Level 1, Unit 3, Section 5 – Pressure–Enthalpy Diagram

Unit 3 — Refrigeration System Fundamentals & Maintenance

Section 5 — Pressure–Enthalpy Diagram

Section 5 Overview

The pressure–enthalpy (P–h) diagram is the refrigeration technician’s most powerful analysis tool. Every process in the vapour compression cycle — compression, condensation, expansion, evaporation — can be drawn, measured, and understood on a single chart. This section builds that skill from the ground up.

5.0.1 — General Learning Outcomes

Upon successful completion of this section, the apprentice will be able to:

Identify and describe each region and constant-property line on a P–h diagram: saturation dome, subcooled liquid region, superheated vapour region, and lines of constant temperature, entropy, quality, and specific volume.
Plot the four state points and four processes of a standard vapour compression cycle on a P–h diagram and label each with the correct refrigerant condition.
Calculate refrigeration effect, heat of compression, heat rejected, and COP from enthalpy values read off a P–h diagram.
Calculate flash gas quality at the expansion device outlet and explain its significance for system capacity.
Use the P–h diagram to explain how changes in condensing temperature, evaporating temperature, superheat, and subcooling shift the cycle and affect system efficiency and capacity.

5.0.2 — Section 5 — Lessons at a Glance

Section 5 builds from the anatomy of the chart through full cycle plotting and calculation, then finishes with how real-world operating conditions shift the cycle on the diagram.

5.1

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Elements of the P–h Diagram

The axes, the saturation dome, the three regions, and all the constant-property lines — what each one means and how to read it.

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5.2

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Plotting the Vapour Compression Cycle

Four state points, four processes, enthalpy-based calculations for capacity and COP, flash gas quality, and fully worked R-410A examples.

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5.3

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Factors Affecting Performance

How changing condensing and evaporating conditions, airflow, subcooling, superheat, and system load shift the cycle and change capacity and COP.

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5.0.3 — Key Terms — Section 5 Preview

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Specific Enthalpy (h)

Heat content per unit mass (kJ/kg or BTU/lb). The horizontal axis of the P–h diagram. The difference in enthalpy between two points tells you exactly how much heat was added or removed per kilogram of refrigerant.

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Saturation Dome

The curved boundary on the P–h diagram separating the two-phase (liquid + vapour) region from the subcooled liquid region (left) and superheated vapour region (right). The peak of the dome is the critical point.

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Refrigeration Effect

h₁ − h₄: the enthalpy absorbed by the evaporator per kg of refrigerant. The horizontal distance on the P–h diagram between the expansion device outlet and the evaporator outlet.

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Heat of Compression

h₂ − h₁: the work added to the refrigerant by the compressor. Represented by the horizontal distance from compressor inlet to compressor outlet on the P–h diagram.

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Flash Gas

Vapour produced instantly when liquid refrigerant is throttled through the expansion device. The fraction that flashes does not contribute to useful cooling in the evaporator.

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Isentropic Compression

Ideal, frictionless, adiabatic compression following a constant-entropy line on the P–h diagram. Real compressors deviate due to friction and heat transfer, producing higher discharge enthalpy and temperature.

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COP

Coefficient of Performance = (h₁ − h₄) ÷ (h₂ − h₁). Dimensionless ratio of useful cooling effect to compressor work input. Higher COP = more efficient cycle.

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Vapour Quality (x)

The mass fraction of vapour in a two-phase mixture, expressed as a decimal (0 = all liquid, 1 = all vapour). Lines of constant quality run vertically inside the saturation dome.

5.0.4 — Why the P–h Diagram Matters

Gauge pressures tell you where in the cycle you are. The P–h diagram tells you why those pressures matter — and what would happen to efficiency and capacity if they changed. A technician who can sketch a rough P–h diagram for a system in the field can instantly predict whether raising the condensing temperature by 5°C will drop capacity by 3% or 10%, and why cleaning the condenser coil improves COP more than any other single action.

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How Section 5 Builds on Sections 2, 3, and 4

Section 2 introduced latent heat and enthalpy — the P–h diagram puts those on an axis you can measure. Section 3 explained how pressure controls boiling temperature — the P–h diagram puts that on the other axis. Section 4 described the vapour compression cycle in words — Section 5 draws it precisely, assigns numbers to every point, and turns the cycle description into a calculation tool.

By the end of Section 5, a pressure reading on a manifold gauge will no longer be just a number — it will be a point on a diagram with a known enthalpy, an identifiable refrigerant state, and a set of performance consequences you can quantify.