P-H diagram shows characteristics of various refrigerants with pressure on vertical axis and enthalpy on horizontal axis.
Change of state change from gas to liquid is said condensation and that from liquid to gas said evaporation. The boundary state of each change is said saturation, and the temperature generating saturation is said saturation temperature.
Saturation temperature depends on kinds of refrigerant and pressure. The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants, which is called saturation curve.
The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams, which is called “isothermal lines”. By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included, the information on the state of refrigerant can be provided. The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point.
As to single refrigerants (R22, R134A etc.), isothermal line has no gradient in the saturated area ,that is, the saturation temperature under certain pressure is same at both liquid side and gas side. As to mixed refrigerants (R407C, R410A etc.), in which plural refrigerants with different boiling points are mixed, their isothermal lines have gradients in the saturated area, so the saturation temperatures under certain pressure are different at liquid side and gas side. They are called zeotropic refrigerants, with the exception that R410A is called false azeotropic refrigerant.
States of refrigerants are classified following 3 categories.
? Superheated vapor: state that refrigerant is existing as gas
? Saturated vapor: state that is mixture of liquid and gas (this is also called wet vapor)
? Subcooled liquid: state that refrigerant is existing as liquid
