| Cycle Parameter | Value | Note |
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Reading the refrigeration cycle on a P-H diagram
The pressure-enthalpy diagram maps every state point in the refrigeration cycle. The horizontal axis is enthalpy in BTU per pound of refrigerant — how much energy each pound carries. The vertical axis is absolute pressure. The dome-shaped curve in the middle is the saturation envelope: everything to the left is liquid, everything to the right is superheated vapour, and inside the dome is a two-phase mixture.
A standard vapour-compression cycle traces 4 state points. Point 1 is the evaporator outlet — superheated vapour at low pressure. Point 2 is the compressor discharge — superheated vapour at high pressure and high temperature. Point 3 is the condenser outlet — subcooled liquid at high pressure. Point 4 is after the metering device — low-pressure two-phase mixture entering the evaporator.
Refrigeration effect: what the evaporator actually delivers
Refrigeration effect (RE) is the enthalpy difference between Point 1 and Point 4: the heat each pound of refrigerant absorbs as it passes through the evaporator. A larger RE means more cooling per pound of refrigerant circulated, which means the compressor moves less refrigerant mass to achieve the same capacity. Superheat slightly increases RE because the refrigerant continues absorbing heat as a vapour after fully evaporating. This is why a small amount of superheat (8-15°F) is beneficial — it increases RE without significantly harming compression efficiency.
Heat of compression: the compressor's contribution
Heat of compression is the enthalpy rise across the compressor — Point 2 minus Point 1. This represents the electrical energy added to the refrigerant by the compressor. A high discharge temperature relative to the condensing saturation temperature signals high heat of compression, often caused by high compression ratio from either low suction pressure (undercharge, restriction) or high discharge pressure (condenser problem, dirty coil, high ambient). The heat of rejection calculator converts this into condenser design load.
Using field measurements for P-H analysis
This calculator derives approximate enthalpy values from your field measurements using simplified refrigerant property correlations. For precision engineering work, use ASHRAE REFPROP or manufacturer software. For field diagnosis — comparing actual cycle efficiency to nameplate ratings, identifying the source of capacity loss, or confirming the effect of a dirty condenser — this tool gives you the cycle analysis you need without a laptop full of refrigerant data tables. Pair it with the superheat calculator and subcooling calculator to gather all the inputs before running the cycle analysis.
Frequently Asked Questions
A P-H diagram plots refrigerant state points on axes of pressure (vertical) and enthalpy in BTU/lb (horizontal). The refrigeration cycle traces 4 points: evaporation at low pressure, compression to high pressure, condensation, and expansion. The horizontal distance at low pressure is the refrigeration effect — heat absorbed per pound of refrigerant. The vertical rise on the right is the heat of compression. Technicians use P-H analysis to calculate COP, diagnose capacity loss, and compare actual cycle performance to nameplate ratings. Start by gathering suction superheat from the superheat calculator and subcooling from the subcooling calculator, then enter them here.
Refrigeration effect equals the enthalpy at the evaporator outlet (Point 1, determined by suction pressure and superheat) minus the enthalpy of liquid entering the metering device (Point 4, determined by condensing pressure and subcooling). In BTU/lb, multiply by mass flow rate in lb/hr to get total capacity in BTU/hr. This calculator estimates these enthalpy values from your gauge readings using refrigerant property correlations. For R-410A at typical summer conditions (118 PSIG suction, 12°F SH, 400 PSIG discharge, 12°F SC), refrigeration effect is approximately 70-75 BTU/lb.