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Homework Help: Airflow and hair dryer efficiency

  1. Jun 11, 2015 #1
    • Poster has been reminded to posts their work in the first post of schoolwork threads.
    1. The problem statement, all variables and given/known data
    Can the power efficiency of the domestic hair dryer be improved by reducing the heat energy and increasing airflow?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 11, 2015 #2


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    Probably not. Think about drag force and see what you come up with.
  4. Jun 11, 2015 #3
    Thanks for your reply. I've actually been trying to find something that relates heat energy supply, evaporation rate, partial pressure, relative humidity, water(liquid), water(vapour in air), temperature, surface area of hair to be dried and airflow! With nearly all these variables both in constant flux and affected by each other it's starting to appear an impossible job to get an accurate mathematical model of the process.

    So far the most useful sources have been the Illinois Physics Dept forum (see link), and Modern Drying Technology Vol 4, Energy Savings (Ian. C. Kemp) 2012 ch1.


    The EU (European Union) is looking to put a limit on drying rate to power consumption of hair dryers of 5.2 Wh/g/min (ref: The EU Ecodesign Directive/125/EC Draft Task Report (2014, p.126). I'm investigating where energy use can be improved for my final degree dissertation, hence the question regarding increasing the air flow rate to dry hair.
  5. Jun 11, 2015 #4


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    I doubt an accurate model exists, in all honesty. You're probably going to have to break it down to its fundamental principles, and formulate your model that way. So you plug it in, do coils heat up? What's the relationship between the current (or voltage, if you prefer) and the temperature of the coils? Well, that'll depend on the size, number of turns, etc...

    I actually don't know how a hair dryer works, it might not be coils.

    So now you have heat, and you need to pump it out, the faster you pump it out, the greater the drag force, (v^2 most likely) the greater the energy loss via heating between the hot air and your environment, as well as the air molecules coliding with each other and not even getting to the hair. So how does the angle of incidence change with velocity?

    If you want a working model, it will be a long process, not that it should discourage you in any way, however. How does a hair dryer work, and what is your motive to produce this model?
  6. Jun 11, 2015 #5


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    Hehehe, I think we could get really stupid with this if we wanted to... How would you describe the water density within the hair? How hot are the molecules of air? How many molecules will it take to evaporate 1 molecule (might be better to deal with mols) of water?

    When do you consider the hair dry?
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