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In this video, I'll show how I built a cryocooler out of salvaged air conditioner parts that's able to make liquid nitrogen at 196C.

The system operates on a process known as the MixedGas Joule Thomson cycle. A mixture of refrigerant gases are compressed, cooled to ambient temperature, and then expanded inside a long counterflow heat exchanger. When the gases expand, they cool. Cold, lowpressure gas cools the incoming highpressure gas causing it to drop its temperature even further, creating a snowball effect that drives the cycle down to cryogenic temperatures. The lowest temperature I've achieved is 180C with a 30/70 mixture of Propane and Argon.

For higher performance, a separate vapor compression system is used to precool the gas mixture to around 30C before it enters the regenerative heat exchanger. This adds complexity, but nets a performance gain, since total cooling power goes up by 3050% with only a 1520% increase in energy consumption.

The super cold gas travels through a heat exchanger pipe in a small tank. High pressure nitrogen is pumped into the tank at up to 30 bar and liquefies at around 150C. Once the tank is filled with liquid nitrogen, it's discharged through an electronic expansion valve at the bottom of the tank into a thermos or dewar.

This is different from the puregas joule thomson cycle in which nitrogen is expanded from 200300 bar to atmospheric pressure to create a cooling effect. In the mixedgas cycle, the refrigerant gases and the process gases being liquefied are two different streams.

The gas mixture being used is:
25% Propane
15% Ethylene
20% Methane
20% Argon
20% Nitrogen

The typical operating pressure is 400 psiG on the high pressure side and about 20 psiG on the low pressure side. Power consumption is approximately 1.4 kW, and my current production rate is about 130cc/hour of liquid nitrogen, though I'm confident this can be improved close to 300cc/hour with some adjustments. The current energy cost is about $1.29 per liter produced.

For this video I've used nitrogen gas from a tank I purchased, but in a future video I'll show the process of separating out oxygen/nitrogen.

Music Used:

Kevin MacLeod Groove Groove
Kevin MacLeod George Street Shuffle