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U values W/M2/K

  1. Apr 29, 2009 #1
    u values ....W/M2/K

    I am getting confused by the definition of the u-value .
    I want to do some calculations for heat loss but I do not understand the timescale for the U- value calcs
    I understand that a U value describes the energy in watts that is lost per Sq Mtr for each degree (K) diferential , but is that per hour , day ,year ?
    I may have missed something obvious but any advice would be gratefully accepted
    Kev
     
  2. jcsd
  3. Apr 29, 2009 #2

    russ_watters

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    Re: u values ....W/M2/K

    A watt is a joule per second.
     
  4. Apr 30, 2009 #3

    Andy Resnick

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    Re: u values ....W/M2/K

    If the 'U-value' is the concept as the 'R-value' for insulation, then there is no time dependence.

    http://en.wikipedia.org/wiki/R-value_(insulation [Broken])

    Heat transfer (as a thermodynamic application) has no time dependence. The amount of heat flow depends only on the temperature differential.
     
    Last edited by a moderator: May 4, 2017
  5. May 5, 2009 #4
    Re: u values ....W/M2/K

    Thanks for the help
    I have sussed now that the figure relates to the rate of energy transfer and that the amount of energy transfered is calculated by the the timescale .
    I probably was being thick, but at the time I just couldn't see it .
    Kev
     
  6. May 5, 2009 #5

    russ_watters

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    Re: u values ....W/M2/K

    They are inverses of each other.
    What? That's like saying velocity has no time dependence!? Quite obviously, for a given piece of insulation, "the amount of heat flow" in an hour is 60 times larger than "the amount of heat flow" in a minute.
     
    Last edited by a moderator: May 4, 2017
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