Part I - Outline
1. Introduction & History Notes
2. Energy & Temperature Notions
3. Production of Cold: Liquefaction and Refrigeration
4. Application of Cold
1. Introduction & History Notes
Cryogenics does many things in the world around us. There are many uses, seen and often unseen, of cryogenics in the real world. Cryogenics has to do with cold temperatures. Temperature has to do with energy. Therefore, cryogenics is all about energy.
Cryogenics is a part of refrigeration which has to do with anything below ambient temperature. There is natural refrigeration (ice and cold due to weather and climate) from the beginning of the world. Artificial refrigeration can be traced to 1755 when William Cullen built the first machine to that made some ice by evaporation of water at reduced pressure in a bell jar (vacuum vessel). Several ingenious, pioneering efforts in the 19th century led to the beginning of industrial refrigeration (by artificial means) around 1875. The previously called "permanent" gases were about to be liquefied, thus starting the era of cryogenics.
2. Energy & Temperature Notions
It takes a lot of "hot" to make a little "cold." That is, much heat energy input (hot power) is required to provide a little cold power. For example, if you would like a nickle's worth of liquid nitrogen (a bucket full) it may cost you about a dollar's worth of heat energy (power bill). So that bucket full cost $1.05: five cents for the nitrogen (and shipping & handling costs) plus one dollar for the power bill involved in making it. Cold costs money! Below are some basic figures on cold powers and the corresponding hot powers needed to make that cold happen. The temperatures are some popular ones in cryogenics, starting with the normal boiling point of liquid nitrogen (77 K).
Temperature (K)
|
Cold Power (W)
|
Hot Power - Ideal (W)
|
Hot Power - Actual (W)
|
77
|
1
|
3
|
10
|
20
|
1
|
14
|
100
|
4
|
1
|
74
|
1000
|
1
|
1
|
300
|
10,000+
|
0.1
|
1
|
3,000
|
100,000++
|
Those are a lot of watts for one watt. For one single watt of cooling at liquid helium temperature, about one thousand watts of power are needed. Wow. And for what? A cold watt, that's what.
As another illustration of the extremes of cryogenics, consider that you have a really good oven at home (a 700 degree F commercial pizza oven, of course) and you want to put a jug of water in there are keep it for a few months. Water boils at 212 degrees F and the oven is kept a just above 700 degrees F, a temperature difference of 500 degrees F. Crazy, impossible? Not at all. This same situation is rountinely handled by a vacuum-jacketed, multilayer insulated liquid hydrogen storage tank. The hydrogen boils at -423 degrees F and the oven is kept at about 80 degrees F (hotter in Florida at the Kennedy Space Center's launch pads); again, the same temperature difference as our jug of water in the pizza oven.
The large temperature differences, no matter what the cryogen, is the reason that thermal insulation is the single enabling factor for everything on Earth (the "oven") in the world of cryogenics.
3. Production of Cold: Liquefaction and Refrigeration
The production of cold is needed for both liquefaction and refrigeration. Both things are refrigeration but the motivations are uniquely different. In any case, the same thermodynamics are at work and the process cycles are the key. Most refrigerators use gas for the working fluid. The steps include either compression or expansion of the gas in a manner that is either adiabatic (thermally isolated) or isothermal (thermally connected). The first and second laws of thermodynamics define the possibilities and describe the terms of work, heat energy, and enthalpy.
Large systems such as air separation plants can work at efficiencies in the range of 30 to 40 percent. Small systems such as cryocoolers for point cold application will have much less efficiency. The Carnot efficiency determines the idealized best possible efficiency. Turbo-Brayton refrigerators with helium gas as the working fluid scale over a wide range and are very useful for cooling in superconducting applications. Popular types of cryocoolers today include G-M, J-T, and pulse tube types. Dilution refrigeration systems are used to obtain temperatures in the range of 0.1 K.
4. Application of Cold
Cryogens or cryocoolers can be used to apply the cold for a given process, or application. The cold is applied for energy density, as in liquid hydrogen for space launch vehicles or automobiles, or for using the cold temperature to do something useful, as in freezing strawberries, performing brain surgery, recycling tires, or keeping cool a superconducting power line.
Part I - Questions:
1. What is the definition of cryogenics? hint: there is one key word.
2. What are the two main types of work within the area of cryogenic engineering?
3. What are the normal boiling points of oxygen, nitrogen, hydrogen, and helium? (K and degF)
4. In general, where do oxygen, nitrogen, hydrogen, and helium come from?
5. What are the two main things (applications) that one can do with cryogenics?
6. What is the central or dominant theme of cryogenics? hint: one word.
7. What is a cryocooler and how can one be used?
8. What are the practical “hot watt per cold watt” ratios for a cryocooler operating at 77 K, 20 K, and 4 K? Also, what would be the ratio as 0 K is approached?
9. What are the two chief problems in cryogenic engineering?
10. Why is vacuum technology important to cryogenics? (How does it come into play for operations, testing, manufacturing, etc.?)