Step 1: Evaporator

The refrigerant entering the evaporator is a cold, partial liquid-vapor mixture. The operating pressure and temperature of the evaporator is called the suction pressure and suction temperature. The suction line is the piping that routes refrigerant gas from the evaporator to the compressor. It is important to note that in the mix region, the pressure and temperature are dependent variables.

For example, if a compressor operates at a suction pressure of 36.8 psia, then the corresponding evaporator pressure is 36.8 psia and the corresponding evaporator temperature is 25 °F, see below figure for points A and B (Values are for Refrigerant R-134a). If the compressor operates at a suction pressure of 49.7 psia, then the corresponding evaporator pressure is also 49.7 psia and the evaporator temperature is 40 °F. See below figure for points A' and B'(Values are for Refrigerant R-134a).

Pressure Enthalpy Diagram with Evaporator Step Shown for the Mechanical PE Exam

The evaporator moves the refrigerant from point A (partial liquid-vapor mixture) to point B, a fully saturated vapor refrigerant. As the evaporator transfers heat to the refrigerant, there is no gain in temperature, since all the heat is used to convert the remaining liquid to a gas. In an ideal evaporator, there is just enough heat transfer to convert all the liquid to gas and nothing more. Thus, the output of an ideal evaporator is 100% vapor at the same entering temperature, refer to figure below. In this figure, we see that as the refrigerant moves through the evaporator, the temperature remains the same and the percentage of vapor increases, until it reaches saturation at 100%.

Movement through Evaporator with Zero Superheat

Also introduced in the figure above is the term superheat. If additional heat were to be added to the 100% vapor refrigerant, then the heat would be used to increase the temperature and it is this increase in temperature that is called superheat.

In the figure below, an evaporator with 15 °F superheat is shown. The refrigerant reaches 100% vapor prior to leaving the evaporator. All the additional heat from this point is used to increase the temperature of the refrigerant until it reaches a temperature of 40 °F. This refrigerant has a superheat of 15 °F because the final temperature is 15 degrees passed the saturation temperature of 25 °F. It is important to note that the pressure remains constant throughout the evaporator.

Movement through Evaporator with Superheat

On the pressure-enthalpy diagram superheat is shown as horizontal movement along the suction pressure line passed the 100% vapor curve. The figure on the following page shows the difference between 0 °F and 15 °F superheat. Point B is the 100% vapor point at a constant evaporator/suction pressure of 36.8 psia and a temperature of 25 °F. Point B' results from additional heat/enthalpy added to the refrigerant. The refrigerant moves from point B to point B', where the resulting temperature is 40 °F.

P-H Diagram with an Evaporat with/without Superheat