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Monkeyboy
08-11-2010, 04:40 PM
Dear all,

I'm hoping to put together an ultra-low temperature two-stage cascade system that can go down to -90 degrees C. I'm planning on running it routinely at -80 degrees C so just want the extra 10 degrees for tolerance!

Additionally, I need a system that supplies a cooling capacity of 200 W at -80 degrees, which equates to a cooling capacity of ~ 5 kW at 20 degrees.

At present, I think I need a two-stage cascade unit using R404A and R23 refrigerants. However, I don't know what size compressors are required. Can anyone give me an idea of how I could calculate that or if they have experience in this area?

I've heard that Danfoss compressors are well respected in this area?

I know that you can buy similar products that do the job. Julabo make a unit (FPW91-SL) that do the job but I don't need all of the control hardware that surrounds it - nor can I afford the price!

Thanks and look forward to your comments. :D

NoNickName
08-11-2010, 04:48 PM
Julabo buys compressors from TEKO GmbH which in turn are modified Frascold compressor, built on their specs.
Basicly the biggest displacement possible in conjunction with the smallest motor. Additionally, the thermistors are disabled to ignore the extremely high discharge temperature when abating.
Also, the oil is TEKO specs, and it looks like a very high multigrade naphtenic.
I have stripped dozens of these.

norseman
09-11-2010, 02:04 PM
Hi Monkeyboy.
I would have used R 508B on the low stage anyway. It gives a much better discharge temperature. R23 is one of the worst in that matter. I have seen a lot of "burned" compressors over the Years. My field is mostly ultradeepfreezers and the brand I am into and most of the competition are using either 508B or some flamable stuff like R170 or R1170. Most of them go for a AB oil mostly Zerol 150 which are very good to lubricate under the stress on the low stages. Never had a oil log of the capillary tubes, but it is wise to add a little R290 propane to get the oil through.
Compressors I am into is mostly US types. Bristol had some good ones, but this days Copeland is the most used brand. The old Copelamatic (Noisy guys) with an oilpump added in the front is also a hard working choice. Some in Europe use Embraco/Aspera and they also seems to be good. Danfoss have made good and bad compressors for the duty, but it is not a bad choice. I have been into many units of many kinds, but to make one from scratch is quite a task, but sure it would make you feel good if you take your time to make a set up like this. If you make a chamber, you get a certain space for the low stage gas to give a certain capacity, but if you cool a small spot it would be more tricky. What are you up to?
PS: A cascade I know of using 2x1hp Copelands gives a reserve capacity of 130watts at -80C. That given in a pretty big chamber. Look for some manuals like for ult freezers, they can give you some ideas about the cap.tube sizes. The hx could be a simple tube in tube type. Several makers of expensive ult units does it that simple way. Most makers use a low stage start up by temperature control of the hx so a simple thermostate to handle it gives a safe and good way to secure the working condition for the low.

GCAP CoolCast
09-11-2010, 05:07 PM
have you heard of mycom's PascalAir system? gets to -100 degrees C using AIR!

Air to Air, Pure and Simple!
PascalAir is an air cycle refrigeration system. Air used as the working fluid is referred to as the ultimate natural refrigerant because it is friendly to the environment with both ozone depletion potential (ODP) and global warming potential (GWP) of zero.

PascalAir recirculates the warehouse ultra-low temperature air as the working refrigerant. Air is compressed by the turbo type compressor, cooled in the air to air heat exchanger and then expanded in the turbine to temperatures lower than those in the warehouse. The expanded air blown into the warehouse keeps the temperatures at ultra low levels.

Features

The newly developed all-in-one turbo type compressor-expander uses the expansion energy of the expander as auxiliary power for the compressor thereby increasing efficiency.
Since air is directly blown into the warehouse, there is no need for an air cooler, therefore, no energy loss caused by defrosting and the temperatures in the warehouse are stable.



Applications

Cold storage for tuna/bonito, freeze-drying, frozen milling in home appliance recycling process, semiconductors production process, medical and chemical industry.

Specification



Temperature range : -100℃~-50℃
Refrigerant : Air
Refrigeration capacity : 30kW (at -60℃)
Compressor power : 60kW
Max. working pressure : 0.2MPa
Compressor type : turbo type compressor
Expander type : turbine expande
Heat exchanger : aluminum plate-fin type (counter flow type)
After cooler : stainless pipe aluminum fin

mad fridgie
09-11-2010, 10:19 PM
A couple of different options for you,

an air cycle , using joule thompson effect, (i built my own -196C for metal processing), but you do not need this cold.
Or a project I completed 13 years ago and succcessful
R404a, cascading on the peltier devices, cheaper to build aand removes the low stage problems.
Basically your cabinet, 3 skin, middle shin half way through your insulation, your run refrigeration piping (much like a chest freeze)
Your make some little flooded boxes (I used electrical buss bars) after brazing machine flat. The hot side of the peltier on to the flooded boxes.
Large piecies of alluminium which inside the chamber (mounted to the cold side of the peltier)
Peltier is DC device so need DC power supply!
So liquid - expansion valve- flooded boxes- cabinet piping- compressor, and so on.

Monkeyboy
10-11-2010, 01:17 PM
Thanks for all the replies!

Norseman: You mention a cascade using 2x1hp Copelands that gives a reserve capacity of 130watts at -80C. Is this a commercially available cascade? If so, could you please give me the details?

mad fridgie: I've never thought of building a cascade with a peltier device! I thought that peltiers were not sufficiently powerful or capable of the dT's? Don't you have to stack them, and as a result lose effciency?

mad fridgie
10-11-2010, 08:44 PM
Thanks for all the replies!

Norseman: You mention a cascade using 2x1hp Copelands that gives a reserve capacity of 130watts at -80C. Is this a commercially available cascade? If so, could you please give me the details?

mad fridgie: I've never thought of building a cascade with a peltier device! I thought that peltiers were not sufficiently powerful or capable of the dT's? Don't you have to stack them, and as a result lose effciency?
R404a or R507 -40C box temp -80C dT 40C, most peltiers have max dt of approx 80C(no energy flow)
You need for allow for driving force so dt 55C td on peltier. You will need multiple peltiers yo meet your load, in the same way you need larger swept volume for larger loads. This type of design is not the sort of thing you find in a book, so one you have to play with.