Split System Explorer
Follow the refrigerant. See where the heat goes.
Power on. Set the temperature. Watch it cycle.
1. Close QF1 → 2. Close QS1 → 3. Press Start. Cooling stops at the setpoint and restarts 1°C above it.
Lower setpoint = more cooling demand.
Higher room temperature = more cooling demand.
Restart protection: 180 simulated seconds. Initial fan pre-run: 20 simulated seconds.
Thermal response is an illustrative model, not a calculation of an actual room.
Conceptual fixed-speed system: QF1 → QS1 supplies the controller and motor branches; KM1 + motor protection feed M1; K2 switches the fan branch. Run capacitors support applicable single-phase motors. Inverter splits normally use electronic drives and different wiring. This is not an installation wiring diagram.
Recent temperature: cyan = room · amber = current setpoint · range 16–38°C · last 120 samples
The tour follows one refrigerant parcel. In a running system, all four processes occur continuously and simultaneously. Stop removes the run command. At the setpoint, the compressor and fans stop automatically in this selected AUTO-fan model. Pressure equalisation is not simulated.
Engineering notes · controls, temperatures & real-world arrangements
Read the model correctly
Cooling transfers heat from the room to outdoors. The outdoor coil rejects the room heat plus the compressor's energy input. Refrigerant stays in a sealed circuit; it does not mix with either air stream.
Example only: indoor return air 27°C and outdoor air 35°C. Evaporating saturation temperature may be around 5–10°C and condensing saturation temperature around 40–50°C. Discharge and suction labels show line temperatures, not saturation temperatures. Actual values depend on refrigerant, load, humidity, airflow and equipment design.
HP and LP are relative pressure zones. No universal pressure in bar is shown: pressure must be linked to refrigerant type and saturation temperature. Superheat = suction temperature minus evaporating saturation temperature; subcooling = condensing saturation temperature minus liquid temperature, at the relevant measurement pressure.
Controls & installation variants
This teaching arrangement puts the expansion device at the indoor evaporator inlet, with a high-pressure liquid interconnecting line. Many inverter mini-splits place an electronic expansion valve outdoors; their small interconnecting line can then carry a low-pressure two-phase mixture in cooling. Consult the unit's actual circuit.
A TXV regulates evaporator outlet superheat through its sensing bulb. A capillary is a fixed restriction. An EEV is electronically positioned. They are alternatives, not three devices installed in series.
Room temperature demand drives the controller; a fixed-speed compressor cycles, while an inverter can modulate. Coil sensors, motor protection and pressure protection depend on the model. This is a simulated fixed-speed control demonstration, including an illustrative overload trip; it is not connected to real equipment.
Heat-pump heating requires a reversing valve and suitable metering: the indoor coil becomes the condenser and the outdoor coil becomes the evaporator. Cooling-only equipment cannot simply reverse its compressor.
Further reading: Natural Resources Canada — Air conditioning your home · Trane — Expansion valve
Mollier diagram: why does expansion cool the refrigerant?
A diagram, not a theorem: in refrigeration, the pressure–enthalpy chart (log p–h) is often called a Mollier chart. Strictly, the classical Mollier diagram uses enthalpy and entropy (h–s). Here we use log p–h to follow the split-system cycle.
A → B: work added. B → C: heat rejected. C → D: pressure falls at constant enthalpy. D → A: room heat absorbed.
Enthalpy is not temperature. Specific enthalpy h = u + pv combines internal energy and flow work per kilogram. During a phase change, the same enthalpy can correspond to different temperatures, pressures and liquid/vapour proportions.
Follow the expansion in three clicks
Warm, high-pressure liquid
The condenser has made liquid. Upstream of the valve, its high pressure allows it to remain liquid at a relatively warm temperature.
The key: lowering pressure lowers the refrigerant's boiling temperature. Some liquid immediately evaporates (“flash gas”). Creating vapour requires latent energy, supplied by the fluid's own energy redistribution, so the remaining liquid and vapour reach a colder equilibrium.
For an insulated valve with no shaft work and negligible inlet/outlet kinetic-energy change, h before ≈ h after. Heat does not need to leave the valve to make the temperature drop. In the evaporator downstream, the remaining liquid then absorbs heat from the room.
Does every expanding gas cool down?
No — the process matters. In a throttling valve, an ideal gas keeps the same temperature because its enthalpy depends only on temperature. A real gas may cool or warm depending on its state: this is the Joule–Thomson effect, with μJT = (∂T/∂p)h. Since pressure falls, a positive coefficient gives cooling; a negative coefficient gives warming.
Expansion through a turbine or against a piston can cool a gas by producing work, even for an ideal gas when heat input is absent. Free expansion into a vacuum is another process again: an ideal gas in an insulated system does not cool. “More space automatically means colder” is therefore an unreliable explanation.
For this air conditioner, the valve inlet is mainly liquid. The most useful explanation is the lower saturation temperature and partial flashing at approximately constant enthalpy, rather than imagining only a gas spreading out.
Technical references: Danfoss — Refrigeration process and pressure/enthalpy diagram · NIST — Thermodynamic properties and Joule–Thomson coefficient
From the schematic to real components
Reference examples from Wikimedia Commons, embedded for offline use. Equipment sizes and arrangements vary. Assembly photographs may show several components; they are not an exact bill of materials for the simulated split.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Unmodified photograph, displayed at reduced size.
Control reference: Carrier — compressor restart protection. The chosen 1°C differential and fan sequence are teaching settings, not universal manufacturer values.