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Commercial refrigeration
Refrigeration and Air conditioning

Refrigeration is a process of removing heat from a low-temperature reservoir and transferring it to a high-temperature reservoir. The work of heat transfer is traditionally driven by mechanical means, but can also be driven by heat, magnetism, electricity, laser or other means. Refrigeration has many applications, including, but not limited to: household refrigerators, industrial freezers, cryogenics and air conditioning. An important reason for having a house hold refrigerator is preservation of food. Refrigeration has had a large impact on industry, lifestyle, agriculture, and settlement patterns.

A refrigeration cycle describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a refrigerator. Heat is removed from a low–temperature space or source and rejected to a high–temperature heat sink with the help of external work. It is also applied to heating, ventilation and air conditioning. Insulation is used to reduce the work and energy needed to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration cycle was described mathematically by Carnot in 1824 as a heat engine.

Magnetic Refrigeration works with principle MAGNETO CALORIC EFFECT which is caused due to varying magnetic entropy of material when it is subjected to varying magnetic field.
Types of refrigeration

The most common types of refrigeration systems use the vapour compression refrigeration cycle. In this cycle, a circulating refrigerant such as Freon enters the compressor as a vapour. The vapour is compressed at constant entropy and exits the compressor as a vapour at a higher temperature. The vapour then travels through the condenser which cools the vapour until it starts condensing and then condenses the vapour into a liquid by removing additional heat at constant pressure and temperature. The liquid refrigerant goes through the expansion valve where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid. That results in a mixture of liquid and vapour at a lower temperature and pressure. The cold liquid–vapour mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapour returns to the compressor inlet to complete the thermodynamic cycle.

Thermoelectric refrigeration which uses the Peltier effect to create a heat flux between the junction of two types of material. This effect is commonly used in camping and portable coolers and for cooling electronic components and small instruments. Magnetic refrigeration, or adiabatic demagnetization, is a cooling technology based on the magneto caloric effect , an intrinsic property of magnetic solids. The refrigerant is often a paramagnetic salt, such as cerium magnesium nitrate. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms. Applications however are limited to cryogenics.

Other methods of refrigeration include the air cycle machine used in aircraft; the vortex tube used for spot cooling, when compressed air is available; thermo acoustic refrigeration using sound waves in a pressurized gas to drive heat transfer and heat exchange and thermo elastic cooling using a smart metal alloy stretching and relaxing.

Parts of a refrigerator

A refrigerator consists of a few key components that play a vital role in the refrigeration process:

Expansion valve

Also referred to as the flow control device, an expansion valve controls the flow of the liquid refrigerant into the evaporator. It is actually a very small device that is sensitive to temperature changes of the refrigerant. The refrigerant in a liquid state, passes through the expansion valve and turns into a cool gas due to the sudden drop in pressure. It enables flow of refrigerant into the evaporator, or cooling coil, as a low-pressure, low-temperature liquid and gas mixture of refrigerant. The expanding refrigerant evaporates further as it goes through the evaporator, where it removes the heat from the substance or space in which the evaporator is located.

Evaporator

This is the part that actually cools the stuff kept inside a refrigerator. It consists of finned tubes which are made of metals with high thermal conductivity to maximize heat transfer, absorb heat blown through a coil by a fan. As the cool refrigerant liquid and gas mixture flows through the heat exchange coil/tubes inside the refrigerator cabinet, it absorbs the heat from the food items inside the fridge.

Compressor

The compressor consists of a motor that ‘sucks in’ the refrigerant from the evaporator and compresses it in a cylinder to make a hot, high-pressure gas.

Condenser

The condenser consists of a coiled set of tubes with external fins and is located at the rear of the refrigerator. It helps in the liquefaction of the gaseous refrigerant by absorbing its heat and subsequently expelling it to the surroundings.

Condenser coils

As the heat of the refrigerant is removed, its temperature drops to condensation temperature, and it changes its state from vapour to liquid.

Refrigerant

Refrigerants are chemical compounds that are alternately compressed and condensed into a liquid and then permitted to expand into a vapour or gas as they are pumped through the mechanical refrigeration system to cycle. Ideal Refrigerant must have low specific heat and high latent heat. Refrigerants evaporate at much lower temperatures than water, which permits them to extract heat at a more rapid rate.

Most refrigerants used are substances obtained by chemical synthesis and consequently have ideal properties for the specific purpose. Many of these, however, have negative effects on the environment, in particular they tend to destroy the ozone layer (i.e. have a high ODP, Ozone Depletion Potential) or directly contribute to the greenhouse effect (i.e. have a high GWP, Global Warming Potential). Types of refrigerants include CFC (chlorofluorocarbon, high ODP),HCFC (hydrochlorofluorocarbon, low ODP, high GWP), HFC (hydrofluorocarbon, zero ODP, high GWP), HFO (hydrofluoroolefin, zero ODP, low GWP), HC (hydrocarbons, zero ODP, negligible GWP) and natural refrigerants Ammonia & CO2 have zero ODP, zero or negligible GWP. Environment friendly coolants such as R–290, R–600A etc are being used.

Air conditioning

An air conditioning system consists of compressor, condenser, metering device and evaporator in addition to refrigerant. The compressor pumps the refrigerant through the air conditioning system at a designed flow rate and pressure after compressing the vapour as it is being pumped through it. The pressure and temperature of that vapour increases. The high temperature refrigerant passes into a condenser coil. As the vapour refrigerant travels through the coil, air from a fan passes over the coil to cool the vapour refrigerant. As the vapour cools, it condenses and becomes a liquid.

The metering device controls the flow of the liquid refrigerant to the next component which is the evaporator. This is a dividing point between the high pressure and low pressure sides of the system. As this high pressure liquid is passing through the metering device and into the evaporator the pressure drops. After leaving the metering device the refrigerant immediately enters a coil called the evaporator. This coil or evaporator has a fan blowing across it. As the refrigerant enters the coil at a lower pressure it begins to bubble and boil and “change state” back to a vapour.

During this process of changing state, energy in the form of heat is being removed from the air passing over the coil and is being absorbed by the refrigerant. The heat that was in the air is transferred into the refrigerant. Since heat was removed from the air blowing over the evaporator coil, the air leaving the evaporator coil is cold. The process of the refrigerant “changing states” from vapour to liquid (releasing heat through the condenser) and from liquid to vapour (absorbing heat in the evaporator) is how an air conditioner works.