Every time you open the door to get milk or put in a freshly prepared dinner, you are taking advantage of one of the greatest engineering achievements of the 20th century. But have you ever wondered what really happens inside that metal box? Many people mistakenly believe that a refrigerator somehow magically “adds cold” to a space.
In fact, the physics of the process is diametrically opposite: the device does not produce cold, it diligently and continuously removes heat from the inner chamber and throws it out into your kitchen. This process is based on the fundamental laws of thermodynamics and the properties of gases to change their temperature with changes in pressure. Understanding these principles will help you not only better understand technology, but also operate it more efficiently.
In this article, we will examine the closed refrigeration cycle, the role of each component of the system, and the physical phenomena that make it possible to keep food fresh for a long time. We will abandon complex formulas in favor of understandable analogies, so that physics becomes accessible to everyone.
Fundamental principle: where does the heat go?
To understand, how a refrigerator worksyou need to understand the basic law of physics: heat always spontaneously transfers from a hotter body to a colder one. The refrigerator causes heat to move in the opposite direction - from the cold interior to the warm exterior. To carry out this “violent” work, energy is required, which is supplied by electric current.
The key element here is a special substance - refrigerant (often called freon). This is a gas with unique properties: it can easily pass from a gaseous state to a liquid state and back again at temperatures accessible in everyday conditions. It is the phase transitions of this substance that underlie the entire cooling process.
⚠️ Attention: The refrigerant in modern models can be fire hazardous (for example, isobutane R600a). Never try to re-solder pipes yourself or release gas in a closed room without professional equipment.
The process of heat extraction occurs due to the evaporation of liquid. Remember the feeling when you get out of the shower: you feel cool because the water on the surface of your skin evaporates, taking your body heat with it. The same thing happens inside the refrigerator: the liquid refrigerant evaporates in a special heat exchanger (evaporator), actively absorbing heat from the food and air of the chamber.
Four main components of the refrigeration system
Circulation of the refrigerant and changes in its state are provided by four main units connected by copper tubes in a closed loop. Each of them performs a strictly defined function, and the failure of any element stops the entire process.
The first and noisiest participant in the process is compressor. This is the “heart” of the refrigerator, which works like a powerful pump. It compresses the refrigerant gas coming from the evaporator, sharply increasing its pressure. As physics says, during compression the gas heats up, so at the outlet of the compressor the temperature of the refrigerant can reach 80-90°C.
Then the hot gas enters condenser - this is the same black lattice radiator, which is usually located on the back wall of the device (although in modern models it can be built into the side walls). Here heat transfer occurs: the hot gas cools down in contact with the kitchen air and condenses, turning into liquid. You can feel the heat if you pass your hand near the rear grill of a running unit.
After the condenser, the liquid passes through capillary tube (or choke). This is a very narrow channel that serves as the boundary between an area of high and low pressure. Passing through it, the refrigerant expands sharply, its pressure drops, and it partially turns into vapor, which leads to strong cooling of the mixture.
Finally, the cooled mixture enters evaporatorlocated inside the freezer or refrigerator compartment. Here the refrigerant boils at a low temperature, actively removing heat from the internal volume. Having turned into gas, it returns to the compressor again, and the cycle repeats.
Thermodynamic cycle: from compression to expansion
The physical process occurring inside a refrigeration machine is called the reverse Carnot cycle. Although real devices do not work perfectly, the principle remains the same: alternating adiabatic compression, isobaric cooling, throttling and isothermal expansion.
Let's follow the path of one refrigerant molecule. In the compressor it is compressed, its kinetic energy increases, and the temperature rises. In the capacitor, it gives off this energy to the surrounding air, turning into a liquid state. This is a critically important point: condensation is a process in which a substance gives up a large amount of latent heat of vaporization.
Passing through a capillary, the liquid sharply loses pressure. In physics, this phenomenon is associated with the Joule-Thomson effect. The sudden expansion causes the fastest molecules to evaporate, taking energy with them, and the remaining liquid becomes very cold. When it enters the evaporator, it boils even at subzero temperatures, taking heat from the chamber.
The role of the thermostat and automation
The refrigerator cannot work endlessly, otherwise it would turn the kitchen into a sauna, and the food into ice blocks. To control the process, thermostat or an electronic control board is used. This sensor constantly monitors the temperature inside the chamber.
When the temperature drops below a set value (for example, +4°C for the main chamber), the thermostat opens the electrical circuit and the compressor turns off. At this moment, the refrigerant stops circulating, and the temperature inside begins to slowly rise due to heat inflows from the outside (through the walls and when the door is opened).
As soon as the temperature rises above the set threshold, the thermostat closes the circuit again, starting the compressor. The difference between the on and off temperatures is called hysteresis. Correct setting of this parameter is important for food safety and energy saving.
Comparison of cooling systems: static, No Frost and Low Frost
The physics of the heat extraction process is the same for all refrigerators, but the methods of cold distribution may differ. This determines whether you have to defrost the unit manually and how quickly the bread dries in it.
In classic models with drip system (Direct Cool), the evaporator is hidden behind the back wall of the refrigerator compartment. The air is cooled by natural convection: cold air sinks, warm air rises. Moisture condenses on the cold wall, freezes, and when the compressor is turned off, it flows into a special tray.
The system No Frost ("without frost") uses forced circulation. The fan drives air through a hidden evaporator, usually located in the freezer. The air is dried by passing through a cold radiator and then distributed among the chambers through channels. This eliminates the formation of ice, but can dry out the food more.
| Characteristics | Drip system | No Frost system | Low Frost |
|---|---|---|---|
| Ice formation | Requires defrosting 1-2 times per year | Does not require defrosting | Frost forms slowly, defrosting is rare |
| Humidity | High, food does not dry for a long time | Low, food can ventilate | Optimal humidity |
| Noise | Quiet operation (no fan) | Audible fan noise | Quiet operation |
| Useful volume | Maximum | Less due to the channel system | Close to the drip system |
The system Low Frost is a compromise solution. The evaporator here is made in the form of a plate built into the walls of the chamber (usually a freezer). This avoids the complex system of No Frost air channels, maintaining volume and humidity, but at the same time the ice freezes much slower than in older models.
Energy efficiency and physical losses
Why does one refrigerator consume 200 kWh per year, and another - all 400? The answer lies in the quality of thermal insulation and compressor efficiency. The main enemy of the refrigerator is heat inflows from the environment.
The walls of modern units are filled polyurethane foam. This material contains millions of closed cells with gas, which has very low thermal conductivity. The thicker the insulation layer and the better the foaming, the less frequently the compressor will have to turn on to maintain the temperature.
The energy efficiency class (A, A+, A++, etc.) also plays an important role. It shows how much cooling capacity the device receives per 1 W of electricity consumed. Inverter compressors, which do not turn off completely, but only reduce the speed, allow you to achieve better performance, as they eliminate energy-consuming starting currents.
⚠️ Attention: If you notice that the rubber door seal is worn out or does not fit tightly, heat inflows increase sharply. The compressor will work continuously, trying to compensate for the losses, which will lead to overheating and breakdown.
Frequently asked questions about the physics of the refrigerator
Why does the refrigerator sometimes make sounds of “gurgling” or “water flowing”?
This is an absolutely normal physical process. The sounds occur due to the movement of refrigerant through the system pipes. When liquid freon mixes with gaseous or changes its state of aggregation in the evaporator and condenser, a mass of substance flows. In modern quiet models, these sounds can be heard even better due to the absence of the noise of the old compressor.
Is it possible to place a refrigerator close to the wall?
No, and the physics here is categorical. The condenser (radiator) requires an influx of cold air for effective heat exchange. If the air circulation around the rear grill is disrupted, the condensation temperature will rise, the pressure in the system will increase, and the compressor will work overload, which will shorten its service life.
Why should you not put a hot pan in the refrigerator?
In addition to the fact that this creates unnecessary stress on the compressor, the hot object creates a powerful upward flow of warm air. This disrupts natural convection inside the chamber, the temperature sensor may not immediately respond to local heating, and moisture from a hot dish will instantly turn into moisture.