Decoding HFO Numbering

 Undoubtedly you have seen news articles mentioning HFO refrigerants with names like 1234yf, 1234ze(Z), or 1234ze(E). Although these names look like a secret code, there is method in the madness. The good news is that technicians probably don’t need to know exactly how to read this secret code to do their job. However, telling me I don’t need to know what’s behind the curtain just encourages me to pull the curtain back. So here goes.

What is an HFO

First, you need to understand what an HFO is. An HFO is essentially an HFC with a double bond between two carbon atoms. You might remember from high school chemistry that carbon has a valence of 4. Think of the valence as the number of Velcro tabs on the atom. The carbon atoms in a normal hydrocarbon molecule are joined by single bonds, just one set of Velcro tabs joined between each carbon atom in the chain. They are called saturated because they are connected to largest number of atoms possible. Unsaturated hydrocarbon molecules, like HFOs, have a double bond between two of the carbon atoms. They use two sets of Velcro tabs between two of the carbon atoms. The double bond means there is one less atom in the molecule since two bonds are used between a pair of carbon atoms. Thus the designation as unsaturated.

Secret Code

The first four numbers of the secret refrigerant numbering code identify, in order: the number of double bonds, the number of carbon atoms, the number of hydrogen atoms, and the number of fluorine atoms in the molecule. However, there are many ways those atoms can be arranged, and different arrangements of the same components create different refrigerants with different physical properties. The last two or three letters describe how the atoms are arranged in the molecule.

First Number

The first number in the HFO numbering system describes the number of double bonds. At present, I am not aware of any HFO refrigerants that have more than one double bond. Currently all HFO refrigerants start with the number 1.  The 1 at the start of R1234ze(Z) indicates that the molecule has one double bond.  

Second Number

The second number is equal to the number of carbon atoms minus one.  The 2 in R1234ze(Z) indicates that the molecule has three carbon atoms: (#Carbons (3) - 1 = 2).

Third Number

The third number is equal to the number of hydrogen atoms plus one. The 3 in R1234ze(Z) indicates that the molecule has two hydrogen atoms (#Hydrogens (2) + 1 = 3).

Fourth Number

The fourth number is equal to the number of Fluorine atoms. The 4 in R1234ze(Z) indicates that the molecule has four Fluorine atoms.

First Lower Case Letter

HFO refrigerants are based on propylene, which has three carbon atoms. The first lower case letter identifies the atom connected to the middle carbon atom: x for chlorine, y for fluorine, and z for hydrogen. The lower case z in R1234ze(Z) indicates that the atom bonded to the middle carbon is hydrogen.

Second Lower Case Letter

The way the atoms are arranged on the ends of the molecule can vary. The second lower case letter describes the arrangement of the atoms on the end carbon containing the double bond. The letters are defined as 

a: 2 chlorine atoms

b: 1 chlorine atom and 1 fluorine atom

c: 2 fluorine atoms

d: 1 hydrogen atom and 1 chlorine atom

e: 1 hydrogen atom and 1 fluorine atom

f: 2 hydrogen atoms

The lower case e in R1234ze(Z) indicates that the end carbon with the double bond is connected to 1 hydrogen atom and 1 fluorine atom.

Upper Case Letter in Parenthesis

In some instances, there are two ways to connect the remaining hydrogen atoms. (Z) indicates the hydrogen atoms are on the same side of the double carbon bond. Z stands for zusammen: German for together. (E) indicates the hydrogen atoms are on opposite sides of the double carbon bond. E stands for entgegen: German for opposite. The (Z) on the end of R1234ze(Z) indicates that the two hydrogen atoms are located on the same side of the carbon double bond.

Although R1234yf, R1234ze(E), and R1234ze(Z) are all built out of the exact same type and number of atoms, the difference in how the atoms are arranged makes them three different refrigerants with different physical properties.  

 

 

 

Unlocking the Secret Refrigerant Numbering Code

I have wondered why the new HFO refrigerant numbers look like an internet password. In short, the numbering system describes the chemical makeup of the refrigerant. But that is also true of the much simpler numbers, such as HCFC 22. So why does HFO 1233zd(E) look like a secret code? Mainly because the chemical is a bit more complicated. HFC 22 is a methane based molecule, with only one carbon. All that is needed to describe it is a way to determine how many fluorine, chlorine, and hydrogen atoms surround the single carbon atom. There is really only one way to put the molecule together.

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.

Alphabet Soup

Daikin just announced the release of R407H and the US EPA has added it to their SNAP list of acceptable refrigerants for both new and retrofit uses. 407H is designed to be a lower GWP refrigerant to replace R404A and R22 in commercial refrigeration applications. I confess, I did not know there was a 407G. I am often asked where all these numbers and letters come from.

The numbers for refrigerants which are mixtures of two or more refrigerants start with either a 4 or a 5. All zeotropic refrigerant numbers start with a 4 while azeotropic refrigerants numbers start with a 5. Zeotropic refrigerants separate when boiling; azeotropic refrigerants do not separate when boiling. The number after the 4 indicates the order that mixture of chemicals was tested by ASHRAE. For example, R401A was the very first. The letter after a zetropic refrigerant designates the order of testing for that specific mix of chemicals. For example, 407A was the first mixture of R32, R125, and R134a to be tested while 407H is the eighth. Please note that the letters for 400 series refrigerants should be upper case.

So what is the difference between 407A, 407C, 407H, and all the other 407 refrigerants? Just the percentage mix of the three ingredients. All eight versions of 407 have slightly different mixtures of the same three constituent refrigerants. A lot of this is done to tweak performance for a specific application or improve a particular characteristic, such as lowering the refrigerant’s GWP. 407H has a GWP of 1500 compared to 404A of 3922.

So what about the other refrigerant numbers, such as 22, or 134a, or (gasp) 1234yf? These describe the chemical construction of the molecules in these refrigerants. These refrigerants all consist of just one chemical compound. Compounds such as R12 or R22 are simple enough to be described without a trailing letter because there is only one way to build them. On the other hand, refrigerants 134a and 1234yf can be built many ways because they have more than one carbon atom. The trailing letters describe how the atom is constructed, which makes a difference in how it behaves. Note that these letters are lower case.