The subcooler cools down the nitrogen from the tank in order to avoid a lot of gas while expanding the nitrogen into the extrusion die and to get a high amount of liquid nitrogen. The main reason for cooling is caused by evaporation energy, this means to change the nitrogen from liquid to gaseous phase, and not by using the cold gas. The nitrogen in a tank at 5 bar has approximately -180°C and is “warmer” than the nitrogen at ambient conditions (-196°C). By expanding the nitrogen of -180°C directly into the die the hole space would be covered by gaseous nitrogen due to the tremendous change of volume. While changing the condition from liquid to gas the volumes increase 600 times.
For this reason a part of the nitrogen would be expanded into a so called subcooler. This is a vessel without pressure which contains a simple heat exchanger. The expanded nitrogen has now a temperature of -196°C and surrounded the heat exchanger. The “warm” nitrogen flows through this heat exchanger and at the end the nitrogen has 5 bar and almost -196°C, the so called subcooled nitrogen. Expanding subcooled nitrogen into the extrusion die, the part of liquid is much higher which cools by means of evaporation very efficient the die.