R407C is a zeotropic blend containing 23% R32, 25% R125 and 52% R134a by weight. Because its pressures and capacity are very close to those of R22 under air conditioning conditions, in the 2000s it became the main refrigerant replacing R22 in air handling units, rooftop units, chillers and heat pumps. It is non-flammable and of low toxicity (A1). It does not affect ozone; its GWP is 1,774. Because it does not mix with mineral oil it is used with POE (polyolester) oil. It begins to boil at about −44 °C at atmospheric pressure.
Typical pressures: at 0 °C about 3.6 bar g at the dew point and 4.7 bar g at the bubble point; 15–16 bar g at 40 °C condensing. This difference of 1 bar between the two columns at the same temperature is the pressure equivalent of the 7 K glide. The inlet of the evaporator is a few degrees colder than its outlet; this is why, in chillers cooling water, the freeze protection is set according to the lowest temperature at the inlet. It is not used in flooded evaporators because the components separate.
On the cold room side its place is limited: as the evaporating temperature falls below −10 °C its capacity and efficiency drop noticeably compared with R22, and it is not suitable for frozen storage. It is found in positive-temperature rooms, in process cooling and in chillers. In a retrofit from R22 the mineral oil is drained and replaced with POE; the residue of the old oil must be brought below 5%. The filter drier, seals and expansion valve are also renewed or readjusted. This procedure usually requires two or three oil changes.
In service the glide rules are followed: the refrigerant is taken from the cylinder as liquid, superheat is calculated from the dew value and subcooling from the bubble value. Using the wrong column means an error of 5–7 K and leads to the system being needlessly undercharged or overcharged. In new air conditioners and heat pumps it has largely been replaced by R32; service gas is available for existing R407C equipment. Our refrigerant pressure–temperature chart gives both columns for R407C.