This page covers a number of intersting breakthroughs I have developed to create cryogengic temperatures using very modest air compressors. Additional innovations cover the separation of different gasses with similar boiling points (nitrogen and oxygen).

The cryogenic heat pump

Everything on this page makes use of a device that continuously draws large quantities of heat from one end. I have never seen such a device described in the context of cryogenic cooling. I've tried out several names for it and I've settled on calling it the cryogenic heat pump.

The cryogenic heat pump is a single thickness coil of hot and cold air line coiled around an insulated core. Room temperature high pressure gas enters the warm end, is released at the cool end (the release of a high pressure gas causes a substantial temperature drop) and cycled back into the heat pump where it is warmed back up to room temperature as it gives up all of its cold to the warm high pressure line. The warm end of the exhcanger can never be too far from the 300 K (room temperature) high pressure air being pumped into that end, and the cold end of the heat pump can never be too far from the colder temperature of the released gas being pumped in from that end. As the cold air cools that end of the heat pump, the high presure air is cooled before it is released and a positive feedback cycle starts in which the lower temperature is only limited by how good of a heat exchanger we can make and how well insulated we can make it. You can see from the thermaldynamics that air at 300 k at 8 atm pressure keeps entering a system and air at 300 K at 1 atm keeps exiting that system. Energy is being constantly pulled from the cold end of this system at a rapid rate and will reach a point where we can liquify or freeze any gas we desire.

This differs from the classical method of blowing the cool air over the high pressure lines for a last moment decrease in temperature. In the classical system the cold air is just released, and the remaining temperature differential is wasted.

Let's look at this system in operation.

The primary quation to understand is:
This just means that the ratio of the the ending temperature T2 to the starting temperature T1 (in Kelvin or celcius) is equal to the ratio of the ending pressure P2 to the starting pressure P1 raised to a constant(k - 1)/k, and since k = 1.4 for air (oxygen or nitrogen) this simplifies to 1/3.5

for simplicity sake all math will be in integer units of 1 atmosphere (atm) absolute pressure and will assume room temperature is 300 kelvin (80 F). You and I are currently at 1 atm of pressure. The compressors that you and I can salvage for cheap achieve a maximum of maybe 11 atm absolute (10 atm relative, about 150 psi) if you are lucky. That doesn't mean you should run the pump at 150 psi any more than you should run your car at 150 mph, yes you can do it but not for the long haul. If you have a 150 psi pump you should probably run it at closer to 100 psi for long term use. 103 psi relative is 117 psi absolute whcih is 8 atm absolute pressure, so we'll use that for this experiment.

Because we are starting with everything at room temperature, the high pressure gas will start out by exiting the "cold" end of the heat pump at 300 K room temperature and then be released back down to 1 atm pressure. This will cause a temperature drop of (T2/300) = (1/8)^(1/3.5), T2 = 166 K which is routed back into the heat pump until it exchanges all its energy differential with the warm line and exits the system at room temperature 300 kelvin.

If you let this run for a few minutes you can see that we actually get the cold end down to 166 K as the ambient pressure cold gas pulls heat out of the that end of the pump until it exits the system at room temperature. The positive feedback awesomeness of this heat pump becomes apparent as we continue to try to figure out how cold the high pressure air can get before we release it down to 1 atm.

If we assume our heat pump is perfectly insulated (it won't be) and that it is simply long enough to exchange most of that heat, then after a short time the high pressure air is being chilled down to 166 K. When we release it back to 1 atm the temperature will drop to (T2/166) = (1/8)^(1/3.5), T2 = 91 K.

Now the 8 atm cold end is at 91 K, we release that down to 1 atm and get 50 K. You get the idea, it just keeps getting colder and colder with no end in sight (positive feedback). This actually turns into a real problem, we don't want the cold end to get so cold that we freeze the nitrogen inside the tube, so several of our designs put a warm air bypass in place vented through an automated temperature controlled valve. For the valve I'm thinking perhaps we can select numbers and placement of the thermostat such that the boiling point of argon or krypton or something is used to force a valve open or close.

Actual construction of the cryogenic heat pump: take 1/4" copper tubing (I'm going to start with 50' of it for $30 and see where that gets me) and put the entire length of copper inside suitably sized reinforced plastic tubing, say 1/2" or a bit smaller. You can't ask for better surface area exposure than that between the warm copper pipe and the cool returning air!! Now wrap the whole thing around a stiff core, you want a fairly tight core to keep like temperatures near each other on the coil, I'm thinking about something in the 4" to 6" diameter range. Now insulate the crap out of it, start with reflective foil on the inside and outside of the coil, then I'm thinking about obsene amounts of that yellow foamy stuff with reflective vapor barrier between each layer (unless of course you can pull off a vaccume sealed barrier between the tubing and a vaccume container, maybe something in a sealed 6" or 8" PVC pipe with the coils kept from touching the sides).

Devices that can be made with the cryogenic heat pump

A huge part of the challenge of this application is to remove unwanted oxygen, CO2, and water from the air. First lets start with a much simpler problem of achieving cryo temperatures to maintain a storage tank well below the boiling point of nitrogen.

keeping your existing tank below nitrogen boiling point

If you are using nitrogen to store biological material at cryo temperatures, your tank fill would last forever if the nitrogen could be kept cooler than boiling temperature of 77k. Here is a simple system to do that:

Liquid nitrogen maintainer to keep your cryo tank well below the boiling point of nitrogen

re-liqifying escaped nitrogen (or just make a cryo mix of O2 & N)

Let's say you are trying to maintain the level of liquid nitrogen in your storage tank, you don't have to go through all the work of creating clean nitrogen, just capture the professionally cleaned, dried, purified nitrogen gas as it escapes and re-liquify it when you have filled your capture container. Nitrogen has a liquid-to-gas expansion ratio of 1:694, so that 0.1 liter per day that you lose from your tank will need a couple cubic feet to store it. You would want to kick the pump off automatically when the gas storage was full and kick it back off when gas storage was near empty. Here is a diagram of how to make liquid nitrogen if you supply pure nitrogen as an input gas:

capture and re-liquify your escaping liquid nitrogen

Personally I began this project trying to create liquid nitrogen for AI semen storage. Liquid oxygen is just 15 kelvin warmer than liquid nitrogen and the first scientists to try to separate the two through normal distilation failed. When one boils off, the other does too, you need more than 25 degrees difference to separate liquids like that. That means that if you just liquify the oxygen and nitrogen together you would get a safe mix of gasses, you would not be left with dangerous pure oxygen after the nitrogen boiled, and the AI straws are all sealed air tight, no oxidation concerns there. If you do it that way you can build the simple device in the first diagram and not worry about that first fill or about capturing escaping nitrogen, you just need to filter out water and CO2. Personally that's the way I'm going to go.

The following are boiling (and freezing) temperatures of common atmospheric gasses at 1 atm.

Carbon dioxide 216K (freezes at 195K) 0.035%
Xenon 165K (freezes at 161k)
Krypton 120K (freezes at 114K)
Oxygen 90K (freezes at 54K) 21%
Argon 87K (freezes at 83K) 1%
Nitrogen 77K (freezes at 63k) 78%
Hydrogen 20K (freezes at 14k)
Helium 4k (freezes at 1k)
and don't forget the huge amount of water vapor that will freeze out.

Since oxygen liqifies at a higher temperature than nitrogen it is a comlete nuisance when trying to liqify nitrogen. On the other hand if you are trying to liquify oxygen, things couldn't get much simpler! Just change the thermometer in the above diagram to something like 154 kelvin, that will de-pressurise to 85 kelving, colder than liquid oxygens boiling point but warm enough not to collect liquid nitrogen.

doing it all

so you want to start with regular old air and make liquid nitrogen? You have to remove most of the unwanted elements. Good luck finding nitrogen membranes or enough oxidizing material to eat vast quantities of oxygen (remember the expansion rations? we are talking vast quantities of oxygen removal). The big boys do it with special towers and lots of different pumps, but we are going to take a different approach. Here is my single pump method for first liquifying the oxygen and then in a second pressure drop I liquify the nitrogen. Then I feed the fresh air through an ice box that is cold enough to freeze out everything I don't want to keep (water, CO2) but not so cold that we liquify O2 or nitrogen. This is a substantially harder project than the nitrogen reclaimer.

creating liquid nitrogen and liquid oxygen from regular air

I don't know how much of the oxygen will really participate out in a single pass like I have designed here, the actual percipitation box will run the air through a radiator to offer plently of tubing for liquid to condense in and then after release from that the air flow will gradually pass through the radiator fins offering a final chance for oxygen to parcipitate. Nothing is 100%, and I don't know what the actual percentage will be, but if I am dissatisfied with the resulting nitrogen purity I will simply put a valve into the system such that the air flow bypasses the nitrogen tank whenever oxygen levels are above a minimum threshold. In that design I would have the output of the pump go into a standard compressor storage tank, which I would then tap and monitor for oxygen content.

So I've given you all sorts of inovations that I personally came up with in order to use ordinary household air compressors to make as much liquid nitrogen as you could ever use. Now we get down to where the cheese binds. The truth of the matter is that the US patent and trademark system is broken. It favors big corporations and does not give the individual inventor much of a fighting chance. Check out http://SSUBA.COM for another of my inventions. I swear to God after I got rejected for the third and final time the patent clerk called me up personally to ask why I let the appeal process expire. I said "You rejected my claim" and her response was "yes, becaues I thought you would appeal it". That's right, she recognized the value and originality of my invention and fully expected the process to continue after a third and final rejection. No way am I putting myself through that wringer again for naught. So here is the unfortunate truth, the government isn't going to protect my inventions. Since they forfeit that right, I get to enforce my rights myself. Please forgive me gentle reader for what I am about to say, but I do say it in all honesty - you are absolutely free to use my inventions for your personal use / gifts / etc to your hearts content, no problem at all. BUT if you are using my intelectual property commercially without negotiating appropriate licensing fees, let's just say that would be a tactical error on your part and leave it at that. The claims I specificlly choose to enforce are the use of the positive feedback cryogenic heat pump to liquify gasses, the use of multiple pressure drops in order to selectivly separate multiple gasses, and the selective valving of colder storage tanks if warmer boiling gasses are not adequatly reduced.

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