Hi Vincent,

There is quite a bit to be concerned with on a cascade system. The lowest stage normally uses a very high-pressure refrigerant (at ambient temperatures) to allow the system to operate in a positive pressure at the ultra low temperatures.

Oil and refrigerant compatability and also solubility and miscibilty are some of the issues to be concerned with. The high-pressure refrigerants require a fade-out (expansion) drum so that the mass of refrigerant in the cascade system (low temp. system) can expand in volume without exceeding the system design pressure.

The suction gas coming from the low-temperature evaporator can contain a lot of suction superheat that increases the specific volume of the refrigerant. The superheat is a combination of the evaporator superheat (the normal expected) plus all of the parasitic heat gain through the suction line, etc.

Most of this superheat is not useful, therefore you need higher mass flows to produce the same capacity.

If the system is to be started and run continously at low temperature this one operational concern. If the system is required to do frequent hot starts and pulldowns that can be another issue (high suction pressure pull-down conditions and overloading).

If this is for a single use (medical storage, research test, etc.) I would recommend you look into buying one from a manufacturer who specializes in this type of work. If it is for a production item you are selling, the design and research effort may be worth your time.

I have seen more problems with cascade systems than any other type of system. Even those built by the manufacturers. This a highly specialized and engineered system. They are also very expensive.

It can be done however.

Some systems I have seen use ethane, ethylene, R-23, and others. Some of these refrigerants are flammable, so this raises another level of requirements (explosion-proof electrical design- NEMA 7, Group D, Div. 1 or 2). Sometimes you can use NEMA 4nitrogen purged panels, but only with the insurers blessing.

I hope this information is useful to you for your initial reviews.

Best Regards,
US Iceman