Thermodynamics, Heat Transfer, And Fluid Flow. V.1. Thermodynamics, страница 11

The energy exchangesare never 100 percent efficient, as already discussed. The degree of efficiency obtained by thesystem depends upon the process through which the system has passed. Generally, the efficiencyof a component depends upon how much friction exists in the flow of the substance, the pressuredrops within the system, the inlet and outlet temperatures, and various other factors. Theproperties affecting the efficiency of the system are determined by use of the charts and diagramsmentioned in this section.When power cycles are utilized for large systems, the efficiency of each component should bemaximized in order to have the highest possible overall efficiency for the system. Eachcomponent affects the system efficiency in a different manner.

To maximize efficiency, thepractical approach to large systems is to have multistage expansion with reheat between stagesand regenerators in the system where applicable.Rev. 0Page 51HT-01PROPERTY DIAGRAMS AND STEAM TABLESThermodynamicsSummaryThe important information from this chapter is summarized below.Property Diagrams and Steam Tables Summary•The Mollier diagram can be used to determine various properties of a fluid.Mollier diagram is an h versus s plot.Can only be used when quality is greater than 50% and for superheatedsteam.Contains a series of constant temperature, constant pressure, constantmoisture content, and constant superheat lines.•The steam tables can be used to determine various properties of water using thefollowing equations.v = xvg + (1 - x)vfh = xhg + (1 - x)hfs = xsg + (1 - x)sfx=x=x=•HT-01vvfvfghhfhfgssfsfgThe change in enthalpy of a fluid as it passes through a component can bedetermined using a Mollier diagram on steam tables.Page 52Rev.

0ThermodynamicsFIRST LAW OF THERMODYNAMICSFIRST LAW OF THERMODYNAMICSThe First Law of Thermodynamics is a balance of the various forms of energy asthey pertain to the specified thermodynamic system (control volume) being studied.EO 1.19STATE the First Law of Thermodynamics.EO 1.20Using the First Law of Thermodynamics, ANALYZEan open system including all energy transfer processescrossing the boundaries.EO 1.21Using the First Law of Thermodynamics, ANALYZEcyclic processes for a thermodynamic system.EO 1.22Given a defined system, PERFORM energy balanceson all major components in the system.EO 1.23Given a heat exchanger, PERFORM an energybalance across the two sides of the heat exchanger.EO 1.24IDENTIFY the path(s) on a T-s diagram thatrepresents the thermodynamic processes occurring ina fluid system.First Law of ThermodynamicsThe First Law of Thermodynamics states:Energy can neither be created nor destroyed, only altered in form.For any system, energy transfer is associated with mass and energy crossing the controlboundary, external work and/or heat crossing the boundary, and the change of stored energywithin the control volume.

The mass flow of fluid is associated with the kinetic, potential,internal, and "flow" energies that affect the overall energy balance of the system. The exchangeof external work and/or heat complete the energy balance.Rev. 0Page 53HT-01FIRST LAW OF THERMODYNAMICSThermodynamicsThe First Law of Thermodynamics is referred to as the Conservation of Energy principle,meaning that energy can neither be created nor destroyed, but rather transformed into variousforms as the fluid within the control volume is being studied.

The energy balance spoken of hereis maintained within the system being studied. The system is a region in space (control volume)through which the fluid passes. The various energies associated with the fluid are then observedas they cross the boundaries of the system and the balance is made.As discussed in previous chapters of this module, a system may be one of three types: isolated,closed, or open. The open system, the most general of the three, indicates that mass, heat, andexternal work are allowed to cross the control boundary. The balance is expressed in words as:all energies into the system are equal to all energies leaving the system plus the change in storageof energies within the system. Recall that energy in thermodynamic systems is composed ofkinetic energy (KE), potential energy (PE), internal energy (U), and flow energy (PL); as well asheat and work processes.Σ (all energies in) = Σ (all energies out) + ∆(energy stored in system)Σ EinΣ Eout∆E storageFor most industrial plant applications that most frequently use cycles, there is no change instorage (i.e.

heat exchangers do not swell while in operation).In equation form, the balance appears as indicated on Figure 14.where:Q̇=heat flow into the system (Btu/hr)ṁin=mass flow rate into the system (lbm/hr)uin=specific internal energy into the system (Btu/lbm)Pinνin =pressure-specific volume energy into the system (ft-lbf/lbm)2Vin2gc=kinetic energy into the system (ft-lbf /lbm) whereVin = average velocity of fluid (ft/sec)gc = the gravitational constant (32.17 ft-lbm/lbf-sec2)gZgc in=ZinggcHT-01potential energy of the fluid entering the system (ft-lbf/lbm) where= height above reference level (ft)= acceleration due to gravity (ft/sec2)= the gravitational constant (32.17 ft-lbm/lbf-sec2)Page 54Rev. 0ThermodynamicsFIRST LAW OF THERMODYNAMICSẆ= work flow out of the system (ft-lbf/hr)ṁout= mass flow rate out of the system (lbm/hr)uout= specific internal energy out of the system (Btu/lbm)Poutνout= pressure-specific volume energy out of the system(ft-lbf/lbm)2Vout2gcgZgc out= kinetic energy out the system (ft-lbf/lbm)= potential energy out of the system (ft-lbf/lbm)Figure 14Rev.

0First Law of ThermodynamicsPage 55HT-01FIRST LAW OF THERMODYNAMICSThermodynamicsHeat and/or work can be directed into or out of the control volume. But, for convenience andas a standard convention, the net energy exchange is presented here with the net heat exchangeassumed to be into the system and the net work assumed to be out of the system. If no masscrosses the boundary, but work and/or heat do, then the system is referred to as a "closed"system.

If mass, work and heat do not cross the boundary (that is, the only energy exchangestaking place are within the system), then the system is referred to as an isolated system. Isolatedand closed systems are nothing more than specialized cases of the open system.

In this text, theopen system approach to the First Law of Thermodynamics will be emphasized because it ismore general. Also, almost all practical applications of the first law require an open systemanalysis.An understanding of the control volume concept is essential in analyzing a thermodynamicproblem or constructing an energy balance. Two basic approaches exist in studyingThermodynamics: the control mass approach and the control volume approach. The former isreferred to as the LeGrange approach and the latter as the Eulerian approach. In the control massconcept, a "clump" of fluid is studied with its associated energies. The analyzer "rides" with theclump wherever it goes, keeping a balance of all energies affecting the clump.Figure 15HT-01Control Volume ConceptsPage 56Rev. 0ThermodynamicsFIRST LAW OF THERMODYNAMICSThe control volume approach is one in which a fixed region in space is established with specifiedcontrol boundaries, as shown in Figure 15. The energies that cross the boundary of this controlvolume, including those with the mass crossing the boundary, are then studied and the balanceperformed.

The control volume approach is usually used today in analyzing thermodynamicsystems. It is more convenient and requires much less work in keeping track of the energybalances. Examples of control volume applications are included in Figures 16-18.Figure 16 Open System Control VolumesRev. 0Page 57HT-01FIRST LAW OF THERMODYNAMICSFigure 17Figure 18HT-01ThermodynamicsOpen System Control Volumes (Cont.)Multiple Control Volumes in Same SystemPage 58Rev. 0ThermodynamicsFIRST LAW OF THERMODYNAMICSThe forms of energy that may cross the control volume boundary include those associated withthe mass (m) crossing the boundary.

Mass in motion has potential (PE), kinetic (KE), andinternal energy (U). In addition, since the flow is normally supplied with some driving power(a pump for example), there is another form of energy associated with the fluid caused by itspressure. This form of energy is referred to as flow energy (Pν-work).

The thermodynamicterms thus representing the various forms of energy crossing the control boundary with the massare given as m (u + Pν + ke + pe).In open system analysis, the u and Pν terms occur so frequently that another property, enthalpy,has been defined as h = u + Pν. This results in the above expression being written as m (h +ke + pe). In addition to the mass and its energies, externally applied work (W), usuallydesignated as shaft work, is another form of energy that may cross the system boundary.In order to complete and satisfy the conservation of energy relationship, energy that is causedby neither mass nor shaft work is classified as heat energy (Q).

Then we can describe therelationship in equation form as follows.ṁ(hinpeinkein)Q̇ṁ(houtpeoutkeout)Ẇ(1-22)where:Rev. 0ṁ=mass flow rate of working fluid (lbm/hr)hin=specific enthalpy of the working fluid entering the system (Btu/lbm)hout=specific enthalpy of the working fluid leaving the system (Btu/lbm)pein=specific potential energy of working fluid entering the system (ft-lbf/lbm)peout=specific potential energy of working fluid leaving the system (ft-lbf/lbm)kein=specific kinetic energy of working fluid entering the system (ft-lbf/lbm)keout=specific kinetic energy of working fluid leaving the system (ft-lbf/lbm)Ẇ=rate of work done by the system (ft-lbf/hr)Q̇=heat rate into the system (Btu/hr)Page 59HT-01FIRST LAW OF THERMODYNAMICSThermodynamicsExample 1 illustrates the use of the control volume concept while solving a first law probleminvolving most of the energy terms mentioned previously.Example 1: Open System Control VolumeThe enthalpies of steam entering and leaving a steam turbine are 1349 Btu/lbm and 1100Btu/lbm, respectively.