Air Conditioning is perhaps one of the most wonderful scientific inventions that provided relief to the people living in a hot climate. What started as a device for cooling homes was later extended to vehicles that have a closed structure. For a layman, it appears that an air conditioner cools the air. However, scientifically speaking, the air conditioner provides cooling by transferring heat from the air inside the thermally insulated place to the air present outdoor by the medium of “refrigerant”. The phenomenon of heat transfer involves three physical processes: evaporation, compression, and condensation (“How Does an Air Conditioner Work?”). Firstly, heat from the air inside the room is absorbed by the refrigerant and turns into a vapor by a phenomenon called evaporation. Evaporation is defined as a phenomenon by which a liquid absorbs the heat from the surrounding, changing its state from liquid to gaseous at a temperature lower than its vaporization point. Since the heat is taken out of the surroundings, it produces a cooling effect. Subsequently, cold air is blown into the room for cooling. Now, the heat absorbed by the refrigerant needs to be released into the outdoor surroundings. For this purpose, the temperature of the hot refrigerant vapor has to be higher than the outside temperature, in accordance with the second law of thermodynamics. This necessitates the compression of the refrigerant vapor, which in turn, raises its pressure and temperature. Compression is a physical process by which a gas is squeezed into a small space of a lower volume. This process is accompanied by the increase in pressure and temperature of the gas. Once the refrigerant’s heat is rejected outside, it is allowed to condense, thereby turning into liquid again, which is used to complete the same cycle again. Condensation is a process, in which, a gas is cooled down by rejecting heat to surroundings. It is the reverse of evaporation as it produces a heating effect. The whole process continues until the temperature of the room reaches a pre-set level, after which, the thermostat fitted in the air conditioner switches off the cooling.
As mentioned above the cooling by airconditioning takesn place in four different steps.
Step 1: Transfer of heat from indoor air to the cold coil containing refrigerant.
This step involves the intake of warm air from the room. The air conditioner is fitted with a blower that pulls the warm air from the room into it. Subsequently, this warm air comes into contact with the cold coil filled with cool refrigerant. Now, as per the second law of thermodynamics, heat flows from the warm air to the cold refrigerant, making air cold and refrigerant hot (“How An Air Conditioner Works”). The refrigerant, on the other hand, receives so much heat that it starts changing into vapor following the phenomenon of evaporation. Thus, this step facilitates the transfer of heat from the air in the room to the refrigerant in the indoor unit of the air conditioner.
Step 2: Compression of refrigerant
In this step, the refrigerant in the vapor form moves out through the coil so that it can shed the heat to the outside surroundings. However, the problem is that the refrigerant is still not heated enough to transfer its heat to the outside air because of its temperature being less than that of outdoor air. This is why it needs to be heated further to raise its temperature more than that of the temperature prevailing outside. For this purpose, the combination of gas laws is used, as per which, the ratio of the product of pressure and volume and temperature is equal to constant (Helmenstine), Mathematically, the combined gas law is given by,
Where, P= Pressure, V= Volume, T= Absolute temperature.
Now, as per the above equation, if the volume of a gas is reduced, the other two factors (Pressure and Temperature) will increase so that the value of the left-hand side of the above equation remains constant. This is what is done in the second stage. The volume of the vapor refrigerant is compressed by injecting it into the compressor. This raises the temperature of the refrigerant, enabling it to go above the temperature of the outside atmosphere.
Step 3: Transfer of heat from the regrigerant to outside
Once the temperature of the refrigerant goes above the temperature of the outdoor air, then again by the second law of thermodynamics, heat from the refrigerant is transferred to the outside air. Thus, the heat obtained by refrigerant from the inside air is finally shed by rejecting it to the outside atmosphere. This is the reason when someone passes through the outdoor unit of the air conditioner, he/she can feel the heat is being pushed out of the unit.
Yet, it is still high, due to which, it is subjected to condensation in the indoor unit where the refrigerant goes back from a gaseous state to a cold liquid state. Once the refrigerant loses its heat, the temperature automatically comes down. Once condensed and cooled, the refrigerant is ready to cool the warm air by following the same cycle over and over again. For this to happen, it is allowed to expand, thus increasing its volume and further reducing its temperature.
The basic working principle behind the air conditioner is that the refrigerant absorbs heat from the warm indoor air, carries it to the outdoor unit, and rejects it into the outside surrounding. Primarily, the heat transfer happens due to the second law of thermodynamics that states that the heat flows from the hotter body to the colder body. The whole cycle is performed in four steps: transfer of heat from indoor air to refrigerant, compression of refrigerant vapor and increase in its temperature, transfer of heat from the refrigerant to the outside environment, and condensation of refrigerant to regain its cold liquid state. Thus, during the whole cycle, three physical processes take place: evaporation, compression, and condensation- in the same sequence. While evaporation produces the cooling effect to the indoor air, compression raises the temperature of the refrigerant, and condensation is responsible for removing heat from it and changing it back to liquid. In the whole cycle, refrigerant acts as a carrier of heat from the indoor unit to the outdoor one. In other words, contrary to the common belief, the air conditioner takes away heat from indoor air and releases it outside. Once the temperature of indoor air achieves stability, the thermostat present inside the air conditioner suspends the whole mechanism for some time until the indoor air rises again.
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Helmenstine, Anne M, “The Combined Gas Law”, Thoughtco.com, ThoughtCo, 6 May 2019, Web, 5 Dec 2020, https://www.thoughtco.com/definition-of-combined-gas-law-604936#:~:text=The%20combined%20gas%20law%20combines,is%20equal%20to%20a%20constant.&text=The%20constant%20k%20is%20a,the%20gas%20doesn't%20change.
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