Ask A Scientist© General Science Archive

 
name         Douglas
status       educator
age          30s

Question -   I am a transport jet aircraft maintenance
instructor.  During a class on turbine engines, a figure involving
"pounds of air" came up.  One of my students promptly asked me what the
volume of a pound of air in cubic feet is.  Hmmmm.  No idea.  Can youhelp?
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For an ideal gas PV=mRT, where P is the pressure, V is the volume, m is the
mass, R is the gas constant, and T is the temperature.  As you can see, the
volume of a pound of air is dependent on the pressure and temperature.  Air
has a mass of 28.97 lbm, and R for air is 53.34 ft-lbf/lbm-R.  So for air at
14.7 psi and a temperature of 70 F (530 R):

V=mRT/P = (28.97 lbm*53.34 ft-lbf/lbm-R*530 R)/ (14.7 lbf/in^2*144in^2/ft^2)
V=387 ft^3=2894 gallons

So, for any given pressure and temperature, you can find the volume (be sure
to keep track of your units).  Most values for m and R can be found in
thermodynamic tables, so you can find volumes of any gas when you know the
temperature and pressure.
Hope this helped.
Dr. C. Murphy
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