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.