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Toward a Safer Refrigerant
Traditionally, refrigerants were judged on their toxicity, flammability, and physical hazards when evaluating their safety risks. In this analysis, depletion or damage to the Earth's protective ozone layer was left out. You may wonder why this was the case. Probably because there was nothing wrong with the ozone layer at the time and any safety analysis must have its boundaries.
In the late 1800's to early 1900's, common refrigerants included ether, naphtha, methyl chloride, dichloroethene, and ammonia. Nearly all flammable, toxic or both. By 1926, investigations into fluorinated organic chemical led to the use of dichlorodiflouromethane or R-12 as a refrigerant. This substance was found to be nearly non-toxic and non-flammable, a seemingly good fit for what the industrial world sought at the time. Ironically, it was the same stability that made chlorofluorocarbons (CFCs) good refrigerants that allowed them to deliver ozone-depleting chlorine to the stratosphere. All of the sudden, refrigerants needed to be judged on another basis in addition to toxicity, flammability, and physical hazards. The world needed to find a refrigerant that does not carry chlorine to the ozone layer. But, considering that chlorine based chemicals make very good refrigerants in all other aspects, where do we begin this search?
It quickly became apparent that currently, no refrigerant seems to economically meet of four criteria for a good refrigerant. Attention started to shift to partially halogenated hydrocarbons (HCFCs). These refrigerants have less chlorine and fluorine, but are generally not viewed as a long term solution because they cause less damage to the ozone than CFCs, but are not ozone-safe. The advantage to HCFCs is that they can be used with current refrigeration systems with low to moderate economical changes.
Thinking in terms of long term refrigeration solutions, people have begun to question the need for non-flammable refrigerants. Some say that with proper training and system design, flammable refrigerants may be used safely. That brings halogen-free hydrocarbons (HCs) into the mix. These can include many classes of hydrocarbons. Most likely, fairly significant changes to current systems would have to be made to existing systems to use these chemicals.
Another option that would also require significant system changes and may offer a long term solution are the use of chemicals such as air, water, nitrogen, helium, and carbon dioxide as refrigerants. Obviously these systems would require higher pressures and thus will be most expensive, but their use has already been demonstrated on industrial scales in some case. For example, Air Products on Allentown, PA developed a cryogenic system that uses air as the refrigerant and it has been quite successful.
While it's clear that finding the "perfect" refrigerant in not a simple tasks, many are willing to challenge conventional thinking. Personally, I cannot imagine a better refrigerant than air or water. However, using such inert substances on all refrigeration scales will require some more time and innovation. Safe options are available, but as with any major change, it will take some time to get where we need to be.