Refrigerators are amazing appliances that enable us to store our food and ensure that it doesn’t become spoilt. The function of a refrigerator is to preserve food by cooling it to just above the freezing point of water to prevent the growth of bacteria. However, it is important to know how a refrigerator works; because it helps you understand the processes within the refrigeration process, as well as any parts that require maintenance.
Underlying Physics – Thermodynamics
The main physical concept that is used to make a refrigerator is known as thermodynamics. Thermodynamics is basically a branch of science that focuses on the relationships between heat, temperature, energy and work.
In thermodynamics, a cyclic process starts and returns to the same thermodynamic state, i.e. pressure, volume and temperature do not change in the process, whereas a non-cyclic process starts and returns to a different thermodynamic state.
The internal energy of a system is the energy contained within the system, excluding kinetic and potential energy contributions and it is directly related to the temperature of a system. In a non-cyclic system, the change in internal energy of a system is equal to the heat added or removed from a system minus the work done by the system. This statement is known as the 1st Law of Thermodynamics. If we define ΔU as the change in internal energy, Q as the heat added or removed, and W as the work done, then
ΔU = Q – W
When a system expands in a process that is in thermal equilibrium, the work done is equal to a product of the pressure, p, and the change in volume, ΔV. Using this, we get
ΔU = Q – pΔV
How Does A Refrigerator Work?
To start off with, a refrigerator consists of a compressor, a condenser, an expansion valve, an evaporator, the refrigerator box and some insulation.
Firstly, a refrigerant, in the form of a vapour, is pumped into the compressor, where it is pressurised and heated. It is then sent to the condenser – this is essentially an array of coils at the back of the refrigerator. While in the condenser, the hot vapour expels heat to the surroundings, which cools the refrigerant.
The reason why this happens is that there is no work being done on the refrigerant, since the occupied volume is constant throughout the refrigerator (the refrigerant is in a uniform pipe). In order for a gas to condense to a liquid at constant pressure and volume, it must decrease in temperature. To do this, the refrigerant releases heat to the surroundings, i.e. the kitchen. This process continues until it reaches the expansion valve.
The way that the expansion valve works is that it throttles a liquid into a very small opening, which causes the pressure to decrease dramatically. The key to this process is that the heat content of the system remains constant before and after the throttling process. Because of this, it means that when the pressure decreases, the temperature decreases. This is known as the Joule-Thomson process.
So, when the refrigerant is passed through the expansion valve, it instantly evaporates and turns into a very cold gas. This then passes into the evaporator – an array of coils on the inside of the refrigerator. However, now that the refrigerant has changed from a liquid to a gas, it needs to take heat from its surroundings. It does this by extracting heat from the inside of the refrigerator, which cools the inside. The refrigerant vapour then flows back to the compressor, where it is compressed into a liquid and the process is repeated.
In order to turn a gas into a liquid, work must be done on the system. To do that, energy is taken from an external source, which in this case is electrical energy from the mains supply of electricity.
We understand the importance of keeping your refrigerator in optimum working condition, especially in the food industry. If you’ve noticed any issues with regards to your refrigeration unit, then please don’t hesitate to contact us. Give us a call on 0131 210 0024 or 07926 529476 and we will be more than happy to help!