Managed to find the humidity ratio

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    Humidity Ratio
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Discussion Overview

The discussion revolves around determining the specific enthalpy of moist air given specific conditions (760 mmHg, 80% relative humidity, and 30 degrees C). Participants explore various methods for calculating specific enthalpy based on the humidity ratio already found.

Discussion Character

  • Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant has calculated the humidity ratio of moist air as 21.65 g/kg and seeks to find the specific enthalpy.
  • Another participant suggests assuming the water vapor is at saturation and looking up the enthalpy in a steam table for the given temperature and pressure.
  • A different participant claims that the specific enthalpy should be 85.25 kJ/kg, noting that this value is not found in the steam table for the specified conditions, implying a need for an alternative equation.
  • Another participant recommends using a psychometric chart to find the specific enthalpy corresponding to the given temperature and relative humidity, asserting that it yields the same value of 85.25 kJ/kg.
  • One participant proposes a formula involving the humidity ratio, suggesting that specific enthalpy can be expressed as a sum of enthalpy of dry air and the product of humidity ratio and enthalpy of water.

Areas of Agreement / Disagreement

Participants express differing views on the methods to determine specific enthalpy, with no consensus on the correct approach or final value. Multiple competing methods and values are presented.

Contextual Notes

There are unresolved assumptions regarding the applicability of steam tables and psychometric charts, as well as the specific conditions under which the proposed equations hold true.

TSN79
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Moist air has these values; 760mmHg, 80%, 30 degrees C. I have managed to find the humidity ratio (21,65 g/kg), but how can I determine the specific enthalpy?
 
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TSN79 said:
Moist air has these values; 760mmHg, 80%, 30 degrees C. I have managed to find the humidity ratio (21,65 g/kg), but how can I determine the specific enthalpy?

Can you assume the water vapor is at saturation, then look up the enthalpy in a steam table for that temp/press combination?
 
No, the answer should be 85.25 kJ/kg, and this value is not in the table for this temp/pressure comb., so there must be some equation in use to figure it out...I guess.
 
Use a psychometric chart for water in air (One is presented in " Basic Principles and Calculations in Chemical Engineering" -David Himmelblau ,Fourth Edition- Prentice Hall, Inc. ) and intercept temperature ( 30 C) and relative humidity ( 80 %). Then you read the corresponding specific enthalpy 85. 25 kJ/kg.
 
What about if you prove with:

[tex]h=h_{air}+\omega h_{water}[/tex]

where [tex]\omega[/tex] is the humidity ratio?
 

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