What is Refrigeration?
Refrigeration and cooling developed from the human requirement for food and comfort and have a centuries-old history. Refrigeration is characterised as the most common way of accomplishing and keeping a temperature beneath surroundings, fully intent on cooling an item or space to the necessary temperature. It is likewise characterised as artificial cooling. To define refrigeration, we can say it removes heat and cools the object to a low temperature.
The first electrical refrigerator was invented in 1894 in Budapest, Hungary, by Ganz works, but these refrigerators were unsafe for domestic use because of toxic refrigerants. Furthermore, this machine was modified, and in 1913 the first electric refrigerator for domestic use was invented by Fred W. Wolf.
Refrigeration Principles
The refrigerator or fridge is the machine utilised to separate heat from a low-temperature body and afterwards dismisses this heat to a high-temperature body. The main reason for the machine is to cool some items; the machine is named a refrigerator. As per the second law of thermodynamics, heat doesn't stream from a low-temperature body to a high-temperature body without the guidance of external work. Thus, external work is expected to drive a refrigerator.
A heat engine can be worked in a reversible cycle with the guidance of an external source. In this cycle, heat is drawn from the cold body and dismissed to a hot body. Subsequently, the engine is known as a heat pump. In the refrigerator, too, heat is drawn from the cold body and dismissed to the hot body. Consequently, the refrigerator operates on the reversed heat engine cycle.
Refrigeration Cycle
While there are many strategies for warming and cooling, the fundamental capability is unchanged and utilised in a few structures across endless industries and processes. Yet, how can it work? In easy terms, a refrigeration cycle's main goal is heat retention and rejection. The refrigeration cycle, sometimes called a heat pump cycle, is a method for steering heat away from the area you need to cool. This is achieved by controlling the tension of the functioning refrigerant (air, water, refrigerants, and so on) through a pattern of compression and expansion.
There are four key components of a fundamental cycle is as per the following:
The compressor
The condenser
The expansion device
The evaporator
Various key components of refrigeration cycles
The Compressor
Compression is the most important phase in the refrigeration cycle. A compressor is a piece of hardware that expands the pressure of the functioning gas. Refrigerant enters the compressor as a low-pressure, low-temperature gas and leaves the compressor as a high-pressure, high-temperature gas.
The Condenser
The condenser, or a condenser curl, is one of two heat exchangers utilised in an essential refrigeration circle. This part is provided with high-temperature, high-pressure, vaporised refrigerant falling off the compressor. The condenser eliminates heat from the hot refrigerant gas vapour until it condenses into an immersed fluid state, known as condensation.
The Expansion Device
These parts arrive in maybe one or two plans. Popular configurations incorporate fixed orifices, thermostatic expansion valves (TXV), and the further developed electronic expansion valves (EEVs). The occupation of a framework's extension gadget is similar to making a drop-in pressure after the refrigerant leaves the condenser. This pressure drop will make a portion of that refrigerant quickly boil, making a two-phase mixture. This quick stage change is called flashing.
The Evaporator
The evaporator is the second heat exchanger in a standard refrigeration circuit, and like the condenser, it's named for its fundamental capability. It gives the final result of a refrigeration cycle, considering that it does what we expect a cooling machine to do - absorb heat.
Types of Refrigeration
The following types of refrigeration are explained below:
Mechanical compression refrigeration
Evaporative cooling
Absorption refrigeration
Thermoelectric refrigeration
Vapour compression refrigeration
Vapour absorption refrigeration
Gas Refrigeration
Similarly, as the vapours are utilised for cooling in the vapour compression cycle and vapour retention cycle, the gas is utilised for cooling in the gas refrigeration cycle. At the point when the gas is choked from exceptionally high pressure to low pressure in the choking valve, its temperature decreases abruptly while its enthalpy stays steady. This standard is utilised in gas refrigeration systems. The gas used in refrigerators is Freon gas.
Working of the Gas Refrigeration Cycle
When gas moves through the compressor, its pressure and temperature become extremely high. It then, at that point, streams into the heat exchanger, which carries out the role of the condenser in the vapour pressure cycle; then again, there is no change in the phase of air or gas. In the heat exchanger, the air surrenders heat; however, its pressure remains consistent.
The high-pressure and medium-temperature air enters the choking valve (an expander), where its pressure is decreased abruptly. Because of this, its temperature additionally turns out to be exceptionally low. The low-temperature and low-pressure gas then enter the other heat exchanger (likewise called the refrigerator), which carries out the role of the evaporator in the vapour pressure cycle.
The gas absorbs the heat from the substance to be cooled and becomes hotter, while the substance becomes cooler. There is no change in the gas phase in this heat exchanger. The high-pressure and high-temperature gas then enters the compressor, where the cycle repeats.
The TS - the diagram below represents the refrigeration cycle
TS diagram
Summary
Refrigeration is the process of removing heat from a body and making it cold. We use this method in air conditioning and heat pumps. This is an essential thermodynamic feature. There are different types of refrigeration which we use in scientific labs, industrial and domestic purposes.
FAQs on Refrigeration: A Detailed Summary
1. What is the fundamental principle of refrigeration?
The fundamental principle of refrigeration is to move heat from a low-temperature space (a cold reservoir) to a higher-temperature surrounding (a hot reservoir). Since heat naturally flows from hot to cold, this process is non-spontaneous and requires external work to be done on the system. This entire process is governed by the Second Law of Thermodynamics.
2. How does a typical household refrigerator work?
A household refrigerator operates on the vapour compression refrigeration cycle, which involves four main components:
- Compressor: It increases the pressure and temperature of the refrigerant vapour.
- Condenser: Located at the back of the fridge, it allows the hot, high-pressure refrigerant to release heat into the room and condense into a liquid.
- Expansion Valve: The high-pressure liquid refrigerant passes through this valve, causing a sudden drop in its pressure and temperature.
- Evaporator: Inside the freezer compartment, the cold, low-pressure liquid refrigerant absorbs heat from the interior, causing it to evaporate back into a gas. This absorption of heat is what makes the inside of the refrigerator cold.
This cycle repeats continuously to maintain the desired low temperature.
3. What is the main difference between a refrigerator and a heat pump?
While both a refrigerator and a heat pump operate on the same thermodynamic cycle, their primary purposes are different. A refrigerator is designed to extract heat from a cold space to keep it cool (the desired effect is cooling). In contrast, a heat pump is designed to deliver heat to a warm space to keep it heated (the desired effect is heating). Essentially, a refrigerator cools its interior, while a heat pump warms its surroundings.
4. What does the Coefficient of Performance (COP) of a refrigerator represent?
The Coefficient of Performance (COP) is a measure of a refrigerator's efficiency. It is defined as the ratio of the heat extracted from the cold reservoir (the cooling effect) to the amount of external work input required to achieve that cooling. A higher COP indicates a more efficient refrigerator, as it removes more heat for every unit of work done by the compressor.
5. What are some important applications of refrigeration beyond keeping food fresh?
Refrigeration has many critical applications beyond domestic use. These include:
- Medical and Pharmaceutical: Storing vaccines, blood plasma, and certain medicines at controlled low temperatures.
- Industrial Processes: Cooling processes in chemical plants, oil refineries, and manufacturing facilities.
- Air Conditioning: Cooling air for residential, commercial, and industrial buildings.
- Cryogenics: The process of achieving extremely low temperatures to liquefy gases like nitrogen and oxygen for scientific and industrial use.
6. Why does leaving a refrigerator door open in a closed room actually make the room warmer?
This is a classic misconception. A refrigerator works by pumping heat from its interior to the exterior through its condenser coils at the back. When the door is open, it continuously works to cool the air entering it, while simultaneously releasing a larger amount of heat into the room. This released heat includes both the heat extracted from the air and the waste heat generated by the compressor motor. According to the First Law of Thermodynamics, the net result is an increase in the total energy (and thus temperature) of the closed room.
7. How is the Clausius statement of the Second Law of Thermodynamics directly related to refrigeration?
The Clausius statement says, "It is impossible for a self-acting machine to transfer heat from a colder body to a hotter body without the aid of an external agency." This is the core principle that makes refrigeration necessary. Heat will not spontaneously flow from the cold interior of a fridge to the warmer room. A refrigerator is the 'external agency' that uses work (supplied by electricity) to force this heat transfer to happen, thereby proving the Clausius statement.
8. Why are older refrigerants like Freon (CFCs) being phased out for modern alternatives?
Older refrigerants, particularly chlorofluorocarbons (CFCs) like Freon, were found to be extremely damaging to the Earth's ozone layer. When these gases leak into the atmosphere, they break down the ozone molecules that protect us from harmful ultraviolet (UV) radiation. Due to international agreements like the Montreal Protocol, CFCs have been phased out and replaced by less harmful substances like hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs).
