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Note for Refrigeration and Air Conditioning - RAC by Rajesh Panda

  • Refrigeration and Air Conditioning - RAC
  • Note
  • Biju Patnaik University of Technology Rourkela Odisha - BPUT
  • Mechanical Engineering
  • B.Tech
  • 13 Topics
  • 3122 Views
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1. Heat Pump and Refrigeration Cycles and Systems Refrigeration is the science of the producing and maintaining temperature below that of the surrounding atmosphere. Production of refrigeration: (a) By melting of a solid. (b) By sublimation of a solid. (c) By evaporation of a liquid. Refrigeration system: 1. Ice refrigeration system. 2. Air refrigeration system. 3. Vapour compression refrigeration system. 4. Vapour absorption refrigeration system. 5. Special refrigeration system. i. ii. iii. iv. v. vi. Adsorption refrigeration system. Cascade refrigeration system. Mixed refrigeration system. Votex tube refrigeration system. Thermoelectric refrigeration system. Steam jet refrigeration system. Heat Engine, Heat Pump Heat engines, Refrigerators, Heat pumps: • A heat engine may be defined as a device that operates in a thermodynamic cycle and does a certain amount of net positive work through the transfer of heat from a high temperature body to a low temperature body. A steam power plant is an example of a heat engine. • A refrigerator may be defined as a device that operates in a thermodynamic cycle and transfers a certain amount of heat from a body at a lower temperature to a body at a higher temperature by consuming certain amount of external work. Domestic refrigerators and room air conditioners are the examples. In a refrigerator, the required output is the heat extracted from the low temperature body. • A heat pump is similar to a refrigerator, however, here the required output is the heat rejected to the high temperature body. Page 1 of 263

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Hea at Pump & Refriigeratio on Cycle es and Systems Cha apter 1 Fig. (a) Heat Eng gine (b) Re efrigeratio on and heat pump c cycles Fig g. Comparison of he eat engine, heat pum mp and refrigeratin ng machin ne C Carnot’s t theorem ms for hea at engine es: Th heorem 1: 1 It is imp possible to construct a heat en ngine that operates b between two thermal reeservoirs and a is morre efficient than a reversible r engine e opeerating bettween the same two reeservoirs. Th heorem 2:: All reverssible heat en ngines operrating betw ween the same two theermal reservoirs have th he same theermal efficieency. Th he two theeorems can n be proved d by carryiing out a thought t exp periment a and with th he help of seecond law. Carnot’s th heorems ca an also be formed f for refrigeratoors in a ma anner simillar to heat en ngines. Carnot efficiency: Th he Carnot efficiencies e are the effficiencies off completelly reversib ble cycles op perating beetween twoo thermal reservoirs. r According to Carnott’s theorem ms, for any given two th hermal reseervoirs, thee Carnot effficiency rep presents the maximum m possible eefficiency. Page 2 of 263

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He eat Pum mp & Refrigerattion Cyc cles and d System ms Ch hapter 1 Thermall efficiency y for a hea at engine ( ηHE ) is de efined as: ηHE = Wcycle QH =1− QC QH Where Wcycle is the net n work ou utput, QC an nd QH and are the heat rejected to the low temperatu ure c reservoir and heat added a (heatt input) from m the high temperatu ure reservoir, respectiively. ⎛Q ⎞ It follows from Ca arnot’s theeorem tha at for a reversible r f of cycle ⎜ C ⎟ is a function ⎝ QH ⎠ Q he two reserrvoirs only. i.e. C = φ (TC , TH ) . temperatures of th QH vin) temperrature scale e then: If we chooose the absolute (Kelv QC T = C QH TH hence, ηCarnot,HE = 1 − QC T = 1− C QH TH The efficiency of reffrigerator and a heat pu ump is called as Coeffficient off Performa ance (COP P). ngines, Ca arnot coefficcient of pe erformance for heat p pump and refrigeratoors Similarly to heat en (COP) HP and a (COP) R can be wrritten as; COPCarnot,HHP = COPCarnot,RR = Where Wcyccle = QH = QC = TH = QH QH TH = = Wcycle QH − QC TH − TC QC QC TC = = TH − TC Wcyycle QH − QC work k input to the t reversib ble heat pu ump and reffrigerator heatt transferreed between n the system m and the hot h reservoiir heatt transferreed between n the system m and cold reservoir temp perature off the hot reeservoir. Page 3 of 263

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Heat Pump & Refrigeration Cycles and Systems Chapter 1 TC = temperature of the cold reservoir. Although we have treated them as the same to this point, refrigeration and heat pump cycles actually have different objectives. The objective of a refrigeration cycle is to cool a refrigerated space or to maintain the temperature within a dwelling or other building below that of the surroundings. The objective of a heat pump is to maintain the temperature within a dwelling or other building above that of the surroundings or to provide heating for certain industrial processes that occur at elevated temperatures. Since refrigeration and heat pump cycles have different objectives, their performance parameters, called coefficients of performance, are defined differently. These coefficients of performance are considered next. Refrigeration Cycles The performance of refrigeration cycles can be described as the ratio of the amount of energy received by the system undergoing the cycle from the cold body, Qin, to the Net work into the system to accomplish this effect, Wcycle. Thus, the coefficient of performance, (COP ) R is (COP )R = Qin Wcycle ( For Refrigeration cycle ) Coefficient of performance: refrigeration Introducing an alternative expression for (COP ) R is obtained as (COP )R = Qin Qout − Qin ( For Refrigeration cycle ) For a household refrigerator, Qout is discharged to the space in which the refrigerator is located. Wcycle is usually provided in the form of electricity to run the motor that drives the refrigerator. Heat Pump Cycles The performance of heat pumps can be described as the ratio of the amount of energy discharged from the system undergoing the cycle to the hot body, Qout, to the net work into the system to accomplish this effect, Wcycle. Thus, the coefficient of performance, (COP )HP is (COP )HP = Qout Wcycle ( For heat pump cycle) Coefficient of performance: pump heat Introducing an alternative expression for this coefficient of performance is obtained as (COP )HP = Qout Q out − Qin ( For heat pump cycle ) From this equation it can be seen that the value of (COP )HP is never less than unity. For residential heat pumps, the energy quantity Qin is normally drawn from the surrounding atmosphere, the ground, or a nearby body of water. Wcycle is usually provided by electricity. The coefficients of performance (COP )R and (COP )HP are defined as ratios of the desired heat transfer effect to the cost in terms of work to accomplish that effect. Based on the definitions, it is desirable thermodynamically that these coefficients have values that are as large as possible. Page 4 of 263

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