Enthalpy of Air: Calculating It With & Without Steam Tables

PDF download Download Article
A step-by-step guide on air enthalpy calculation (what formula to use and more)
PDF download Download Article

Psychrometry is the study of properties of air, which is an important part of the field of Air-conditioning. One important property is the enthalpy, which is a measure of the energy present in the substance at a certain state.

Things You Should Know

  • The equation for enthalpy is h = ha + H*hg where ha is the specific enthalpy of dry air, H is the humidity ratio, and hg is the specific enthalpy of water vapor.
  • You can look online for a steam table and use that to determine hg (the specific enthalpy of water vapor).
  • If you have to calculate the specific enthalpy of water vapor manually without a steam table, use the formula 2501 kJ/kg + cpw*T .
Method 1
Method 1 of 2:

With Steam Tables

PDF download Download Article
  1. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/0\/0e\/Calculate-the-Enthalpy-of-Moist-Air-Step-1-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-1-Version-2.jpg","bigUrl":"\/images\/thumb\/0\/0e\/Calculate-the-Enthalpy-of-Moist-Air-Step-1-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-1-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    Specific enthalpy (enthalpy per kg of dry air) of moist air is the sum of the specific enthalpy of dry air and the specific enthalpy of  the water vapor in the air. This is given by the equation: h = ha + H*hg ; where ha is the specific enthalpy of dry air, H is the humidity ratio, and hg is the specific enthalpy of water vapor.
  2. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/a\/a9\/Calculate-the-Enthalpy-of-Moist-Air-Step-2-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-2-Version-2.jpg","bigUrl":"\/images\/thumb\/a\/a9\/Calculate-the-Enthalpy-of-Moist-Air-Step-2-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-2-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    This is given by the equation cpa*T; where cpa is the specific heat of air, and T is the temperature of air.
    • You have to memorize the value of cpa which is 1.006 kJ/kgC since this is constant. T must be given in the problem, or it can simply be measured with a thermometer or other temperature measuring devices.
    Advertisement
  3. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/0\/06\/Calculate-the-Enthalpy-of-Moist-Air-Step-3-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-3-Version-2.jpg","bigUrl":"\/images\/thumb\/0\/06\/Calculate-the-Enthalpy-of-Moist-Air-Step-3-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-3-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    Humidity ratio is the ratio of the mass of water vapor in the moist air and the mass of dry air. In layman's terms, it gives the wetness of air. Along with the temperature of air, this must be given in the problem.
  4. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/1\/18\/Calculate-the-Enthalpy-of-Moist-Air-Step-4-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-4-Version-2.jpg","bigUrl":"\/images\/thumb\/1\/18\/Calculate-the-Enthalpy-of-Moist-Air-Step-4-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-4-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    Using a steam table, this can easily be found. Once you have the temperature, bring out your saturated steam table and look for the value of hg. This value has the units kJ/kg.
  5. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/0\/09\/Calculate-the-Enthalpy-of-Moist-Air-Step-5-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-5-Version-2.jpg","bigUrl":"\/images\/thumb\/0\/09\/Calculate-the-Enthalpy-of-Moist-Air-Step-5-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-5-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    Find the specific enthalpy of moist air at 25°C with 0.02 kg/kg moisture. These are the T and H, respectively.
    • Get ha. Use the equation given above and the given T in the problem.
    • Get hg. Use the given T and your steam table.
    • Get h. Use your calculated values of ha and hg, and the given value of H. If your answer is 76.09 kJ/kg or something very close to that, you got the right answer!
    6. Advertisement
Method 2
Method 2 of 2:

Without Steam Tables

PDF download Download Article
  1. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/b\/ba\/Calculate-the-Enthalpy-of-Moist-Air-Step-6-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-6-Version-2.jpg","bigUrl":"\/images\/thumb\/b\/ba\/Calculate-the-Enthalpy-of-Moist-Air-Step-6-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-6-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    Start with the original equation h = ha + Hhg .
  2. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/c\/c3\/Calculate-the-Enthalpy-of-Moist-Air-Step-7-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-7-Version-2.jpg","bigUrl":"\/images\/thumb\/c\/c3\/Calculate-the-Enthalpy-of-Moist-Air-Step-7-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-7-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    If you have no access to a steam table, hg can be expressed as: hg = 2501 kJ/kg + cpw*T.
    • You must memorize the value 2501 kJ/kg, which is the specific enthalpy of water vapor at 0°C.
    • Therefore, to get the specific enthalpy at any temperature, you must add the term cpw*T; where cpw = 1.84 kJ/kg°C, which is the specific heat of water vapor at constant pressure; and T is the temperature of air.
  3. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/1\/18\/Calculate-the-Enthalpy-of-Moist-Air-Step-8-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-8-Version-2.jpg","bigUrl":"\/images\/thumb\/1\/18\/Calculate-the-Enthalpy-of-Moist-Air-Step-8-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-8-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    h = cpa*T + H*(2501 kJ/kg + cpw*T). As you can see, you only need T and H from the problem, and memorize the values of cpa, cpw and 2501 kJ/kg to solve any problem.
  4. {"smallUrl":"https:\/\/www.wikihow.com\/images\/thumb\/b\/b5\/Calculate-the-Enthalpy-of-Moist-Air-Step-9-Version-2.jpg\/v4-460px-Calculate-the-Enthalpy-of-Moist-Air-Step-9-Version-2.jpg","bigUrl":"\/images\/thumb\/b\/b5\/Calculate-the-Enthalpy-of-Moist-Air-Step-9-Version-2.jpg\/v4-728px-Calculate-the-Enthalpy-of-Moist-Air-Step-9-Version-2.jpg","smallWidth":460,"smallHeight":345,"bigWidth":728,"bigHeight":546,"licensing":"<div class=\"mw-parser-output\"><p>License: <a target=\"_blank\" rel=\"nofollow noreferrer noopener\" class=\"external text\" href=\"https:\/\/creativecommons.org\/licenses\/by-nc-sa\/3.0\/\">Creative Commons<\/a><br>\n<\/p><p><br \/>\n<\/p><\/div>"}
    Find the specific enthalpy of moist air at 25°C with 0.02 kg/kg moisture. If your answer is 76.09 kJ/kg or something very close to that, you got the right answer!
    5. Advertisement


Community Q&A

Search
Add New Question
  • Question
    How did you get H=0.02 kg/kg?
    Han
    Community Answer
    H=0.02 kg/kg is part of the given data for the specific example problem in the article.
    Thanks! We're glad this was helpful.
    Thank you for your feedback.
    If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow
    Yes No
    Not Helpful 1 Helpful 14
Ask a Question
200 characters left
Include your email address to get a message when this question is answered.
Submit
      Advertisement

      Tips

      Submit a Tip
      All tip submissions are carefully reviewed before being published
      Submit
      Thanks for submitting a tip for review!

      You Might Also Like

      How to
      Calculate Theoretical Yield
      How to
      Calculate the Concentration of a Solution
      How to Calculate Molarity: Easy Step-by-Step Guide
      How to
      Calculate Electronegativity
      How to
      Calculate Percent Yield in Chemistry
      How to
      Calculate the Enthalpy of a Chemical Reaction
      How to
      Calculate Molar Mass
      How to
      Calculate Vapor Pressure
      How to
      Calculate Molar Absorptivity
      How to
      Convert Grams to Moles
      How to
      Calculate Bond Energy
      How to Calculate Average Atomic Mass (and Use the Result)
      How to
      Calculate Bond Order in Chemistry
      How to
      Calculate Mass Percent
      Advertisement

      Expert Interview

      Thanks for reading our article! If you’d like to learn more about chemistry, check out our in-depth interview with Dr. Vivek Prakash .

      References

      1. http://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.html

      About This Article

      Thanks to all authors for creating a page that has been read 70,797 times.

      Did this article help you?

      Advertisement