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Originally Posted by redleader
Typically refrigeration systems are vacuumed at much lower pressure then this, often for hours and I've not heard of valves leaking.
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What kind of valves are used?
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R134a might be a good choice because its so cheap, nonflammable and comes in pure form. Pressure would be a bit high though, so you'd have to braze or at least use flare fittings on everything.
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I was just looking at boiling points - too bad pentane's flammable (
and a suspected
neurotoxicant 
) a boiling point of ~36C at 1ATM looks pretty good...
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R134a might be a good choice because its so cheap, nonflammable and comes in pure form. Pressure would be a bit high though...
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What about
SP34E boiling point of -15 vs R134a at -26 - which should mean lower pressures needed. I know they make a big deal about R12 computability (mostly ester oil vs mineral oil, right?) but for a heatpipe-like-thing with a target temp around, say 22 to 26 then a boiling point closer to that target temp is a good thing as there'd be need for less pressure (plus or minus). Or have I missed something important? Other than playing with heat-driven refrigeration in the late '70's (arab oil embargo, energy panic) I have no experience with this stuff.