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geokonst
10-03-2016, 04:59 PM
Dear all,
First of all I am not a refrigeration engineer so please be gentle with me. I knew nothing about refrigeration (electronics engineer myself) but I am getting more and more fascinated the deeper I dive into the subject.

I recently bought a used laboratory cold plate (or tissue cooling plate) off ebay, for the sole purpose of making a cloud chamber (https://en.wikipedia.org/wiki/Cloud_chamber). It's a Medite COP30, using R-134a. It is designed to go down to -15C but after I bypassed its thermostat I was able to reach -20C. The compressor run hot but I don't intent to run the chamber continuously for hours anyway.

Unfortunately, I need temperatures below -30C, which I realised is not achievable without at least changing the refrigerant. Could you please give me your opinion, whether there is a worthwhile way to modify the cold plate to reach such low temperatures? I've got nothing to loose; I am not planning to use the device for anything else, and cannot afford a new one (like the MEDIT TKF 22 which uses R-404a and can go down to -35C).

Any help and ideas would be very welcome.

Thank you
George

al
11-03-2016, 04:29 PM
In short i don't think so, 134a was never wide spread used for freezing but with the stat shorted was -20 as low as it would go? compressor would get very hot but that is not unusual for these slicing machines anyway. Welcome to forum btw and that looks like a fairly mental application, post pics if you get it working!! changing the gas would most likely involve changing compressor and condenser etc, if the chamber suits your purpose then it might be possible to pipe in a seperate condensing unit to do the job?

geokonst
12-03-2016, 05:50 PM
In short i don't think so, 134a was never wide spread used for freezing but with the stat shorted was -20 as low as it would go? compressor would get very hot but that is not unusual for these slicing machines anyway. Welcome to forum btw and that looks like a fairly mental application, post pics if you get it working!! changing the gas would most likely involve changing compressor and condenser etc, if the chamber suits your purpose then it might be possible to pipe in a seperate condensing unit to do the job?

Thanks for your reply and your warm welcome, Al.

-20 is the the lowest it would get after running for about a half an hour but it was on open air without any short of chamber to insulate. But, to my understanding, you are restricted by the boiling point of your refrigerant (−26.3 C ) anyway, no?

I am considering all options right now (nobody so far has suggested that simply changing the refrigerant might work). I am also seriously considering changing the whole condensing unit, but my budget is very limited plus I wouldn't know which one I should get. Would for instance something like this http://www.ebay.co.uk/itm/Embraco-Refrigeration-condensing-unit-NJ2212GK-R404a-/231855907244?hash=item35fbb061ac:g:aY4AAOSwo6lWLimY work? And is there a source for cheaper used units, ideally in the UK.

The plate itself seems to be perfect for the application so I would like to keep that. I am making the cloud chamber for an art installation and won't be running for too long at a time, so I am even considering hacking off the condenser and piping in CO2 cannisters. I could release/regulate the gas during the shows. I know it's not sustainable, but it's fairly low budget. Is it a terrible idea?

Thank you everyone for taking the time to read my posts

P.S. I started by using peltier elements which didn't work for such size and blew up my budget :)

geokonst
12-03-2016, 05:57 PM
BTW here's my small scale prototype based on a watercooled 4 stage thermoelectric element:
https://www.youtube.com/watch?v=w6Yltp9aw6Y

geokonst
18-03-2016, 05:55 PM
Dear All,
Do you think the CO2 idea above would work? If I removed the refrigeration unit connected a CO2 cylinder (like a paintball tank or something bigger) to one side of the plate and exhaust to environment via the other, do you think that could bring the temperature to sub -30C levels? Assuming the plate doesn't clog and the I have reasonable flow regulation?

Any pointers to a reasonably priced replacement condesnser unit that could work for my application? Any help would be very very welcome.

Have a nice weekend!

RANGER1
19-03-2016, 10:45 PM
[QUOTE=geokonst;311422]Dear All,
Do you think the CO2 idea above would work? If I removed the refrigeration unit connected a CO2 cylinder (like a paintball tank or something bigger) to one side of the plate and exhaust to environment via the other, do you think that could bring the temperature to sub -30C levels? Assuming the plate doesn't clog and the I have reasonable flow regulation?


If the current system is set up to how you intend on using it, then run it longer to see what happens.
At 0 pressure it boils at -26 deg C, so it would run in a vacuum if was to reach -30 deg C.
Short term it might last, blow a fan over compressor.
CO2 -78 deg C boiling point, need to vent it outside.

geokonst
20-03-2016, 12:14 PM
[QUOTE=geokonst;311422]Dear All,
Do you think the CO2 idea above would work? If I removed the refrigeration unit connected a CO2 cylinder (like a paintball tank or something bigger) to one side of the plate and exhaust to environment via the other, do you think that could bring the temperature to sub -30C levels? Assuming the plate doesn't clog and the I have reasonable flow regulation?


If the current system is set up to how you intend on using it, then run it longer to see what happens.
At 0 pressure it boils at -26 deg C, so it would run in a vacuum if was to reach -30 deg C.
Short term it might last, blow a fan over compressor.
CO2 -78 deg C boiling point, need to vent it outside.

Thank you for your reply Ranger,
It might be worth trying to run the system for longer with some sort of insulation around it, before hacking the system apart.

RANGER1
20-03-2016, 08:22 PM
[QUOTE=RANGER1;311435]

Thank you for your reply Ranger,
It might be worth trying to run the system for longer with some sort of insulation around it, before hacking the system apart.

The compressor should have inbuilt protection that will stop it from overheating, so you have nothing to lose, hopefully works.

geokonst
20-03-2016, 09:47 PM
[QUOTE=geokonst;311446]

The compressor should have inbuilt protection that will stop it from overheating, so you have nothing to lose, hopefully works.

Dear Ranger, could you tell me some more about the inbuilt protection? Is it a current overload protection? An overheating protection? Or some short of mechanical regulator? Excuse my ignorance and probably stupid questions, but I find this new to me field fascinating.

P.S. I don't mind ruining it as I only got it for building a cloud chamber and I have no other use for it - other than maybe making a silent air compressor. This thing is surprisingly silent.

RANGER1
21-03-2016, 08:20 AM
Without knowing much about these units, I'm assuming it has a Klixton type overload.
This type of protection usually used on hermetic compressors.
If it does not have a Klixon it would be something similar.
No guarantee, but think you have nothing to lose.
I see on a site there is another model with R404A going down to -35 C,
what the design difference is who knows, might be compressor, metering device for cold plate etc

http://www.sensata.com/klixon/motor-protector-phenolics.htm

geokonst
22-03-2016, 03:42 PM
Without knowing much about these units, I'm assuming it has a Klixton type overload.
This type of protection usually used on hermetic compressors.
If it does not have a Klixon it would be something similar.
No guarantee, but think you have nothing to lose.
I see on a site there is another model with R404A going down to -35 C,
what the design difference is who knows, might be compressor, metering device for cold plate etc

http://www.sensata.com/klixon/motor-protector-phenolics.htm


Thanks for the link Ranger. Looks like the klixon protector is just a bi-metallic switch. That's cool.

I was aware of their other Medite's other model (mentioned in my original post). I contacted them and the quality manager (no engineer unfortunately) responded to my queries and was very helpful. He suggested that if I were to use 404 i would have to swap most of the components. I was hoping they would go with the same design for mass production's shake, but it doesn't look like that's the case.

For further information and for your viewing pleasure I am attaching some photos of the unit
14020140211402214023

thanks

geokonst
23-03-2016, 12:06 AM
and here it is after running for while with hi-tech insulation installed.
Good new is that the compressor didn't get any hotter than that even after a couple of hours of running.
14024

Would just changing the refrigerant make any difference on how cold I can get it? Is it even possible? If so it might be worth after all. Can somebody please explain to me what the process would be for changing to R404A for instance?

Thanks!