Carnot Vapour Compression Refrigeration cycle

Have you ever thought how does cooling or refrigeration in your fridge happen?

Ever wonder how does your home air-conditioner gives you cool air?

How does your freezer work?

All these questions I will try to explain it in a way that is easier, relevant, practical way. In all cases, I tried not to use complex terminologies but have choosen layman's terms in an ordinary plain simple language so that people without engineering background can understand. It is my hope that this article might impart some useful knowledge as well people with higher or better understanding of the principles and processes here are encouraged to comment in order to improve or correct deficiencies.

Residential, domestic, building, commercial, industrial air conditioners, refrigerators, and freezers of the past few decades have used ozone layer-depleting compounds such as R-12 and R-22. CFC (chlorofluorocarbon) refrigerants such as R-12 and HCFC (hydrochlorofluorocarbon) such as R-22 contains chlorine which is responsible for the depletion of the ozone layer. International treaty and regulations have these harmful chemicals to be banned and being totally phased out soon. New eco-friendly refrigerants such as R-410A and R-407C have less or minimal impacts to the environment and are now being used in modern air conditioners, refrigerators, and freezers.

In order to provide air-conditioning, refrigeration, and freezing, these equipment uses a vapor-compression refrigeration and air-conditioning system . A cooling or freezing substance called a refrigerant is circulated within the system and is used to absorb heat from the space to be cooled or freezed. The Carnot Vapour Compression Refrigeration cycle is the thermodynamic cycle that is widely used by commercial manufacturers and producers of refrigerators, air-conditioners, freezers, and other equipment for the same cooling and freezing applications. There are four major components involved in the Carnot vapor compression refrigeration and air conditioning cycle in which a refrigerant is used as a cooling medium that flows through these components.

Four main components of a Vapor-compression Refrigeration System:

1. Evaporator
2. Compressor
3. Condenser
4. Thermal Expansion Valve

Evaporator
The low pressure, low temperature, liquid state refrigerant enters the Evaporator. The Evaporator through the refrigerant is used to absorb heat. In the Evaporator is where the cooling of the items that you put inside your fridge take place. Heat exchange will take place between the refrigerant and the heat sources. The food items will then be cooled down while the previously cold refrigerant will be heated up and be turned to vapor. The boiling point (saturation temperature) of the refrigerant must be lower than the temperature of the heat source (such as food, drinks, beverage, vegetables, fruits that you put inside your fridge) so that by the 2nd Law of Thermodynamics, "Heat flows from a hot body to a cold body", the heat will flow from the heat source to the refrigerant. Upon exiting the Evaporator and absorbing the heat, the previously liquid refrigerant is now turned into a low pressure, high temperature, gas/vapor state refrigerant.

Compressor
The low pressure, high temperature, gas/vapor state refrigerant enters the Compressor. In order for the refrigerant to reject the heat (in order for the refrigerant to get cooled down) in the condenser, the boiling point or saturation temperature of the refrigerant must be higher than the condenser (heat sink) temperature. The purpose of the Compressor is to raise the saturation temperature of the refrigerant as well as to increase the refrigerant's pressure. Upon exiting the Compressor, the previously low pressure, high temperature, gas/vapor state refrigerant is now turned into a gas or vapor state refrigerant with high pressure, and higher temperature than when it entered the Compressor.

Condenser
The high pressure, high temperature, gas state refrigerant enters the Condenser or Heat Sink. The purpose of the Condenser is to cool down and condense the vapor state refrigerant and turn it into a liquid state refrigerant. Typical condensers used in refrigeration and air-conditioning are Air-cooled Condensers which are basically made up of multiple thin coils that are used to liquefy the refrigerant and discharge the heat to the surrounding air or atmosphere in the environment. When the refrigerant exits the Condenser, it is now high pressure, high temperature, and in liquid form.

Thermal Expansion Valve
The high pressure, high temperature, liquid state refrigerant enters the Thermal Expansion Valve (TEV). The purpose of the TEV is to lower down the pressure and bring back the refrigerant to its original saturation temperature so that the cycle is completed and prepares the refrigerant to start another cycle. Another important function of the TEV is to regulate the amount of flow of liquid refrigerant to the evaporator. This is determined by how much food and drink items were place in your refrigerator and how cold you want them to be. The thermostat, (the usually circular button that has numbers indicating Cold, Colder, Coldest) that you turn inside your fridge is where you control to the desired settings. On the exit to the Thermal Expansion Valve, the refrigerant is now turned into low pressure, low temperature, liquid refrigerant that is now again ready to absorb heat from whatever food, drink, milk, fruits, veggies, cheese, soda, and other heat sources that you place inside your fridge or refrigerator. Thus completes the Carnot Vapour-Compression Refrigeration cycle.

Carnot Cycle

According to the Second Law of Thermodynamics, the maximum thermal efficiency attainable is the Carnot cycle efficiency. Also, according to the 2nd Law of Thermodynamics, the most efficient heat engine that operates in a thermodynamic cycle is the Carnot Heat Engine operating in a Carnot cycle.
The Carnot cycle has four thermodynamic processes, two of them are constant temperature (isothermal) processes and the other two are adiabatic processes (an adiabatic process is a thermodynamic process is which the system does not gain nor loss any heat). Carnot cycle is a reversible cycle which means that all the four processes are reversible. A reversible adiabatic process is an isentropic process. An isentropic process is a constant entropy process. Entropy is a measure of the amount of energy which can not be used or converted to perform work (e.g. move a piston, etc.). Entropy is also a measure of the degree of the disorder of a system. The second law of thermodynamics states that in all the natural processes, the entropy (amount of disorder) will always increase. In the molecular level, when all the molecules are more arranged and stay in one place, the entropy is lesser or at minimum. However, if the molecules are more scattered or far away from one another, they are more disorderly, and therefore have greater entropy.

Four Processes of the Carnot cycle:

1. Isothermal Heat Addition
2. Isentropic Compression
3. Isothermal Heat Rejection
4. Isentropic Expansion

Comparison of the IDEAL (theoretical) Carnot cycle vs. the ACTUAL Vapor Compression Refrigeration system:

1. Heat Addition - heat is added (absorbed) by the refrigerant at the Evaporator

2. Compression - the refrigerant in gas form (saturated vapor) is compressed by the Compressor

3. Heat Rejection - the refrigerant is condensed or liquefied at the Condenser

4. Expansion - the refrigerant expands or becomes less pressurized at the Thermal Expansion Valve (TEV)

As an example, Cars and other automotive applications have an ideal Carnot efficiency of 70%, and an actual thermal efficiency of 25-35% (rough estimates). The efficiency of Refrigerators, Air conditioners, and Freezers are measured by COP or Coefficient of Performance. The COP (Coefficient of Performance) is the ratio of the Heat Removed to the Input Electric Energy. COP is greater than 1. Typical values of COP is between 3 to 8. The Energy Efficiency Ratio (EER) is the ratio of the Cooling Power of the Equipment and the Electric Energy Input. Typical values of EER is between 5 to 10.