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Types of compressors rotary vs recip - OLD fridges

I'm curious what the advantages and disadvantages of reciprocating and rotary compressors are?

As many know, I have been restoring two 1930s refrigerators for a while now and I've studied everything I can find on them.

One thing that had me wondering for a bit was why GE switched from SO2 to methyl formate for 2 years and then back to SO2. From what I've found it was due to non-condensibles causing problems due to moisture breaking the refrigerant down. This lead to service calls where repairmen would "bleed" the system and let the non-condensibles out without letting much if any refrigerant out. Either way it lead to angry customers.

What I don't understand is why GE used reciprocating compressors with SO2 but used a rotary design with the methyl formate.

I'm hoping someone may have an idea why? What I can tell you is SO2 is similar to R12 as far as operating pressures where methyl formate operates under a substantial vacuum on the evaporator side. My guess has been a rotary is better at low pressure operation but I have no clue honestly. Both use what appear to be called flooded evaporators where refrigerant flow is regulated with a highside float.

I have a 1933 and 1934 unit which are both methyl formate \rotary units and all I can say is I have never heard such a beautiful sounding pump anywhere else.

On a side note, both reciprocating and rotary compressors used on these have a unloader than holds the intake valve open until oil pressure reaches a certain point. This actually allows the pump to start with a high head pressure and I have personally tested it by unplugging the fridge for a second, allowing it to stop and plugging it back in. It started right up.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
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