Pressure-Enthalpy Path

The refrigeration industry didn't always have today's modern analysis tools. For many decades, manufacturers and service technicians relied on tabulated and graphical presentations of refrigerant properties and expected equipment performance. One of their favorite tools was the pressure-enthalpy diagram which defines the thermodynamic properties for the refrigerant in use and the performance of equipment.

This chart shows the pressure expressed in psia (pounds per square inch absolute) along the vertical axis. The energy content (enthalpy) of one pound of the refrigerant is shown on the horizontal axis on a Btu/lb basis. {The refrigerant shown here happens to be R-22, the most common refrigerant used in packaged rooftop and other small packaged cooling equipment). Notice there are three distinct regions on this chart and two "boundary lines" separating them.

The region on the left is subcooled liquid; basically refrigerant liquid at a temperature cooler than the equivalent boiling point for the pressure noted. At the example pressure of 74.7 psia (approximately 69 psig), this boiling point is 40°F.

The region inside the ''dome" is a liquid-vapor mixture. If the liquid is at the boiling point, but just hasn't begun to boil, it is defined as saturated liquid. Adding any heat to this liquid will vaporize a portion of it. Adding more heat to the liquid-vapor mixture eventually evaporates all of the liquid. At that precise point (G), the vapor is fully saturated. Adding any more heat to the vapor will cause it to rise in temperature further; this is referred to as superheated vapor.

Do not associate the term superheat with being "hot." Superheated vapors can be cold. They are simply above their corresponding saturated vapor point. Similarly, subcooled liquid can be fairly warm. It just means that the liquid is cooler than the saturation line for that pressure.

The typical refrigeration cycle is shown superimposed on this chart, along with pictures of the typical equipment components associated with that step in the cycle.

Let's start our review at point A. To get this point after leaving the evaporator (G), the refrigerant vapor is superheated slightly, and crosses the compressor suction valve to point A. The compressor elevates the refrigerant's pressure to a point at which it can push the discharge valve open and flow to in the condenser. The refrigerant vapor leaves the compressor at point B, desuperheats to point C, and then begins to condense. After the vapor is completely condensed at point D, it is subcooled a bit further (E), at which time is still at a much higher pressure than the evaporator.

Controlling the flow to the evaporator and throttling this pressure to that of the evaporator is the job performed by the expansion device, a thermal expansion (TX) valve in this illustration. This pressure reduction step vaporizes a portion of the liquid which cools (called flash gas) the remaining liquid going to point F -- the "average" mixture of vapor and liquid crossing the valve doesn't change in energy content -- it simply separates into liquid and vapor at the reduced temperature and pressure according to its precise thermodynamic properties. The liquid at point F is then ready to pick up heat in the evaporator and form vapor at point G where the cycle repeats.

Each step in this process follows precise thermodynamic laws.