crazyboy - 21-7-2011 at 14:51
I am thinking about building a small propane refrigeration system based on this design: http://www.sparkbangbuzz.com/els/refrig-el.htm
My question is can the fridge compressor be replaced with a HVAC rotary vane vacuum pump? My concern is that it might not be able to compress the
propane enough or that the exhaust might not be sealed well or that the exhaust might contain vaporized oil. What do you think?
Mildronate - 21-7-2011 at 16:15
You can use freon and compresor from old refrigerator its cheap.
[Edited on 22-7-2011 by Mildronate]
crazyboy - 21-7-2011 at 16:25
Right, but I don't have freon or a refrigerator compressor. I do have propane and a vacuum pump.
peach - 21-7-2011 at 16:36
Garage Chemist would be the man to reference, since he has done all of this.
There are also forums like Xtreme Systems, which is mass populated with guys who build these things like the world will end tomorrow. They're so into
it, they have their avatars set to their lowest thermocouple readings and a 'sub -100C' group of members. Patrick Cloud posts on there, who's reached
liquid nitrogen temperatures with a really, really big set up; he used ethylene towards the end if I remember correctly (there were 5 stages in
total). Grey Mole managed about -96C using one stage and weird mix of gases (there was a competition thread going for who could get the lowest with
one stage, with prizes. Mr Mole missed the closing date, and then -100 by a gnats whisker). But both he and Patrick know precisely what they're doing
and are very well equipped to do it.
There's this PDF as well. I believe the person who wrote that is a member on Xtreme Systems.
I think the membership is currently full, but there are spin off forums with similar things going on.
The exhaust of a fridge compressor also contains oil, that's something they have to think about when designing the coolers.
I wouldn't mess around with the vacuum pump personally, grab one of the piston or rotary AC compressors from eBay, or scavenge one from a window AC
unit or old fridge / freezer. You also want a fan cooled condenser to get some decent watts out of it.
I'm going to have a go at a single stage at some point soon. I scored most of the parts from an old dehumidifier. The unit was cosmetically mucky, so
it cost £20, but it's still perfectly functional. From that I have the condenser, fan, a high pressure solenoid, the pump, a mechanical timer and a
filter / drier bulb. The pump is rated at about 100w from memory. I checked the Co-efficient Of Performance by dunking the coil into a portable ice
box full of water, then recorded the temperature drop over time, found the energy from the volume of water present, then the wattage from the time
taken to achieve the change. It came out at about 350W. About the same as a hotplate puts out.
[Edited on 22-7-2011 by peach]
bfesser - 22-7-2011 at 13:17
Simple answer: No.
Absolutely not. Rotary vane vacuum pumps are not designed to handle pressure over atmospheric at the outlet. You'll quickly destroy the pump.
I am interested in your progress on this, though. I've been tinkering with propane and phase change systems myself, lately.
bwpatton1 - 24-7-2011 at 11:17
If you need a fridge compressor then try the salvage yards and the dumpsters behind local stores and schools (Stores and schools are always throwing
away stuff). Finding an empty system would be ideal so the salvage yard will be better but if all else fails try the store method, you might just get
lucky and find a unit that is already empty. Ask around. If I remember right propane is compatible with R22 compressors (Someone correct me if I'm
wrong).
smuv - 24-7-2011 at 22:14
Technically propane/butane mixtures are the drop in replacement, but propane seems to work well enough on its own.
Edit: So yeah, compatible.
[Edited on 7-25-2011 by smuv]
Wizzard - 25-7-2011 at 07:48
Butane has a much higher boiling point than propane. But many compressed air containers and airhorns have R134A in them 
I have a small project of my own, building one of these to attach to the hot end of a Stirling 40W cryocooler, maybe produce some LN!!
peach - 25-7-2011 at 12:14
I want pictures and details Wizzard!
{edit}Actually, I'm demanding them. 
[Edited on 25-7-2011 by peach]
bwpatton1 - 25-7-2011 at 15:42
Hmmm... Well I may just use propane in a refrigeration system im looking into building...
Oh and I'm with Peach. PICTURES! PICTURES! *pretty please?*