ASHRAE FUNDAMENTALS SI CH 2-2013 Thermodynamics and Refrigeration Cycles.pdf

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1、2.1CHAPTER 2THERMODYNAMICS AND REFRIGERATION CYCLESTHERMODYNAMICS 2.1Stored Energy. 2.1Energy in Transition. 2.1First Law of Thermodynamics . 2.2Second Law of Thermodynamics . 2.2Thermodynamic Analysis of Refrigeration Cycles. 2.3Equations of State 2.4Calculating Thermodynamic Properties 2.5COMPRESS

2、ION REFRIGERATION CYCLES 2.6Carnot Cycle 2.6Theoretical Single-Stage Cycle Using a Pure Refrigerant or Azeotropic Mixture. 2.8Lorenz Refrigeration Cycle 2.9Theoretical Single-Stage Cycle Using Zeotropic Refrigerant Mixture 2.10Multistage Vapor Compression Refrigeration Cycles . 2.10Actual Refrigerat

3、ion Systems . 2.11ABSORPTION REFRIGERATION CYCLES 2.13Ideal Thermal Cycle. 2.13Working Fluid Phase Change Constraints. 2.14Working Fluids . 2.15Effect of Fluid Properties on Cycle Performance 2.16Absorption Cycle Representations . 2.16Conceptualizing the Cycle 2.16Absorption Cycle Modeling 2.17Ammon

4、ia/Water Absorption Cycles 2.19ADSORPTION REFRIGERATION SYSTEMS . 2.20Symbols 2.21HERMODYNAMICS is the study of energy, its transforma-Ttions, and its relation to states of matter. This chapter covers theapplication of thermodynamics to refrigeration cycles. The first partreviews the first and secon

5、d laws of thermodynamics and presentsmethods for calculating thermodynamic properties. The second andthird parts address compression and absorption refrigeration cycles,two common methods of thermal energy transfer.THERMODYNAMICSA thermodynamic system is a region in space or a quantity ofmatter boun

6、ded by a closed surface. The surroundings includeeverything external to the system, and the system is separated fromthe surroundings by the system boundaries. These boundaries can bemovable or fixed, real or imaginary.Entropy and energy are important in any thermodynamic system.Entropy measures the

7、molecular disorder of a system. The moremixed a system, the greater its entropy; an orderly or unmixed con-figuration is one of low entropy. Energy has the capacity for produc-ing an effect and can be categorized into either stored or transientforms.STORED ENERGYThermal (internal) energy is caused b

8、y the motion of moleculesand/or intermolecular forces.Potential energy (PE) is caused by attractive forces existingbetween molecules, or the elevation of the system.PE = mgz (1)wherem =massg = local acceleration of gravityz = elevation above horizontal reference planeKinetic energy (KE) is the energ

9、y caused by the velocity of mol-ecules and is expressed asKE = mV2/2 (2)where V is the velocity of a fluid stream crossing the system boundary.Chemical energy is caused by the arrangement of atoms com-posing the molecules.Nuclear (atomic) energy derives from the cohesive forces hold-ing protons and

10、neutrons together as the atoms nucleus.ENERGY IN TRANSITIONHeat Q is the mechanism that transfers energy across the bound-aries of systems with differing temperatures, always toward thelower temperature. Heat is positive when energy is added to the sys-tem (see Figure 1).Work is the mechanism that t

11、ransfers energy across the boundar-ies of systems with differing pressures (or force of any kind), alwaystoward the lower pressure. If the total effect produced in the systemcan be reduced to the raising of a weight, then nothing but work hascrossed the boundary. Work is positive when energy is remo

12、ved fromthe system (see Figure 1).Mechanical or shaft work W is the energy delivered or absorbedby a mechanism, such as a turbine, air compressor, or internal com-bustion engine.Flow work is energy carried into or transmitted across thesystem boundary because a pumping process occurs somewhereoutsid

13、e the system, causing fluid to enter the system. It can be moreeasily understood as the work done by the fluid just outside the sys-tem on the adjacent fluid entering the system to force or push it intothe system. Flow work also occurs as fluid leaves the system.The preparation of the first and seco

14、nd parts of this chapter is assigned toTC 1.1, Thermodynamics and Psychrometrics. The third and fourth partsare assigned to TC 8.3, Absorption and Heat-Operated Machines. Fig. 1 Energy Flows in General Thermodynamic System2.2 2013 ASHRAE HandbookFundamentals (SI)Flow work (per unit mass) = pv (3)whe

15、re p is pressure and v is specific volume, or the volume dis-placed per unit mass evaluated at the inlet or exit.A property of a system is any observable characteristic of thesystem. The state of a system is defined by specifying the minimumset of independent properties. The most common thermodynami

16、cproperties are temperature T, pressure p, and specific volume v ordensity . Additional thermodynamic properties include entropy,stored forms of energy, and enthalpy.Frequently, thermodynamic properties combine to form otherproperties. Enthalpy h is an important property that includes inter-nal ener

17、gy and flow work and is defined ash u + pv (4)where u is the internal energy per unit mass.Each property in a given state has only one definite value, andany property always has the same value for a given state, regardlessof how the substance arrived at that state.A process is a change in state that

18、 can be defined as any changein the properties of a system. A process is described by specifyingthe initial and final equilibrium states, the path (if identifiable), andthe interactions that take place across system boundaries during theprocess.A cycle is a process or a series of processes wherein t

19、he initialand final states of the system are identical. Therefore, at the conclu-sion of a cycle, all the properties have the same value they had at thebeginning. Refrigerant circulating in a closed system undergoes acycle.A pure substance has a homogeneous and invariable chemicalcomposition. It can

20、 exist in more than one phase, but the chemicalcomposition is the same in all phases.If a substance is liquid at the saturation temperature and pressure,it is called a saturated liquid. If the temperature of the liquid islower than the saturation temperature for the existing pressure, it iscalled ei

21、ther a subcooled liquid (the temperature is lower than thesaturation temperature for the given pressure) or a compressed liq-uid (the pressure is greater than the saturation pressure for the giventemperature).When a substance exists as part liquid and part vapor at the sat-uration temperature, its q

22、uality is defined as the ratio of the mass ofvapor to the total mass. Quality has meaning only when the sub-stance is saturated (i.e., at saturation pressure and temperature).Pressure and temperature of saturated substances are not indepen-dent properties.If a substance exists as a vapor at saturati

23、on temperature andpressure, it is called a saturated vapor. (Sometimes the term drysaturated vapor is used to emphasize that the quality is 100%.)When the vapor is at a temperature greater than the saturation tem-perature, it is a superheated vapor. Pressure and temperature of asuperheated vapor are

24、 independent properties, because the temper-ature can increase while pressure remains constant. Gases such asair at room temperature and pressure are highly superheated vapors.FIRST LAW OF THERMODYNAMICSThe first law of thermodynamics is often called the law of con-servation of energy. The following

25、 form of the first-law equation isvalid only in the absence of a nuclear or chemical reaction.Based on the first law or the law of conservation of energy, forany system, open or closed, there is an energy balance asorEnergy in Energy out = Increase of stored energy in systemFigure 1 illustrates ener

26、gy flows into and out of a thermodynamicsystem. For the general case of multiple mass flows with uniformproperties in and out of the system, the energy balance can bewritten(5)where subscripts i and f refer to the initial and final states, re-spectively.Nearly all important engineering processes are

27、 commonly mod-eled as steady-flow processes. Steady flow signifies that all quanti-ties associated with the system do not vary with time. Consequently,(6)where h u + pv as described in Equation (4).A second common application is the closed stationary system forwhich the first law equation reduces to

28、Q W = m(uf ui)system(7)SECOND LAW OF THERMODYNAMICSThe second law of thermodynamics differentiates and quantifiesprocesses that only proceed in a certain direction (irreversible) fromthose that are reversible. The second law may be described in sev-eral ways. One method uses the concept of entropy f

29、low in an opensystem and the irreversibility associated with the process. The con-cept of irreversibility provides added insight into the operation ofcycles. For example, the larger the irreversibility in a refrigerationcycle operating with a given refrigeration load between two fixedtemperature lev

30、els, the larger the amount of work required to oper-ate the cycle. Irreversibilities include pressure drops in lines andheat exchangers, heat transfer between fluids of different tempera-ture, and mechanical friction. Reducing total irreversibility in acycle improves cycle performance. In the limit

31、of no irreversibili-ties, a cycle attains its maximum ideal efficiency.In an open system, the second law of thermodynamics can bedescribed in terms of entropy asdSsystem= + misi mese+ dI (8)wheredSsystem= total change within system in time dt during processmisi= entropy increase caused by mass enter

32、ing (incoming)mese= entropy decrease caused by mass leaving (exiting)Q/T = entropy change caused by reversible heat transfer between system and surroundings at temperature TdI = entropy caused by irreversibilities (always positive)Equation (8) accounts for all entropy changes in the system. Re-arran

33、ged, this equation becomesQ = T (mese misi) + dSsys dI (9)Net amount of energyadded to systemNet increase of storedenergy in system=minupvV22- gz+inmoutupvV22- gz+outQW+mfuV22- gz+fmiuV22- gz+isystem=mhV22- gz+all streamsenteringmhV22- gz+all streamsleaving QW+0=QT-Thermodynamics and Refrigeration C

34、ycles 2.3In integrated form, if inlet and outlet properties, mass flow, andinteractions with the surroundings do not vary with time, the generalequation for the second law is(Sf Si)system= + I (10)In many applications, the process can be considered to operatesteadily with no change in time. The chan

35、ge in entropy of the systemis therefore zero. The irreversibility rate, which is the rate ofentropy production caused by irreversibilities in the process, can bedetermined by rearranging Equation (10):(11)Equation (6) can be used to replace the heat transfer quantity.Note that the absolute temperatu

36、re of the surroundings with whichthe system is exchanging heat is used in the last term. If the temper-ature of the surroundings is equal to the system temperature, heat istransferred reversibly and the last term in Equation (11) equals zero. Equation (11) is commonly applied to a system with one ma

37、ssflow in, the same mass flow out, no work, and negligible kinetic orpotential energy flows. Combining Equations (6) and (11) yields(12)In a cycle, the reduction of work produced by a power cycle (orthe increase in work required by a refrigeration cycle) equals theabsolute ambient temperature multip

38、lied by the sum of irreversibil-ities in all processes in the cycle. Thus, the difference in reversibleand actual work for any refrigeration cycle, theoretical or real, oper-ating under the same conditions, becomes(13)Another second-law method to describe performance of engi-neering devices is the c

39、oncept of exergy (also called the availabil-ity, potential energy, or work potential), which is the maximumuseful work that could be obtained from the system at a given statein a specified environment. There is always a difference betweenexergy and the actual work delivered by a device; this differe

40、ncerepresents the room for improvement. Note that exergy is a propertyof the system/environment combination and not of the systemalone. The exergy of a system in equilibrium with its environmentis zero. The state of the environment is referred to as the dead state,because the system cannot do any wo

41、rk.Exergy transfer is in three forms (heat, work, and mass flow), andis given byXheat= Xwork= Xmass= mwhere = (h h0) T0(s s0) + (V 2/2) + gz is flow exergy.Exergy balance for any system undergoing any process can beexpressed asTaking the positive direction of heat transfer as to the system andthe po

42、sitive direction of work transfer as from the system, the gen-eral exergy balance relations can be expressed explicitly asTHERMODYNAMIC ANALYSIS OF REFRIGERATION CYCLESRefrigeration cycles transfer thermal energy from a region of lowtemperature TRto one of higher temperature. Usually the higher-temp

43、erature heat sink is the ambient air or cooling water, at temper-ature T0, the temperature of the surroundings.The first and second laws of thermodynamics can be applied toindividual components to determine mass and energy balances andthe irreversibility of the components. This procedure is illustra

44、ted inlater sections in this chapter.Performance of a refrigeration cycle is usually described by acoefficient of performance (COP), defined as the benefit of thecycle (amount of heat removed) divided by the required energyinput to operate the cycle:COP (14)For a mechanical vapor compression system,

45、 the net energy sup-plied is usually in the form of work, mechanical or electrical, andmay include work to the compressor and fans or pumps. Thus,COP = (15)In an absorption refrigeration cycle, the net energy supplied isusually in the form of heat into the generator and work into thepumps and fans,

46、orCOP = (16)In many cases, work supplied to an absorption system is verysmall compared to the amount of heat supplied to the generator, sothe work term is often neglected.Applying the second law to an entire refrigeration cycle showsthat a completely reversible cycle operating under the same con-dit

47、ions has the maximum possible COP. Departure of the actualcycle from an ideal reversible cycle is given by the refrigeratingefficiency:R= (17)The Carnot cycle usually serves as the ideal reversible refrigera-tion cycle. For multistage cycles, each stage is described by a revers-ible cycle.Xin Xout X

48、destroyed= Xsystem(general)Net exergy transfer by heat, work, and massExergy destructionChange in exergyQT-revmsinmsout+ImsoutmsinQTsurr-=ImsoutsinhouthinTsurr-=WactualWreversibleT0I+=1T0T-QWWsurr (for boundary work)W (for other forms of work)=dXsystem/dt(general, in rate form)Rate of net exergy transfer by heat, work, and massRate of exergy destructionRate of change in exergyXinXout Xdestroyed1T0Tk-QkWP0V2V1 min+ moutXdestroyedX2X1=Useful refrigerating effectNet energy supplied from external sources-QevapWnet-QevapQgenWnet+