Constant volume heat value vs. constant pressure heat value

  • Thread starter lalbatros
  • Start date
  • #1
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Hello

My story
Till today, I had never imagined that the High Heat Value (HHV) of fuels or their Lower Heat Value (LVH) have themselves (at least) two variants:

- the constant volume heat value
- the constant pressure heat value

These quantities can be most directly measured in respectively constant volume or constant pressure calorimeters.
Of course, conversions from one to the other values can also be performed, but may need additional information.

Till today, I implicitely assumed that the Lower Heat Values I used were on constant pressure basis.
I always made this assumption without knowing!

My question
I would like to know what the usual practices are.
I could guess that constant volume calorimeters are more often used, for greater simplicity.
However, many industrial processes are almost at constant pressure on the combustion side.
This is the case for coal combustion in power plant, or also in all industrial kilns.
Therefore, the constant pressure heat value is easier to use.

Would you know about traditions or legal obligations or other reasons for chosing constant volume or constant pressure heat values in the industry?
This is specially important when official reporting is involved.
For engineering heat balances we only need to know what we are doing.

Would you know some industrial or legal standards regarding this?
At the least, we should know the kind of HV we are using, in order to use them properly.
For official reporting (like heat consumption), this information should at least be specified.
By looking documents on the web, I found our that this information is NEVER indicated.
How can we make sense from unclearly defined data?

Thanks for your comment and suggestions.

Michel
 

Answers and Replies

  • #2
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Typically, heats of reaction are presented in thermodynamics as changes in enthalpy between reactants and products at standard state. The standard state is 1 atm pressure (or 1 bar) and 25 C. The effects of temperature and pressure in actual processes are then calculated from knowledge of the heat capacities of the reactants and products, and the equations of state for the gases.
 

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