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Homework Help: Find heat transfer

  1. Oct 10, 2015 #1
    1. The problem statement, all variables and given/known data
    One kg of Refrigerant 22 initially at p1=0.9 MPa, u1=232.92 kJ/kg, is contained within a rigid closd tank. The tank is fitted with a paddle wheel that transfers energy to the refrigerant at a constant rate of 0.1kW. Heat transfer from the refrigerant to its surroundings occurs at a rate Kt, in kW, where K is a constant, in kW per minute, and t is time, in minutes. After 20 minutes of stirring, the refrigerant is at p2=1.2MPa. u2=276.7kJ/kg. Neglect kinetic and potential energy. Determine the value of the constant K appearing in the given heat transfer relation in kW/min.

    2. Relevant equations

    3. The attempt at a solution
    I've already found the work to be W= -120 kJ and the heat to be Q= -76.25 kJ

    What I'm struggling with is how to find the constant K.
    Bounds are from 0 to 20 minutes.
    Q=∫ Qdt
    =∫-Kt dt

    And now I'm stuck, I'm not sure what to do at this point.
    The next step shows K=(-2Q)/(t2) I have no idea how they got this.
  2. jcsd
  3. Oct 10, 2015 #2
    It looks like just a rearrangement of your last equation, no?
  4. Oct 10, 2015 #3
    Haha oh my god, thanks! I forgot it was equal to Q :P
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