Zeotropic Refrigerant Terms

Zeotropic refrigerants, or blends, are now the most common refrigerants available. Any refrigerant whose ASHRAE number is 400 something is zeotropic. A whole list of somewhat confusing terms are used to describe the characteristics of these refrigerants. The first issue to tackle is simply what zeotropic means. I find remembering multisyllabic tehno-jargon much easier if I understand the derivation of the word. In ancient Greek zeo means “to boil” while tropo means “to change”.  Putting the two together zeotrope means that something changes when it boils. That is why refrigerant mixtures whose percentage composition charges when they boil are referred to as zeotropic. Also in Greek, the prefix a means “not”. It basically inverts the meaning. That is why refrigerant mixtures that do NOT change when they boil are known as azeotropic.

Dew-point, Bubble-point, and Glide

Because zeotropic refrigerants change percentage composition as they change state, the temperatures at which they start to evaporate and condense are different for any given pressure. Dew-point describes the temperature at which the first liquid droplets start to form in a saturated vapor, and bubble-point describes the temperature at which the first bubbles start to appear in a saturated liquid. For a “normal” refrigerant these are the same temperature. For zeotropic refrigerants, at any given pressure the dewpoint is a little higher than the bubble-point. Glide is the difference between the bubble-point temperature and the dew-point temperature at any given pressure. Low glide blends typically have a glide of less than 2°F, while high glide blends have glides as high as 10°F. You may also have seen the term near-azeotropic. Personally, I think near-azeotropic sounds like something coined by the marketing department. It literally means “nearly does not change when it boils”. I believe a more useful description is a low-glide zeotropic mixture.

Zeotropic PT Charts

When calculating evaporator and condenser pressures, superheat, or subcooling I admit I have to think a bit when figuring out if I need to know the dew-point, bubble-point, saturated liquid temperature, or saturated vapor temperature. I find the terminology can be confusing. I would like to offer a way to keep these terms straight. Refrigerant pressure temperature charts typically have one column for pressure and two columns for temperature. The temperature columns will either be the dew-point and bubble-point, or saturated vapor and saturated liquid.

Saturated Vapor and Saturated Liquid

Since superheat involves determining the temperature of a gas in excess of its saturation temperature it makes sense to use the saturated vapor temperature when calculating superheat. Similarly, since subcooling involves determining the temperature of a liquid below its saturation temperature, it makes sense to use the saturated liquid temperature when calculating subcooling.

Dew-point and Bubble-point

The terms here can lead you astray because they describe the beginning of condensation or evaporation. However, they also describe the very last stage of any amount of vapor in a saturated liquid (bubble-point) or liquid in a saturated vapor (dew-point). The key here is to pay attention to the state the bulk of the refrigerant. For example, at bubble-point, most of the refrigerant is a liquid because the first bubbles are just starting to form. So, the bubble-point temperature is used when calculating subcooling. Bubble-point and the saturated liquid temperature describe the same condition. At dew-point, most of the refrigerant is a vapor because the first liquid droplets are just starting to form. So, the dew-point temperature is used when calculating superheat. Dew-point and saturated vapor temperature describe the same condition. I hope this helps you sort out all the techno-babble surrounding 400 series refrigerants.

No More R-22 Production

No more manufacturing or importing R-22 beginning January 1, 2020. That day has come and gone. It is time to take a hard look at how you service R-22 equipment. If the equipment needs charging there is something else wrong. There are two common practices that should end: seasonal topping off and “diagnostic” refrigerant additions.   

Topping Off

Simply adding refrigerant has not fixed the problem. Adding refrigerant to a leaky system is like giving aspirin to someone with a fever: it temporarily decreases the pain but does nothing to solve the underlying problem, which will return soon enough. If a system that has been in service truly needs charging, it either has leak or a recreational refrigerant inhaler resides nearby. You need to locate and repair the leak and/or the recreational user. In cases where a leak cannot be found, the addition of dye might help with both issues. Place a large warning sticker on the equipment near the service valves notifying future service techs that “indelible dye” has been added. The recreational inhaler will most likely find another source. If there are leaks, they should show up by the time more refrigerant is needed. Then you can determine if repair is feasible or if the system needs replacing.

“Diagnostic” Refrigerant Charging

I use the term diagnostic loosely here. A “diagnostic” refrigerant charge involves adding refrigerant to see what happens. You are adding refrigerant because the unit is not cooling and you don’t know what else to do. This is a poor practice at best. It is an especially poor practice when servicing R-22 systems.   

R-22 Replacements

Although there are many R-22 replacement refrigerants available, there are no replacements that can legally simply be added on top of existing R-22. Fortunately, new and reclaimed R-22 is still readily available at fairly reasonable prices because the demand for it has decreased. I recommend staying with R-22 as long as it is available at reasonable prices, which it is. Converting an R-22 system to another refrigerant is time consuming, costly, and carries with it the possibility of killing the unit. The difference in the refrigerant cost just does not justify the time and expense required to correctly and legally perform the refrigerant conversion.  

A2L Refrigerants and Codes

In the last post I talked about what it really means for a refrigerant to be classified as an A2L refrigerant. One practical ramification in the United States in 2019 is that an A2L refrigerant cannot be used in a direct expansion system in most buildings in the United States. Most building codes refer to ASHRAE Standard 34 Designation and Classification of Refrigerants and Standard 15 Safety Standard for Refrigeration Systems. Until the most recent revision in 2019, Standard 15 forbid the use of flammable refrigerant in what it describes as “direct systems.” And until the most recent revision in 2019, Standard 15 made no distinction between levels of flammability. Flammable is flammable. Because nearly all mechanical, building, and fire safety codes use ASHRAE as their refrigerant safety reference, no codes presently allow A2L refrigerants in direct systems. A direct system is one in which the building air is directly exposed to the refrigeration components, as in a normal direct expansion evaporator coil. However, now that ASHRAE has revised Standard 15, look for states and code agencies to begin adopting the revised standard. Washington state has already done that. Beginning July 1, 2020 direct expansion A2L systems will be allowed in Washington State subject to the stipulations of the revised 2019 Standard 15. There will be industry pressure for adoption of the new standards. Major refrigerant manufacturers such as Honeywell and Chemours have invested heavily in developing lower GWP refrigerants, many of which are rated A2L. Equipment manufacturers have invested heavily in designing equipment that uses R32, a lower GWP A2L refrigerant. If you would like to read more details of the revised 2019 Standards 15 and 34, ASHRAE here is a link where you can view them on-line https://www.ashrae.org/technical-resources/standards-and-guidelines/read-only-versions-of-ashrae-standards

Of course, ASHRAE will also sell you a downloadable pdf which is really better for extenedreading and studying.

What Does Mildly Flammable Mean?

I confess that I have always thought of flammability as an either or question: it either burns or it doesn’t. So the concept of different levels of flammability was a hard one for me to grasp. I wondered: what is the difference between 3,2, and 2L refrigerant designations? What follows is a somewhat lengthy discussion of what I learned.

First off, I found that it is not all that simple. There are several flammability characteristics that can be compared: lower flammability limit, upper flammability limit, auto ignition temperature, minimum ignition energy, heat of combustion, and flame velocity. The table at the bottom of the article shows these different specifications for a small selection of flammable refrigerants. Note that pressure and temperature also play a part. For the ASHRAE safety tests, a temperature of 140°F at atmospheric pressure is specified. You get different results when applying higher pressures and temperatures.

The original three classifications (1,2,3) were determined by the lower flammability limit and the heat of combustion. Later, ASHRAE added a 2L category for refrigerants with burning velocities less than 10 centimeters per second. The table below summarizes the different flammability classifications.

Classification Lower Flammability Limit % by volume Heat of Combustion Burning Velocity
1 Does not support combustion at atmospheric pressure
2L Greater than 3.5% Less than 19 kj/g 10 cm/s or less
2 Greater than 3.5% Less than 19 kj/g Greater than 10 cm/s
3 3.5% or less 19 kj/g or more NA

Lower flammability limit (LFL) is the minimum percentage required in air to be combustible. For example propane (R290) has an LFL of 2.1% by volume while ammonia (R717) has an LFL of 15%. Notice that propane only requires 2.1% while ammonia requires 15%. So that is one difference – the amount that must build up before it can burn.

The upper flammability limit (UFL) describes the maximum concentration which will still burn. If the concentration of flammable vapors exceeds the UFL, it will not ignite. It is more difficult to draw a straight line comparison using the UFL. However, you can say that refrigerants whose LFL and UFL are closer together are generally a bit safer simply because the conditions for a flammable mixture are less likely to occur.

The auto ignition temperature is the temperature which the flammable mixture will ignite. With the exception of 1234yf, refrigerants with a lower flammability have higher auto ignition temperatures than the more flammable refrigerants.

The minimum ignition energy is a bit different than the auto ignition temperature. It is the amount of energy that must be used to ignite a flammable mixture, measured in megajoules. Note that in this case R1234yf stands out because the minimum ignition energy is so high compared to the other refrigerants. Also note that the class 2L refrigerants all have minimum ignition energy ratings in the hundreds of megajoules or higher while propane’s minimum ignition energy is a very small 0.25 megajoules. Basically, this means it takes a lot more energy to ignite a class 2L refrigerant than a highly flammable class 3 refrigerant such as propane. Again, this means that the chance of having the right condition for combustion is much lower for class 2L refrigerants.

The heat of combustion is a measure of the amount of heat created when the refrigerant burns. Note that the class 2L and class 2 refrigerants have a heat of combustion in the single digits per gram while propane jumps to 46 kilojoules per gram. This means that the heat produced by combustion of a class 2L or class 2 refrigerant is far less than a class 3 refrigerant. Indeed, it would be possible for a class 2L refrigerant to burn and not ignite other nearby flammable materials.

Burning velocity is the characteristic which distinguishes class 2 and 2L refrigerants. It is the speed with which the flame advances. Note that the 2L class refrigerants have a burning velocity in the single digits while 152a, a class 2 refrigerant, has a burning velocity of 23 cm/sec. Propane’s burning velocity is twice that of 152a. The take home point here is that the flames from higher flammability refrigerants spread faster.

So wrapping it up, my general impression is that

1. Lower flammability refrigerants (2L) are less likely to burn in the first place.

2. When class 2L refrigerants do burn, the flames are not as hot as higher flammability class 3 refrigerants.

3. The flames from burning  2L refrigerant do not spread as quickly as the flames from higher flammability class 3 refrigerants.

Refrigerant R1234yf R32 717 Ammonia 152a 290 Propane
Safety Group A2L A2L B2L A2 A3
Lower Flammability LImit 6.5% 14.4% 15% 3.9% 2.1%
Upper Flammability Limit 12.3% 33.3% 28% 16.9% 10%
Auto Ignition Temperature 405°C 648°C 651°C 440°C 455°C
Minimum Ignition Energy 5,000 – 10,000 mJ 30 – 100 mJ 100 – 300 mJ 0.38 mJ 0.25 mJ
Heat of Combustion 9.5 kJ/g 9 kJ/g 22.5 kJ/g 6.3 kJ/g 46.3 kj/g
Burning Velocity 1.5 cm/sec 6.7 cm/sec 7.2 cm/sec 23 cm/sec 46 cm/sec
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