Liquid Nitrogen (LN2)
Liquid nitrogen is what most cryogenic processing companies use to cool the parts in their machines. Here are some facts about liquid nitrogen that may be useful.
1. The use of liquid nitrogen does not pollute the air. The atmosphere is about 78% nitrogen anyway. Liquid nitrogen is produced by cooling air down until it liquefies. So using liquid nitrogen just puts the nitrogen back where it came from.
2. No, you cannot recycle and use the liquid nitrogen over. The way nitrogen cools a part is by absorbing the heat from the part and becoming a gas.
3. The boiling point of LN2 is -320.4 degrees F at 1 atm. (195.8 degrees C, 77.4 degrees K, and 139.3 degrees R)
4. The heat of vaporization is 38.8 K Cal/Liter or 579 BTU/Gallon
5. 1 Liter of LN2 makes 22.8 ft3 of gas at Standard Temperature and Pressure (STP)
6. LN2 weighs 6.745 pounds/gallon
7. LN2 should be used in a ventilated area. Too much used in a closed area will dilute the oxygen in the room and can cause asphyxiation.
8. Never touch LN2, it is extremely cold and can cause extreme frostbite.
9. Get training on the use of LN2 and always wear protective gear when using it.
10. Nitrogen was discovered by Daniel Rutherford in 1772
11. Nitrogen has a specific heat of 2.04 kj/kg K
12. Nitrogen is the fifth most abundant element in the universe
13. Nitrogen freezes at -345.9 degrees F. (-209.9C or 63.2 degrees K)
Liquid Nitrogen is usually sold by the "Standard Cubic Foot". This is the amount of space the gas produced by evaporating the liquid would take up at 1 atmosphere and at 68 degrees F. This is done because a great majority of the users get liquid nitrogen for use as a gas. One liter of LN2 will make 22.8 cubic feet of gas.
LN2 is delivered in two ways. It can be delivered in dewars or in bulk. The picture at the right shows a CP-500vi connected to a portable dewar. When the dewar is empty the supplier comes out and picks it up and leaves another. Note that there are wheels on this dewar, but many dewars just sit on the floor. They are heavy, so for safety you need some means of moving them.
Most LN2 suppliers charge rent for the dewars on a day to day basis. They also charge for delivery. Another charge that will show up is a "Hazardous Materials" charge. This is basically a nonsense charge that makes their profit more.
With bulk deliveries, you have a large dewar permanently located at your facility. A truck with a huge dewar on it comes and pumps your dewar full. This has the attraction that you need not be present in order to receive the LN2.
Note that LN2 is stored and delivered in dewars. Dewars are the most efficient means of storing LN2. Dewars still lose a little LN2 over time.