Thermodynamics, Heat Transfer, And Fluid Flow. V.1. Thermodynamics, страница 4

0Page 1HT-01THERMODYNAMIC PROPERTIESThermodynamicsAccording to Newton’s Second Law of Motion, force (F) = ma, where a is acceleration. Forexample, on earth an object has a certain mass and a certain weight. When the same object isplaced in outer space, away from the earth’s gravitational field, its mass is the same, but it isnow in a "weightless" condition (that is, gravitational acceleration and, thus, force equal zero).The English system uses the pound-force (lbf) as the unit of weight. Knowing that accelerationhas the units of ft/sec2 and using Newton’s second law, we can determine that the units of massare lbf-sec2/ft. For simplification, 1 lbf-sec2/ft is called a slug. The basic unit of mass in theEnglish system is the slug.

However, the slug is an almost meaningless unit for the averageindividual. The unit of mass generally used is the pound-mass (lbm). In order to allow lbm tobe used as a unit of mass, we must divide Newton’s second law by the gravitational constant (gc).32.17 lbm ft lbf sec2 gcNewton’s second law can be expressed by Equation 1-2.Fmagc(1-2)Use of the gravitational constant, gc, adapts Newton’s second law such that 1 lbf = 1 lbm at thesurface of the earth.

It is important to note that this relationship is only true at the surface of theearth, where the acceleration due to gravity is 32.17 ft/sec2. However, because all of ourdiscussions will be based upon experiences and observations on earth, we will use the lbm as theunit of mass.NOTE:In Equation 1-2, acceleration "a" is often written as "g" because, in this case, theacceleration is the gravitational acceleration due to the earth’s gravitational field(g = 32.17 ft/sec2).Example:Using Equation 1-2, prove that 1 lbf = l lbm on earth.Solution:F1 lbf1 lbfHT-01mggc(1 lbm) (32.17 ft/sec2)(lbm ft)32.17(lbf sec2)1 lbf ( an equality)Page 2Rev. 0ThermodynamicsTHERMODYNAMIC PROPERTIESSpecific VolumeThe specific volume (ν) of a substance is the total volume (V) of that substance divided by thetotal mass (m) of that substance (volume per unit mass).

It has units of cubic feet perpound-mass (ft3/lbm).νVm(1-3)where:ν=specific volume (ft3/lbm)V=volume (ft3)m=mass (lbm)DensityThe density ( ρ ) of a substance is the total mass (m) of that substance divided by the totalvolume (V) occupied by that substance (mass per unit volume). It has units of pound-mass percubic feet (lbm/ft3). The density ( ρ ) of a substance is the reciprocal of its specific volume (ν).ρmV1ν(1-4)where:Rev. 0ρ=density (lbm/ft3)m=mass (lbm)V=volume (ft3)ν=specific volume (ft3/lbm)Page 3HT-01THERMODYNAMIC PROPERTIESThermodynamicsSpecific GravitySpecific gravity (S.G.) is a measure of the relative density of a substance as compared to thedensity of water at a standard temperature.

Physicists use 39.2°F (4°C) as the standard, butengineers ordinarily use 60°F. In the International System of Units (SI Units), the density ofwater is 1.00 g/cm3 at the standard temperature. Therefore, the specific gravity (which isdimensionless) for a liquid has the same numerical value as its density in units of g/cm3. Sincethe density of a fluid varies with temperature, specific gravities must be determined and specifiedat particular temperatures.HumidityHumidity is the amount of moisture (water vapor) in the air. It can be expressed as absolutehumidity or relative humidity. Absolute humidity is the mass of water vapor divided by a unitvolume of air (grams of water/cm3 of air). Relative humidity is the amount of water vaporpresent in the air divided by the maximum amount that the air could contain at that temperature.Relative humidity is expressed as a percentage.

The relative humidity is 100% if the air issaturated with water vapor and 0% if no water vapor is present in the air at all.Intensive and Extensive PropertiesThermodynamic properties can be divided into two general classes, intensive and extensiveproperties. An intensive property is independent of the amount of mass. The value of anextensive property varies directly with the mass. Thus, if a quantity of matter in a given stateis divided into two equal parts, each part will have the same value of intensive property as theoriginal and half the value of the extensive property.

Temperature, pressure, specific volume,and density are examples of intensive properties. Mass and total volume are examples ofextensive properties.HT-01Page 4Rev. 0ThermodynamicsTHERMODYNAMIC PROPERTIESSummaryThe important information from this chapter is summarized below.Thermodynamic Properties SummaryThe following properties were defined:•Specific volume (ν) is the total volume (V) of a substance divided by thetotal mass (m) of that substance.•Density (ρ) is the total mass (m) of a substance divided by the totalvolume (V) occupied by that substance.•Specific gravity (S.G.) is a measure of the relative density of a substanceas compared to the density of water at a standard temperature.•Humidity is the amount of moisture (water vapor) in the air.

It can bemeasured in absolute or relative units.The following classifications of thermodynamic properties were described:•Intensive properties are those that are independent of the amount of mass.•Extensive properties are those that vary directly with the mass.Rev. 0Page 5HT-01TEMPERATURE AND PRESSURE MEASUREMENTSThermodynamicsTEMPERATURE AND PRESSURE MEASUREMENTSSeveral types of temperature and pressure measurements are used duringdiscussions of thermodynamics. Operators must recognize the different types andtheir interrelationships in order to understand thermodynamics.EO 1.3DEFINE the thermodynamic properties temperatureand pressure.EO 1.4DESCRIBE the Fahrenheit, Celsius, Kelvin, andRankine temperature scales including:a.Absolute zero temperatureb.The freezing point of water at atmospheric pressurec.The boiling point of water at atmospheric pressureEO 1.5CONVERT temperatures between the Fahrenheit,Celsius, Kelvin, and Rankine scales.EO 1.6DESCRIBE the relationship between absolutepressure, gauge pressure, and vacuum.EO 1.7CONVERT pressures between the following units:a.Pounds per square inchb.Inches of waterc.Inches of mercuryd.Millimeters of mercurye.Microns of mercuryTemperatureTemperature is a measure of the molecular activity of a substance.

The greater the movementof molecules, the higher the temperature. It is a relative measure of how "hot" or "cold" asubstance is and can be used to predict the direction of heat transfer.Temperature ScalesThe two temperature scales normally employed for measurement purposes are the Fahrenheit (F)and Celsius (C) scales. These scales are based on a specification of the number of incrementsbetween the freezing point and boiling point of water at standard atmospheric pressure. TheCelsius scale has 100 units between these points, and the Fahrenheit scale has 180 units. Thezero points on the scales are arbitrary.HT-01Page 6Rev. 0ThermodynamicsTEMPERATURE AND PRESSURE MEASUREMENTSThe freezing point of water was selected as the zero point of the Celsius scale.

The coldesttemperature achievable with a mixture of ice and salt water was selected as the zero point of theFahrenheit scale. The temperature at which water boils was set at 100 on the Celsius scale and212 on the Fahrenheit scale. The relationship between the scales is represented by the followingequations.°F = 32.0 + (9/5)°C(1-5)°C = (°F - 32.0)(5/9)(1-6)It is necessary to define an absolute temperature scale having only positive values. The absolutetemperature scale that corresponds to the Celsius scale is called the Kelvin (K) scale, and theabsolute scale that corresponds to the Fahrenheit scale is called the Rankine (R) scale.

The zeropoints on both absolute scales represent the same physical state. This state is where there is nomolecular motion of individual atoms. The relationships between the absolute and relativetemperature scales are shown in the following equations.°R = °F + 460(1-7)°K = °C + 273(1-8)Figure 1Rev.

0Comparison of Temperature ScalesPage 7HT-01TEMPERATURE AND PRESSURE MEASUREMENTSThermodynamicsThe conversion of one temperature scale to another is sometimes required at nuclear facilities,and the operator should be acquainted with the process. The following two examples will behelpful.Example 1: Temperature Scale ConversionWhat is the Rankine equivalent of 80°C?Solution:°F= (9/5) °C + 32= (9/5)(80) + 32= 176 °F°R= °F + 460= 176 + 460= 636 °RExample 2: Temperature Scale ConversionWhat is the Kelvin equivalent of 80°F?Solution:°C= (5/9) (°F - 32)= (5/9) (80 - 32)= 26.7°C°K= °C + 273= 26.7 + 273= 299.7 °KHT-01Page 8Rev. 0ThermodynamicsTEMPERATURE AND PRESSURE MEASUREMENTSPressurePressure is a measure of the force exerted per unit area on the boundaries of a substance (orsystem). It is caused by the collisions of the molecules of the substance with the boundaries ofthe system.