Refrigerators are one of the most commonly used home appliances. From keeping our food fresh for days to providing us with the leisure of cold beverages during hot summers, they make our lives comfortable every day. The use of the refrigerator in our daily life is so common that we hardly ever wonder how this technological marvel work at its core. In general, the lower temperature in refrigerators slows down down the activity of the bacteria present in the food, which in turn prevents the food from spoiling. Before the invention of electric refrigerators, fortunes were made shipping large blocks of ice to achieve cold temperature for storage of food and beverages. Today, thanks to the refrigerators, we do not have to go through such struggles to save our food from spoiling. The process that refrigerators use today is based on the work of the 19th century American inventors Oliver Evans and Jacob Perkins. In 1805, Evans came up with the vapor-compression refrigeration cycle, which we now employ to make refrigerators, but he was never able to build a refrigerator. In the 1830s, Perkins made the first refrigeration system based on Evan’s ideas. Although Perkin’s system wasn’t a commercial success, it was the first step toward the modern refrigerators that we use today. Before we try to understand how this refrigeration cycle works, let’s first understand what components of a refrigerator are involved in the cooling process.
Components of Refrigerator
Refrigerant fluid is usually a gas or a liquid that facilitates the conversion of thermal energy into mechanical energy by undergoing a phase change. In refrigerators, the refrigerant fluid can also be addressed as a coolant that flows throughout the refrigerator to maintain the temperature inside it. The most commonly used refrigerant fluids have extremely low boiling points, e.g., hydrochlorofluorocarbon chemicals that are generically referred to as freons.
The compressor is considered the heart of the refrigeration system. In technical terms, a compressor is a device that increases the pressure of a gas by reducing its volume. In refrigerators, the compressor mechanically increases the temperature of the refrigerant fluid by forcing it to move through a low-volume pipe. The refrigerant fluid enters the compressor as a low-pressure and low-temperature gas, and then it leaves the compressor as a high-pressure and high-temperature gas. It also acts as an electric pump that facilitates the flow of the refrigerant throughout the refrigerator in a closed system. The compressor is usually located at the lower part of the backside of a refrigerator.
A condenser, or condensing coils, is a mesh of pipes that one can find on the backside of the refrigerator. The main function of the condenser is to remove the inner heat of the refrigerator, just like a radiator. It is called a condenser because the removal of heat is achieved by the condensation of the refrigerant fluid from vapor to liquid state inside the condensing coils. Heat is given off as temperature drops to the condensation temperature.
Thermostatic Expansion Valve
A thermostatic expansion valve acts as a regulator that controls the flow of the liquid refrigerant into the evaporator. A thermostat (a regulating device component that senses the temperature and gives an output accordingly) controls the cooling process by switching the compressor on and off. When the sensor senses that it’s cold enough inside a refrigerator, it turns off the compressor. If it senses too much heat, it switches the compressor on and begins the cooling process again. This is the reason why we hear a humming sound at regular intervals coming from the refrigerator.
This is the part of the refrigeration system that does the cooling inside the refrigerator. Its function is to absorb heat into the refrigeration system from the inside, which then radiates out of the system through the condenser. The evaporation coils are present inside the refrigerator where we put our food items. The refrigerant fluid is let into these coils by the thermostatic expansion valve.
The receiver, or liquid receiver as it is commonly known, is a pressure vessel designed to hold liquid refrigerant. The function of a receiver is to store liquid refrigerant and provide a continuous flow of refrigerant to the expansion device. The receiver has inlet and outlet connections and a threaded port for the installation of an over-pressure safety-relief device. The main purpose of a receiver is to act as a buffer for the refrigerant fluid during the refrigeration cycle.
Working Principle of a Refrigerator
The basic principle behind the working of a refrigerator is the second law of thermodynamics; more specifically, Clausius’s statement of the second law of thermodynamics, which states, “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.” This statement can be understood as follows: to transfer heat from a region with lower temperature to a region with higher temperature, mechanical work must be done on the system. This is evident in a refrigerator as heat flows from a cold environment (inside of the refrigerator) to a hot environment (outside of the refrigerator), but only when forced by an external agent, i.e., the refrigeration system. In other words, refrigeration is a reverse operation of a heat engine. A heat engine takes heat from a hot body, converts a part of it into mechanical work, and rejects the rest to a colder environment. On the contrary, a refrigerator employs mechanical work to transfer heat from a region with a lower temperature to a region with a higher temperature. This periodic delivery of heat is known as the refrigeration cycle or the vapor-compression refrigeration cycle. Let’s discuss how this process is carried out inside a refrigerator.
In this cycle, a volatile refrigerant fluid is pumped through the cooling coils of the evaporator to the indoor compartment of a refrigerator. Inside these coils, the refrigerant evaporates due to the latent heat provided by the food items. This latent heat causes a change of phase in the refrigerant fluid’s state from liquid to vapor, causing the temperature inside the refrigerator to fall. These vapors are then passed to an electrically driven compressor that raises the temperature of these vapors by changing their pressure from low to high. These high-temperature and high-pressure vapors are then passed to the condenser coil, where they again go under a phase change from vapor to liquid by the process of liquefication. The heat is released to the outer environment by this second change of phase. The liquid refrigerant then enters a storage vessel called a receiver, or liquid reservoir, before finally passing through an expansion valve, which reduces its pressure before the beginning of another cycle. This cycle repeats over and over again until the temperature reaches the desired value (about 2-4 °C in the food chamber of a domestic refrigerator and < 0-1 °C in the freezing compartment). Some high-efficiency refrigerators also make use of fans to increase the efficiency of evaporator and condenser coils.