Temperature glide
Also known as: glide, boiling range
Short definitionThe difference between the temperature at which a blended refrigerant starts to boil and the temperature at which it is fully vapour, at the same pressure. About 0.7 K for R404A and 7 K for R407C.
A pure gas boils at one temperature at a given pressure. A zeotropic blend starts boiling at the bubble point and finishes a few degrees higher at the dew point. For R449A and R448A this range is 4–6 K, for R454C 6–8 K. As a result the evaporator inlet is colder than its outlet, and the tubes at the inlet collect more frost.
When selecting an evaporator, the mean evaporating temperature is the basis; looking at the dew value alone gives a wrong difference between room and coil. On site, superheat is read from the dew column and subcooling from the bubble column. Mixing up the columns gives a 4–5 K error and the system ends up undercharged or overcharged. When moving from R404A to R449A the expansion valve and pressure switch settings are redone; this is the step most often skipped.