Calculate Energy of Compression for Hydrogen

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Discussion Overview

The discussion revolves around determining the appropriate equation to calculate the energy required to compress hydrogen gas using a pump, specifically whether to use isothermal or adiabatic models. Participants explore the conditions under which each model applies and the implications of heat transfer during the compression process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether to use isothermal or adiabatic equations for calculating the energy of compression.
  • Another participant suggests that the choice depends on the pump's operation, indicating that if the temperature is fixed, an isothermal model is appropriate, while an adiabatic model applies if little heat is lost.
  • A participant notes that real systems may exhibit behavior between isothermal and adiabatic processes, emphasizing the need to choose the best model based on available information.
  • Further inquiry is made about the formula for isothermal compression and the effects of heat transfer during or after compression.
  • One participant explains that the work done varies based on when heat transfer occurs and mentions the relationship between efficiency improvements in gas turbines and isothermal processes.
  • The same participant describes the general work formula and highlights the differences in application for isothermal versus adiabatic processes, noting that the path of integration affects the outcome.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of isothermal versus adiabatic models, indicating that no consensus has been reached regarding the best approach for the specific scenario of compressing hydrogen.

Contextual Notes

Participants acknowledge that the choice of model may depend on specific conditions such as heat transfer and the efficiency of the pump, which are not fully defined in the discussion.

aboElFod
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I am having a problem determining the equation to use here. I want to find the energy to compress hydrogen using a pump from a low pressure to a high pressure. Do I use an isothermal or an adiabatic equation?

thanks
 
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It depends on how your pump is working.

If it ensure that the temperature is fixed, then it would be isothermal.
This would be a good model if there was an excellent heat exchange between the gas and the pump, and if the pump itself was maintained at a fixed temperature for example by an efficient cooling system.

If, on the contrary, we can assume there is only little heat lost from the gas, then it would adiabatic.

In a real system, it would probably be somwhere in between.
In a real process there could be still other differences.

In summary, you have to chose the best model.
This choice may depend on other information available to you.
 
Thanks for the info the problem is starting to clear up now.
What's the formula for Isothermal compression?. and would it be isothermal if heat was transferred to the system after or during compression of the gas?

Thanks
fady
 
The work would be totally different depending on when the heat transfer occurs.
In gas turbines, approaching an isothermal process leads to efficiency improvements.
(and the transformation is close to adiabatic too ... no heat losses, both together makes it reversible!)

The formula to use is the same for all processes:

Work = integral (dW) = integral (- p dv)

The difference occurs simply because of path of integration.
For a perfect gas, you also always have pv=nRT.
For an isothermal process T is constant.
For an adiabatic process, the work done heats up the gas: dE = n cv.dT = dW .

From this you see that the formulas are -in a sense- the same, but the way to use them is different: either T is fixed, or T increases.
 

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