Basics of Heat Engineering: How Does a Refrigerator Work?

Refrigerators play a crucial role not only in private households but also in the retail food industry. They ensure that fresh foods such as dairy products, meat, fruits, and vegetables can be safely stored and offered for sale over longer periods. Without the efficient cooling of goods, many products would spoil, leading to both economic losses and increased waste. But how do refrigerators that are used daily in supermarkets and stores work? To understand this, it is necessary to explore the fundamentals of heat engineering and examine the functioning of a cooling system in detail. In this article, we will explain the physical principles behind refrigeration, analyze the structure of a refrigerator, and comprehensively describe the process behind cooling food.

What is Heat Engineering?

Heat engineering is a subfield of thermodynamics and deals with the conversion and transfer of heat. It focuses on how heat is transferred between different bodies or systems and what processes take place. There are three fundamental types of heat transfer: conduction, convection, and radiation.

Conduction: Heat is transferred through direct contact between molecules. This process is particularly efficient in metals, as freely moving electrons conduct heat well.
Convection: Heat is transferred through the movement of liquids or gases. Warm air rises because it is less dense than cold air, creating a circulation of heat.
Radiation: Heat is transferred in the form of electromagnetic radiation (such as infrared rays) without the need for physical contact.

In a refrigerator, controlling heat transfer plays a critical role. The main task of a refrigerator is to transfer heat from the inside (where the food is stored) to the outside, keeping the interior at a lower temperature.

The Structure of a Refrigerator

A modern refrigerator consists of various components that work together to lower the temperature inside. The main parts are:

Evaporator: This is where the refrigerant evaporates and absorbs heat from inside the refrigerator.
Compressor: The compressor is the heart of the cooling system. It compresses the gaseous refrigerant, increasing its pressure and temperature.
Condenser: After being heated in the compressor, the refrigerant passes through the condenser (a system of tubes at the back of the refrigerator), where it releases heat to the surroundings and liquefies.
Expansion valve: The refrigerant finally passes through an expansion valve, which significantly reduces its pressure, cooling it before it returns to the evaporator.

All these components work in a cycle to transfer heat out of the refrigerator and keep the temperature low.

The Principle of Heat Transfer in Refrigerators

The functioning of a refrigerator is based on the refrigerant's ability to absorb and release heat while transitioning between different states (liquid and gaseous). This occurs in a closed loop. The refrigeration process can be divided into four main phases:

Evaporation of the refrigerant in the evaporator:
- Inside the refrigerator is the evaporator, where the refrigerant, which was significantly cooled by the expansion valve, evaporates. As the refrigerant transitions from liquid to gas, it absorbs heat from the refrigerator's interior, causing the temperature to drop. The evaporator is often hidden behind the back wall of the refrigerator.
Compression of the gaseous refrigerant by the compressor:
- After the refrigerant evaporates in the evaporator, it is drawn into the compressor. The compressor compresses the refrigerant gas, causing its pressure and temperature to rise. This increase in temperature is necessary to efficiently release heat to the environment later.
Condensation in the condenser:
- The hot, compressed refrigerant is then directed to the condenser, located at the back of the refrigerator. Here, the refrigerant releases the absorbed heat to the surroundings, aided by the condenser tubes, which are often equipped with fins to improve heat dissipation. As the refrigerant cools, it liquefies again.
Expansion of the refrigerant in the expansion valve:
- The liquid refrigerant now flows through an expansion valve, which rapidly reduces its pressure. This pressure drop causes the refrigerant's temperature to decrease significantly. The cold refrigerant then returns to the evaporator, where the cycle starts again.

This process repeats continuously to maintain a consistently low temperature inside the refrigerator.

Refrigerants: The Engine of the Cooling System

The refrigerant is the substance used in a refrigerator to transport heat from the inside to the outside. It undergoes the cycle described above, changing between gaseous and liquid states.

In the past, chlorofluorocarbons (CFCs) were commonly used as refrigerants, but they are harmful to the environment because they contribute to the depletion of the ozone layer. Modern refrigerators use more environmentally friendly refrigerants such as isobutane (R600a) or tetrafluoroethane (R134a). These substances have less impact on the environment and are more effective in the refrigeration process.

Energy Efficiency and Environmental Aspects

Over the past few decades, refrigerators have evolved significantly, especially in terms of energy efficiency. Modern devices consume far less energy than older models. This is partly due to improved insulation materials and more efficient compressors, but also advancements in refrigerants.

The energy efficiency of a refrigerator in Europe is classified by an energy label, ranging from A+++ (very efficient) to G (less efficient). An energy-efficient refrigerator can significantly reduce household energy consumption and contribute to lowering CO₂ emissions.

Another aspect of refrigerator sustainability is proper recycling at the end of its life. Since refrigerators often contain hazardous refrigerants, they must be disposed of properly to avoid environmental damage. Many countries have specific recycling programs for household appliances such as refrigerators.

The Impact of Temperature and Humidity

For a refrigerator to function optimally, it is important that it is placed in a well-ventilated room and not exposed to direct sunlight or high ambient temperatures. High temperatures make it harder for the refrigerator to dissipate heat, leading to higher energy consumption.

Humidity inside the refrigerator also plays a role. Foods with high water content evaporate in the refrigerator, which can increase humidity levels and affect the efficiency of the evaporator. However, modern refrigerators often have humidity regulators that help control the interior humidity and keep food fresh.

Common Problems and Solutions

Like any technical device, refrigerators can develop problems over time. To detect and address these issues early, the use of data loggers is an effective method. With data loggers, temperature and humidity fluctuations in the refrigerator can be continuously monitored and analyzed, enabling quick diagnosis and resolution of problems. Here are some of the most common problems and potential solutions where data loggers can help:

Refrigerator not cooling adequately: This could be due to a dirty condenser or an inaccurate temperature setting. A data logger can precisely record the temperature trends in the refrigerator over time. Deviations from the target values can be detected early, and appropriate measures can be taken before major failures occur.
Compressor running constantly: If the compressor runs continuously, it could indicate a leaky door seal or a problem with the thermostat. A data logger can help monitor the exact timing and duration of compressor activity to determine if the refrigerator is overcooling. This allows potential causes, such as a leaky door seal or a faulty thermostat, to be identified more quickly.
Ice buildup in the freezer: This problem can be caused by high humidity or a faulty defrost heater. Data loggers with integrated humidity measurement allow continuous monitoring of humidity levels inside the refrigerator, helping to determine if they are within the optimal range. This can prevent excessive ice buildup and improve the efficiency of the cooling system.

The use of data loggers provides a valuable way to monitor the performance and efficiency of cooling systems over the long term. They not only help to detect problems early but also improve maintenance scheduling and avoid unnecessary costs from malfunctions. Data loggers offer a reliable and precise monitoring solution suitable for both retail food applications and industrial refrigeration systems.

Future Technologies in Refrigeration

Cooling technology is constantly evolving to reduce energy consumption and minimize environmental impact. In the future, magnetic refrigeration systems could present a promising alternative to conventional compression refrigeration. These systems use magnetic fields for cooling, which could eliminate the need for refrigerants, further reducing the environmental footprint.

Another area of research concerns smart refrigerators connected to the Internet of Things (IoT). These devices can monitor food, place orders automatically, or optimize their efficiency by adjusting to household energy needs.

Conclusion

A refrigerator may seem like a simple household appliance, but its operation is based on complex physical principles and advanced technology. Through the interaction of evaporation, compression, and condensation, heat can be transferred from the inside of the refrigerator to the outside, keeping food fresh. Thanks to constant innovations in refrigeration technology, refrigerators are becoming more energy-efficient and environmentally friendly, benefiting both consumers and the planet.

Heat engineering, which underlies cooling technology, remains a fascinating and important topic that will continue to improve as technologies advance.

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