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Car Not Viii | Heat Pump | Heat

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Model Processes: When we extend the engineering analysis to discrete engineering devices working under steady flow conditions such as turbines, compressors, nozzles, we examine the degree of degradation of energy in these devices as a result of irreversibilities. But first we need to define some process close to an ideal process, which will serve as a model for the actual processes. Let us first examine the ideal adiabatic and isothermal processes in which output work is obtained through a numer
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  Model Processes: When we extend the engineering analysis to discrete engineering devices working under steadyflow conditions such as turbines, compressors, nozzles, we examine the degree of degradation of energy in these devices as a result of irreversibilities. But first we need to define some processclose to an ideal process, which will serve as a model for the actual processes. Let us firstexamine the ideal adiabatic and isothermal processes in which output work is obtained through anumerical.Turbines and piston engines convert kinetic energy or enthalpy of flowing fluid, or the heatadded to the fluid, into work. The control surface around the device is usually chosen so thatthere is no change in potential energy and the flow is SSSF If Δz is negligible the analysis yields( ǭ )   cv + ₥   ( h i + pe i + ke i ) = ₥   ( h e + pe e + ke e ) + ( ẅ )   cv The power can be divided by the mass flow rate to obtain the work per unit mass of fluidflowing.( ẅ )   cv ₥ = w cv = h i - h e +C i2 –C e2 2+ q cv Example 1 : Determine the work obtained per unit mass of air flowing in SSSF of air through aturbine where the inlet conditions are P1 = 1 MPa and T1 = 500 K and the discharge pressure is0.1 MPa. First conduct the computation for a reversible adiabatic process, then for reversibleisothermal process, assuming a negligible change in ke in both cases.1.Reversible adiabatic processw cv = h i - h e +(C i2 –C e2 )/2 + q cv = Cp ( T i - T e ) …………………………………..1 T L T H = (T L T H ) ( γ 1) = 0.1 0.4/ 1.4 Thus T L = 0.5179 (500) =258.9 K  Eq 1 gives w cv = 1.004 (500 – 258.9) = 242.1 kJ/kg .2.Reversible isothermal processw cv = h i - h e +(C i2 –C e2 )/2+ q cv = Cp ( T i - T e ) + q cv = q cv = ∫Tds = ∫ (dh –vdp) = ∫ -vdp =RT ∫ (-dp/p) = -RT ln (P L / P H ) = RT ln (P H /P L )= 0.287 (500) ln 10 = 330.4 kJ/kg There are two points that merit comment in this example. First, the reversible isothermal work isobserved to be greater than the reversible adiabatic work for the expansion of an ideal gas (air) between fixed pressures. This is true in general. If the process had been a compression process between fixed pressures, the isothermal process would involve less work input than thereversible adiabatic process. Thus, in both cases the reversible isothermal process would be preferable over the reversible adiabatic process. However, in practice the isothermal process isdifficult, if not impossible, to achieve because of the high rate of heat transfer.The second point is that here we have a    process (reversible isothermal process) where all heatadded to the control volume at 500 K has been converted to work. This is not a violation of thesecond law since this is a  process , not a cycle . This process required air at 1 MPa and 500 K. If there happened to be infinite supply of such air, this process would have tremendous practical  significance, but such an energy source does not exist. In practice, the air would have to becompressed to the high pressure and the compression process would require a work input,reducing significantly the net work output. Note that in a process there is no prohibition againstconverting all the heat supplied into work. In fact, the reversible adiabatic process had zero heatsupplied, yet work was produced.Although, some heat transfer between these devices and the surrounding medium is unavoidablein actual practice, most steady state devices are intended to operate under adiabatic conditions.Therefore the model process for these devices should be an adiabatic one, and units are usually provided with insulation even if the processes of work transfer happen so rapidly that there is notime available for heat transfer to occur. Furthermore, an ideal process should involve noirreversibilities since the effect of these irreversibilities is always to downgrade the performanceof engineering devices. Thus the ideal process that can serve as model for most steady flowdevices is the isentropic process.What is the difference between an adiabatic process and an isentropic process? or else state oneset of condition whenI.An adiabatic operation is not isentropic, andII.An isentropic operation is not adiabatic.In adiabatic process the system does not exchange any heat with the surroundings, i. e. no heatenters or leaves the working fluid externally or pass across the system boundaries.But, if during an adiabatic process heat is generated internally by friction, there will be gain inentropy, e.g. during adiabatic compression in a rotary compressor or during an adiabaticexpansion in a nozzle as there is friction between the molecules of fluid , between the fluid and passages when fluid pass through these units.These factors cause internal generation of heat and consequently at any point of time the properties like temperature, specific enthalpy could be more than that for ideal adiabatic process.This results in a progressive increase in entropy along fluid flow. Such a process thoughadiabatic is not isentropic.On the other hand, an isentropic process is one during which there is no change of entropy. For any process involving friction, if heat generated by friction be removed continuously at a rate soas to offset the extent of heat generated, net result would be no entropy change. The process thenwould be still isentropic but non-adiabatic as heat transfer occurs at the system boundaryrequired for removing extra heat generated.An ideal reversible adiabatic process, of course is also an isentropic, because in ideal processirreversibilities such as friction, eddies formation etc would be absent and in this process entropyremains constant.In our consideration of the I Law, we initially stated the law in terms of a cycle, but then defineda property internal energy, which enabled us to use I law quantitatively for processes .Similarly, we stated the II Law for a cycle, and we found that the II law leads to a property,entropy, which enabled us to treat the second law qualitatively for processes . To determinethe efficiency of a devices (e.g. turbine or compressor) in which processes take place, wecompare the actual performance of the device under given conditions and the performance itwould have in ideal conditions. It is the definition of this ideal process that the second law becomes major consideration. REVERSED CYCLES: Reversible processes are the ideal prototypes (models, samples) that actual processes approach;or ones that we wish they would closely approach. Now since reversible engine (- constituting of reversible processes) is reversible, it can be reversed in the thermodynamic as well as themechanical sense. That is the same engine can be operated as a heat pump, a refrigerator or an air   conditioner with no changes in magnitudes of energies flowing across the boundaries in thefollowing manner.The backward-running heat engine appears to ‘pump’ heat from low temperature region to a hightemperature region. However, since heat  is really a thermal energy transfer phenomena and not afluid, it is somewhat misleading to refer to it as being ‘pumped’. Yet it is common practice inHVAC industry to refer to these devices as heat pumps when they are used to provide transfer of thermal energy from a cold a environment to rooms that are maintained at higher temperaturesthan the surroundingIf we imagine a cycle performed in a direction opposite to that of an engine, the outcome would be absorption of some heat at low temperature, the rejection of a larger amount at a higher temperature, the net amount of work done on the working substance. In order to express therelative performance of these reversed cycle equipment, some criterion comparable to efficiencyused for engines is desirable. The index of performance is characterized by COP № and theeffectiveness of the system is expressed as the ratio of desired result to the energy required toachieve that result.Coefficient of Performance =Heat RemovedWork input=WQ H Looking at the magnitudes of energy entering and leaving the device, it is obvious numerator isalways greater than the denominator for such devices. Consequently efficiency is always greater than 100 %. No basic laws are violated here; the intent of the heat engine cycle is conversion of low grade heat energy to high grade mechanical energy, where as the intent of refrigeration or reversed cycle is only transfer of energy. Yes, conversion of energy take place in units such asmechanical-compression refrigeration machines, but the compressors used act energizers whichconvert high-grade mechanical work input into low grade thermal energy. Such conversion isalmost perfect.It is true that in case of a heat pump you get more thermal energy out of it than energy put into it.This makes a heat pump much more attractive for domestic heating than, say, a purely resistiveelectric heater. Electric heaters convert all their electrical energy into thermal energy andtherefore have energy conversion efficiencies of 100 %; whereas most heat pumps have energyconversion efficiencies far in excess of 100 % for the same electrical input energizing units.To do away with the laymen’s confusion that would arise by quoting efficiencies in excess of 100%, the less suggestive phrase coefficient of performance, COP, is used to express the-is concept.Another favored criterion that expresses the approach of the cycle to that of an ideal reversedrefrigeration cycle is called figure of merit (COP/COP rev ; Carnot cycle serves the ideal reversible  cycle). The COP is simply the pure efficiency number that is not converted into a percentage.The COP of a heat pump with an energy conversion efficiency of 400 % is 4.5. The energyconversion efficiency of a refrigerator or air conditioner are also greater than 100 % and they tooare commonly represented with a pure number of COP.When the purpose of machine is to cool some space to a lower temperature than its surrounding,the m/c is called as a refrigerator where food is stored in the cooled space and is called an air-conditioner when people occupy the cooled spaces. The heat pump is applied to a m/c whoseobjective is to heat medium which may already be warmer than its surrounding.Although the Carnot (reversed) cycle is the mostefficient between the fixed temperature limitsand therefore useful as a criterion of perfection,it possesses undesirable characteristics, particularly with gaseous refrigerant such as air which renders it objectionable from the practicalviewpoint.It would be highly desirable, however, to approximate the constant temperature Heat transfer  processes, in order to achieve the higher COP possible, this is accomplished in a large degree byoperating a refrigerating device only on a vapor compression cycle. As may be seen later, theCarnot cycle may be applied to vapors with comparatively few drawbacks, but the use of a permanent gas as the working fluid produces both extreme pressures and extreme volumes andwould thus require a very large machine per unit of refrigeration capacity. For a gas, certain portions of the cycle, notably the isothermals, would be difficult to accomplish. Isothermal heattransfer processes require that the specific heat of the air should approach infinity and or themachine operating at excessively low speed. Some type of air-refrigeration plants operate by the principle of adiabatic expansion of air, so that the temperature of the air decreases and it canserve as a source of cold. Such air-cycle systems in the form of an open cycle are employed for the purpose of aircraft cooling, since it has a definite weight advantage over vapor-compressionsystems. Such cycles operate on reversed Bryton cycle and the Joule Thomson Coefficient is avaluable coefficient for determining whether or not the gaseous-fluid can be used for cooling processes by dropping the pressure under adiabatic conditions in the expander (controlledexpansion).
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