# How Air Conditioning Works

How a vapor\-compression air conditioner moves heat out of a room through a closed refrigerant loop, and why it can reverse to heat a house\.

By merry\-otter\-13418f14

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/sqfY8y0dZPwm)

## Transcript

### 1. Pumping heat uphill

An air conditioner is a heat pump: it moves heat out of a 24 °C room into 35 °C outdoor air, against the temperature gradient\. The second law allows that only with work input, so the heat rejected outdoors equals the heat absorbed indoors plus the compressor work\. This 3\-ton \(10\.5 kW\) split system does it with a vapor\-compression cycle running R\-410A\.

Drag to turn the diorama\. Use the arrows to step through the cycle\.

### 2. One closed refrigerant loop

The refrigerant circulates without being consumed: compressor \(1 to 2\), condenser \(2 to 3\), expansion valve \(3 to 4\) and evaporator \(4 to 1\)\. The compressor and valve split the loop into a high side near 2\.8 MPa and a low side near 1\.0 MPa\. Particles are spaced by transit time, so dense, slow liquid crowds together while fast vapor thins out\.

State points follow the textbook numbering of the ideal cycle\.

### 3. Evaporation absorbs heat indoors

At about 1\.0 MPa, R\-410A boils near 7 °C, so 24 °C return air crossing the coil gives up heat as latent heat of vaporization at nearly constant refrigerant temperature\. The air leaves near 13 °C\. Because the fins sit below the air’s dew point, water vapor condenses and drains away: the coil removes latent load as well as sensible heat\.

The expansion valve meters flow to hold roughly 5 to 8 K of superheat at the coil outlet, so no liquid reaches the compressor\.

### 4. Compression raises pressure and temperature

The scroll compressor traps vapor between a fixed and an orbiting spiral; each orbit pushes the pockets inward and shrinks them until the gas discharges at the centre\. Pressure rises nearly threefold, from about 1\.0 to 2\.8 MPa, and the vapor leaves strongly superheated\. This shaft work is the system’s main electrical input\.

The ideal cycle assumes isentropic compression; real losses add entropy and raise the discharge temperature\.

### 5. Condensation rejects heat outdoors

On the high side the refrigerant condenses about 10 to 15 K above the outdoor air, near 47 °C on a 35 °C day\. The fan pulls air through the coil, which desuperheats, condenses and then subcools the refrigerant by about 5 K\. The coil rejects the indoor heat plus the compressor work, about 13\.5 kW, warming the air by roughly 8 K\.

Saturation pressure grows roughly exponentially with temperature, so a hotter day raises the head pressure steeply\.

### 6. Throttling at the expansion valve

Warm, subcooled liquid is forced through a narrow orifice\. Throttling is essentially isenthalpic: no work or heat crosses the boundary, so the pressure drop makes part of the liquid flash to vapor, and that evaporation chills the mixture to the low\-side saturation temperature\. Roughly a quarter of the mass leaves the valve as vapor, which is why extra subcooling raises capacity\.

### 7. Coefficient of performance

Performance is the coefficient of performance: heat absorbed divided by compressor work\. A reversible Carnot cycle between the evaporating and condensing temperatures sets the upper bound, T\_L over T\_H minus T\_L in kelvin, about 7 here\. Real systems reach roughly half that, typically 2 to 5, because heat transfer needs finite temperature differences and compression has losses\.

Every kelvin of extra temperature lift lowers both the Carnot limit and the real COP\.

### 8. Running the cycle in reverse

A reversing valve swaps the coils’ roles: the outdoor coil evaporates and the indoor coil condenses, so the same hardware heats the house\. At the 8\.3 °C outdoor, 21 °C indoor rating point it delivers several times more heat than the electricity it uses\. The simple model breaks down with line pressure drops, real compressor losses, outdoor\-coil frost and the shift from R\-410A to lower\-GWP refrigerants such as R\-32 and R\-454B\.