You might have wondered how the stuff inside the refrigerator gets cooled. What is the science behind it? How does the refrigerator works? Right? To know the answer to it, first, you should know the concept of heat flow during cooling as well as heating.
The process of cooling fruits and vegetables in a refrigerator involves transferring the heat from the food items to the surrounding environment.
Cooling the food items in the refrigerator below room temperature is made possible by applying the second law of thermodynamics.
Which states that the transfer of heat from a lower temperature to a higher temperature through a “system” is possible only when work is added to the system(e.g. refrigerator system).
Refrigerators utilize the vapour compression cycle, using several components such as a compressor, condenser, evaporator, expansion valve, and refrigerant to cool food items below room temperature according to the second law of thermodynamics.
To gain a deeper understanding of how does the refrigerator works by applying the second law of Thermodynamics and using the vapour compression cycle read further!
Concept of heat flow
In order to understand the second law of thermodynamics, it is important to have a grasp of the concept of heat flow. To illustrate this, let’s consider two different scenarios.
Figure 1 explaining direction of heat flow
Firstly, in Case 1, if the temperature of an object is 50 degrees Celsius while the room temperature is 28 degrees Celsius. In this condition placing the object in the room will cause it to cool down gradually until its temperature reaches the same as that of the room i.e. 28 degree celsius.
It is important to note that during this process, heat is transferred from the higher-temperature object to the lower-temperature environment.
On the other hand, in Case 2, if the temperature of the same object is now 10 degrees Celsius while the room temperature is 28 degrees Celsius. Now placing the object in the room will cause it to warm up instead of cooling down.
. This will continue until the temperature of the object becomes the same as that of the room, i.e., 28 degrees Celsius. In this case, heat is transferred from the higher-temperature environment (the room) to the lower-temperature object.
Conclusion
By analyzing the above-mentioned cases, we can come to a few conclusions.
Firstly, heat flows from a higher temperature to a lower temperature body or environment.
Secondly, when the temperature of the object and the room becomes equal, the transfer of heat between them ceases (Case 3). This indicates that a temperature difference is necessary for heat to be transferred naturally between two bodies.
Lastly, it is important to note that heat does not flow from a low-temperature to a high-temperature body or environment on its own (Case 4).
Figure 2 explains conditions where heat transfer is not possible
How does the Refrigerator works?
From the conclusion it is clear that food items which are normally at room temperature will not get cooled to below room temperature naturally!
If the food items are warm or above room temperature, then they can cool up to room temperature naturally but after this, the food items will not get cooled further!
It is not possible to cool an object below room temperature (as demonstrated in Case 4). because heat naturally flows from an object or environment with a higher temperature to one with a lower temperature.
If So, then how do the food items inside a refrigerator get cooled below room temperature? is it interesting right?
The answer lies in the second law of thermodynamics, which states that if work is added to a “system,” heat can be made to move from a cold body to a hot body in the system.
In other words, the cooling of food items inside a refrigerator is possible due to the application of the second law of thermodynamics in the working of the refrigerator.
How does the refrigerator works according to the 2nd law of thermodynamics?
Fig 3 Schematic diagram explaining the 2nd law of thermodynamics
In a refrigerator, the “system” refers to the “refrigerator” itself. The “work” is added through an electric motor that powers the compressor of the refrigerator.
This allows the refrigerator to transfer heat from the food items inside to the surrounding room environment. Ultimately resulting in the cooling of the food items below room temperature.
Parts of a household Refrigerator
The main parts of a refrigerator are 1. compressor 2. condenser 3. evaporator and 4. expansion valve
Refrigerant is a fluid through which the heat is transferred from the stuff inside the fridge to an environment using a compressor, condenser, evaporator and expansion valve
The compressor and condenser are placed behind the refrigerator, whereas the evaporator is located inside the freezer and the capillary tube or expansion valve is kept in between the evaporator and the condenser line.
How does the refrigerator works using the Vapour compression cycle?
In the household refrigeration system, usually vapour compression cycle is utilized to achieve the cooling.
The Vapour compression cycle consists of four processes viz 1. Evaporation, 2. compression, 3. condensation and 4. expansion Process.
Let’s briefly discuss each process to know how exactly it is practically possible to transfer heat from food items to an environment in order to cool the food items below the room temperature.
Evaporation
Fig 5 Schematic diagram of an evaporator
During this process, the cold liquid refrigerant which is flowing inside the evaporator tube of the refrigerator will absorb the heat of the stuff inside the refrigerator and thereby refrigerant gets vaporized.
Compression
Fig 6 Schematic diagram of a Compressor
In this process, the compressor sucks the vapour refrigerant from the evaporator and compresses it to high pressure and temperature (above room temperature), thus it becomes a superheated vapour refrigerant and is delivered to the condenser!
The compressor is the reason for the circulation of refrigerant in the refrigeration system, just like our heart which circulates blood throughout the body
Condensation
Fig 7 Schematic diagram of a condenser
Here, Superheated Vapour refrigerant from the compressor gives off heat (which is absorbed from the stuff inside the refrigerator) to an environment and the refrigerant starts to condensate (transforming into liquid) in the condenser tube.
Expansion
Fig 8 Schematic diagram of an Expansion valve
Here saturated liquid refrigerant with high pressure and temperature from the condenser is expanded by using the expansion valve and due to this refrigerant gets cooled up and becomes low-pressure cold liquid refrigerant. Here cycle completes
Now again this cold liquid refrigerant is ready to take up the heat from the stuff inside the refrigerator further and the cycle repeats!
Fig 9 Schematic diagram of a Vapour Compression Cycle
How does the refrigerator works In general?
In the refrigerator, Real magic happens at the compressor and in the expansion valve to cool the stuff inside the refrigerator according to the second law of thermodynamics.
In the expansion valve, refrigerant is cooled to a temperature below the temperature of the stuff which is kept inside the refrigerator.
This will make it possible for refrigerant to absorb the heat from the stuff inside the refrigerator using an evaporator. Since heat naturally flows from high temperature to low temperature body.
Whereas a compressor increases the temperature of the refrigerant above the room temperature to help in discharging the heat(which is absorbed by the refrigerant from the food items inside the refrigerator) to the environment in a condenser!
To know the operation of the refrigerator in the Kannada language then click/tap here
Meaning and definition of some of the terms used in this article
Energy
It is the capacity to do work. Energy can’t be seen and doesn’t have substance. The presence of energy is indicated through the properties of matter such as its pressure, temperature etc.
Heat
It is energy in transition, which means it is one of the modes of energy transfer
Work
This is also an energy in transition, which means it is one of the modes of energy transfer.
Temperature
It is the measure of the hotness or coldness of a body
System
The system is any prescribed and identifiable collection of matter whose behaviour is being investigated.
References
- Thermal Science & Engineering by M.L. Mathur, F.S. Mehta
- Refrigeration and Air Conditioning by C.P Arora
- work | Definition, Formula, & Units | Britannica
- Thermodynamic system – Wikipedia
- The Physics Classroom Tutorial