The P-H diagram is a graphical representation of thermodynamic properties on axes of pressure (vertical) and specific enthalpy (horizontal), fundamental to refrigeration cycle analysis. Also called the pressure-enthalpy diagram, the P-H diagram remains the primary diagnostic tool for HVAC technicians throughout troubleshooting and system commissioning.
Technical Details
The two-phase region between saturation curves contains lines of constant dryness fraction showing refrigerant quality percentages from 0 to 100 percent. The critical point appears at the top of saturation curves. Refrigeration cycles appear as closed loops on P-H diagrams with corners at the compressor outlet, condenser exit, expansion device inlet, and evaporator outlet. Modern systems operating at pressures to 450 psi and enthalpies from 0 to 200 Btu/lbm fit comfortably within standard diagram ranges.
System Analysis and Optimization
Plotting measured compressor discharge pressure and temperature determines the actual cycle shape compared to theoretical performance. Deviations indicate problems like poor heat transfer, refrigerant undercharge, or excessive pressure drop in lines. The diagram quickly reveals whether a system operates in the correct regions for design efficiency. Technicians compare calculated points to ideal cycle paths to quantify performance losses and prioritize corrective actions for improving coefficient of performance values.