How Cooling Is Produced By an Air-Conditioner Machine?
Filling of Gas in an Air-Conditioner
A Life without an Air Conditioner
During a hot day of summer, it is difficult to remain comfortable without an air conditioner. This is because our body temperature is much lower (about 370 C) than the temperature of surrounding in summer which may be ranging between 380 C to 450 C.
As per the first law of thermodynamics, spontaneous flow of heat takes place from higher level to lower level. In this case, as the temperature of surrounding is more it is at higher level while our body is at lower level. Hence as per natural phenomenon of heat exchange, heat from surrounding flows into our body. This results in increase in temperature of our body.
But gaining temperature higher than 370 C is harmful to our body. This is because most of the physiological processes taking place in our body need a temperature of 370 C. Do you remember we take special treatment while suffering from fever?
How to protect our body from this problem?
Fortunately, nature has installed a great device which automatically brings down body temperature to normal.
This device is sweating. In order to bring the temperature down, body exudes moisture on our skin which is called sweat or perspiration.
As evaporation of sweat takes place naturally, cooling is produced which helps to bring the temperature of body down.
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Necessity of Air-Conditioner
However, under following two situations the natural cooling produced by sweating is not adequate and helpful.
(a) When the temperature of surrounding is much higher than 370 C.
If the temperature of surrounding is only little higher than temperature of our body, the natural phenomenon of evaporation of sweat is adequate to maintain the body temperature to normal. However, if the temperature of surrounding is much higher than temperature of our body, the rate of heat gained by body from surrounding is higher than that of heat lost by body through sweating.
(b) When the humidity level of surrounding is much higher. Under such a condition the rate of evaporation is very slow to maintain the body temperature.
During above two situations, natural phenomenon of evaporation of sweat can not produce enough cooling which is required.
Here we need some special device to maintain our body temperature to normal.
The name of this device is, “Refrigeration Cooling”.
Fundamental Chemical Principle of Refrigeration Cooling
Refrigeration cooling works on the principle,
“Formation of a chemical bond is an exothermic process (means heat releasing process), while breaking of a chemical bond is an endothermic process (means heat absorbing process)”.
It is interesting to note here that there are some chemical processes in which both formation and breaking of chemical bonds occur alternately and continuously. Such processes are called, "Cyclic processes". These processes can produce continuous heating as well as equal amount of cooling simultaneously.
If heat produced during such process is disposed and cooling produced is utilized, then it is a device of our interest.
This is called, "chemistry of cooling"!
Example of exothermic and endothermic chemical reactions
Importance of Refrigerant Gas in produce of cooling
It is the refrigerant gas filled inside the air conditioner, molecules of which continuously undergo the phenomenon of bond formation (while coming closer during compression) and bond breaking (while going apart during expansion).
This cyclic exothermic and endothermic processes taking place inside the air conditioner, produce equal amount of heating and cooling. The heating produced is emitted into the atmosphere while cooling produced is utilized to solve our purpose to cool the room.
The role of gas in air conditioner is like blood in our body. No blood no life; no gas no cooling!
Major Parts and Contents of a Refrigeration System
Any refrigeration system includes following parts/contents:
(a) A refrigerant gas of fixed quantity:
In old times the gas used for this purpose was ammonia. But due to its corrosive effect on various metallic parts and also due to pollution resulted from its leakage, nowadays it is not in use.
Then various C. F. C. means chlorofluorocarbons (like Freon-12) were used for this purpose. Chlorofluorocarbons were widely used till it was learned that they are responsible for depletion of “Ozone layer” in the upper atmosphere. Their use is limited due to this reason.
Nowadays more efficient and environmentally safe (also known as, "eco friendly") gases are used for this purpose.
The gas filled in refrigeration system is confined in a closed chamber which is airtight. It is due to this reason that quantity of gas remains constant. The gas is continuously circulated through the machine inside the closed system. As the amount of cooling produced directly depends upon the quantity of refrigerant gas present, some threshold quantity of this gas must be present. It is due to this reason that some extra gas needs to be supplied to the system in case of its leakage.
(b) A motorized compressor along with a fan:
The role of compressor is to compress the gas inside the closed system. On compressing the gas, the molecules of gas come closer and form a chemical bond with each other. As the formation of bond is invariably and unexceptionally an exothermic reaction, heat is generated in side the compressor. It is due to this reason that compressor is always hot when unit is in operation.
(To confirm this try to touch carefully the compressor fitted in the rare lower portion of a domestic refrigerator).
In order to keep the compressor cool, a fan is fitted behind it which continuously throws the heat produced into the surrounding. Obviously this unit is installed outside the room so that hot air can not enter in the room.
(c) A cooling coil inside the room:
The compressed gas obtained thus, containing gas molecules chemically bonded with each other, is now allowed to pass into a coil within which gas will expand naturally due to absence of pressure as there is no compressor here.
On expansion of gas inside the cooling coil, the molecules of gas will go apart from each other resulting in breaking of bonds between them.
Thus the phenomenon taking place inside the cooling coil is exactly opposite to that taking place inside the compressor.
As the breaking of chemical bonds is endothermic process, heat is absorbed in this step resulting in cooling of surrounding.
Obviously this portion of air conditioner is installed inside the room, so that room becomes cool.
Functioning of An Air Conditioner
It is clear from the above discussion that if 100 number of chemical bonds are formed during the phenomenon of compression, similar number of bonds will be broken during the phenomenon of expansion.
This means if “x” joules of heat is removed from the room, same “x” joules of heat is emitted into the atmosphere.
As such, as a whole there is neither cooling nor heating is produced actually!
We feel cooling only due to design of air conditioner. The unit producing cooling is installed inside the room while the unit producing same quantity of heat is installed outside the room.
It is due to this reason that an air conditioner can be used as a heater in winter, if installed in an opposite manner!
Unit Containing Cooling Coil.
Unit Containing Condenser Coil.
Understanding the Tonnage Capacities (like: 1 ton, 1.5 ton, 2 ton etc.) of Various Air Conditioner machines
What is meant by capacity of an air conditioner machine?
The capacity of an air conditioner machine means its capacity to produce cooling within a given time interval.
In case of an air conditioner, cooling produced by it is measured in terms of heat removed by it. This is because, heat removed by air conditioner is exactly identical to the cooling produced by it. This means, removal of 100 joule of heat from the room will result in production of 100 joules of cooling!
Though common units of heat are joule and calorie, an uncommon unit of heat called, “B. T. U.” is used here.
The meaning of different units of heat is discussed below.
(a) In British system of units (which is also known as imperial system):
In this system, heat is measured in terms of B. T. U., which is abbreviation of, "British Thermal Unit".
As per this system, 1 B. T. U. heat is defined as, “amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit”.
But the problem is, “how to understand the amount of heat content in 1 B. T. U.”?
To solve this problem, the unit of B. T. U. is standardized by defining the latent heat of ice in terms of B. T. U. as follows:
“Amount of heat required to melt exactly 1 pound of ice, kept at its freezing point (means at 320 F) is taken as 143 B. T. U.”. This means, in order to melt 1 ton (approximately 2000 pounds) of ice, the amount of heat required is: 143 x 2000 = 2,86,000 B.T.U. If this much B. T. U. of heat is supplied in 24 hours, then heat supplied in 1 hour = 286000/24 = 11917 B. T. U., say approximately 12000 B. T. U. per hour.
This is called, "1 ton capacity of an air conditioner machine"!
In short, an air conditioner having capacity of 1 ton can remove 12000 B. T. U. of heat from the room within a period of 1 hour!
In the similar way capacities of 1.5 ton air conditioner and that of 2 ton air conditioner can be easily worked out.
(b) In S. I. system of unit:
In international system of units, heat is measured in terms of Joules (J).
As 1 B. T. U. = 1055 J, 12000 B. T. U. = 12260000 J = 12260 kilo Joules. This means, 1 ton air conditioner can remove 12260 kilo Joules of heat from the room within a period of 1 hour!
(c) In Calorie unit:
As 4.184 J = 1 calorie, the capacity of 1 ton air conditioner can be worked out as:
12260/4.184 = 2930 kilo Calorie per hour.
Misunderstanding Regarding Tonnage Capacity of an air conditioner machine
There are some misunderstandings prevailing among general people regarding tonnage capacities of various air conditioners.
(1) Is there any relation of tonnage capacity of air-conditioner with the size of room?
Remember, the tonnage capacity of air-conditioner is not about quantity of cooling it will offer, but it is about speed of cooling it will work through!
If an air-conditioner with capacity of "1 ton" takes 5 minutes to cool your room, another one with "1.5 ton" capacity may take 4 minutes or so!
As the price of air conditioner with higher tonnage is more, company's salesman will always insist you to purchase the costlier one (for the reason best known to everybody).
However, you can decide as per your waiting convenience!
(2) Is there any relation of tonnage capacity of air conditioner with the electricity bill?
As it is all regarding speed, there is no appreciable difference in consumption of electricity.
If an air-conditioner with capacity of 1 ton consumes less electricity, it will run for longer time to achieve desired level of cooling. And another one with 1.5 ton capacity will consume more electricity but will run for shorter time to achieve the similar level of cooling!
Which one consumes more electricity?
Of course both will consume almost same electricity, it is only the time in which they differ!
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