How does a fridge keep things cold? The vapour-compression refrigeration cycle

Hold your hand near the back or the bottom of a fridge and you will feel warm air. If the inside is at 4 °C, why is the back warm? Because a fridge does not really make cold. Its job is to collect the heat inside and carry it out into the kitchen. The warmth at the back is heat taken out of the milk, the eggs and yesterday’s leftovers and released into the room.

In this post we open up the machine that does this carrying, part by part: the compressor, the condenser, the capillary tube and the evaporator. One substance flows through all of them: the refrigerant. Follow its journey and the secret of the fridge comes undone.

There is no such thing as cold: heat is carried

On its own, heat always flows from hot to cold. A cup of tea left on the table cools down, a drink taken out of the fridge warms up; the reverse never happens by itself. In physics this is the everyday face of the second law of thermodynamics.

A fridge moves heat “uphill”: from a 4 °C compartment to a 22 °C room. It is like a pump that lifts water from a low tank to a higher one. Water does not climb on its own, but a pump can lift it by using electricity. The fridge’s pump is the compressor, and what it carries is not water but heat. That is why fridges and air conditioners are sometimes called heat pumps too.

The trick: take in heat by boiling, give it out by condensing

The most effective way to carry heat is a change of state. A liquid takes in a lot of heat from its surroundings as it evaporates; that is why we shiver when we come out of the sea and why cologne feels cool (more in Why does evaporation cool?). When the vapour condenses back into a liquid, it gives the same heat back.

A fridge makes these two things happen in two different places:

• Inside, in the tubes around the freezer box, the refrigerant boils and takes heat from the compartment.
• Outside, in the grille-shaped tubes at the back, the refrigerant condenses and gives that heat to the room.

But there is a problem: the same substance has to boil at around −20 °C and condense at around 40 °C. What makes this possible is pressure. The boiling temperature of a liquid depends on pressure; the lower the pressure, the lower the temperature at which it boils, and the higher the pressure, the higher that temperature (the same idea makes a pressure cooker work: Why does a pressure cooker cook faster?).

The common refrigerant in household fridges today is isobutane (called R600a in the refrigeration world). At normal air pressure, isobutane boils at about −12 °C. Lower its pressure to about 0.7 bar and it boils at around −20 °C; raise it to about 5.3 bar and it condenses at 40 °C. The fridge moves the refrigerant back and forth between these two pressures: low pressure inside, high pressure outside.

The four parts of the cycle

The refrigeration cycle is made of four main parts and a closed tube that connects them. The refrigerant travels the same path inside this tube for years; it is not used up and not replaced.

Compressor. The black, sealed can at the back, near the bottom of the fridge. Inside are an electric motor and a piston. It draws in the cold, low-pressure vapour coming from the evaporator and squeezes it. Both the pressure and the temperature of the compressed vapour rise; the vapour leaving the compressor can reach 50–70 °C. The “running noise” of a fridge is mostly this motor.

Condenser. The black grille at the back: a zigzagging tube with thin wires welded across it. The wires add surface area, so the hot tube can pass its heat to the room air more easily. The refrigerant condenses here as it cools, and by the end of the tube it has turned into a warm liquid. The warmth you feel at the back comes from here. In some models this tube is hidden inside the outer metal skin; then the sides of the fridge feel warm instead.

Capillary tube (expansion device). The liquid leaving the condenser first passes through a small filter drier, then enters a thin tube a few metres long with an inner diameter of about a millimetre. This narrow, long tube resists the flow, so the pressure of the liquid falls as it moves along. As the pressure drops, some of the liquid boils on its own, and that boiling cools the rest of it. What comes out of the capillary is a cold mixture of liquid and vapour at around −20 °C. In air conditioners and larger refrigeration systems this job is usually done by an adjustable expansion valve.

Evaporator. The cold surface inside the compartment: in single-door fridges a tube wrapped around the freezer box, in many models a plate hidden behind the inner back wall. Here the cold refrigerant takes heat from the air and the food, boils and turns completely into vapour. Then it returns to the compressor through the thick suction line and the lap begins again.

One lap of the refrigerant

In the drawing above, the side wall of the fridge has been cut away. The colour of the tube shows the refrigerant temperature: hot parts gold, cold parts purple. A solid line stands for liquid, flowing dots for vapour. At the numbered points you can read the pressure and the temperature. Turn the thermostat, open the door, switch the compressor off and watch what changes. You can also type the temperature of your own kitchen and a compressor power into the boxes and run the numbers with your own values.

1. Compressor outlet: Hot, high-pressure vapour (about 5 bar, 60 °C). The refrigerant is hotter than the room, so heat can flow from it into the room.
2. Condenser outlet: The pressure is still high, but the refrigerant has given its heat to the room and become a warm liquid (about 30 °C).
3. Capillary outlet: The pressure has fallen below 1 bar and part of the refrigerant has boiled; what is left is a liquid–vapour mixture much colder than the inside of the fridge (about −20 °C). Now heat can flow from the compartment into the refrigerant.
4. Evaporator outlet: The refrigerant has taken the compartment’s heat and evaporated completely; cold, low-pressure vapour returns to the compressor.

The logic of the cycle fits in one sentence: make the refrigerant colder than the place you want to take heat from, and hotter than the place you want to give heat to. The compressor and the capillary tube do exactly that by changing the pressure.

Why does the back give out more heat than the inside loses?

The heat budget in the drawing shows one thing clearly: while the compressor runs, the heat given to the room is always larger than the heat taken from inside. That is because the room receives both the heat taken from the compartment and the electrical energy used by the compressor. Energy does not disappear; the electricity that goes into the compressor’s motor also leaves the back as heat in the end.

In the simple model of the drawing, with the default values (a 22 °C room and a 4 °C thermostat), the compressor draws 60 W of electricity, takes about 120 J of heat per second (120 W) from inside and gives about 180 W of heat to the room at the back. The ratio here, the heat removed divided by the electricity used (120 / 60 = 2), is the coefficient of performance of the refrigerator (COP). In real fridges this number depends on the model and the conditions; it gets smaller as the temperature difference between inside and outside grows. That is why a fridge pushed against the wall with no air around it, or placed next to a stove, uses more electricity.

The thermostat: why does a fridge keep starting and stopping?

A fridge does not run all the time. A thermostat watches the temperature inside: when it rises a little above the setting, it starts the compressor, and when it falls a little below, it stops it. Without this small margin between the setting and the two limits (±1 °C in the drawing), the compressor would switch on and off every few seconds and wear out quickly. While the compressor rests, the refrigerant does not circulate; heat leaking through the walls slowly warms the inside, and when the limit is passed the cycle starts again. That “run, pause, run” rhythm you hear in the kitchen is this.

If we leave the door open, does the kitchen get cooler?

No, it gets a little warmer. With the door open, the fridge releases the heat it takes from inside back into the same room, and it adds the electricity used by the compressor on top. The heat taken from the room and the heat given to it cancel out; what is left, the electrical energy, warms the room. Leave the door open in the drawing and watch the room temperature to see this. That is why air conditioners put their hot side (the outdoor unit) outside the building.

When the door opens, the warm air that pours in heats the compartment quickly; the compressor cannot keep up and runs non-stop. Cold air is heavier, so it spills out at the bottom of the door while warm air enters at the top. Opening the door briefly and letting hot food cool before putting it in protects both the food and the electricity bill.

The same machine turned around: the heat pump

Now turn the fridge around in your head: put the evaporator outside the house and the condenser inside. The machine now takes heat from the outdoor air and carries it into the house. This is the heat pump that heats homes. Even in the middle of winter the outdoor air holds heat; the cycle collects it with an even colder refrigerant. Many air conditioners also reverse the direction of flow with a valve, cooling in summer and heating in winter.

The nice thing about a heat pump is that the heat it delivers to the house is the heat it takes in plus the electricity it uses. So for each unit of electricity it uses, it often carries several times as much heat into the house. Turning the same electricity directly into heat in an electric heater gives only one unit of heat. Some tumble dryers and water heaters work on the same cycle.

A little history: from ammonia to isobutane

The first patent for vapour-compression refrigeration was granted in London in 1834 to the American inventor Jacob Perkins. In the late 19th century, large refrigerating machines running on ammonia spread through breweries and slaughterhouses. Early household fridges used toxic refrigerants such as ammonia, sulphur dioxide and methyl chloride, and leaks were dangerous.

In the 1930s, non-toxic and non-flammable CFCs (chlorofluorocarbons) solved this problem and spread around the world. In the 1970s, however, scientists showed that CFCs escaping into the atmosphere thin the ozone layer high above us. Under the Montreal Protocol, signed in 1987, CFCs were phased out step by step. Some of the refrigerants that replaced them are strong greenhouse gases, so the Kigali Amendment of 2016 decided to cut their use as well. Today many household fridges circulate isobutane, which does not harm the ozone layer and has a very low greenhouse effect. Because isobutane is flammable, only a small amount is put in a fridge and its tubes are kept sealed.

Safe observations at home

You can observe everything in this post without opening or taking apart a fridge:

1. Feel the back. While the compressor is running (when you hear a soft hum), hold your hand near the grille at the back, without touching it. You will feel that the air is warm. Try again after the compressor has been off for a long time.
2. Time the rhythm. Use a clock to note how long the compressor runs and how long it rests. Then open and close the door a few times and time it again: the running time gets longer.
3. Measure the inside. Put a glass of water on the middle shelf and measure its temperature with a kitchen thermometer a few hours later. Move the thermostat by one step and measure again the next day.
4. Rules: Do not touch or tamper with the tubes, cables or the compressor at the back of the fridge; piercing or bending the tubes is dangerous. If the grille at the back needs cleaning, unplug the fridge first and let an adult do it.

Three common misconceptions

“A fridge makes cold.” Cold is not a substance; a fridge only carries heat from inside to outside. The inside gets cold because its heat has been taken away.

“When the gas runs out the fridge stops cooling, so you have to top it up.” The refrigerant goes around a closed loop and is not used up. If cooling has dropped, there is either a leak or another fault, and that is a job only a technician can find and fix.

“Turning the thermostat to the coldest setting cools the fridge faster.” In most fridges the compressor runs at the same speed whatever the setting; the setting only decides when it stops. The coldest setting does not cool the fridge faster, it cools it for longer and uses more electricity.

Summary

• A fridge does not make cold; it takes heat from inside and carries it out of its back into the room. The heat given to the room is the heat taken from inside plus the electricity used.
• The refrigerant takes in heat by boiling at low pressure inside and gives out heat by condensing at high pressure outside. The compressor raises the pressure; the capillary tube (or an expansion valve) lowers it.
• The four parts of the cycle: compressor, condenser, capillary tube, evaporator. The thermostat keeps the temperature within a range by switching the compressor on and off.
• Built the other way round, the same cycle becomes a heat pump that can heat a home with several times as much heat as the electricity it uses.