Refrigeration is defined as the science of providing and maintaining temperature below that of atmospheric temperature. It means continuous extraction of heat from a body which is already below atmospheric temperature. Refrigeration involves cooling a space, substance or system to lower its temperature or maintain that temperature below its ambient temperature. Energy in the form of heat is removed from a low temperature reservoir and transferred to a high temperature reservoir. The energy transfer process is done by mechanical means but can also be achieved by electricity, magnetism, heat, laser. Refrigeration is applied to various uses such as domestic refrigerators, industrial freezers,cryogenics and air conditioners. The refrigeration process is achieved by working fluids known as refrigerants, the first of such being ammonia.
Methods of Refrigeration.
The methods include, cyclic, non- cyclic thermoelectric and magnetic.
This consist of a refrigeration cycle where heat is removed from a low temperature space and rejected to a high temperature sink with the help of external work and its inverse the thermodynamic power cycle. In the power cycle heat is supplied from a high temperature source to the engine, part of the heat used to produce work and the rest being rejected to a low temperature sink. This satisfies the second law of thermodynamics. Cyclic refrigeration can be classed as
- Vapour Cycle
- Gas Cycle
- Vapour Cycle
Vapour refrigeration cycle can further be classed into: Vapour compression refrigeration and sorption refrigeration. Sorption refrigeration can be classed into: vapour absorption refrigeration and adsorption refrigeration.
The vapour compression cycle is mostly used for domestic purposes and industries where it is applied for commercial purposes. In the vapour compression cycle the refrigerant in this case Freon enters the compressor as a vapour. The vapour is compressed to a higher temperature and exits the compressor at a higher temperature but still below the vapour pressure. The vapour then enters the condenser where its temperature is lowered and the vapour condenses into a liquid. The liquid refrigerant then passes through the expansion valve or throttle valve where its pressure abruptly decreases, causing flash evaporation and auto refrigeration of less than half of the liquid. This results on a mixture of liquid and vapour at a lower temperature and pressure. The cold liquid – vapour mixture then travels through the evaporator coils and is completely vaporized by cooling the warm air from the space being refrigerated blown by the fan across the evaporator coils. The refrigerant vapour then returns to the compressor inlet to complete the thermodynamic cycle.
The absorption cycle is similar to the compression cycle except for the method of raising the pressure of the refrigerant vapour. In the absorption system the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which drives off the refrigerant vapour from the high pressure liquid. In an absorption refrigerator a combination of refrigerant and absorbent is used. The most common combinations are; ammonia ( refrigerant) and water( absorbent) and water (refrigerant) with lithium bromide (absorbent).
In adsorption cycle the refrigerant could be ammonia, water or methanol while the absorbent is solid such as silicon gel, activated carbon or zeolite unlike the absorption cycle where the absorbent is liquid.
Non Cyclic Refrigeration
This method cools a contained area by melting ice or by sublimation of dry ice. An example is the portable cooler where items are placed inside and ice is poured over them. Dry ice is used for cooling and it effectively brings down the temperature below water freezing point.
The Gas Cycle
When the working fluid is a gas that is compressed and expands but does not change phase the refrigeration cycle is called a gas cycle. Air is often used as the working fluid. The gas cycle is less efficient than the vapour compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. Thus the fluid does not receive or reject the heat at constant temperature. Because of their lower capacity and large size, air cycle coolers are not used nowadays for terrestrial cooling devices. However they find applications in gas turbine jet engines where they are used as cooling and ventilation units.
Thermoelectric cooling utilizes the Peltier effect to create a heat flux between the junction of two types of material. This method is commonly used in camping and portable coolers and for cooling electronic components and small instruments.
Magnetic refrigeration is based on the magnetocaloric effect, an intrinsic property of magnetic solids, the refrigerant is a paramagnetic salt, such as cerium magnesium nitrate. A strong magnetic field is applied to the refrigerant, forcing an alignment of its various magnetic dipoles, putting the refrigerant in a lower state of lowered entropy. A heat sink then absorbs the heat released by the refrigerant due to loss of entropy. Thermal contact with the heat sink is then broken so that the system is insulated and the magnetic field is switched off. This increases the heat capacity of the refrigerant thus decreasing its temperature below that of the heat sink. This method of refrigeration has been limited to cryogenics and for research purposes.