Thermodynamics, Heat Transfer, And Fluid Flow. V.1. Thermodynamics, страница 7

In general, three types of units are used to measure energy: (1) mechanical units, suchas the foot-pound-force (ft-lbf); (2) thermal units, such as the British thermal unit (Btu); and (3)electrical units, such as the watt-second (W-sec). In the mks and cgs systems, the mechanicalunits of energy are the joule (j) and the erg, the thermal units are the kilocalorie (kcal) and thecalorie (cal), and the electrical units are the watt-second (W-sec) and the erg. Although the unitsof the various forms of energy are different, they are equivalent.Some of the most important experiments in science were those conducted by J.

P. Joule in 1843,who showed quantitatively that there was a direct correspondence between mechanical andthermal energy. These experiments showed that one kilocalorie equals 4,186 joules. These sameexperiments, when performed using English system units, show that one British thermal unit(Btu) equals 778.3 ft-lbf. These experiments established the equivalence of mechanical andthermal energy. Other experiments established the equivalence of electrical energy with bothmechanical and thermal energy.

For engineering applications, these equivalences are expressedby the following relationships.1 ft-lbf = 1.286 x 10-3 Btu = 3.766 x 10-7 kW-hr1 Btu = 778.3 ft-lbf = 2.928 x 10-4 kW-hr1 kW-hr = 3.413 x 103 Btu = 2.655 x 106 ft-lbfRev. 0Page 23HT-01ENERGY, WORK, AND HEATThermodynamicsThere is one additional unit of energy encountered in engineering applications. It is thehorsepower-hour (hp-hr).

It is a mechanical unit of energy defined by the following relationship:1 hp-hr = 1.980 x 106 ft-lbfThese relationships can be used to convert between the various English system units for thevarious forms of energy.Most computations involving the energy of the working fluid in an energy transfer system areperformed in Btu’s. Forms of mechanical energy (such as potential energy, kinetic energy, andmechanical work) and other forms of energy (such as P-V energy) are usually given infoot-pounds-force. These are converted to Btu’s by using 1 Btu = 778.3 ft-lbf.This conversion factor is often used.

In fact, a constant called the mechanical equivalent of heat,usually denoted by the symbol J and sometimes referred to as Joule’s constant, is defined as:J778ft lbf.BtuPower is defined as the time rate of doing work. It is equivalent to the rate of the energytransfer. Power has units of energy per unit time. As with energy, power may be measured innumerous basic units, but the units are equivalent. In the English system, the mechanical unitsof power are foot-pounds-force per second or per hour (ft-lbf/sec or ft-lbf/hr) and horsepower(hp).

The thermal units of power are British thermal units per hour (Btu/hr), and the electricalunits of power are watts (W) or kilowatts (kW). For engineering applications, the equivalenceof these units is expressed by the following relationships.1 ft-lbf/sec = 4.6263 Btu/hr = 1.356 x 10-3 kW1 Btu/hr = 0.2162 ft-lbf/sec = 2.931 x 10-4 kW1 kW = 3.413 x 103 Btu/hr = 737.6 ft-lbf/secHorsepower is related to foot-pounds-force per second (ft-lbf/sec) by the following relationship:1 hp = 550.0 ft-lbf/secThese relationships can be used to convert the English system units for power.HT-01Page 24Rev.

0ThermodynamicsENERGY, WORK, AND HEATSummaryThe important information from this chapter is summarized below.Energy, Work, and Heat Summary•Heat is described as energy in transit. This transfer occurson a molecular level as a result of temperature differences.The unit of heat is the British thermal unit (Btu).Latent heat•=the amount of heat added orremoved to produce only aphase change.Sensible heat =the heat added or removedthat causes a temperaturechange.The following properties were defined:Specific enthalpy (h) is defined as h = u +Pν, where u is the specific internal energy(Btu/lbm) of the system being studied, P isthe pressure of the system (lbf/ft2), and ν isthe specific volume (ft3/lbm) of the system.Entropy is sometimes referred to as ameasure of the inability to do work for agiven heat transferred.Rev.

0Page 25HT-01THERMODYNAMIC SYSTEMS AND PROCESSESThermodynamicsTHERMODYNAMIC SYSTEMS AND PROCESSESDefining an appropriate system can greatly simplify a thermodynamic analysis.A thermodynamic system is any three-dimensional region of space that is boundedby one or more surfaces. The bounding surfaces may be real or imaginary andmay be at rest or in motion. The boundary may change its size or shape. Theregion of physical space that lies outside the selected boundaries of the system iscalled the surroundings or the environment.EO 1.10DESCRIBE the following types of thermodynamicsystems:a.Isolated systemb.Closed systemc.Open systemEO 1.11DEFINE the following terms concerningthermodynamic systems:a.Thermodynamic surroundingsb.Thermodynamic equilibriumc.Control volumed.Steady-stateEO 1.12DESCRIBE the following terms concerningthermodynamic processes:a.Thermodynamic processb.Cyclic processc.Reversible processd.Irreversible processe.Adiabatic processf.Isentropic processg.Throttling processh.Polytropic processThermodynamic Systems and SurroundingsThermodynamics involves the study of various systems.

A system in thermodynamics is nothingmore than the collection of matter that is being studied. A system could be the water within oneside of a heat exchanger, the fluid inside a length of pipe, or the entire lubricating oil system fora diesel engine. Determining the boundary to solve a thermodynamic problem for a system willdepend on what information is known about the system and what question is asked about thesystem.HT-01Page 26Rev. 0ThermodynamicsTHERMODYNAMIC SYSTEMS AND PROCESSESEverything external to the system is called the thermodynamic surroundings, and the system isseparated from the surroundings by the system boundaries.

These boundaries may either be fixedor movable. In many cases, a thermodynamic analysis must be made of a device, such as a heatexchanger, that involves a flow of mass into and/or out of the device. The procedure that isfollowed in such an analysis is to specify a control surface, such as the heat exchanger tubewalls. Mass, as well as heat and work (and momentum), may flow across the control surface.Types of Thermodynamic SystemsSystems in thermodynamics are classified as isolated, closed, or open based on the possibletransfer of mass and energy across the system boundaries.

An isolated system is one that is notinfluenced in any way by the surroundings. This means that no energy in the form of heat orwork may cross the boundary of the system. In addition, no mass may cross the boundary of thesystem.A thermodynamic system is defined as a quantity of matter of fixed mass and identity uponwhich attention is focused for study. A closed system has no transfer of mass with itssurroundings, but may have a transfer of energy (either heat or work) with its surroundings.An open system is one that may have a transfer of both mass and energy with its surroundings.Thermodynamic EquilibriumWhen a system is in equilibrium with regard to all possible changes in state, the system is inthermodynamic equilibrium.

For example, if the gas that comprises a system is in thermalequilibrium, the temperature will be the same throughout the entire system.Control VolumeA control volume is a fixed region in space chosen for the thermodynamic study of mass andenergy balances for flowing systems. The boundary of the control volume may be a real orimaginary envelope. The control surface is the boundary of the control volume.Steady StateSteady state is that circumstance in which there is no accumulation of mass or energy within thecontrol volume, and the properties at any point within the system are independent of time.Rev.

0Page 27HT-01THERMODYNAMIC SYSTEMS AND PROCESSESThermodynamicsThermodynamic ProcessWhenever one or more of the properties of a system change, a change in the state of the systemoccurs. The path of the succession of states through which the system passes is called thethermodynamic process. One example of a thermodynamic process is increasing the temperatureof a fluid while maintaining a constant pressure. Another example is increasing the pressure ofa confined gas while maintaining a constant temperature. Thermodynamic processes will bediscussed in more detail in later chapters.Cyclic ProcessWhen a system in a given initial state goes through a number of different changes in state (goingthrough various processes) and finally returns to its initial values, the system has undergone acyclic process or cycle. Therefore, at the conclusion of a cycle, all the properties have the samevalue they had at the beginning.

Steam (water) that circulates through a closed cooling loopundergoes a cycle.Reversible ProcessA reversible process for a system is defined as a process that, once having taken place, can bereversed, and in so doing leaves no change in either the system or surroundings. In other wordsthe system and surroundings are returned to their original condition before the process took place.In reality, there are no truly reversible processes; however, for analysis purposes, one usesreversible to make the analysis simpler, and to determine maximum theoretical efficiencies.Therefore, the reversible process is an appropriate starting point on which to base engineeringstudy and calculation.Although the reversible process can be approximated, it can never be matched by real processes.One way to make real processes approximate reversible process is to carry out the process in aseries of small or infinitesimal steps.

For example, heat transfer may be considered reversibleif it occurs due to a small temperature difference between the system and its surroundings. Forexample, transferring heat across a temperature difference of 0.00001 °F "appears" to be morereversible than for transferring heat across a temperature difference of 100 °F. Therefore, bycooling or heating the system in a number of infinitesamally small steps, we can approximate areversible process. Although not practical for real processes, this method is beneficial forthermodynamic studies since the rate at which processes occur is not important.Irreversible ProcessAn irreversible process is a process that cannot return both the system and the surroundings totheir original conditions. That is, the system and the surroundings would not return to theirHT-01Page 28Rev. 0ThermodynamicsTHERMODYNAMIC SYSTEMS AND PROCESSESoriginal conditions if the process was reversed.

For example, an automobile engine does not giveback the fuel it took to drive up a hill as it coasts back down the hill.There are many factors that make a process irreversible. Four of the most common causes ofirreversibility are friction, unrestrained expansion of a fluid, heat transfer through a finitetemperature difference, and mixing of two different substances. These factors are present in real,irreversible processes and prevent these processes from being reversible.Adiabatic ProcessAn adiabatic process is one in which there is no heat transfer into or out of the system.