Development and Experimental Evaluation of an IoT-Based Multi-Point Thermal Monitoring System for an R134a Domestic Refrigeration System

Every home that keeps food cold relies on a machine that works by moving heat from one place to another. Inside a standard refrigerator, a special fluid called a refrigerant travels in a continuous loop through four main parts: a compressor that squeezes the fluid, a set of coils that release heat to the room, a valve that lowers the pressure, and another set of coils that absorb heat from the food. This cycle is the engine of modern food preservation, but for most people, it remains a black box. We see the cold air inside, but we do not see how the temperature changes as the fluid moves through the machine, nor do we know exactly how the system behaves when it first turns on or how it settles into a steady rhythm. Understanding these hidden movements is crucial because the way heat moves through these components determines how efficiently the machine runs and whether it might fail.

Researchers at Olusegun Agagu University of Science and Technology and the Federal University Oye Ekiti in Nigeria decided to open that black box. They built a new way to watch a refrigerator work in real time, not just by checking the temperature inside the food compartment, but by measuring the heat at seven different spots along the entire path of the refrigerant. They attached a small, low-cost computer to a standard household refrigerator and connected it to a network of sensors that could send data to the internet. This allowed them to see exactly how hot or cold the machine was at every critical point, from the moment the compressor started until the system reached a stable state. Their goal was to create a clear picture of the thermal behavior of a domestic refrigerator using a technology known as the Internet of Things, which simply means connecting everyday objects to the internet so they can share information.

The team used a domestic refrigerator that runs on a common type of refrigerant called R134a. They equipped this machine with a central controller and a collection of digital sensors. Seven of these sensors were placed at specific locations to track the temperature of the refrigerant as it moved through the system. One sensor watched the air inside the evaporator compartment, while others monitored the pipes entering and leaving the heat-releasing coils, the surface of those coils, the area where the pressure drops, and the hot gas leaving the compressor. A separate sensor measured the temperature and humidity of the air in the room where the refrigerator stood. All of this data was collected every two seconds, a speed fast enough to catch the rapid changes that happen when the machine first turns on. The system displayed the numbers on a small screen attached to the unit and simultaneously sent them to a cloud-based platform, allowing the researchers to view the data from anywhere.

When the researchers turned on the refrigerator, the sensors immediately began to reveal the dramatic shifts in temperature that occur during operation. As the compressor started, it pushed the refrigerant into the heat-releasing coils at a very high temperature. The sensor at the exit of the compressor recorded the highest average temperature of the entire system, reaching 53.58 degrees Celsius. This heat is generated by the compression process and must be released into the surrounding air for the refrigerator to work. As the hot fluid traveled through the coils, it began to cool down. The temperature dropped as it moved along the heat-releasing section, falling to an average of 49.11 degrees at the start of the coils and further down to 38.36 degrees at the exit. This drop confirmed that the system was successfully dumping heat into the room. The surface of the coils, which transfers that heat to the air, settled at an average of 42.83 degrees, sitting comfortably between the hot inlet and the cooler outlet.

After passing through the pressure-reducing valve, the refrigerant entered the cooling section of the cycle. Here, the temperature fell significantly as the fluid absorbed heat from the inside of the refrigerator. The sensor inside the evaporator compartment recorded the lowest average temperature of the entire experiment, holding steady at 10.26 degrees Celsius. This created a massive temperature difference of more than 43 degrees between the hottest point of the system and the coldest, illustrating the powerful thermal gradient that drives the cooling process. The room temperature remained relatively constant at 27.44 degrees, providing a stable backdrop against which the machine's internal changes could be measured. The system also tracked humidity, showing that as the air inside the evaporator compartment cooled, its ability to hold moisture decreased, causing the relative humidity near the cooling coils to drop from about 75 percent to a stable 45 percent. This drying effect is a natural part of how refrigerators remove moisture from the air, which helps prevent frost buildup.

The most revealing part of the study was watching how the system behaved during the first few hours of operation. When the refrigerator was first switched on, the temperatures at all points changed rapidly. The hot gas at the compressor exit rose quickly from a starting point of around 15 degrees to nearly 50 degrees within the first two hours before settling into its steady rhythm. Similarly, the cold air in the evaporator compartment warmed up slightly from a starting point of 8 degrees to its stable operating level of 12 degrees. It took about four hours for the entire system to stop fluctuating wildly and reach a state of near-steady operation, where the temperatures only varied slightly. This transition period is often invisible to standard monitoring tools that only check the final temperature, but the new system captured the entire journey, showing exactly how long it took for the machine to stabilize and how the different parts responded to the start-up stress.

The researchers compared their detailed, multi-point approach to other existing monitoring systems, which typically only measure the temperature inside the storage compartment or the general environment. Those older methods provide a single snapshot of the final result but miss the complex interactions happening inside the machine. By placing sensors at the inlet, outlet, and surface of the heat-exchanging parts, as well as the expansion valve and compressor, this new system provided a complete map of the refrigerator's thermal life. It showed that the heat rejection process is not uniform and that the surface temperature of the coils tells a different story than the temperature of the fluid inside them. This level of detail is something that conventional methods cannot achieve without expensive and complex equipment.

The study concluded that this low-cost, internet-connected system is a practical and effective way to monitor the health and performance of a refrigerator. It successfully demonstrated that a simple setup of digital sensors and a small computer could capture the full thermal story of a domestic appliance, from the moment it starts up to its steady daily operation. The data collected proved that the system could resolve the differences in temperature between every major component, offering a clear view of how heat moves through the cycle. While the study focused on a single unit, the method suggests that such systems could be scaled up to monitor many refrigerators at once, providing a foundation for smarter maintenance and better energy use. The ability to see these transient behaviors and component-level details opens the door for future technologies that could automatically detect problems or optimize performance before a failure occurs, turning a simple household appliance into a source of rich, actionable data.