Railways NTPC (Technical Ability) Refrigeration and Air-Conditioning

Refrigeration and Air-Conditioning

Category : Railways

Refrigeration and Air-Conditioning


  • Refrigeration is a process in which work is done to move eat from one location to another. The work of heat transport is traditionally driven by mechanical work, 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.
  • Heat pumps may use the heat output of the refrigeration process, and also may be designed to be reversible, but are otherwise similar to refrigeration units.
  • Refrigeration has had a large impact on industry, lifestyle, agriculture and settlement patterns. The idea of preserving food dates back to the ancient Roman and Chinese empires.
  • However, refrigeration technology has rapidly evolved. In the last century from ice harvesting to temperature controlled rail cars. The introduction of refrigerated rail cars contributed to the westward expansion of the United States, allowing settlement in areas that were not on main transport channels such as rivers, harbors, or valley trails.
  • The history of artificial refrigeration began when Scottish professor William Cullen designed a small refrigerating machine in 1755. Cullen used a pump to create a partial vacuum over a container of diethyl ether, which then boiled, absorbing heat from the surrounding air. The experiment even created a small amount of ice, but had no practical application at that time.
  • In 1758, Benjamin Franklin and John Hadley, professor of chemistry, collaborated on a project investigating the principle of evaporation as a means to rapidly cool an object at Cambridge University, England.
  • They confirmed that the evaporation of highly volatile liquids, such as alcohol and ether, could be used to drive down the temperature of an object past the freezing point of water.
  • They conducted their experiment with the bulb of a mercury thermometer as their object and with a bellows used to 'quicken' the evaporation; they lowered the temperature of the thermometer bulb down to\[7\,\,\text{ }\!\!{}^\circ\!\!\text{ F}\]\[(-14{}^\circ C),\] while the ambient temperature was\[65\,\,\text{ }\!\!{}^\circ\!\!\text{ F}\]\[(18{}^\circ C)\].
  • They noted that soon after they passed the freezing point of water \[(32{}^\circ F),\] a thin film of ice formed on the surface of the thermometer's bulb and that the ice mass was about a quarter inch thick when they stopped the experiment upon reaching \[7{}^\circ F\,\,(-\,14{}^\circ C).\]
  • In 1805, American inventor Oliver Evans described a closed vapor-compression refrigeration cycle for the production of ice by ether under vacuum.
  • In 1820, the English scientist Michael Faraday liquefied ammonia and other gases by using high pressures and low temperatures, and in 1834, an American expatriate to Great Britain, Jacob Perkins, built the first working vapor-compression refrigeration system in the world.
  • In 1842, a similar attempt was made by American physician, John Gorrie, who built a working prototype, but it was a commercial failure. Like many of the medical experts during this time, Gorrie thought too much exposure to tropical heat led to mental and physical degeneration, as well as the spread of diseases such as malaria.
  • He conceived the idea of using his refrigeration system to cool the air for comfort in homes and hospitals to prevent disease. American engineer Alexander Twining took out a British patent in 1850 for a vapor compression system that used ether.
  • The first practical vapor compression refrigeration system was built by James Harrison, a British journalist who had emigrated to Australia. His 1856 patent was for a vapour compression system using ether, alcohol or ammonia.
  • He built a mechanical ice-making machine in 1851 on the banks of the Barwon River at Rocky Point in Geelong, Victoria, and his first commercial ice-making machine followed in 1854.
  • Harrison also introduced commercial vapour-compression refrigeration to breweries and meat packing houses, and by 1861, a dozen of his systems were in operation.
  • The first gas absorption refrigeration system using gaseous ammonia dissolved in water was developed by Ferdinand Carre of France in 1859 and patented in 1860.
  • Air conditioning is the process of altering the properties of air to more comfortable conditions, typically with the aim of distributing the conditioned air to an occupied space to improve thermal comfort and indoor air quality.
  • The basic concept behind air conditioning is said to have been applied in ancient Egypt, where reeds were hung in windows and were moistened with trickling water.
  • The evaporation of water cooled the air blowing through the window. This process also made the air more humid, which can be beneficial in a dry desert climate. In Ancient Rome, water from aqueducts was circulated through the walls of certain houses to cool them.
  • Other techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season.
  • Modem air conditioning emerged from advances in chemistry during the 19th century, and the first large scale electrical air conditioning was invented and used in 1902 by American inventor Willis Carrier.
  • The introduction of residential air conditioning in the 1920s helped enable the great migration to the Sun Belt in the United States.
  • Inductors or choke coils are made of iron core because large valued flux densities can be produced in iron cores and so inductances of large value can be had. Air-cored inductors become too much bulky to provide an inductance of a required value.
  • An inductance draws instantaneous power but no average power. The power drawn by inductance during one quarter cycle is released in another quarter cycle, and so average power drawn by an inductance is zero.
  • Power factor may be defined as the cosine of the phase angle between voltage and current. It may also be defined as the ratio of resistance to impedance or the ratio of true power to apparent power.
  • The current component which is in phase with circuit voltage (i.e.,\[I\cos f\]) and contributes to active or true power of the circuit is called the active (wattfull or in phase) component of current.
  • The current component which is in quadrature (or \[90{}^\circ \]out of phase) to the circuit voltage (i.e.,\[I\,\,sin\,\,\phi \]) and contributes to reactive power of the circuit is called the reactive (or wattless) component of current.
  • The power which is actually consumed or utilized in an ac circuit is called the true or active power of the circuit.
  • Power is consumed only in resistance. It is given by the product of the circuit voltage V, current I and power factor \[\cos \,\,\phi \] i.e., \[P=VI\cos \,\,\phi \]. It is expressed in watts.
  • A pure inductor and a pure capacitor do not consume any power, as in a quarter cycle whatsoever power is drawn from the supply source by these components, the same is returned to the supply source in the other quarter cycle. This power which flows back and forth (i.e., in both directions in the circuit) or reacts itself is called the reactive power. This is also known as wattles power.
  • The reactive power of an ac circuit is given by the product of voltage V, current I and sine of the phase angle f, i.e., Reactive power, \[Q=VIsin\,\,\phi \] it is expressed in reactive volt-amperes.
  • In a purely capactive circuit, current leads the applied voltage by \[90{}^\circ \,\,or\,\,\pi /2\] radians.
  • Inductive reactance of an inductor increases proportionately with the increase in supply frequency i.e., \[{{\text{X}}_{L}}\propto f.\]
  • In 1758, Benjamin Franklin and John Hadley, a chemistry professor at Cambridge University, conducted an experiment to explore the principle of evaporation as a means to rapidly cool an object.
  • Franklin and Hadley confirmed that evaporation of highly volatile liquids could be used to drive down the temperature of an object past the freezing point of water.
  • They conducted their experiment with the bulb of a mercury thermometer as their object and with a bellows used to speed-up the evaporation. They lowered the temperature of the thermometer bulb down to\[-\,14{}^\circ C\,\,(7{}^\circ F)\] while the ambient temperature was \[18{}^\circ \,C\,\,(64{}^\circ \,F).\]
  • In 1820, English scientist and inventor Michael Faraday discovered that compressing and liquefying ammonia could chill air when the liquefied ammonia was allowed to evaporate.
  • In 1842, Florida physician John Gorrie used compressor technology to create ice, which he used to cool air for his patients in his hospital in Apalachicola, Florida.


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Notes - Refrigeration and Air-Conditioning

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