Saturation temperature and saturation pressure
Also known as: saturation pressure, saturated temperature
Short definitionThe temperature at which a refrigerant’s liquid and vapour sit side by side; it changes when the pressure changes. Each pressure has a single saturation temperature, and these pairs make up the refrigerant’s pressure–temperature table.
A service technician looks at the gauge and says “the evaporator is at minus eight”. What is measured is pressure, what is said is temperature. That works because a refrigerant’s pressure and temperature are tied together while it boils or condenses. Water boiling at 100 °C at sea level and sooner on a high mountain is the same rule. For refrigerants these temperatures are very low; R404A boils at −46 °C under 1 bar absolute. Suction pressure gives the evaporating temperature and discharge pressure the condensing temperature.
Superheat and subcooling calculations rest on this value: the difference is taken between the temperature measured on the pipe and the saturation temperature in the table. If the table belongs to another refrigerant, the whole diagnosis comes out wrong. A pure refrigerant has a single curve. For a zeotropic blend the liquid side is read from the bubble value and the vapour side from the dew value; the difference between the two is called glide. A gauge shows gauge pressure, and absolute pressure is 1.013 bar higher. Check which one the catalogue states.