Level 1, Unit 3, Section 5 – 5.1 Elements of the Pressure

5.1.1 — The Two Axes

Every piece of information on a P–h diagram is expressed through just two quantities. Understanding the axes is the first step to reading any cycle plotted on the chart.

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Vertical Axis — Pressure (P)

Represents absolute pressure, typically in kPa or bar (SI) or psia (imperial). The scale is logarithmic — equal vertical distances represent equal ratios of pressure, not equal differences. This compresses the wide pressure ranges of refrigeration cycles into a readable chart and makes constant-entropy compression lines appear nearly straight.

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Horizontal Axis — Specific Enthalpy (h)

Represents heat content per unit mass in kJ/kg (SI) or BTU/lb (imperial). The scale is linear. The horizontal distance between any two points directly measures the heat transferred per kilogram of refrigerant during that process — making energy calculations purely geometric.

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Why a logarithmic pressure axis?

A typical R-410A system operates between about 120 psig (suction) and 400 psig (discharge) — a 3:1 ratio of absolute pressures. On a linear scale, the low-side details would be compressed into a thin strip at the bottom. The log scale spreads both sides out evenly, making the diagram equally readable at any operating range. It also has the useful effect of making isentropic (constant-entropy) compression lines appear nearly vertical and straight.

5.1.2 — The Saturation Dome

The most distinctive feature of the P–h diagram is the dome-shaped curve that divides it into distinct regions. The dome represents the conditions at which liquid and vapour coexist in equilibrium.

Saturated Liquid Line (Left Side)

Boundary where liquid is at its boiling point (zero quality)
All refrigerant is liquid — any heat addition will start boiling
Marked as the bubble point line
Enthalpy at this line = h_f (enthalpy of saturated liquid)
As pressure increases, h_f increases (moving right and up)

Saturated Vapour Line (Right Side)

Boundary where vapour is at its condensing point (quality = 1.0)
All refrigerant is vapour — any heat removal will start condensation
Marked as the dew point line
Enthalpy at this line = h_g (enthalpy of saturated vapour)
As pressure increases, h_g decreases (dome narrows toward critical point)

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Latent Heat of Vaporisation on the Diagram

At any horizontal line (constant pressure) inside the dome, the horizontal distance from the saturated liquid line to the saturated vapour line equals the latent heat of vaporisation at that pressure:

h_fg = h_g − h_f

For R-410A at 40°F / 4.4°C (118.2 psig / 915 kPa abs): h_f ≈ 42.3 BTU/lb (98.3 kJ/kg), h_g ≈ 122.5 BTU/lb (285.0 kJ/kg)
h_fg = 122.5 − 42.3 = 80.2 BTU/lb (186.5 kJ/kg)

This latent heat value decreases as pressure increases and reaches zero at the critical point, where the distinction between liquid and vapour disappears.

5.1.3 — The Three Regions

The saturation dome divides the P–h diagram into three distinct regions. Knowing which region a state point falls in immediately tells you the refrigerant’s physical condition.

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Subcooled Liquid Region

Left of the saturated liquid line. The refrigerant is 100% liquid and its temperature is below the saturation temperature at that pressure. The amount of subcooling = T_sat − T_actual. This is where the condenser outlet and liquid line are located. Subcooled liquid is desirable because it reduces flash gas and improves efficiency.

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Two-Phase (Saturation) Region

Inside the dome. Liquid and vapour coexist at equilibrium. Temperature and pressure are fixed by the saturation relationship (P–T chart). The fraction of vapour by mass is the quality (x). This is where evaporation and condensation occur. The refrigerant absorbs or releases latent heat with no change in temperature.

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Superheated Vapour Region

Right of the saturated vapour line. The refrigerant is 100% vapour and its temperature is above the saturation temperature at that pressure. The amount of superheat = T_actual − T_sat. Compressor suction and discharge states are in this region. Superheated vapour is required at the compressor inlet to prevent liquid slugging.

5.1.4 — Constant-Property Lines

In addition to the saturation dome boundaries, P–h diagrams are overlaid with families of lines representing constant values of specific properties. Each family appears different in each region.

Line Type	Symbol	Appearance	Where Most Useful
Constant Temperature (isotherms)	T = const	Subcooled region: nearly vertical (pressure has little effect on liquid enthalpy). Inside dome: horizontal — same temperature at all qualities for a given pressure. Superheated region: curves downward to the right.	Locating superheat and subcooling; identifying condensing temperature
Constant Entropy (isentropes)	s = const	Nearly vertical in the superheated region, curving slightly to the right with increasing enthalpy. A truly isentropic compression appears as a vertical line from the suction point straight up to condenser pressure.	Finding ideal compressor discharge enthalpy; calculating isentropic efficiency
Constant Specific Volume (isochores)	v = const	Slant upward to the right through the superheated region, steeper than isotherms. In the subcooled region, liquid is nearly incompressible so these lines are nearly vertical.	Compressor sizing; calculating mass flow rate from volumetric displacement
Constant Quality (isoquality)	x = const	Exist only inside the saturation dome. Run diagonally from a point on the saturated liquid line to the corresponding point on the saturated vapour line at the same pressure. Equally spaced fractions (x = 0.1, 0.2, … 0.9).	Determining flash gas fraction at expansion device outlet

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Why Constant-Entropy Lines Matter for Compression

Ideal (isentropic) compression follows a constant-entropy line upward from the suction state. This represents the theoretical minimum compressor work for that pressure ratio and is used as the benchmark for calculating isentropic efficiency:

η_s = (h_2s − h₁) ÷ (h₂ − h₁)

Where h_2s is the ideal isentropic discharge enthalpy and h₂ is the actual discharge enthalpy. A typical hermetic scroll compressor has η_s ≈ 0.70–0.78 (70–78%). The lower the efficiency, the further the actual discharge point lies to the right of the isentropic point at the same discharge pressure — meaning more heat is added to the refrigerant, raising discharge temperature and the heat rejected at the condenser.

5.1.5 — The Critical Point

The critical point is the peak of the saturation dome — the highest pressure and temperature at which liquid and vapour can coexist as distinct phases. Above the critical temperature, no amount of pressure can cause the refrigerant to condense into a distinct liquid. The latent heat of vaporisation is zero at the critical point.

Refrigerant	Critical Temperature	Critical Pressure	Notes
R-410A	72.1°C (161.8°F)	4,902 kPa (711 psia)	Common residential A/C; replaced R-22. Being phased toward R-32 blends.
R-22	96.1°C (205.0°F)	4,974 kPa (721 psia)	Legacy refrigerant; HCFC phase-out complete in most markets. Wider dome than R-410A.
R-134a	101.1°C (214.0°F)	4,060 kPa (589 psia)	Common in automotive A/C, commercial refrigeration, and chillers.
R-404A	72.1°C (161.8°F)	3,735 kPa (542 psia)	Commercial refrigeration blend; high GWP, being phased out in many regions.
CO₂ (R-744)	31.1°C (88.0°F)	7,377 kPa (1,070 psia)	Very low critical temperature. Most A/C and heat pump applications operate above the critical temperature — called transcritical operation. No condensation occurs on the high side; heat is rejected by gas cooling, not condensation.

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CO₂ systems operate transcritically — the cycle looks different

Because CO₂’s critical temperature (31°C) is below typical summer ambient air temperatures, CO₂ heat pumps and refrigeration systems reject heat above the critical pressure in a “gas cooler” instead of a condenser. On the P–h diagram, the high-side process does not cross into the saturation dome — there is no condensation. This changes how subcooling and capacity are calculated and is why CO₂ systems require different design and service procedures.

5.1.6 — How to Read a State Point off the Chart

Given any two independent properties (pressure + temperature, pressure + quality, or pressure + superheat), a state point can be located on the P–h diagram and its enthalpy read directly from the horizontal axis.

Find the pressure value on the vertical (log) axis and draw a horizontal reference line across the chart at that pressure.
Determine which region the state is in. If pressure + quality are given, the point is inside the dome. If pressure + temperature are given, compare T to T_sat to decide: T < T_sat = subcooled (left); T = T_sat = on the dome boundary; T > T_sat = superheated (right).
Subcooled: Move left of the saturated liquid line along the pressure line. Use isotherms to locate the exact temperature. Read h from the horizontal axis.
Two-phase: Move along the pressure line between the two dome boundaries. The quality lines (x = 0.1…0.9) indicate the fraction. Read h between h_f and h_g.
Superheated: Move right of the saturated vapour line along the pressure line. Use isotherms to locate the exact temperature. Read h from the horizontal axis.

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Worked Example — Locating the Suction State (R-410A)

A technician measures 118 psig on the low-side gauge of an R-410A system. From the P–T chart: 118.2 psig ≈ 133 psia ≈ 915 kPa abs → saturation temperature ≈ 40°F (4.4°C).

The suction line surface temperature is measured at 50°F (10°C). Since 50°F > 40°F, the state is superheated vapour with 10°F (5.6°C) of superheat.

On the P–h diagram at 133 psia, move right of the saturated vapour line until the 50°F isotherm is reached. Read the enthalpy from the horizontal axis:

h₁ ≈ 124.5 BTU/lb (289.7 kJ/kg)

Compare this to h_g at 40°F ≈ 122.5 BTU/lb: the 2.0 BTU/lb difference represents the sensible heat added during the 10°F of superheat. This is exactly what the superheat calculation from Section 4 quantifies — the P–h diagram gives you the enthalpy value that goes with it.