Introduction to Refrigeration :
- Refrigeration may be defined as the process of achieving and maintaining a temperature below that of the surroundings, the aim being to cool some product or space to the required temperature.
- One of the most important applications of refrigeration has been the preservation of perishable food products by storing them at low temperatures. Refrigeration systems are also used extensively for providing thermal comfort to human beings utilizing air conditioning.
- Air Conditioning refers to the treatment of air so as to simultaneously control its temperature, moisture content, cleanliness, odor, and circulation, as required by occupants, a process, or products in the space.
Types Of Refrigeration System :
1. Natural Cooling :
- Art of Ice making by Nocturnal Cooling
- Evaporative Cooling
- Cooling by Salt Solutions
2. Artificial Refrigeration
- Vapour Compression Refrigeration Systems
- Vapour Absorption Refrigeration Systems
- Solar energy based refrigeration systems
- Gas Cycle Refrigeration
- Steam Jet Refrigeration System
- Thermoelectric Refrigeration Systems
- Vortex tube systems
1. Natural Cooling :
In olden days refrigeration was achieved by natural means such as the use of ice or evaporative cooling. In earlier times, ice was either:
1. Transported from colder regions,
2. Harvested in winter and stored in ice houses for summer use or,
3. Made during the night by the cooling of water by radiation to the stratosphere.
Art of Ice making by Nocturnal Cooling:
- The art of making ice by nocturnal cooling was perfected in India. In this method ice was made by keeping a thin layer of water in a shallow earthen tray, and then exposing the tray to the night sky.
- Compacted hay of about 0.3 m thickness was used as insulation. The water loses heat by radiation to the stratosphere, which is at around -55°C and by early morning hours the water in the trays freezes to ice. This method of ice production was very popular in India.
- As the name indicates, evaporative cooling is the process of reducing the temperature of a system by evaporation of water.
- The water permeates through the pores of the earthen vessel to its outer surface where it evaporates to the surrounding, absorbing its latent heat in part from the vessel, which cools the water.
- Evaporative cooling by placing wet straw mats on the windows is also very common in India.
Cooling by Salt Solutions:
- Certain substances such as common salt, when added to water dissolve in water and absorb its heat of solution from water (endothermic process).
- This reduces the temperature of the solution (water+salt). Sodium Chloride salt (NaCl) can yield temperatures up to -20°C and Calcium Chloride (CaCl2) up to – 50°C is properly insulated containers. However, as it is this process has limited application, as the dissolved salt has to be recovered from its solution by heating.
Refrigeration as it is known these days is produced by artificial means. Though it is very difficult to make a clear demarcation between natural and artificial refrigeration, it is generally agreed that the history of artificial refrigeration began in the year 1755, when the Scottish professor William Cullen made the first refrigerating machine, which could produce a small quantity of ice in the laboratory. Based on the working principle, refrigeration systems can be classified as vapor compression systems, vapor absorption systems, gas cycle systems, etc.
Vapour Compression Refrigeration Systems:
- Vapour compression cycle is an improved type of air refrigeration cycle in which a suitable working substance, termed as refrigerant, is used.
- The refrigerants generally used for this purpose are ammonia (NH3), carbon dioxide (CO2), and sulfur-dioxide (SO2).
- The refrigerant used does not leave the system but is circulated throughout the system alternately condensing and evaporating. In evaporating, the refrigerant absorbs its latent heat from the solution which is used for circulating it around the cold chamber and in condensing; it gives out its latent heat to the circulating water of the cooler.
- The vapor compression cycle which is used in the vapor compression refrigeration system is nowadays used for all-purpose refrigeration. It is used for all industrial purposes from a small domestic refrigerator to a big air conditioning plant.
Vapour Absorption Refrigeration Systems
- The vapor absorption refrigeration is a heat operated system. It is quite similar to the vapor compression system. In both systems, there are evaporators and condensers.
- The process of evaporation and condensation of the refrigerant takes place at two different pressure levels to achieve refrigeration in both cases. The method employed to create the two pressure levels in the system for evaporation and condensation of the refrigeration makes the two processes different. Circulation of refrigerant in both the cases is also different.
- In the absorption system the compressor of the vapor compression system is replaced by the combination of „absorber‟ and „generator‟.
- A solution known as the absorbent, which has an affinity for the refrigerant used, is circulated between the absorber and the generator by a pump (solution pump).
- The absorbent in the absorber draws (or sucks) the refrigerant vapor formed in the evaporator thus maintaining a low pressure in the evaporator to enable the refrigerant to evaporate at low temperatures.
- In the generator the absorbent is heated. Thereby releasing the refrigerant vapor (absorbed in the absorber) as high-pressure vapor, to be condensed in the condenser. Thus the suction function is performed by absorbent in the absorber and the generator performs the function of the compression and discharge.
- The absorbent solution carries the refrigerant vapor from the low side (evaporator–absorber) to the high side (generator-condenser). The liquefied refrigerant flows from the condenser to the evaporator due to the pressure difference between the two vessels; thus establishing the circulation of the refrigerant through the system.
Solar energy based refrigeration systems:
Attempts have been made to run vapor absorption systems by solar energy with concentrating and flat plate solar collectors. Several small solar absorption refrigeration systems have been made around the 1950s in several countries. Professor G.O.G. L f of America is one of the pioneers in the area of solar refrigeration using flat plate collectors. A solar refrigeration system that could produce 250 kg of ice per day was installed in Tashkent, USSR in 1953. This system used a parabolic mirror of 10 m2 area for concentrating the solar radiation. F. Trombe installed an absorption machine with a cylinder-parabolic mirror of 20 m2 at Montlouis, France, which produced 100 kg of ice per day.
Gas Cycle Refrigeration :
Just as the vapors are used for cooling in the vapor compression cycle and vapor absorption cycle, the gas is used for cooling in the gas refrigeration cycle. When the gas is throttled from very high pressure to low pressure in the throttling valve, its temperature reduces suddenly while its enthalpy remains constant. This principle is used in a gas refrigeration system.
In this system instead of using Freon or ammonia as the refrigerant, the gas is used as the refrigerant. Throughout the cycle there are no phase changes of the gas, which are observed in the liquid refrigerants. Air is the most commonly used gas, also called as refrigerant in this case, in the gas refrigeration cycles.
Steam Jet Refrigeration System:
- This system uses the principle of boiling the water below 100 Degree C. If the pressure on the surface of the water is reduced below atmospheric pressure, water can be made to boil at low temperatures.
- Water boils at 6 degrees C, when the pressure on the surface is 5 cm of Hg and at 10 degrees C when the pressure is 6.5 cms of Hg. The very low pressure or high vacuum on the surface of the water can be maintained by throttling the steam through jets or nozzles.
Thermoelectric Refrigeration Systems:
- Thermoelectric cooling uses the Peltier effect to create a heat flux between the junctions of two different types of materials. This effect is commonly used in camping and portable coolers and for cooling electronic components and small instruments.
- Applying a DC voltage difference across the thermoelectric module, an electric current will pass through the module, and heat will be absorbed from one side and released at the opposite side. One module face, therefore, will be cooled while the opposite face simultaneously is heated.
Vortex tube systems:
- It is one of the non-conventional type refrigerating systems for the production of refrigeration.
- The vortex tube is a device that separates a high-pressure flow entering tangentially into two low-pressure flows, thereby producing a temperature change. The vortex tube has no moving parts and generally consists of a circular tube with nozzles and throttle valve. High-pressure gas enters the vortex tube tangentially through the nozzles which increase the angular velocity and thus produce a swirl effect.
- There are two exits in the vortex tube. The hot exit is located in the outer radius near the far end of the nozzle and the cold exit is in the center of the tube near the nozzle. The gas separated into two layers. The gas closer to the axis has a low temperature which comes out through the hot exit.
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