Cascade Refrigeration System

[Vincent Yu]

Thank everyone for the valuable suggestions. Currently our company builds 2-stage cascade systems. Normally they are used in vacuum chamber and testing chamber. Two ¼ HP compressors are used. The system can reach -100 degree Celsius. We do have some experience to design and manufacture two-stage cascade systems.

Recently, a customer has an application which needs the temperature goes down to -130 degree Celsius. We plan to develop a 3-stage system based on our current 2-stage systems. Following is an outline of my design. Please give me your opinions:

Three ¼ HP compressors are used. For two stage systems, it is a good choice to use 2 compressors having the same power. I am not sure if this principle still works for three stage systems.

First Stage: condensing temperature: 35C; evaporating temperature: -30C. Refrigerant: R404A or ISCEON89. A fined tube air cooled condenser is used.

Second Stage: condensing temperature: -25C; evaporating temperature: -75C. Refrigerant: R23 or (R508 + R14).

Third Stage: condensing temperature: -70C; evaporating temperature: -140C. Refrigerant: R14.

By my rough calculation, if the refrigerants are selected as above, the condensing and evaporating temperatures in each stage should be the values I give. But I am not sure if the system is optimal in these conditions. It is very difficult to get the optimal solution just by calculation. I will adjust the system based on test result.

Plate heat exchangers (WTT) are used between first and second stage, and second and third stage. Five degree temperature difference is enough for efficient heat transfer.

The metering device in first stage is a thermal expansion valve. Capillary tubes are used in second and third stages.

Cylinders are used as fade-out drums in second and third stages. They are place in the low side between the evaporator and the compressor. The size of the tank will be determined by experiments. There is no harm that the fade-out drum is a little bit oversize. Is it correct?

Coalescent oil separators (Temprite) are used in second and third stages for oil separation.

The principle of cascade system is very simple. But to select parts and build it is very challenge. I start to test the performance of the first and second stages, and hope it can provide about 300 ~ 350 watts cooling capacity at -80C for the third stage.

Now I still have trouble to find a suitable compressor for the third stage. And I also need to confirm that R14 is the best choice for the third stage. Any information and suggestions about the compressor and the refrigerant for the third stage are welcome.

Ken, Could you give me more information about the consultant you talked about? If he is really very professional in cascade system design, and if we really have trouble to develop it by ourselves, maybe my boss is willing to pay him for his help.

Thanks again. I am willing to share all the information with you, and will report you guys my work on this project regularly.