The specific volume (v) is expressed in cubic feetper pound.For any weight of a gas this equation maybemodified as follows:W = weight of the gas in pounds,V = volume of W pounds of the gas in cubic feet.The volume of 1 pound would then be V/W.If we substitute this for v in equation 11-3, it thenbecomesSolving equation 11-4 for pressure,In chapter 2 we defined density as the massper unit volume. In equation 11-5,represents density. (Notice that this is the reverseof the specific volume.) We can now say thatpressure is equal to the density of the gas timesthe gas constant times the absolute temperatureof the gas. (The gas constant varies for differentgases.) From this equation we can show howdensity varies with changes in pressure andtemperature. Decreasing the volume, with theweight of the gas and the temperature heldconstant, causes the pressure to increase.NOTE: During the compression of the gas,the temperature will actually increase; however,the explanation is beyond the scope of this text.a decrease in volume with the weight held constantwill cause density to increase.TEMPERATUREAs indicated previously, temperature is adominant factor affecting the physical propertiesof gases. It is of particular concern in calculatingchanges in the states of gases.Three temperature scales are used extensivelyin gas calculations. They are the Celsius (C), theFahrenheit (F), and the Kelvin (K) scales. TheCelsius (or centigrade) scale is constructed byidentifying the freezing and boiling points ofwater, under standard conditions, as fixed pointsof 0° and 100°, respectively, with 100 equaldivisions between. The Fahrenheit scale identifies32° as the freezing point of water and 212° as theboiling point, and has 180 equal divisionsbetween. The Kelvin scale has its zero point equalto –273°C, or –460°F.Absolute zero, one of the fundamentalconstants of physics, is commonly used in thestudy of gases. It is usually expressed in terms ofthe Celsius scale. If the heat energy of a gassample could be progressively reduced, sometemperature should be reached at which themotion of the molecules would cease entirely. Ifaccurately determined, this temperature couldthen be taken as a natural reference, or as a trueabsolute zero value.Experiments with hydrogen indicated that ifa gas were cooled to –273.16°C (–273° for mostcalculations), all molecular motion would ceaseand no additional heat could be extracted. Sincethis is the coldest temperature to which an idealgas can be cooled, it is considered to be absolutezero. Absolute zero may be expressed as 0°K,–273°C, or –459.69°F (–460°F for mostcalculations).When you work with temperatures, always besure which system of measurement is being usedand how to convert from one to another. Theconversion formulas are shown in figure 11-1. Forpurposes of calculations, the Rankine (R) scaleillustrated in figure 11-1 is commonly used to11-2
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business