Please advise how a Air Conditioning Unit actually works in terms of technology and sunstances used ?

Answer 1

See below.

Answer 2

An air conditioner has two major components, the compressor and the evaporator with associated heat transfer coils on both the hot and cold side. The material flowing through most air conditioners is a Freon (in a compressor oil to keep the compressor parts lubricated) which is a trademark name for a clorofluorocarbon (CFC). Fully halogenated CFC's are bad for the ozone layer because the catalytically degrade upper atmosphere ozone. The newer class of refrigerants are HCFC's where one of the halogens has been replaced by a hydrogen atom. They tend to degrade in the atmosphere before reaching the upper levels so they are safer for ozone than traditional CFC's.

Anyway, the CFC is compressed to a high pressure by the compressor. This process heats the CFC. At this point there is usually a series of heat transfer tubes with large surface area to reduce the temperature of the CFC. The coils on the back of refrigerator are an example of this. This generated heat is put into the room. For an air conditioner there is a highly packed series of fins like in your car radiator to reject the heat. These fins are on the outside of the house and give up the heat to the air.

This compressed gas is then allowed to evaporate or expand and it cools as it does so. This creates a low temperature area. Room air is circulated over this cold coil so that the temperature of that air drops. In humid conditions, the cold air can no longer hold all of it's moisture and water will condense and drop out of the air. It is for this reason that the evaporator has to be well designed so that the water is removed safely and so that the evaporator coil doesn't freeze up.

The gas is then ready for another cycle and process continues.

By the way, this is why opening the refrigerator door won't make the room cooler. It is cold in the refrigerator, but the cycle to cool the refrigerator air isn't 100% efficient and the heat from the compressor is dumped into the room. Leaving the refrigerator door open will actually raise the temperature of the room!

Answer 3

Intro to Refrigeration.
Refrigeration is the Science of the Production of 'Coldness'.
Refrigeration is a cooling process used to remove heat energy from a substance by the evaporation principle, to produce a temperature below that of its surroundings.
Simple examples of refrigeration are: -
1. In desert locations, the Nomads throw water over their tents. As the water evaporates, the heat required for the evaporation process is removed from the interior of the tent thereby causing cooling.
2. If you dip your finger in alcohol, and then blow gently on it, your finger feels colder - again this is due to the evaporation process which removes heat from the finger.
In order to produce the cooling effect required, a substance called a 'Refrigerant' is used.
The desirable properties of a refrigerant are listed below: -
*...A Low Boiling Point - below the temperature it is expected to maintain.
*...A Low Freezing Point - below the minimum temperature the system can reach - to prevent solidification of the refrigerant.
*...A High Latent Heat - Will require more latent heat of vaporization which will remove more sensible heat from the substance or space being cooled.
*...A High Critical Temperature - Can be easily condensed. A Low Critical Pressure - Will condense at relatively low pressure, thereby needing low energy to produce the condensation pressure.
*...Non-Corrosive, Non-Toxic, Non-Flammable.
However, some refrigerants used in industry do not meet all of these conditions.
For example:
*...Ammonia (BP –28 °F) is a good refrigerant, but it is toxic and, in the presence of water, it is corrosive.
*...Propane (BP –44°F), is also a good refrigerant but, it is highly flammable.
Generally, refrigerants used in cooling systems are: -
*...Arcton and Freon - trade names for a group of refrigerants called 'ChloroFluoro-Carbons' (CFC's), are used in some industries and particularly in domestic fridges, and meet the requirements listed above.
*...Liquid CO2 - (BP = –78°C), can also be used as a refrigerant.

However, attempts to decrease the amount of 'Chlorofluorocarbons' (CFC's) are being made, in an effort to prevent the deterioration of the Ozone layer around the earth with helps to protect us from high levels of Ultra-violet light from the sun. Modern refrigerants are being developed which are not based on CFC's.

The purpose of a refrigeration system is to maintain a material or an inside environment (fridge, freezer, office, home, car …etc) at a desired temperature below that of the atmosphere.

To accomplish this, the refrigeration substance, (refrigerant), must continuously remove the heat from the material or the space being cooled. The removed heat will then be dissipated into the atmosphere OUTSIDE of the space being cooled.
When your fridge or freezer at home is running, you will note that there is quite a lot of heat coming from the back of the unit. This heat is due to the compression of the refrigerant gas PLUS the heat removed from the food in the fridge / freezer. Due to this heat, the room in which the fridge / freezer is kept, will be at a higher temperature than the rest of the building interior, (unless the building is air conditioned, which includes the room containing the fridge - usually the kitchen).

Refrigeration Principles.
Large, modern refrigeration units use the evaporation principle to produce the low temperatures necessary to do the job required. The process consists of a cycle of compression, cooling and condensation, then the expansion of the liquid, evaporation and re-compression of the vapour.

The Vapour Compression Refrigeration Cycle.
The principles used in a vapour compression refrigeration system are: -

A...Compression of a gas causes its temperature to increase. When the gas is cooled and sensible and latent heat removed, the temperature decreases and the gas condenses to liquid which is also the boiling point of the liquid. (The compression also increases the temperature at which the liquid boils). The liquid is then further cooled to around atmospheric temperature.
B...When the liquid is expanded (volume increased) into a lower pressure system, it will boil and cause the liquid temperature to decrease rapidly as it gives up sensible heat to provide the latent heat of partial vaporization of the liquid. The cold liquid and vapour, (the latent heat does not increase the vapour temperature), now pass through the coils inside the 'Cold Box' (or Evaporator) of the system.
Exchange of heat between the refrigerant and the material or space being cooled, adds more heat to the refrigerant liquid which continues to evaporate. The refrigerant, on leaving the cooling system is now all cool vapour and passes to the suction of the compressor to begin the cycle again.
The sequence of the refrigeration cycle is as follows: -
1...Compression of the refrigerant gas.
2...Cooling and condensation of the refrigerant to liquid.
3...Expansion and partial evaporation of the liquid into a lower pressure which causes cooling.
4...Continued evaporation of the liquid in the 'cold box' - further heating by removal of sensible heat from the item being cooled, to provide the latent heat of vaporization of the refrigerant.
5...Re-compression of the vapour to begin the cycle again.

The type of compressor used in refrigeration systems may be reciprocating or centrifugal in operation. Large industrial units may have multi-stage compression systems with inter-stage cooling, in order to achieve the required refrigerant pressure without excessive temperature increases.

The cooling & condensing unit of a system can consist of cooling by natural convection - as in a household fridge, or by forced air cooling - i.e. a fan or fans to force the air over the cooling coils (similar to a car radiator), or by water cooled heat exchange equipment.
Control of a refrigeration unit can be by a thermostatic system which will start and stop the compressor - (a bi-metal strip switch or mercury switch), or, in large units, by control of the expansion valve.
When the unit shuts down, the expansion valve will also close by activation of a solenoid valve.

Following are some typical uses of refrigeration in industry and everyday life.
*...To Reduce the Rate of Chemical Reactions - and Storage of Food and Other Perishable Goods
*...To Store Flammable Materials
*...To Condense the Vapour of Low Boiling Point Liquids
*...For Air Conditioning
*...For Freeze Drying of Materials
*...For Separation and Recovery of Process Fluids
*...To Produce Cryogenic Temperatures