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Chapter 11 Pneumatics
Compressibility and Expansion of Gases

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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The specific volume (v) is expressed in cubic feet per  pound. For any weight of a gas this equation maybe modified  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 then becomes Solving  equation  11-4  for  pressure, In chapter 2 we defined density as the mass per  unit  volume.  In  equation  11-5, represents  density.  (Notice  that  this  is  the  reverse of  the  specific  volume.)  We  can  now  say  that pressure is equal to the density of the gas times the gas constant times the absolute temperature of the gas. (The gas constant varies for different gases.)  From  this  equation  we  can  show  how density   varies   with   changes   in   pressure   and temperature.  Decreasing  the  volume,  with  the weight   of   the   gas   and   the   temperature   held constant, 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 constant will cause density to increase. TEMPERATURE As   indicated   previously,   temperature   is   a dominant factor affecting the physical properties of gases. It is of particular concern in calculating changes in the states of gases. Three  temperature  scales  are  used  extensively in gas calculations. They are the Celsius (C), the Fahrenheit  (F),  and  the  Kelvin  (K)  scales.  The Celsius  (or  centigrade)  scale  is  constructed  by identifying  the  freezing  and  boiling  points  of water, under standard conditions, as fixed points of   0°   and   100°,   respectively,   with   100   equal divisions  between.  The  Fahrenheit  scale  identifies 32° as the freezing point of water and 212° as the boiling   point,    and   has   180   equal   divisions between.  The  Kelvin  scale  has  its  zero  point  equal to  –273°C,  or  –460°F. Absolute   zero,   one   of   the   fundamental constants  of  physics,  is  commonly  used  in  the study of gases. It is usually expressed in terms of the  Celsius  scale.  If  the  heat  energy  of  a  gas sample   could   be   progressively   reduced,   some temperature  should  be  reached  at  which  the motion of the molecules would cease entirely. If accurately  determined,  this  temperature  could then be taken as a natural reference, or as a true absolute  zero  value. Experiments with hydrogen indicated that if a  gas  were  cooled  to  –273.16°C  (–273°  for  most calculations),  all  molecular  motion  would  cease and no additional heat could be extracted. Since this is the coldest temperature to which an ideal gas can be cooled, it is considered to be absolute zero.  Absolute  zero  may  be  expressed  as  0°K, –273°C, or    –459.69°F   (–460°F   for   most calculations). When  you  work  with  temperatures,  always  be sure which system of measurement is being used and  how  to  convert  from  one  to  another.  The conversion  formulas  are  shown  in  figure  11-1.  For purposes  of  calculations,  the  Rankine  (R)  scale illustrated  in  figure  11-1  is  commonly  used  to 11-2







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