Understanding Cryo Cooling, or Refrigeration

A primer on basic cooling options – Reine Gill

I thought I should share my thoughts about cryo cooling or refrigeration.
Eventually I wanted to make an environmentally friendly refrigeration unit
for a CPU or some other electronics.

To be environmentally friendly, we can not use any of the old coolants
CFC:s (CloroFlouroCarbons), or the so called environmentally friendly
HFC:s(Hydroflourocarbons). The worst ozone eater used was R11 and R12, or
Trichlorofluoromethane (CFCl3)and Dichlorodifluoromethane (CF2Cl2).
The problem is that they destroy ozone (O3) without being consumed themselves, at least not in the first place; they are kind of catalysts to the ozone destroying process.

There are, however, a lot of replacements. I see now that Tetrafluoroethane
(CH2FCF3) or R134a is currently being used as a coolant in the
Prometeia cooling unit. But let’s say we don’t trust that stuff and want to do something really safe.

But let’s hold the discussion about coolants for a while and instead
discuss what kind of cooling methods there are and their pros/con’s
for use in cooling electronics.

1. Gas compression
2. Vapour compression cooling
3. Absorption cooling
4. Sorption cooling
5. Thermoelectric cooling (Peltier)
6. Vortex Cooling.

1. In the gas compression model, we don’t have any phase shifts and we only work with a gas, though this system needs a turbine. I don’t now much more about this cooling method, so let’s leave it.

2. Vapour compression cooling is the way our fridges and freezers are cooled. Except for your cooler in your caravan or summer house, they most certainly use absorption cooling.

Vapour compression works as follows: You first compress the coolant (gas) in a compressor and then you cool it (take away heat) in a condenser/radiator, so it becomes a liquid. Then you let it pass through a narrow tube or expansion valve so that you can achieve a pressure gradient – this is important!

The now liquid and cold gas goes into the evaporator – it’s just an enclosed volume on top of the heat sink on the CPU. In the evaporator, the gas now expands and heat from the surroundings force the liquid gas to (phase change) boil into gas vapour; again, and this is what lowers the temperature.

Now the gas goes back into the compressor again. If we didn’t have the narrow tube, the expansion in the evaporator would cause a lower pressure in the condenser and that would ruin the liquid phase we achieved in the condenser.

Now to enhance the effect, one can exchange heat from the narrow tube (expansion valve) to the outlet from the evaporator. This has two effects: it cools (super cooling) the liquid gas before it goes into the evaporator and it heats (super heating) the gas going into the compressor, removing any excess liquid gas – this increases performance. This is a good way to cool your CPU if you have a good (and friendly) coolant.

3. Absorption cooling is, in my view, too complicated to do yourself, but here’s it is in a brief outline:

You heat up a solution of water and ammonia in the so called generator. The ammonia vapour is then transferred to a radiator/condenser->expansion valve->evaporator, as with vapour compression. But after the evaporator, the ammonia gas is again mixed with weak ammonia solution in an absorber (just an enclosed volume) to obtain strong ammonia solution.

We have a pump instead of a compressor, pumping strong ammonia solution from the absorber to the generator. There we again heat it to get ammonia gas.

Now you may wonder where did the weak ammonia solution come from (or you wonder if I was insane when I wrote this). Now since we have a higher pressure in the generator (because we heat it up), the remaining weak ammonia solution is now driven by higher pressure through an expansion valve (thin pipe or similar) into the absorber; there it recombines with the ammonia gas.

The conclusion we can draw from absorption cooling is that it’s a bit complex and copper tubing can’t be used, since ammonia is corrosive to copper!

4. Sorption cooling is very interesting and it’s used in space crafts and satellites to cool electronics to a few kelvin. But they only need to cool a few 100 uW and we need to cool ~90 W to 300 W. (And in space, you also have a perfect insulation – you don’t have any heat transfer through vacuum, but of course you loose energy by thermal radiation.) But this technique still holds potential, as I will discuss later on.

Sorption cooling works similar to vapour compression; the difference is in the compressor. A sorption cooling compressor does not have any moving parts at all! Except for valves.

Now imagine a steel cylinder containing a porous material and with one connection pipe. Let the pipe be divided into two pipes with one back valve each, facing opposite ways – one will be the high pressure outlet, the other the low pressure inlet.

The cylinder is filled with a gas – in space applications they use 3He and kevlar as the porous material; I believe that carbon dioxide and carbon would work well for our use. The cylinder is heated and cooled in cycles 10 sec to 20 sec long. High pressure carbon dioxide will be pushed out through the high pressure back valve, then heating it up, and low pressure carbon dioxide will return to the cylinder through the low pressure back valve during cooling.

The problem is the very low flow rates and the long heating/cooling cycle. But we can get around both problems easily by putting a lot of cylinders in parallel and then cycle through them, so we always have at least one cylinder or more heating up and the rest absorbing.

For cooling with carbon dioxide, we need a pressure above 74 bar to cool it below 31 C in the condenser (the so called critical point NOT! same as triple point). This is high pressure! But it is easily achieved with a sorption cooling compressor.

The conclusion about this method is that it might take a lot of space and it will possibly not produce enough heat transfer. But the good thing about it is that it’s environmentally friendly and it has a great longevity, since it can go on forever without moving parts!

5. I don’t know much about peltiers, only that they are made of semiconductors and that by applying a current, they work as a heat pump, though they do make their own heat, so you always need some other cooling system. So let’s leave them out.

6. Vortex cooling is really interesting. It’s drawback is that you need a continuous supply of compressed air – a standard 8 bar, 200 l/min home compressor is not enough.

A vortex cooler works as follows: Air at high pressure enters at the end of a hollow cylinder. There it’s made to spin (by entering at an angle) up along the outer rim of the inside of the cylinder. When it reaches the top, it’s forced to turn around and spiral down inside the middle of the outer spiral.

The inner and the outer spiral do turn at the same speed, contrary to what one might think. Due to conservation of momentum, the lost momenta has to go somewhere. And yes, then the air turns around the momenta it looses, is converted to heat and follows the outer spiral out through the end. The inner spiral is thus now much cooler and exits at the other end of the tube.

Great it’s small too! And they are capable of a lot of heat transfer – but you need a big compressor! Go HERE for vortex coolers and see this link for a guy who built one!

Now the cooling method I decided to think a bit more about is the simple Vapour Compression technique used in all home equipment – but without any dangerous coolant. What do we have to choose from? We have ammonia (NH4), carbon dioxide (CO2), sulfur dioxide (SO2).

Now ammonia is corrosive and needs aluminium tubing. Sulfur dioxide is also corrosive if any water is around and it’s not healthy. So it leaves CO2. Although it enhances the green house effect, it won’t do it in our system since it’s closed and we don’t give a net increase, since only burning carbon based fuel does that.

But now we have a high pressure problem. We need, as I have said
above, to get above 74 Bar and at the same time, cool down less than 31 C. Three cascaded compressors with condensers/radiators in between them should be enough. If you do some calculations, you find that one doesn’t need such a great flow to get the heat transfer we need. Test the free simulation software from coolpack HERE –
they have a built in example for CO2. I haven’t done that much calculations, but the example below might work.

This is quite a lot of inflow, though I might have overdone it, and if someone has the urge to make a better calculation, please let me know.
I also estimated the stroke length diameters of the cylinders, power and torque. But since it’s not that well done, I’m reluctant to put it on the web.

Now I realized it’s quite a project to build a compressor like this! So I looked on the web for one – and yes, there exist large industrial ones!
For example:

intltec.com: I think this one’s for liquids only.
bauer-kompressoren.de: Too monster large, heavy and expensive!
indoair.com: Promising, but pressure is a little too low.

So that leaves scuba compressors! They can easily produce above 200 bar and compressors exist that can run continuously!. However, they are bloody expensive – $10,000 dollars for a new stationary one. So what about old ones? They seem to be highly sought for and they are still expensive!!!

So this is there my cryo cooling project – sort of stranded! I hope the basics can be of help to someone else, though!

Reine Gill