第二代制冷劑是HCFC（氫氯氟烴），它是含有氫、氯、氟和碳原子的合成化合物。 它們的通式為 C<sub>n</sub>H<sub>x</sub>Cl<sub>y</sub>F<sub>z</sub>，其中 n、x、y 和 z 是整數1。
最常用作制冷劑的 HCFC 是 R21（二氯氟甲烷）、R22（一氯二氟甲烷）、R31（氟氯甲烷）、R123（2,2-二氯-1,1,1-三氟乙烷）、R124（1-氯-1,2、 2,2-四氟乙烷)、R141b(1,1-二氯-1-氟乙烷)和R142b(1-氯-1,1-二氟乙烷)2。
HCFC 是在 20 世紀 20 年代和 1930 年代開發的，作為第一代制冷劑 CFC（氯氟烴）的替代品，CFC 被發現會導致嚴重的臭氧消耗。 HCFC 的臭氧消耗潛值 (ODP) 低于 CFC，但它們仍然具有一些負面環境影響，例如高全球變暖潛值 (GWP) 和大氣壽命3。
HCFC作為制冷劑具有許多優點，如穩定性高、毒性低、不易燃、沸點低、汽化潛熱高以及與潤滑油和金屬相容等。 它們還廣泛用于其他應用，例如發泡劑、溶劑、滅火器和其他化學品的中間體。
全球 HCFC 的生產和消費量在 20 世紀 90 年代達到頂峰，然后由于科學證據的不斷增加和公眾對其環境危害的認識而開始下降。 1987年，197個國家簽署了《蒙特利爾議定書》，同意在具體期限內逐步淘汰氟氯化碳和其他消耗臭氧層物質的生產和使用。 1992年，《蒙特利爾議定書》的哥本哈根修正案將HCFC列入受控物質清單，并制定了逐步淘汰的時間表。
《蒙特利爾議定書》被認為是最成功的國際環境協定之一，有效減少了氟氯化碳等消耗臭氧層物質的排放，為臭氧層的恢復做出了貢獻。 根據聯合國最新評估，臭氧層預計到本世紀中葉將恢復到1980年的水平。
HCFC 的淘汰還刺激了替代制冷劑的開發和采用，例如 HFC（氫氟碳化物）、HFO（氫氟烯烴）和天然制冷劑（如氨、二氧化碳、碳氫化合物和水）。 這些替代品的臭氧消耗潛勢較低或為零，但它們可能具有其他缺點，例如全球變暖潛勢高、易燃性、毒性或效率低。
因此，制冷劑的選擇和開發是一個復雜的動態過程，涉及熱力學、物理、化學、安全、經濟和環境等多個因素。 理想的制冷劑應該具有高性能、低環境影響、低成本和廣泛的可用性。
The second generation of refrigerants are HCFCs (hydrochlorofluorocarbons), which are synthetic compounds that contain hydrogen, chlorine, fluorine and carbon atoms. They have the general formula of C<sub>n</sub>H<sub>x</sub>Cl<sub>y</sub>F<sub>z</sub>, where n, x, y and z are integers1.
The most common HCFCs used as refrigerants are R21 (dichlorofluoromethane), R22 (chlorodifluoromethane), R31 (fluorochloromethane), R123 (2,2-dichloro-1,1,1-trifluoroethane), R124 (1-chloro-1,2,2,2-tetrafluoroethane), R141b (1,1-dichloro-1-fluoroethane) and R142b (1-chloro-1,1-difluoroethane)2.
HCFCs were developed in the 1920s and 1930s as alternatives to the first generation of refrigerants, CFCs (chlorofluorocarbons), which were found to cause severe ozone depletion. HCFCs have lower ozone depletion potential (ODP) than CFCs, but they still have some negative environmental impacts, such as high global warming potential (GWP) and atmospheric lifetime3.
HCFCs have many advantages as refrigerants, such as high stability, low toxicity, non-flammability, low boiling point, high latent heat of vaporization and compatibility with lubricants and metals. They are also widely used in other applications, such as foam blowing agents, solvents, fire extinguishers and intermediates for other chemicals.
The global production and consumption of HCFCs peaked in the 1990s and then began to decline due to the growing scientific evidence and public awareness of their environmental hazards. In 1987, the Montreal Protocol was signed by 197 countries, which agreed to phase out the production and use of CFCs and other ozone-depleting substances by specific deadlines. In 1992, the Copenhagen Amendment to the Montreal Protocol added HCFCs to the list of controlled substances and set a timetable for their phase-out.
The Montreal Protocol is considered one of the most successful international environmental agreements, as it has effectively reduced the emissions of CFCs and other ozone-depleting substances, and has contributed to the recovery of the ozone layer. According to the latest assessment by the United Nations, the ozone layer is expected to return to its 1980 levels by the middle of this century.
The phase-out of HCFCs has also stimulated the development and adoption of alternative refrigerants, such as HFCs (hydrofluorocarbons), HFOs (hydrofluoroolefins) and natural refrigerants (such as ammonia, carbon dioxide, hydrocarbons and water). These alternatives have lower or zero ozone depletion potential, but they may have other drawbacks, such as high global warming potential, flammability, toxicity or low efficiency.
Therefore, the selection and development of refrigerants is a complex and dynamic process that involves multiple factors, such as thermodynamic, physical, chemical, safety, economic and environmental aspects. The ideal refrigerant should have high performance, low environmental impact, low cost and wide availability.
