I think this is a chemistry question... Due to the Montreal Protocol (https://en.wikipedia.org/wiki/Montreal_Protocol) and subsequent phase-out schedule for all the “conventional” refrigerants, many/most of us in the HVAC service industry are consequently working with “interim” alternatives for R-22 (refrigerant 22, “freon” 22) which has been the most common refrigerant in air conditioners and heat pumps for decades. All the alternative refrigerants I’m familiar with are zeotropic “blends” with two or more refrigerant components. The two primary components of the blends are R-125 and R-134A. R-125 has a boiling point considerably less than that of R-22, and R-134A has a boiling point considerably higher. The mixture ends up performing with saturated temperatures and pressures similar to R-22. Since the blends are zeotropic, they have a bubble point for saturated liquid conditions and dew point for saturated vapor conditions, in contrast to R-22’s single pressure-temperature (P-T) relationship. The blend I’m working with is R-421A…58% R-125; Pentafluoroethane (CHF2CF3) and R-134A; 42% Tetrafluoroethane (CF3CH2F). The gist of vapor compression refrigeration revolves around controlling the boiling point of the refrigerant in the evaporator section of the equipment. Cooling equipment is designed to run an evaporator temperature in the 40-50˚ F range. The liquid enters the evaporator in a saturated condition at a vapor pressure (psig) corresponding to the refrigerant’s 40˚ F boiling point. The liquid absorbs latent heat of vaporization from the air flowing through the coil assembly, and eventually vaporizes completely as it nears the exit point of the evaporator coil circuits. There is also some superheat designed into the system operation, so the liquid is typically completely vaporized before exiting the evaporator section and the vapor picks up some additional sensible heat. With R-22’s single P-T characteristic, the boiling/vaporizing phase change takes place at a constant temperature and pressure. The zeotropic blends, having two (or more) components with different boiling points, increase in temperature as they pass through the evaporator. The R-125 (lower boiling point) component boils at a faster rate than the R-134A, so the initial ratio of the blend begins to change, moving towards a greater proportion of the R-134A component. Typically for the blends, the change in temperature, or “glide”, is 8˚-10˚ F. Said another way, the liquid at the entrance of the evaporator coil is at the 40˚ F “bubble point” (saturated liquid) temperature, and increases to the 50˚ “dew point” (saturated vapor) temperature. When I try to “visualize” the evaporator/vaporizing process, I see all the lower boiling point R-125 eventually boiling to vapor, leaving only some amount of liquid phase R-134A. But at the operating evaporator pressure, which is some 60-70 psig, the boiling point of R-134A is 60-70˚ F, which would result in a saturated vapor temperature of 60-70˚. Which can’t be the case, since the published dew point/saturated vapor temperature for the blend, is 50˚± F (bubble point + 8±) at the same pressure. The only red neck theory I’ve come up with, is some kind of “bonding” going on between the molecules of the two blends. I recently learned azeotropic blends, which behave as a single component refrigerant, experience a “special attraction” between the two components which essentially results in a compound (?) where both refrigerants vaporize at the same rate. Is it possible there is some degree of bonding taking place with the R-125(CHF2CF3) / R-134A(CF3CH2F) blend? Any explanation will be welcomed…thanks in advance.