Step 2: Compressor

The compressor is characterized by the refrigerant suction and discharge conditions. Horizontal lines are drawn across the refrigerant's pressure enthalpy diagram for the suction and discharge pressures. Then the incoming temperature of the compressor, as determined by the leaving temperature of the evaporator, is used as the starting point of the compressor, as shown by point B' on the figure below. The compressor then increases the pressure of the refrigerant up to the discharge pressure. Compression occurs at constant entropy, also known as isentropic compression. Therefore the intersection of the constant entropy line and the discharge pressure line will identify the final condition of the refrigerant gas leaving the compressor, as shown by point C' in the figure below.

Compressor Movement on Pressure Enthalpy Diagram for the Mechanical PE Exam

A common skill, that is required of a professional engineer, is to determine the work done by the compressor. This work is shown in the figure above as the difference between the compressor entering enthalpy (H1) and the leaving enthalpy (H2). The equation to determine the work of the compressor is shown below. This equation multiples the refrigeration flow rate by the change in enthalpy between the discharge and suction conditions.

Pressure Enthalpy Diagram for the Mechanical PE Exam with Constant Temperature Lines

The x-y axes of the P-H diagram are the pressure lines running from left to right. The enthalpy lines are the vertical lines. The skeletal graph shown below shows the pressure-enthalpy lines.

Compressor Work Equation for the Mechanical PE Exam