Refrigeration is the process by which heat is removed from a low-temperature level and rejected at a relatively higher temperature level.
The Americans define refrigeration in a somewhat different way, thus, refrigeration is the process by which heat is removed from a place where it is not required and rejected into a place where it is not objectionable.
Note:- No process can be called “refrigeration” unless the removal of heat is at a temperature than the surrounding temperature.
By nature heat always flows from one body to another body which is relatively at a lower temperature. This law of nature cannot be altered by any means. Transferring heat from a low-temperature level is analogous to transferring water from a lower level to a higher level. Imagine two water tanks one located on the ground floor full of water and another empty, located at the roof level of a building. If water from the ground floor tank is to be transferred to the roof tank, then the only thing to do is to bring a bucket, place it at a level lower than the ground floor tank and allow the water to initially drain into the bucket according to the law of nature. The second step would be to lift this bucket full of water to a level above the roof tank and now allow the water from the bucket to drain into the roof tank according to the natural flow by gravity.
In the foregoing process, we have used the bucket as the carrier and moved it up and down, first to a level lower than the ground floor tank and then to a level higher than the roof tank. Needless to add that in the process some mechanical work has been performed for lifting the bucket from the lower level to the higher level.
Applying this analogy to the process of refrigeration, it is evident, that we require a substance as the carrier of heat analogous to the bucket. This substance should be first brought to a temperature level that will automatically flow into this carrier substance which has been brought to a still lower level of temperature. After this carrier substance has been fully loaded with heat it has got to be raised to a temperature that is higher than the high-level temperature so that heat from this carrier will automatically flow according to the law of nature. The carrier substance referred to above is what is known in refrigeration parlance as “refrigerant”. We shall now see what a refrigerant is really like. All volatile liquids including water have property whereby the temperature at which they evaporate changes according to the pressure it is subjected to take water for example.
For refrigeration purposes, the most commonly used refrigerants are refrigerant 12 (ban now) and refrigerant 22. But now most of the country bans this refrigerant due to environmental issues.
Now, What are the main refrigerant used?
Main refrigerant being used R410a. As for residential refrigeration, R134a refrigerant mostly used. But in Still for Commercial application (Walk-In refrigerators) prior choice R22, but for mostly commonly used refrigerant is R134a. Other Refrigerant is also used for refrigeration.
Temperature at which the liquid refrigerant would boil (or conversely the refrigerant vopor would condense) when it is subjected to a certain pressure is define as the saturation temperature corresponding to that pressure.
Saturation temperature means boiling point. The Saturation temperature is the temperature for corresponding saturation pressure at which a liquid boils into vopor phase.
It is obvious that at this temperature and pressure, refrigerant in liquid and vopor form kept in a closed container would be in equilibrium with each other.
So long as the pressure inside this container is maintained steady the liquid portion will vaporize if heat is added or the vapor portion will condense if heat is removed. On this basis, saturation temperature for any given pressure is defined as that temperature at which liquid refrigerant and its vapor remain in contact with each other in equilibrium.
In the above paragraph, liquid refrigerant and its vapor will be in equilibrium with each other in a closed container at the saturation temperature corresponding to the pressure. Any addition or removal of heat would only result in liquid vaporizing or the vapor condensing, pressure remaining same. However, if the vaporized refrigerant is separated from liquid portion , then any heat added to this refrigerant in vapor form would only go to raise the temperature of the vapor above its saturation temperature corresponding to its pressure. This is superheated vapor. Superheat is usually expressed in terms of degrees. when we say 10 degree superheat, what we mean is the gaseous refrigerant is at a temperature 10 degree above the saturation temperature corresponding to its pressure.
Definition of Super Heat:-
Superheating:- (Super Heat) is that any temperature of gas above the boiling point of that liquid. Super Heat is the phenomenon in which a liquid is heated to a temperature higher than its boiling point without boiling.
If the liquid portion of the refrigerant is separated and completely isolated from the vapor which is in equilibrium with it then any removal of heat from this refrigerant would lower its temperature to a value below its saturation temperature. Such a liquid is called sub-cooled liquid. When we say the liquid is 15 degrees below the saturation temperature corresponding to its pressure. It is obvious that in the case of sub-cooled liquid when heat is added it will first rise up in temperature till it reached its saturation temperature and thereafter only it will begin to boil as long as it is receiving heat. Likewise, in the case of superheated gas, when heat is removed from the same it will first fall down in temperature till it reaches the saturation temperature corresponding to its condensation pressure (this is generally referred to as de-superheating). Any further removal of heat after this would result in condensation of the vapor into liquid form at a constant temperature, namely, the saturation temperature. One thing that should be borne in mind is neither sub-cooling of liquid nor superheating of the vapor is possible when liquid and its vapor are in contact with each other in equilibrium because as already explained earlier, any removal or addition of heat, in this case, would only respectively result in condensing of the vapor part or evaporating of the liquid part, at a constant temperature.
Note:- Sub-cooling refers to a liquid existing at a temperature below its normal boiling point. Example water boils at 373K, temperature 300K is subcooled.
Enthalpy is the term that denotes the heat content of the refrigerant from a base saturation temperature of 40 Degree F. At this temperature and the corresponding saturation pressure the heat content of the liquid has been arbitrarily fixed at 0. It, therefore, follows that the enthalpy of the liquid above 40 degrees F will be positive and that below 20 degrees F. will be negative. The enthalpy of the refrigerant when it is in vapor form will be equal to the enthalpy of the liquid at the same pressure and saturation temperature.
h = u + pv
Note: Enthalpy relates to the total heat content of the system.
Any process which is performed without the addition of heat to or removal of heat from the process is said to be an adiabatic process. Compression of gaseous refrigerant without addition or removal of heat is called adiabatic compression. Pressure enthalpy diagram of a refrigerant has also lines showing adiabatic compression. It is, therefore, possible to find out the enthalpy and temperature of the gas at various pressure during the course of compression.
Note:- Adiabatic process at which no heat addition or rejection takes place.
Lets understand the P-H Diagram of Refrigeration Cycle