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Can a gas expand and be compressed simultaneously?

  1. Feb 19, 2016 #1
    This is a thermodynamics question.I am a beginner and know only algebra.Here's a description.A cylinder full
    of CO2 gas at 670 psi is compressed by a piston.At the far end of the cylinder is a pipe,through which the CO2 gas flows to push a piston into another cylinder.At one third of the stroke the gas is still 670 psi because the gas that
    flows through the pipe prevents the piston from compressing the gas,or because the gas is expanding and being compressed simultaneously.
    Here are the questions;
    1) It appears that the gas is compressed and expanded at the same time.Is it possible to compress and expand a gas simultaneously?
    2)How would one calculate the entropy,enthalpy and temperature of the gas at the end of the one third of a stroke?Is it just a matter of calculating the these properties for expansion and compression and then taking either the difference or the average of the two?
    3)I have read that increasing temperature causes entropy to increase and increasing pressure reduces entropy.Which influences entropy more,pressure or temperature?In this case at the end of the one third stroke,would the entropy,temperature and enthalpy of the 670 psi gas be more or less than at the beginning of the stroke?
  2. jcsd
  3. Feb 19, 2016 #2


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    It sounds like neither is happening here. If the far piston is adding volume to the space at the same rate as the near piston is subtracting it, the volume is remaining constant, so neither compression nor expansion is occurring.

    In practice, I imagine what would happen is that at first the gas would start to compress a little from the effect of the near cylinder. Once it was compressed enough, it would start to move the far piston, and from that point the pressure would remain slightly higher until the near cylinder stops pushing, then the far piston would continue to retreat for a while as pressure gradually reduced to the original pressure, with the volume changing to match.

    So the pattern is (1) compress (2) stable [while both pistons are moving] (3) expand
    According to my thermo text (Callen), the Entropy is undefined, because entropy is only defined for a system in equilibrium. This system is not in equilibrium because of the moving pistons.
  4. Feb 20, 2016 #3
    The volume of the far cylinder is larger so the gas should expand ,but it is joined by gas at a higher pressure,coming from a reservoir,so this gas
    does not expand, but the gas in the near cylinder should,however it is simultaneously compressed by the near piston.The weight of the gas in the near cylinder at the end of one third of a stroke should give a specific volume for 670 psi.If it is impossible to calculate the entropy is it possible to know whether the entropy would be higher,lower or the same after one third of a stroke? Using this example,I would guess that if you had a piston compressing a gas in a cylinder and gas exiting through a pipe at the far side ,there would be compression and expansion of the gas at both sides of the cylinder with more compression at the piston side and more expansion at the exit side.Do you know the answer to question 3 ?
  5. Feb 22, 2016 #4
    The properties I would like to calculate are temperature,enthalpy and entropy,but especially entropy. Here's some more background info.The one third of a stroke takes one third of a second.I have a book on CO2 properties over a wide range of pressures and temperatures.According to it the properties of CO2 at 670 psi (which is 47.2 bar in IS units) is -42 j/kg/k.At the end of one third of the stroke the pressure is still 670 psi because some of the gas has expanded into the far cylinder while the remainder is being being compressed by the piston.I want to know whether the remaining gas will have (although I would prefer to calculate it) lower,higher, or the same entropy.I thought maybe I could look up these properties for the gas as if there was expansion and no compression,(which would bring the pressure down to 34.85 bar) and then the properties as if there was compression and no expansion (which would bring the pressure up to 56.6 bar) and then take an average of the two sets of properties,but I thought I should ask an expert first.
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