The other evening my wife and I went to a new ice-cream shop in town, and while I was having my usual cup of vanilla I happened to glance at a refrigerated glass-front cabinet where the employees kept decorated cakes for sale. Way down on the lower left corner of the front was a green label that read "NATURAL [followed by a leaf symbol] Refrigerant."
That got me curious. I knew the "natural" label has great consumer appeal these days when applied to food and maybe clothing, but refrigerants? So I did some digging and discovered that there is quite a history behind the various refrigerants that have been used since mechanical refrigeration was developed in the nineteenth century.
For readers who need a review of how mechanical refrigeration works, here's the brief version. When you compress a gas, it gets hotter, and that heat comes from the work you expend in compressing it. If you then remove the heat somehow (that's what the condenser unit does outside every air-conditioned building), and the gas is suitable for use as a refrigerant in the system designed for it, it will turn into a liquid when it's cooled down enough. If you then send the liquid through a small opening into a place where the pressure is lower, it will evaporate back into a gas, and get a lot cooler than it was before. That's what happens in the evaporator inside an air-conditioned building. It's the same basic principle that makes you feel cooler when you're sweating on a hot day and a breeze comes up: water evaporates from your skin and takes some heat with it. Run this process around and around again with the same substance, and you have a mechanical refrigerator.
All right. When large-scale mechanical refrigeration systems operated by steam power became available in first Europe and then the U. S. in the 1880s, they used ammonia gas. The bottled liquid we call ammonia is just a solution of pure ammonia gas (NH3) in water. The gas itself condenses to a liquid at atmospheric pressure at a temperature of -28 F (-33 C), but under moderate pressure it can be persuaded to condense at or above room temperature, and it carries a good amount of heat away when it re-evaporates at lower pressure.
Nineteenth-century ammonia was about as natural and organic as you could get. It was obtained mainly from the waste urine from packing houses, by distillation, and it was therefore fairly expensive. It got a lot cheaper when the Haber-Bosch process for making it from hydrogen (obtained from natural gas) and nitrogen (obtained from air) was perfected in the early 20th century. But leaks in refrigeration machinery were common and ammonia gas is nothing you want to get loose around customers. So the industry sought a non-toxic, non-corrosive substitute, and along came a General Motors chemist named Thomas Midgely Jr.
Midgely was largely responsible for the development of leaded gasoline, which in the 1920s was viewed as vital to the efficient operation of gasoline-powered vehicles. Fresh from this first long-term environmental disaster, Midgely devised a new chemical that used carbon, hydrogen, and fluorine to make what at the time appeared to be the ideal refrigerant, which was trade-named Freon. It became wildly popular, but in the 1970s, just when we were phasing out Midgely's first brainchild, leaded gas, it was discovered that the original type of Freon destroyed ozone in the atmosphere, at a rate that promised to leave us unshielded from the harmful ultraviolet rays that are normally absorbed by the naturally-occurring ozone layer.
Somehow, the world's engineers cooperated in 1987 to agree to the Montreal Protocol, which committed the signatories to replace ozone-destroying refrigerants with some that are less harmful to the atmosphere. Since then, the overarching question asked about new refrigerants is whether they hurt the atmosphere, and if so, by how much? So it turns out "natural" in the context we're talking about means "less harmful to the atmosphere," and not necessarily something that occurs only in nature.
For example, on a website that sells restaurant equipment, I found an article that rates many current types of refrigerant with red (Not Eco-Friendly), yellow (Somewhat Eco-Friendly) and green (Eco-Friendly) labels. Something called R-450A has a green label, and may be what's keeping the ice-cream shop's cakes cool. It's made of a chemical called hydrofluoroolefin (HFO), which is anything but natural in the sense that it's a highly engineered artificial compound. But if it gets loose in the air, the nature of its chemical bonds makes it turn into a reactive acid that gloms onto something or other fairly quickly and leaves the air, never making it into the stratosphere where it could bother the ozone layer.
If you want "natural" to mean "naturally occurring," there is the old standby ammonia, which is still used in large-scale industrial refrigeration where its toxicity and flammability can be kept safely under control. Propane and isobutane, which are distilled from natural gas, can also be used as refrigerants, but they can burn and have to be used in carefully sealed systems for consumer applications. And the bad boy of the climate-change movement itself, carbon dioxide, can also be used as a refrigerant, although it doesn't condense unless the pressure is raised to over sixty times atmospheric pressure, necessitating very sturdy compressors and containment pipes.
As historian Jacques Barzun pointed out in his monumental From Dawn to Decadence, the notion that nature knows best about a wide variety of things has a life of its own, and was one reason the "natural" tribes discovered in North America were of such interest to the Europeans who eventually overwhelmed them. In the context of refrigeration, it looks like a more accurate label than "natural" would be "eco-friendly," but the PR people know what words look good in public view, and they picked "natural."
It's only pedants like me who would even think to quibble with what the word actually means. Without rolling the cabinet out from the wall and looking at the nameplate, I couldn't tell exactly what refrigerant was being called natural. And my curiosity has its limits—it was good ice cream, and I didn't want to cause a scene and get barred from the shop forever. I'm just glad that once we found problems with the refrigerant that at first glance looked ideal, we changed course and developed a whole spectrum of other ones. That's the way engineering should work, and in this case, it has.
Sources: I referred to the website https://www.webstaurantstore.com/article/474/refrigerant-types.html?srsltid=AfmBOorPfBjgu4ChyoyuSMDiNc9uETNLojxoiL_gL91XRkUgPKAPtlym
for the list of eco-friendly-graded refrigerants, and to the Wikipedia articles on HFOs and Thomas Midgely Jr.
