Ideal Gas Rulebreakers
So what do you think of when you hear an “ideal gas”? R22, R12 maybe… Natural? Take a look at the F-18 above… It is breaking the sound barrier and that cloud is a shockwave… This has nothing to do with this article but I think it’s pretty darn cool!
An ideal gas is a gas that obeys the ideal gas law, it’s ideal because it’s good at following rules. These ideal gasses walk in a straight line, they don’t run on the playground and they never fish without a proper permit. More like an ideal gas behaves in a predictable way with changes in volume, pressure, temperature, and mass.
The problem is, a truly “ideal” gas really doesn’t exist.
While many gasses behave close to ideal at normal temperatures there is no gas that obeys the ideal gas laws in all conditions.
The ideal gas law is –
P= Absolute Pressure (gauge pressure + atmospheric pressure)
V = Volume (How much space the gas occupies)
n = Mass measured in “moles” (the number of molecules)
R = The universal gas constant (varies depending on the units of measure being used Example: [lbf ft/(lb mol oR)]= 8.3145 )
T = Absolute Temperature (temperature in a scale that starts at absolute zero like Kelvin or Rankine)
The ideal gas law is really a combination of several different laws into one.
The result is that many gasses that we work with behave in about the same way with changes in mass, volume, temperature & pressure. This is the case because the primary force at play in a nearly ideal gas like nitrogen or CO2 is simply the velocity of the molecules bouncing around in the container and against one another like tiny little ping pong balls.
If the molecules react, or interact with one another through attraction or repulsion due to their intermolecular forces then they can cease to behave as an ideal gas. A perfect example is when a gas is in contact with its liquid form (saturation) it no longer obeys the gas laws. This is why most gasses behave more and more like an ideal gas the hotter they get (within a range) because the hotter they are the greater the force of molecular velocity (temperature) will be relative to the intermolecular interaction of the molecules.
Once the gas gets to the “supercritical” state all bets are off once again. So like most good kids, even the most ideal gasses have their limits where if pushed they become little molecular rebels.
Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade