On Tuesday, June 2, 2009 at 6:50:35 AM UTC-5, wrote:
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

some walkin freezers have a hot gas bypass valve that is usually called a head pressure control. hot gas is bypassed into the compressor to warm it when it is very cold outside. this is the reason for the third port in some cases.