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B Specific heat help -- cooling down a metal bar

  1. May 25, 2016 #1
    I'm a design draftsman, I have bar of 17-4 stainless steel it weighs 6.9 lbs. The Product Data Sheet shows a specific heat 0.11 (BTU/lb/F (32-212F) (www.aksteel.com)

    It's at room temperature or 70 degrees F. I want to cool the bar to 50 degrees F. The surface area of the bar is 245 sq. inches. The bar is then immersed in pure water and the water temperature is 50F.

    How long will it take to cool and how many BTU's are used?

    I'm trying to put the answer in simple terms and so I'm looking to get a simple equation that lets me say "it takes (X) amount of minutes or hours to cool and it takes (X) amount of BTU's".

    Thanks
     
  2. jcsd
  3. May 25, 2016 #2
    Welcome to PF, Don9of11!
    Can't answer that without knowing the heat transfer coefficient between the bar and the water. That, in turn, depends on the conditions: is the water just a small tub, or is the cool water forced over the bar? Is the bar submerged completely? And so on. You could assume a very high coefficient to get a minimum time ("it will take at least XXX minutes"). The shape/thickness of the bar comes into it and you will need the density and the thermal conductivity values as well. Google "Heisler Charts" for a starting point on one way of doing the calculation.

    That you can calculate, its just the temperature change (70 - 50 = 20 degrees) times the specific heat, times the mass of the bar. That assumes that the entire bar reaches a uniform temperature and the water stays at 50 degrees somehow.
     
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