Thermodynamics, Heat Transfer, And Fluid Flow. V.1. Thermodynamics, страница 5

As molecules hit the walls, they exert forces that try to push the walls outward. Theforces resulting from all of these collisions cause the pressure exerted by a system on itssurroundings. Pressure is frequently measured in units of lbf/in2 (psi).Pressure ScalesWhen pressure is measured relative to a perfect vacuum, it is called absolute pressure (psia);when measured relative to atmospheric pressure (14.7 psi), it is called gauge pressure (psig).

Thelatter pressure scale was developed because almost all pressure gauges register zero when opento the atmosphere. Therefore, pressure gauges measure the difference between the pressure ofthe fluid to which they are connected and that of the surrounding air.If the pressure is below that of the atmosphere, it is designated as a vacuum. A perfect vacuumwould correspond to absolute zero pressure. All values of absolute pressure are positive, becausea negative value would indicate tension, which is considered impossible in any fluid. Gaugepressures are positive if they are above atmospheric pressure and negative if they are belowatmospheric pressure.

Figure 2 shows the relationships between absolute, gauge, vacuum, andatmospheric pressures, as do Equations 1-9 and 1-10.Figure 2Rev. 0Pressure RelationshipsPage 9HT-01TEMPERATURE AND PRESSURE MEASUREMENTSThermodynamicsPabs = Patm + Pgauge(1-9)Pabs = Patm - Pvac(1-10)Patm is atmospheric pressure, which is also called the barometric pressure. Pgauge is the gaugepressure, and Pvac is vacuum. Once again, the following examples relating the various pressureswill be helpful in understanding the idea of gauge versus absolute pressures.Example 1: Pressure RelationshipsHow deep can a diver descend in ocean water (density = 64 lbm/ft3) without damaginghis watch, which will withstand an absolute pressure of 80 psia? (P = density • height)Solution:Assume:Patm= 14.7 psiaPabs= Patm + Pgauge80 psia= 14.7 + PgaugePgauge= (80 - 14.7) = 65.3 psigPgauge= density(65.3)(144 in2/ft2)HT-01height = ρH= (64 lbm/ft3)HH= (65.3)(144)/(64)H= 146.9 ftPage 10Rev.

0ThermodynamicsTEMPERATURE AND PRESSURE MEASUREMENTSExample 2: Pressure RelationshipsWhat is the absolute pressure at the bottom of a swimming pool 6 feet deep that is filledwith fresh water? Patm = 14.7 psiaSolution:Pabs= Patm + Pgauge= 14.7 + ρH= 14.7 + [(62.4 lbm/ft3)(6 ft)/(144 in.2/ft2)]= 14.7 + 2.6Pabs= 17.3 psiaIn addition to pounds per square inch, pressure can be measured with reference to the force thatexists in a column of fluid at a certain height. The most common of these are inches of water,inches of mercury, millimeters of mercury, and microns of mercury.

Conversion factors are listedbelow.14.7 psia = 408 inches of water14.7 psia = 29.9 inches of mercury1 inch of mercury = 25.4 millimeters of mercury1 millimeter of mercury = 103 microns of mercuryRev. 0Page 11HT-01TEMPERATURE AND PRESSURE MEASUREMENTSThermodynamicsSummaryThe important information from this chapter is summarized below.Temperature and Pressure Scales SummaryThe following properties were defined as follows.•Temperature is a measure of the molecular activity of a substance.•Pressure is a measure of the force per unit area exerted on the boundaries of asubstance (or system).The relationship between the Fahrenheit, Celsius, Kelvin, and Rankine temperature scaleswas described.•Absolute zero= -460 °F or -273 °C•Freezing point of water = 32 °F or 0 °C•Boiling point of water= 212 °F or 100 °CConversions between the different scales can be made using the following formulas.•°F = 32 + (9/5)°C•°C = (°F - 32)(5/9)•°R = °F + 460•°K = °C + 273Relationships between absolute pressure, gauge pressure, and vacuum can be shownusing the following formulas.••HT-01PabsPabs= Patm + Pgauge= Patm - PvacPage 12Rev.

0ThermodynamicsTEMPERATURE AND PRESSURE MEASUREMENTSTemperature and Pressure Scales Summary (Cont.)Converting between the different pressure units can be done using the followingconversions.•14.7 psia= 408 inches of water•14.7 psia= 29.9 inches of mercury•1 inch of mercury•1 millimeter of mercuryRev. 0= 25.4 millimeters of mercury= 103 microns of mercuryPage 13HT-01ENERGY, WORK, AND HEATThermodynamicsENERGY, WORK, AND HEATHeat and work are the two ways in which energy can be transferred across theboundary of a system. One of the most important discoveries in thermodynamicswas that work could be converted into an equivalent amount of heat and that heatcould be converted into work.EO 1.8DEFINE the following:a.Heatb.Latent heatc.Sensible heatd.Units used to measure heatEO 1.9DEFINE the following thermodynamic properties:a.Specific enthalpyb.EntropyEnergyEnergy is defined as the capacity of a system to perform work or produce heat.Potential EnergyPotential energy (PE) is defined as the energy of position.

Using English system units, it isdefined by Equation 1-11.PEmgzgcPE=potential energy (ft-lbf)m=mass (lbm)z=height above some reference level (ft)g=acceleration due to gravity (ft/sec2)gc=gravitational constant = 32.17 ft-lbm/lbf-sec2(1-11)where:HT-01Page 14Rev.

0ThermodynamicsENERGY, WORK, AND HEATIn most practical engineering calculations, the acceleration due to gravity (g) is numerically equalto the gravitational constant (gc); thus, the potential energy (PE) in foot-pounds-force isnumerically equal to the product of the mass (m) in pounds-mass times the height (z) in feetabove some reference level.Example:Determine the potential energy of 50 lbm of water in a storage tank 100 ft above theground.Solution:Using Equation 1-11PEmgzgcPE(50 lbm) (32.17 ft/sec2) (100 ft)32.17 ft lbm/lbf sec2PE5000 ft lbfKinetic EnergyKinetic energy (KE) is the energy of motion.

Using English system units, it is defined byEquation 1-12.KEmv 22gcKE=kinetic energy (ft-lbf)m=mass (lbm)v=velocity (ft/sec)gc=gravitational constant = 32.17 ft-lbm/lbf-sec2(1-12)where:Rev. 0Page 15HT-01ENERGY, WORK, AND HEATThermodynamicsExample:Determine the kinetic energy of 7 lbm of steam flowing through a pipe at a velocity of100 ft/sec.Solution:Using Equation 1-12.mv 2KE2gcKE(7 lbm) (100 ft/sec)22(32.17 ft lbm/lbf sec2)KE(7 lbm) (10,000 ft 2/sec2)(64.34 ft lbm/lbf sec2)KE1088 ft lbfSpecific Internal EnergyPotential energy and kinetic energy are macroscopic forms of energy. They can be visualizedin terms of the position and the velocity of objects.

In addition to these macroscopic forms ofenergy, a substance possesses several microscopic forms of energy. Microscopic forms of energyinclude those due to the rotation, vibration, translation, and interactions among the molecules ofa substance. None of these forms of energy can be measured or evaluated directly, buttechniques have been developed to evaluate the change in the total sum of all these microscopicforms of energy. These microscopic forms of energy are collectively called internal energy,customarily represented by the symbol U. In engineering applications, the unit of internal energyis the British thermal unit (Btu), which is also the unit of heat.The specific internal energy (u) of a substance is its internal energy per unit mass.

It equals thetotal internal energy (U) divided by the total mass (m).Uu(1-13)mwhere:HT-01u=specific internal energy (Btu/lbm)U=internal energy (Btu)m=mass (lbm)Page 16Rev. 0ThermodynamicsENERGY, WORK, AND HEATExample:Determine the specific internal energy of 12 lbm of steam if the total internal energy is23,000 Btu.Solution:Using Equation 1-13.Uumu23,000 Btu12 lbmu1916.67 Btu/lbmSpecific P-V EnergyIn addition to the internal energy (U), another form of energy exists that is important inunderstanding energy transfer systems. This form of energy is called P-V energy because itarises from the pressure (P) and the volume (V) of a fluid.

It is numerically equal to PV, theproduct of pressure and volume. Because energy is defined as the capacity of a system toperform work, a system where pressure and volume are permitted to expand performs work onits surroundings. Therefore, a fluid under pressure has the capacity to perform work. Inengineering applications, the units of P-V energy, also called flow energy, are the units ofpressure times volume (pounds-force per square foot times cubic feet), which equals foot-poundsforce (ft-lbf).The specific P-V energy of a substance is the P-V energy per unit mass.