In this blog post, we’ll take an easy-to-understand look at the principle behind how refrigerators produce cold air and the operation of the vapor-compression refrigeration cycle.
How Does a Refrigerator Produce Cold Air?
When sweltering summer days with not a breath of wind continue, people look for various ways to cool off. Taking cold water out of the refrigerator to drink, or enjoying ice cream and cold drinks, are some of those ways. So where does that coldness come from? Even when the outside temperature exceeds 30°C, you can feel a cool breeze inside the refrigerator as soon as you plug it in. While it’s relatively easy to understand how an electric heater works—converting electricity into heat to warm up a space—a refrigerator actually cools its interior. Let’s explore the operating principle of a refrigerator, focusing on the vapor-compression refrigeration cycle.
A refrigerator consists of four main components: the compressor, the condenser, the expansion valve, and the evaporator. A refrigerant, which repeatedly changes states between liquid and gas, circulates through the pipes connecting these components. The refrigerant is the core element of the refrigeration system, and the process by which it circulates sequentially through the compressor, condenser, expansion valve, and evaporator to produce a cooling effect is called the refrigeration cycle.
In the refrigeration cycle, the refrigerant begins its journey in the compressor. In the compressor, the refrigerant is compressed using power supplied by an electric motor, and during this process, the refrigerant receives work from an external source. Since work is converted into energy, the compressed refrigerant stores energy, causing both its temperature and pressure to rise. It is important to note that the temperature of the compressed refrigerant becomes much higher than that of the surrounding air. In fact, if you touch the area near the compressor at the bottom rear of a refrigerator, you can feel that it is warm; this is due to the heat generated during the compression process.
The high-temperature refrigerant that has passed through the compressor moves to the condenser. In the condenser, the refrigerant releases the heat it has absorbed to the outside through heat exchange with the surrounding air. During this process, the refrigerant loses heat while maintaining nearly constant pressure and condenses into a liquid state; conversely, the surrounding air becomes warmer. The reason you can feel heat at the back of the refrigerator is precisely because the condenser is releasing heat.
The refrigerant, now in a liquid state, moves from the condenser to the expansion valve. In the expansion valve, the liquid refrigerant passes through a very narrow passage, causing the pressure to drop sharply and the temperature to drop significantly as well. You may have observed this phenomenon as a child: when you fill a syringe with air, block the opening, push the plunger down, and then suddenly release it, a mist forms inside. This occurs because the gas expands rapidly, causing the temperature to drop. A similar principle is at work in the expansion device. The refrigerant, which was a high-temperature liquid, becomes a low-temperature, low-pressure liquid, and its temperature generally drops to between approximately -20°C and -40°C.
The refrigerant, now in a low-temperature, low-pressure state, then moves to the evaporator. The evaporator is the part of the refrigerator that is in direct contact with the freezer compartment; here, the refrigerant exchanges heat with the surrounding air once again. Unlike in the condenser, the temperature of the surrounding air is higher than that of the refrigerant, so heat moves from the air to the refrigerant. As a result, the air is cooled as heat is removed, and the refrigerant, absorbing heat from its surroundings, evaporates from a liquid to a gaseous state at low pressure. The refrigerant, now in its gaseous state, returns to the compressor where its pressure increases. By repeating this cycle, the system continuously keeps the interior of the refrigerator cool.
How is the performance of a refrigeration cycle evaluated?
In a refrigeration cycle, a key performance indicator is the amount of heat absorbed by the evaporator relative to the power supplied to the compressor. This is called the Coefficient of Performance (COP). In other words, it represents the ratio of the cooling effect—that is, the heat absorbed from the surrounding air by the evaporator—to the power supplied to the compressor. Generally, the Coefficient of Performance for household refrigerators varies depending on operating conditions but typically ranges from 2 to 5. A higher Coefficient of Performance means that a greater cooling effect can be achieved using the same amount of power, indicating a highly efficient refrigeration system. Furthermore, while supplying more power to the compressor is one way to achieve a greater cooling effect, it is essential to balance efficiency with power consumption.
Why is correction necessary in an actual refrigeration cycle?
In an actual vapor-compression refrigeration cycle, various corrections are required because, unlike an ideal thermodynamic cycle, there are several practical limitations. To understand this, it is necessary to examine the T-s diagram. Here, T represents temperature, and s represents entropy. Entropy is a thermodynamic state variable that indicates the degree of disorder in a system; generally, as temperature or pressure increases, the energy level rises and entropy also increases. The unit of entropy is J/K, and in a T-s diagram, the area represented by the product of temperature and entropy indicates the amount of energy. Furthermore, the saturation curve on the T-s diagram marks the boundary where phase changes occur. The region to the left represents the liquid phase, the region to the right represents the gas phase, and in the region between the two curves, both liquid and gas coexist. When a line parallel to the entropy axis is drawn in this region, the point where it intersects the left curve is called saturated liquid, and the point where it intersects the right curve is called saturated vapor.
The first factor to consider is superheat. If the refrigerant does not evaporate sufficiently in the evaporator and enters the compressor while still partially in the liquid state, damage to the compressor may occur. Liquid refrigerant can cause mechanical damage by colliding with high-speed rotating components inside the compressor. This principle is similar to the phenomenon where an aircraft flying at supersonic speeds repeatedly collides with water droplets in a cloud, causing damage to the aircraft’s surface. To prevent this problem, the refrigerant exiting the evaporator undergoes additional heating to intentionally bring it to a gaseous state at a temperature higher than that of saturated vapor. The degree to which the refrigerant is heated above the saturation temperature is called the degree of superheat.
