Most of today’s refrigerants are blends of two to four components. Because the components have different boiling points, the blend evaporates at constant pressure not at a single temperature but over a temperature range: the liquid entering the evaporator starts boiling at a lower temperature and the temperature rises towards the outlet. This behaviour is called zeotropic and is designated by the R400 series (R404A, R407C, R449A, R454C). The size of the shift (glide) is 0.7 K for R404A, 4.5 K for R449A and 7 K for R407C.
Azeotropic blends (R500 series: R507A, R513A) behave like a pure substance when combined in a specific ratio; glide is close to zero, and measurement and charging are easy. R507A is an alternative very close to R404A but azeotropic.
Practical consequences: zeotropic refrigerants are charged as liquid (cylinder inverted), otherwise the components evaporate in different proportions and the blend is altered; the composition can also shift in a leak, and in large leaks the whole charge is replaced. Superheat is calculated from the dew point, subcooling from the bubble point. Our refrigerant P–T chart shows both points in separate columns for zeotropic refrigerants.