Hi Xtremer's
I'm having fun designing AutoCascade and Cascade cycles. Gary Mole got me interested to see if I could find a cascade solution for Methane using only 3-stages. I have, but it needs 4 compressors (They should be 410a rotary compressors for the higher pressures needed).
The cooling duty chosen is 350W. The average evaporator temperature is -123C (-133.5C to -112.7C). You need to uses two rotary compressors of at least 10.8 cc/rev (good choice Rechi model 44A281B 11.4cc/rev) and two of at least 4.5 cc/rev (good choice Rechi 39A111A 4.75cc/rev) both these compressors have good reserve motor power output and are able to tolerate 34-bar discharge pressures reached on ASHRAE/T standard AC test-cycle.
The refrigerants used were: 410a on warmest stage, 0.90 ethylene 0.10 Methane (mole fractions) on middle stage and 0.90 Methane 0.10 Ethylyene on the lowest temperature stage. Study the diagram of the system and you can see how I exchange heat between the lower two stages.
It was very difficult to bridge the large boiling point gap between Methane and Ethylene, but this cycle arrangement is about the best solution available (I think).
Hoping Gary might just try and build this.
Regards,
Kevin
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