat at the air-cooled condenser. This should greatly prolong the life of the compressor.
On the other hand if we wanted to run the system closer to R-23 boiling point temperatures (-84C), then the R-22 becomes a detriment, acting like a contaminate in the R-23 condensate leaving the final stage. Basically the more R-22 that is present (in solution) with the R-23, then the warmer will be the boiling point of the condensate. Essentially you are creating a pseudo refrigerant, something that boils off in the range between R-22 and R-23.
So to achieve a more pure form of R-23 condensate, we would either need additional or better phase separation (e.g; 2nd phase separator and cascade), or to keep it simple, a substitute for the R-22 that has a much warmer boiling point like R-123. Of course if this were a conventional cascade, R-123 wouldn't do us much good since it does have such a high boiling point. But with autocascades, we now have the advantage that all the refrigerants are working together in a mixture, and as such presents a whole different picture.
In autocascades as we move upstream in the serially connected heat exchangers, we will see an ever decreasing temperature. We will also see some refrigerants being condensed, perhaps in a sub-cooled state before being separated out, and then go on to be evaporated at the next stage up. The more sub-cooled a refrigerant is, the higher will be the amount of other refrigerant gases that will dissolve into it. When such a sub-cooled condensate is later evaporated, it will do so at a much lower temperature then its own boiling point. So if the conditions are right, R-23 gases will dissolve into sub-cooled R-123 condensate. In fact beyond a doubt, the R-123 circulating in an autocascade will be extremely sub-cooled by the time it is extracted at the first phase separator. This is good for 2 reasons. First of all, being in a such an extremely sub-cooled state, precludes much of it getting past the first phase separator (less chance of contaminating the R-23 gases that are passing through). Secondly, a good amount of R-23 will have dissolved into the sub-cooled R-123, and when evaporated, yield a temperature somewhere between -40 to -50C which should be sufficient to condense the R-23 on its way to the evaporator (assuming we have a high enough discharge pressure).
So hopefully that helps clarify my choice of refrigerants for the cold trap chiller configuration, and also for a means of achieving a -80C system that might be suitable for PC Cooling.
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