- Why the cycle lives on a P-h diagram
- The two axes: log pressure and enthalpy
- The dome, and the three regions it creates
- The four points of every cycle
- One real cycle, every number measured
- Move one input: the subcooling experiment
- Five misreadings that cost you a diagnosis
- The 60-second read
- Questions engineers ask
Why the Cycle Lives on a P-h Diagram
A pressure-enthalpy diagram is a map of a refrigerant, and a refrigeration cycle is a closed loop drawn on that map. Each corner of the loop is a state your gauges and thermometers can find on a real machine. Read the map once and you stop memorising tables: compression ratio, compressor work, refrigerating effect and liquid-line subcooling all become distances on the same picture.
The reason it beats a temperature-entropy diagram for field and design work is what sits on the horizontal axis. Enthalpy is energy per kilogram, so any horizontal distance on the chart is an energy per kilogram directly: the work a compressor adds, the heat a condenser rejects, the cooling an evaporator delivers. No integration, no area under a curve — you measure across.
The one-sentence version: the vertical position of a point is the pressure it is at, the horizontal position is how much energy each kilogram of refrigerant is carrying, and the cycle is four points joined by four lines.
The Two Axes: Log Pressure and Enthalpy
Vertical axis: absolute pressure, logarithmic
Pressure is plotted on a log scale so that the suction side and the discharge side of a real cycle both stay readable on one chart: 933.2 kPa and 2,733.8 kPa are only one decade apart, but on a linear axis the low side would be squashed into the bottom few millimetres. Two conventions to keep straight:
- The chart uses absolute pressure. This cycle's low side is 9.33 bar absolute, which reads roughly one bar lower on a manifold gauge at sea level.
- Horizontal lines are isobars, and inside the dome a horizontal line is also an isotherm — which is exactly why the evaporator and condenser show up as straight, flat lines.
Horizontal axis: specific enthalpy
Moving right means adding energy to each kilogram of refrigerant. That single idea explains the cycle:
- The compressor line moves right — work is added.
- The condenser line moves left — heat is removed.
- The expansion valve line is vertical — it drops pressure without changing enthalpy, which is why you can draw it with a straight edge.
- The evaporator line moves right again — this is the cooling you are paid for.
The Dome, and the Three Regions It Creates
The dome is the two-phase boundary, and it splits the chart into three regions. For R-410A the dome closes at the top at 49.0 bar: that is the critical pressure, and above it there is no liquid-vapour distinction and no condensation at all.
- Left of the dome — subcooled liquid. Liquid colder than its saturation temperature at that pressure. A condenser outlet with real subcooling lands here.
- Inside the dome — two-phase mixture. Liquid and vapour coexist, and the vapour mass fraction (quality) is read off on the same pressure line. The expansion-valve outlet lands in this region, and this is the single biggest reason to use a P-h chart: pressure and temperature together cannot tell you the mixture ratio, but enthalpy can.
- Right of the dome — superheated vapour. Vapour above its saturation temperature. Compressor suction should sit just inside this band: enough superheat to protect the compressor, not so much that you are paying for it.
The dome's two edges are the saturated liquid line and the saturated vapour line. On a single-component or near-azeotropic refrigerant they are sharp lines. On a blend with real glide they open into a bubble-point curve and a dew-point curve, and the gap between them is the glide you have to add into your superheat and subcooling measurements. That is the quantity that makes zeotropic blends harder to service, and it is visible on the chart before you ever touch a manifold.
The Four Points of Every Cycle
Every vapour-compression cycle is the same four states. Learn where each one sits and which measurement proves it is where you think it is.
| Point | What it is | Where it sits | What proves it |
|---|---|---|---|
| 1 | Evaporator outlet / compressor suction | Low-pressure isobar, just right of the saturated vapour line | Suction superheat: suction temperature minus saturation temperature at suction pressure |
| 2 | Compressor discharge | Highest point of the loop, well inside the superheated region | Discharge temperature against the compressor and oil limit, not just the pressure ratio |
| 3 | Condenser outlet / liquid line | High-pressure isobar, on or left of the saturated liquid line | Subcooling: saturation temperature at liquid-line pressure minus liquid-line temperature |
| 4 | Expansion valve outlet / evaporator inlet | Back on the low-pressure isobar, inside the dome | Quality: how much of the mixture has already flashed to vapour |
Points 3 and 4 share the same enthalpy — that is the vertical line on the chart, and it is why you compare the evaporator outlet against point 4, never against point 3.
One Real Cycle, Every Number Measured
Here is the whole thing on a concrete cycle: R-410A, evaporating at 5 °C, condensing at 45 °C, 5 K of suction superheat, 2 K of liquid subcooling, compressor isentropic efficiency 70.0%, and a refrigerant mass flow of 0.10 kg/s. Every number below comes from refrigerant properties, not from a sketch.
Figure 1 — The cycle as drawn by the engine. Numbers 1 to 4 are the four states; the dashed line is the same cycle with 12 K of subcooling instead of 2 K, which is the experiment in the next section. Change any input yourself in the interactive P-h diagram.
| Point | Temperature | Pressure | Enthalpy | State |
|---|---|---|---|---|
| 1 evaporator outlet | 10.0 °C | 933.2 kPa | 428.54 kJ/kg | Superheated vapour, 5 K superheat |
| 2 compressor discharge | 77.0 °C | 2,733.8 kPa | 471.65 kJ/kg | Superheated vapour, 32.0 K above condensing temperature |
| 3 condenser outlet | 43.0 °C | 2,733.8 kPa | 271.78 kJ/kg | Subcooled liquid, 2 K subcooling |
| 4 evaporator inlet | 4.93 °C | 933.2 kPa | 271.78 kJ/kg | Two-phase, 29.9% vapour by mass |
One engine run: R-410A, 5 °C evaporating, 45 °C condensing, 5 K superheat, 2 K subcooling, 70.0% isentropic efficiency, 0.10 kg/s. At this evaporating pressure the refrigerant enters at 207.5 kJ/kg as saturated liquid and leaves at 422.8 kJ/kg as saturated vapour — those two numbers are the endpoints the quality at point 4 is measured against.
Now read the four lines between the points, and the energy each one carries per kilogram of refrigerant:
| Process | On the chart | Energy per kg | Physical meaning |
|---|---|---|---|
| 1 → 2 compression | Rises and moves right | 43.11 kJ/kg | Compressor work in. This is the price of the 40 K lift, on every kilogram circulated. |
| 2 → 3 condensation | Flat at high pressure, moves left | 199.87 kJ/kg | Heat rejected to the outdoor air. Always larger than the cooling delivered, because it also carries the compressor work. |
| 3 → 4 expansion | Straight down, no horizontal movement | 0 kJ/kg | Pressure drops through the valve with enthalpy unchanged — the vertical line. |
| 4 → 1 evaporation | Flat at low pressure, moves right | 156.76 kJ/kg | The cooling actually delivered: energy each kilogram absorbs from the space. |
Consistent by definition: 156.76 + 43.11 = 199.87 kJ/kg. If the numbers you read off a chart do not add up this way, you have misread a point.
The performance that falls out of the same run: 4.31 kW of compressor power, 15.68 kW of cooling, COP 3.64, and 19.99 kW rejected at the condenser. The theoretical maximum (Carnot) for a 40.0 K lift is 5.19, so this cycle is running at 70.0% of that limit — real equipment behaviour, and the gap worth comparing against a machine you actually own. The COP calculator and the COP efficiency guide go deeper on that comparison.
Move One Input: the Subcooling Experiment
A P-h diagram earns its keep when you move one input and watch the loop change. Keep everything from the run above and change only the subcooling, from 2 K to 12 K.
Points 1 and 2 do not move at all. Suction state, discharge pressure and compressor work are identical, because nothing about the evaporator or the compressor changed. What moves is point 3: liquid now leaves the condenser at 33.0 °C instead of 43.0 °C, so it sits further left at 253.08 kJ/kg instead of 271.78 kJ/kg, and the vertical expansion line drags point 4 with it, to 253.08 kJ/kg.
| Quantity | 2 K subcooling | 12 K subcooling | Change |
|---|---|---|---|
| Liquid-line temperature (point 3) | 43.0 °C | 33.0 °C | −10 K |
| Enthalpy at the valve outlet (point 4) | 271.78 kJ/kg | 253.08 kJ/kg | lower: colder mixture |
| Flash gas at the evaporator inlet | 29.9% vapour | 21.2% vapour | 29.1% less flash gas |
| Refrigerating effect | 156.76 kJ/kg | 175.46 kJ/kg | +11.9% |
| Cooling capacity at the same power | 15.68 kW | 17.55 kW | +11.9% |
| COP | 3.64 | 4.07 | +11.9% |
Read that again, because it is the most useful habit the diagram teaches: at the same compressor power, 29.1% of the flash gas at the evaporator inlet is gone and the system moves 11.9% more heat. Point 4 starts further left inside the dome, so every kilogram of refrigerant can absorb more heat before it reaches point 1.
Do not read this as free efficiency. The gain is real, but it comes from somewhere. If the condenser circuit physically delivers that subcooling, you have earned it. If it appears because the system is overcharged, the extra liquid buys you a flooded compressor or a slugged valve later. If you manufacture it with a suction-line heat exchanger, you are trading it against suction superheat, which raises discharge temperature. The diagram shows the trade; it cannot tell you which side you are on — the gauges do.
Run the same experiment in the live tools with your own pressures: the superheat & subcooling calculator for the field measurements, the superheat and subcooling guide for the targets, and the P-h diagram tool for the picture your numbers make.
Five Misreadings That Cost You a Diagnosis
- Plotting gauge pressure instead of absolute. Every P-h chart is absolute pressure, so a manifold reading of 9.33 bar on the low side is plotted at about 9.33 bar absolute plus the atmospheric offset. One bar of error moves every state on the chart.
- Using one saturation temperature for a blend. On a zeotropic blend the bubble point and the dew point differ and the dome is a band, not a pair of crisp lines. Compare the liquid line to the bubble point and suction to the dew point, or superheat and subcooling both read wrong by the glide.
- Measuring temperature and pressure at different places. Suction superheat is a point-1 number only if both values come from the same point. A pressure tap at the compressor with a temperature at the evaporator outlet puts suction-line pressure drop silently inside your superheat.
- Drawing the compression line straight up. A real compressor is not isentropic. At 70.0% efficiency the discharge lands at 77.0 °C instead of the ideal-compression temperature, further right on the chart because friction work is real enthalpy. Plot an isentropic line and you cannot explain a hot-running compressor.
- Taking the refrigerating effect from the wrong point. Cooling per kilogram is point 1 minus point 4, not point 1 minus point 3. Using the liquid-line enthalpy ignores the flash gas that already formed in the valve and overstates capacity — here it would inflate the number by about 29.9%.
The 60-Second Read
On a real system, six checks in this order find most of what is wrong, and each one is a glance on the chart:
- Is the loop closed? Four points in the right order. A missing measurement is a guessed point, and a guessed point hides the fault.
- Compression ratio from the two pressures: 2.93:1 at a 40.0 K lift here. A rising ratio at the same load usually means a condenser problem, not a compressor problem.
- Suction superheat. Small but positive. Zero means liquid is reaching the compressor; a large number with poor capacity means a starved evaporator.
- Liquid-line subcooling. Positive under a TXV. Near zero or negative points at charge, condenser air flow, or a restriction before the valve.
- Evaporator inlet quality. Inside the dome, and not so far right that most of the refrigerant arrives as flash gas — the number the P-h diagram tool prints for you.
- Discharge temperature against the compressor limit: 77.0 °C here at 70.0% efficiency. If the measured value is far higher, look at suction superheat and pressure ratio before blaming the compressor.
Draw Your Own Cycle in Seconds
Set evaporating and condensing temperature, superheat, subcooling and compressor efficiency and the interactive P-h diagram redraws the dome, the loop and all four state points, with every number labelled. Free, browser-based, no install.
Open the Interactive P-h Diagram →Also useful: refrigerant comparison calculator · R-410A property page · superheat & subcooling
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Questions Engineers Ask
What is a P-h diagram used for?
To see a refrigeration cycle as state points and energy distances: compression ratio, compressor work, heat rejection, refrigerating effect, liquid subcooling and evaporator-inlet flash gas on one chart — for design work, and for diagnosing a system that is not performing to design.
How do I find the four state points?
Measure two independent properties per point. Suction and discharge pressures set the vertical positions; superheat, subcooling and the constant-enthalpy expansion rule set the horizontal ones. Points 3 and 4 always share an enthalpy, and points 1 and 3 never do.
Why is the pressure axis logarithmic?
So the low side and the high side of a real cycle both stay readable. On a linear axis a suction pressure of 933.2 kPa collapses against a discharge pressure of 2,733.8 kPa and the evaporator region cannot be resolved at all.
What does the horizontal distance between points mean?
Energy per kilogram of refrigerant. Point 1 to point 2 is the compressor work, point 4 to point 1 is the refrigerating effect, and their sum is the heat the condenser rejects between points 2 and 3.
Can I read a P-h diagram without refrigerant property tables?
Not accurately by hand — the dome and the enthalpies come from an equation of state, and interpolating them manually is where errors creep in. Use the interactive P-h diagram or the refrigerant comparison calculator, which compute the properties live, then read the shape of the cycle exactly as you would on paper.
Related reading: R-454B vs R-410A transition guide · all engineering guides · the live cycle demo.