The idea of mixing two fluorocarbons to provide a refrigerant that behaved like a single fluid but with beneficial properties was commercialized in the 1950s with the introduction of R-500, R-501 and R-502. ASHRAE Standard 34, which was originally published in 1958, classified these three blends as “azeotropes” and considered them to be lower toxicity and non-flammable.
The idea of putting a mixture that did not form an azeotrope into a vapor compression system was slower to develop, although very early experiments in the nineteenth century had used hydrocarbon mixtures like this to good effect. When chemists at the Kinetic Chemical Company were looking for more applications for the newly-developed CFCs in the 1930s they also tried nonazeotropic mixtures and even assigned them code numbers in the 400 range to differentiate from other fluids. However, when the main single component refrigerants for various applications (R-11, R-12, R-13 and R-22) had been identified and were being commercialized, these other experimental oddities with their strangely ambivalent boiling points were quietly forgotten.
By 1978 ASHRAE Standard 34 had been around for twenty years and the committee, recognizing the theoretical concept of a non-constant boiling temperature mixture, added the 400 series to the standard to denote mixtures, although no fluids were listed in that category. Ten years later, when they met again, there were still no 400 series refrigerants in widespread commercial use. The committee felt uncomfortable in having an empty class so they racked their brains for an example that could be used to fill the gap. One of the committee mentioned that a mixture of CFC-12 and CFC-114 was used for air conditioning of operator’s cabins in the overhead cranes used in steel mills—a very arduous, high ambient application. This became R-400, but in general the composition percentages were not quoted and no information was published about the dew and bubble point temperatures. By 2007 it had been removed from Standard 34 and ISO 817 followed suit in 2014.
In the late 1980s the idea that refrigerant mixtures were a way of combining favorable properties to offset the basic disadvantage of non constant boiling really took hold. The term “blend” was adopted as preferred terminology for all 400 and 500 refrigerants at this time because, it was said, this was more suggestive of an intention to achieve a specific desirable effect rather than the random happenstance implied by “mixture.” Several researchers looked at less common components to deliver specific benefits, for example small amounts of hydrocarbon to improve oil miscibility or traces of lower pressure components to lower the overall saturation pressure of the blend. This resulted in intense scrutiny of the science of fractionation and a tidal wave of new applications.In general, when they were used in direct expansion systems these blends performed reasonably consistently, provided the technician setting up the expansion valve superheat knew to “aim off” to account for the effect of temperature glide. They were less successful in flooded evaporators so have not been used much in industrial refrigeration or large chillers.By January 2019 one hundred and five different blends were listed in the 400 series in Standard 34, and more keep appearing. Paradoxically, this can be seen as a great success for this odd idea that was so slow to catch on, but at the same time a great failure because the strenuous efforts of more than thirty years of global development have not yet delivered a universally acceptable replacement for CFCs.