Heat exchanger, liquid nitrogen

In order to reach a low temperature, the sample may be placed inside a metal block which is flushed with a cooling medium. Such devices reach approximately -78°C. Even lower temperatures are achieved if the sample holder is in thermal contact with a heat exchanger. Liquid nitrogen is usually vaporized in the heat exchanger to cool the sample. To maintain the desired temperature, most sample holders can be heated electrically. Vaporization of liquid helium, typically in a closed cycle, also has a cooling effect. To avoid heat loss and to protect the cell windows from condensed water, the cells are surrounded by a vacuum chamber or a chamber containing a dry gas at atmospheric pressure. 

Robertson, J. M., 1983, The Boiling Characteristics of Perforated Plate-Fin Channels with Liquid Nitrogen in Upflow, in Heat Exchangers for Two-Phase Flow Applications, ASME HTD-Vol. 27, pp. 35-40. (4)... 

As shown in Figure 11.8, a loop of nitrogen is realized between a low-pressure zone (flash evaporator EV01, heat exchanger EC01, and bioreactor R01), a medium-pressure zone (gas/liquid separator V04), and a high-pressure zone (final condenser V05). 

Fig. 2.30. Simplified P. I diagram of liquid nitrogen. 1, Chamber and shelves 2, condenser 3, silicone oil circuit 4, heater 5, heat exchanger.

Liquid nitrogen is stored in large vacuum vessels (cryostats). The liquid nitrogen is evaporated through a heat exchanger and the gas used in the factory. The units are normally supplied and maintained by the gas company. The cryostats can be easily topped up by a road tanker. This system is more economic in medium to large units. 

In the column, the vapor is countercurrently contacted with 250 mol/h of liquid propane, which absorbs from the vapor feed 85 percent of its ethane content, 9 percent of its methane, and a negligible amount of nitrogen. The pressure of the bottom stream of liquid propane with its absorbed constituents is raised to 500 psia (3450 kPa), and the stream is heated to 50°F (283 K) in a heat exchanger before being directed to another section of the process. The vapor, leaving the overhead tower at -60°F (222 K), consists of unabsorbed constituents plus some vaporized propane from the liquid-propane absorbing medium. 

---