HFO refrigerants are decidedly more complicated. They are built around a propene molecule. Propene has three carbon atoms surrounded by hydrogen atoms. Propene is similar to propane, except propane has all single bonds between its atoms while propene has a double bond between two of the carbons atoms.
You can think of each carbon atom as having four Velcro hooks. Molecules like propane only use 1 hook for each bond. This allows each carbon to bond to the most possible other atoms. Molecules constructed this way are referred to as saturated. Two of the carbon atoms in a propene molecule use two Velcro straps to bond to each other, which reduces the number of other atoms the carbon molecules can bond with. Molecules built this way are referred to as unsaturated.
To unlock the secret code which describes fluorinated hydrocarbon refrigerants, just add 90 to the number, leaving off the leetrs for now. For example, 1233 + 90 = 1323. Working backwards from the right, the first number describes the number of fluorine atoms. In this case it is 3. The second number from the right describes the number of hydrogen atoms. In this case 2. The third number from the right describes the number of carbon atoms. In this case 3. The fourth number from the right lists the number of double bonds in the molecule. In this case 1. Notice the number of chlorine atoms was not addressed. The number of chlorine atoms is found by subtracting the fluorine and hydrogen atoms from the number of bonds. A propene molecule has 6 bonds. 6- 3 -2 = 1. There is one chlorine atom.
So what are the letters at the end of 1233zd(E)? The short answer is that all the letters following the number describe the particular molecular arrangement. We know that 1233zd(E) contains 3 carbons, 3 fluorines, 1 chlorine, and 2 hydrogens. However, even if you know exactly which atoms there are, you must also describe where they are attached because there are now many places to put them.
Each different arrangement of the same atoms produces different properties, so it is important to specify which arrangement the refrigerant is using. These different arrangements are called isomers. The two lower case letters after the number describe the specific arrangement (isomer). But note that this refrigerant number has yet another upper case letter after the two lower case letters. Some isomers have the same arrangement, but differ in spatial orientation. The upper case letter identifies which spatial orientation.
This is about as deep as I feel I should go in a blog post (maybe even a bit too deep). If you want more detail, it is all explained in the ASHRAE Standard 34-2016.
