An excerpt from the beginning of PART I - INTRODUCTION: THE province of Engineering Thermodynamics is to guide numerical thermal computations dealing with actual substances and apparatus in accordance with the laws of thermodynamic philosophy. In order to do this, numerical values for heat effects must be available for the various substances and materials used in engineering under the varying conditions of practice, and in such units as may readily be applied; these include especially that class of units known as physical ...
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An excerpt from the beginning of PART I - INTRODUCTION: THE province of Engineering Thermodynamics is to guide numerical thermal computations dealing with actual substances and apparatus in accordance with the laws of thermodynamic philosophy. In order to do this, numerical values for heat effects must be available for the various substances and materials used in engineering under the varying conditions of practice, and in such units as may readily be applied; these include especially that class of units known as physical constants which embrace, for example, such quantities as the coefficients of expansion, the specific heats, latent heats of fusion and vaporization, the ratio of the pressure-volume product to absolute temperature, the exponent "s" in adiabatic expansion of gases and vapors, and various other quantities. In addition to the physical constants which are necessary in the work of thermodynamic computation, the solution of numerical problems is greatly facilitated by the use of other correlated tables and diagrams many of which are given in the present book of tables, but to correctly use such aids there should be no ambiguity in regard to the units employed. It should be noted that true pressures are always absolute, that is, measured above a perfect vacuum or counted from zero, while most pressure gages and other devices for measuring pressure, such as indicators, give results measured above or below atmospheric pressure. In all problems involving work of gases and vapors, the absolute values of the pressures must be used; hence, if a gage or indicator measurement is being considered, the pressure of the atmosphere found by means of the barometer must be added to the pressure above atmosphere in order to obtain the absolute or true pressures. When the pressures are below atmosphere the combination with the barometric reading will depend on the record; if the record be taken by an indicator it will be in pounds per square inch below atmosphere and must be subtracted from the barometric equivalent in the same units to give the absolute pressure in pounds per square inch. When, however, a vacuum gage reads in inches of mercury below atmosphere, as such gages do, the difference between its reading and the barometric gives the absolute pressure in inches of mercury directly, which can be converted to the desired units by the proper factors. In general, steam pressures are most commonly stated in pounds per square inch and are designated as either gage or absolute. Pressures of compressed air are commonly expressed in the same units as steam, either gage or absolute, though sometimes in atmospheres. Steam pressures below atmosphere are conveniently stated as a vacuum of so many inches of mercury, or they may be given as a pressure of so many inches of mercury absolute or so many pounds per square inch absolute. The pressures of gases stored in tanks under high pressure are frequently recorded in atmospheres due to the convenience of computation of quantities on this basis. Pressures of air obtained by blowers or fans are sometimes given in ounces per square inch above atmosphere, but such pressures, and also differences of pressure of air due to chimney draught, or forced draught, and the pressure of illuminating gas in city mains are commonly stated in inches of water. In many cases the data are given in other units which must be converted by the use of tables, diagrams or otherwise, before the results can be properly interpreted or intelligently compared. Time is an important item in all engineering work and none the less so in computations, so that convenient tables and diagrams are most essential to the solution of such problems....
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