Understanding Mollier Diagrams (P-H Charts) in Refrigeration

Introduction to P-H Charts

For advanced HVAC engineers, the Mollier Diagram (Pressure-Enthalpy or P-H chart) is the ultimate diagnostic map. It visually represents exactly what the refrigerant is doing inside a chiller or AC unit at any given second.

At Prime Cool, our senior engineers use P-H charts to diagnose complex capacity issues in large industrial plants across Pune. Here is a simplified breakdown for 2026.

Key Factors in Reading a P-H Chart

1. The Dome: The large curve in the middle is the saturation dome. Inside the dome, the refrigerant is a mix of liquid and vapor. To the left is subcooled liquid; to the right is superheated vapor.
2. The 4 Cycles:
   • Compression: Moving up the right side (pressure and heat increase).
   • Condensation: Moving horizontally to the left across the top (pressure constant, heat rejected).
   • Expansion: Dropping straight down the left side (pressure drops, temp drops).
   • Evaporation: Moving horizontally to the right across the bottom (absorbing heat to provide cooling).
3. Finding Enthalpy: The bottom X-axis represents Enthalpy (heat energy in kJ/kg). By measuring the gap between the evaporation line and expansion line, you calculate the exact "Refrigerating Effect" of the system.

Expert Troubleshooting

If a system is short of gas, the entire evaporation line shifts downward on the chart, reducing the refrigerating effect and forcing the compressor to work harder for less cooling.

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