Why 3 Pipes on Fridge/AC etc Compressors?

Every small compressor I've seen in the back of a fridge or A/C or dehumidifier, etc has 3 copper pipes coming out. I can make sense out of 2 of them: hot out to condenser coils, cold in from evaporator coils (or 1 to suck, 1 to blow). But what is the third one for? It's usually crimped closed.
Google's happy to tell me how refrigeration works, but not what the third pipe is for.
Red
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snipped-for-privacy@rocketmail.com wrote:

Think about how the factory introduced the refrigerant charge based on what you are looking at.
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It's called a compressor "process tube" and it's used in the manufacturing process to charge the sealed system with refrigerant, since most small hermetic systems don't have any service valves for access. It can also be opened up in the field for maintenance and then re-pinched. Or, it can have fittings soldered or brazed to it to make for easier for future access.
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The third pipe is called a "process stub", as I've heard it. When they make the fridge, etc. They hook a vacuum pump onto the process stub, and evacuate the air. Then, they fill with a charge weight of refrigerant, and oil. The process stub is pinched off, and brazed shut. If nothing is leaking, the refrigerator, or window / wall AC is good to go for many years. There is a special tool called a "pinch off tool" which is used for this.
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On Jun 2, 9:18 am, "Stormin Mormon"

Ah-ha! A big light bulb just turned on over my head!
Red
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