Step 4: Expansion Device

The expansion device is the counterpart of the compressor. Similarly, the expansion device is characterized by the suction and discharge pressures. Horizontal lines are again drawn on the refrigerant's pressure-enthalpy diagram. The input condition of the expansion device is determined by the condenser output conditions.

There are two entering conditions to the expansion device shown on the following diagram. The first situation has 0 °F of sub-cooling [D’’] and the second situation has 15 °F of sub-cooling [D’’’].

The expansion device expands the high pressure refrigerant gas adiabatically to a low pressure liquid-vapor refrigerant mixture. Adiabatic expansion indicates that there is no change in enthalpy and is characterized by a downward vertical line as shown on the below graph.

Note on the graph below as the refrigerant moves from point D to point A, the refrigerant moves from the liquid phase of the graph to the vapor-liquid mixture region. The amount of gas that is formed during this expansion is called flash gas.

Expansion Device on Pressure Enthalpy Diagram for the Mechanical PE Exam