What is a pressure-enthalpy (p-h) diagram?

A pressure-enthalpy diagram, also called a log p-h diagram, plots a refrigerant's specific enthalpy on the horizontal axis against its pressure on a logarithmic vertical axis. It shows the saturation dome, single-phase liquid and vapor regions, and families of isolines (constant temperature, entropy, density, and quality), which makes it the standard tool for visualizing and analyzing vapor-compression refrigeration and heat-pump cycles.

How do you read a log p-h diagram?

Pressure increases upward on a log scale and enthalpy increases to the right. The bell-shaped vapor dome separates subcooled liquid (left), the two-phase mixture (inside), and superheated vapor (right). Isobars run horizontally, isenthalps run vertically, isotherms are nearly vertical in the liquid region and horizontal inside the dome, and isentropes climb steeply through the superheated region. A horizontal distance across the chart is an enthalpy change, which corresponds to heat transfer or work per unit mass.

How is a refrigeration cycle shown on a p-h diagram?

A vapor-compression cycle is four processes: isentropic compression (a steep line up an isentrope into the superheated region), constant-pressure condensation and heat rejection (a horizontal line to the left across the dome), isenthalpic expansion through the valve (a vertical line down), and constant-pressure evaporation (a horizontal line to the right back to the start). The refrigerating effect is the evaporator enthalpy rise, the compressor work is the enthalpy rise during compression, and COP is their ratio.

Which refrigerants does the OTS p-h diagram support?

The OTS R&D interactive p-h chart covers 40 refrigerants, including R134a, R410A, R32, R1234yf, R1234ze, R290 (propane), R744 (CO2, with transcritical states), R717 (ammonia), and common HFO and HFC blends. You can overlay multiple refrigerant domes to compare them, switch between SI and IP units, and export the chart to SVG or PNG.

P-h Diagram Online — Free Log Pressure–Enthalpy Chart

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Tutorials & how-to videos

Step-by-step tutorials and how-to videos for the p-h diagram are coming soon.

A free, interactive pressure-enthalpy diagram for 40 refrigerants. Plot state points, read the isolines and vapor dome, sketch a complete refrigeration cycle, overlay refrigerants to compare them, and export the result to SVG or PNG.

What is a pressure-enthalpy diagram?

A pressure-enthalpy diagram — usually drawn as a log p-h diagram — maps a refrigerant's specific enthalpy (horizontal axis) against its pressure on a logarithmic vertical axis. Because pressure spans several orders of magnitude over a working cycle, the log scale keeps the whole operating range readable on one chart. It is the standard way to visualize vapor-compression refrigeration and heat-pump cycles, size components, and reason about efficiency.

The vapor dome and the three regions

The bell-shaped vapor dome is the heart of the chart. Its left edge is the saturated-liquid line and its right edge is the saturated-vapor line; the two meet at the critical point at the top. The dome divides the diagram into three regions:

Subcooled liquid — left of the dome, fully liquid.
Two-phase mixture — inside the dome, a coexisting blend of liquid and vapor described by its quality (vapor mass fraction).
Superheated vapor — right of the dome, fully gas.

Reading the isolines

The families of curves on a p-h diagram each hold one property constant:

Isobars (constant pressure) — horizontal lines.
Isenthalps (constant enthalpy) — vertical lines.
Isotherms (constant temperature) — nearly vertical in the subcooled liquid, flat and horizontal inside the dome, then curving down into the superheated region.
Isentropes (constant entropy) — steep curves used to model ideal compression.
Isochores (constant specific volume or density) — shallow curves through the vapor region.
Quality lines (constant vapor fraction) — fan across the inside of the dome from 0 at the liquid line to 1 at the vapor line.

Drawing a refrigeration cycle

A basic vapor-compression cycle traces four processes around the chart:

Compression (1→2): the compressor raises the low-pressure vapor up an isentrope into the superheated region.
Condensation (2→3): heat rejection at constant pressure — a horizontal line left, desuperheating, condensing across the dome, often with some subcooling.
Expansion (3→4): the expansion valve drops the pressure at constant enthalpy — a vertical line straight down into the two-phase region.
Evaporation (4→1): heat absorption at constant pressure — a horizontal line right, back to the suction state, usually with some superheat.

The horizontal spans tell you the energy flows per unit mass: the refrigerating effect is the enthalpy rise across the evaporator (h₁ − h₄), the compressor work is the rise across compression (h₂ − h₁), and the coefficient of performance (COP) is their ratio, (h₁ − h₄) / (h₂ − h₁).

What this tool does

40 refrigerants — including R134a, R410A, R32, R1234yf, R1234ze, R290 (propane), R744 (CO₂, with transcritical states), and R717 (ammonia).
State points and process lines drawn directly on the chart.
Compare refrigerants — overlay multiple vapor domes to evaluate alternatives side by side.
SI and IP units with a single toggle.
Export to SVG or PNG for reports and presentations.

Using the chart controls

The sidebar lets you build an analysis directly on the chart:

Conditions — a named group that sets the color of the state points and process lines you plot. Each condition is one operating scenario (for example "Baseline" and "High Ambient"): create several and switch between them to overlay and compare multiple scenarios on the same p-h chart, each in its own color.
State points — click the chart, or type any two properties (pressure, enthalpy, temperature, entropy, quality), to pin a point and read its full thermodynamic state.
Process lines — connect two state points to draw a process (compression, condensation, expansion, evaporation) and read the enthalpy change across it.
Compare refrigerants — overlay the saturation domes of other refrigerants to evaluate alternatives side by side.

Properties are computed with PreciseFluids.

Free Interactive P-h Diagram for 40 Refrigerants