Low Temperature Special Topic | How to Get Low Temperature (I)

Time: 2020.02.24Source: View: 3700

Vacuum technology and cryogenic technology are both highly versatile sciences, and there is an inseparable relationship between them. There are many ways to obtain low temperatures, which can be roughly divided into two categories: physical methods and chemical methods. Most refrigeration methods are physical methods, among which gas adiabatic expansion refrigeration and phase change refrigeration are commonly used. In addition, there are eddy current refrigeration, adiabatic degassing refrigeration, temperature difference thermoelectric refrigeration, top magnetic salt or adiabatic demagnetization refrigeration, 3He and 4He dilution refrigeration, 3He adiabatic compression refrigeration, adsorption refrigeration, etc. These refrigeration methods are mostly used in ultra-low temperature areas. The following briefly introduces the refrigeration principles of these methods.

1. Phase change refrigeration

This is a method of refrigeration that uses the heat absorption effect of a substance during phase change. Such as the heat of fusion when a solid turns into a liquid, the heat of sublimation when a solid directly turns into a gas, the heat of evaporation when a liquid generates a gas (also including reduced pressure evaporation, such as 3He reduced pressure evaporation), etc., using the heat absorbed during these phase change processes to achieve the purpose of refrigeration. The open-cycle refrigerator designed by Fermion Company uses the heat absorption of vaporization of liquid helium or liquid nitrogen to achieve refrigeration.

【Figure 1】 Common cryogenic liquid working temperature range

【Table 1】Latent heat of vaporization of four cryogenic liquids

【Table 2】 Correspondence between vapor pressure and temperature of liquid nitrogen

【Figure 2】 He-3 Cryostat

2. Gas isenthalpic expansion refrigeration

When a gas is expanded isenthalpically at a certain pressure and temperature through a throttle valve or an expander, its temperature will decrease and may even liquefy. This refrigeration method is most widely used in the liquefaction and separation of gases and gas refrigerators. The JT cold head of the GM-JT refrigerator independently developed by Fermion Company is isenthalpic expansion refrigeration.

[Figure 3] Schematic diagram of JT refrigerator

[Picture 4]JT Refrigerator

[Figure 5] GM Refrigerator

The temperature is changed by three methods: phase change refrigeration, gas isenthalpic expansion refrigeration, and adiabatic degassing refrigeration, so as to achieve the low temperature we need. If you have other methods to achieve low temperature, please leave a message on WeChat public account and share your method of achieving low temperature.

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Low Temperature Special Topic | How to Get Low Temperature (I)