How cold can a compressor chill down to?

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Old 04-13-05, 09:10 AM
lost_ring
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How cold can a compressor chill down to?

I'm having a heck of a time trying to find out how to make a parts chiller for an assembly tool I'm designing. I don't know if this qualifies as a DIY project because I'm getting paid for it. I'm designing an assembly bench parts dispenser with mechanical moving parts and such, it's just the chilling part that I'm having trouble with. The parts being assembled will pass through a copper tube about 6" long by 2" in diameter which I want to put a chilling coil around. The chiller needs to get down to -40F. I would love to go out and get an air conditioner and modify it to have flexible lines running to the chilling coil. But can these compressors get down to -40? What are the limiting factors? Is it the refrigerant? Is it the compressor? Is it the charge pressure? I'm trying to find someone who will design and build this thing (and I'll stick to the mechanical). Or can I buy refrigeration systems that I could hook up to my chiller coils? I found the Ridgid Pipe Freezer online. I was very excited. It's really cool. But after thinking more about it, I'm worried that it won't last under constant use. They say it gets down to -18 to -28F.
 
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Old 04-13-05, 09:48 AM
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lost_ring,

You are right in that you will need someone to help design and build the cooling system for you.

An a/c unit is way off the mark in working at the temps you need.
Compressors are rated generally as a/c, high temp, med temp, low temp and extra low temp.
What you need falls into extra low temp.

There are many considerations in deciding how to go about this. A big one is the length of time you want it to take to lower the temp and how many pieces per hour you will need to do.
In the equation will be defrost time, as the temps you will be working at will frost up the assembly fairly quickly.

The Ridgid pipe freezer may be closer to the mark.
It would likely be more efficient to use liquid Co2 in either a bath or spray to do what you want as the installed cost would be cheaper than mechanical cooling.
There are issues with ventilation but would be easier to overcome than the limitations of going mechanical.
 
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Old 04-13-05, 10:38 AM
lost_ring
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Thanks

Thanks G. That's helpful info. I may still go with the pipe freezer. They specifically said they don't want to use any gas. The parts are small and the BTUs/hr will be minimal. It's less than 3 BTUs per part. Maybe I will get a small -40 freezer and put a heat exchanger inside it with a circulating pump and some heat transfer fluid. Frosting up is something I'm worried about. I may need to make some sort of ice scraping feature built in. Frosty parts is not a problem but if the tube frosts up and won't let parts through, that would be a problem. Thanks for the "med temp, low temp, extra low temp" info. I suspected something like that but I couldn't find any mention of it anywhere.
 
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Old 04-13-05, 06:10 PM
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lost_ring,

You may have trouble finding information on ultra low refrigeration because of the fact that it is a very narrow speciality.
3 btu's per part really doesn't mean anything. Is is 3 btu's/degree of temperature drop/lb?

If indeed you need to lower your parts to -40 deg F I would suggest that the complexities would require someone who specializes in this area.
The user may have said they don't want Co2 but the economics may sway them.
Co2 sublimes at around -78 deg F and would be capable of cooling your parts to the level you need in a proper cooling chamber.
 
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Old 04-13-05, 08:14 PM
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-40 is out of my league but I would think if you could even get something to cool that low and keep it that way for extended amounts of time the surounding humidity would need to be extreemly low so that frosting would be at a minumum.

How cold would it get if something was made to surround the pipe and could house dry-ice ?? I know dry ice alone is like -110 F.
 
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Old 04-14-05, 11:07 AM
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Wink

Might look into the meat companys .How their set up is for flash freezing of the meats.Or could you heat the other parts a little to expand them?????


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Old 04-14-05, 03:34 PM
lost_ring
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Working within the customer's wants and needs is tough. Well, moreso with the wants than the needs. They don't want gasses and they don't want dry ice. Probably good ideas though.

I've talked to a few refrigeration guys around here and they do have flash freezers getting that low but with big industrial compressors. I'm looking at a company now that makes freezers that range between -40 and -86C. Burr! I'm considering putting a heat exchanger inside a standard freezer and circulating some sort of heat transfer fluid.

Now I never said that these chilled parts were being put into any other parts so don't go jumping to conclusions that there are other parts to heat up here. Having said that, I put in my proposal that we should heat the parts that these things go into but they don't like that idea much. Again, I have to deal with the wants as well as the needs. Heating the other parts is a very good idea. Especially since they come out of a parts washer, one at a time at about 100F. I proposed a staging rack to maintain that slightly elevated temperature. I figure 20 degrees here is worth -20 on the other end. *shrug*

Thanks for the advice guys.
 
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Old 04-15-05, 05:08 AM
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Originally Posted by lost_ring
I'm considering putting a heat exchanger inside a standard freezer and circulating some sort of heat transfer fluid.
Your "standard" freezer would have to get down below-40 and your transfer medium would be a very concentrated brine solution or proplyne glycol.
 
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Old 04-16-05, 12:18 PM
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I don't think that you will ever get the job done quickly passing the parts through a 6 inch tube no matter how cold you got the tube. There will be air inside that tube along with the parts. Air (and most other gasses) have a very low specific heat. In order for the parts to give up heat they must be subjected to a large differential temperature for a certain period of time. The air would act more like an insulator an would prohibit a quick transfer of heat from the part to the refrigerating system. The other problem with small parts is that they have a small surface area to volume ratio. Yes, you can get the parts cold, but it will take a significant amount of time. You can only radiate heat from the part surface area and if the surface area is small in relation to the volume there will be a time penality to be paid. Refrigeration compressors have nothing at all to do with making things cold. Getting things cold is mostly about expanding a liquid to a gas in an expansion valve. All the compressor does is then collect and compress that gas so you can pass it to a heat exchanger, condense it back to a liquid so you can use it again. You wouldn't ever need a compressor at all if you were willing to vent the gas to the atmosphere, which of course is illegal and not very cost effective. The only gas that has a high specific heat is hydrogen, but that's explosive. Power companies use that gas to cool their generators, but they also have very expensive heat exchangers to dump the heat to a flowing water source. About the only inexpensive solution to a production problem like yours would be to take a batch of parts to a freezer where they could remain for the length of time necessary to get sufficiently cold. Another possible way would be to immerse those parts in a very cold liquid, but that would also be expensive. It would be wise to consult someone who does cryogenics work and let them explain the thermodynamics of heat transfer.
 

Last edited by jughead; 04-16-05 at 12:36 PM.
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Old 04-16-05, 06:43 PM
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Way to go jughead.

Yes the only way that I can think of is to imerse them in a liquid that is recirculating.
 
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