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Homework Help: Velocity exiting Heat Exchanger

  1. Feb 9, 2016 #1
    1. The problem statement, all variables and given/known data
    Cold air enters a heat exchanger at 400 [K] and exits it at 1200 [K]. At the 0.025 [m2] inlet, the pressure is measured to be 1.0 [MPa] and the velocity is measured to be 3 [m/s]. The area of the exit is also 0.025 [m2] and the pressure here is measured to be 300 [kPa]. What is the velocity exiting the heat exchanger? Use conservation of mass principles.

    2. Relevant equations

    3. The attempt at a solution
    So the mass flow rate going in equals the mass flow rate going out. m1=m2

    I used this formula to calculate the mass flow rate:
    m=(1.225)(0.025)(3)=0.091875 kg/s

    Since its an ideal gas, I substituted the mass flow rate equation into the ideal gas equation:

    P2 =(m/(A2v2))RT2

    I found R to be 0.287 kJ/(kg*K)

    v2=[(0.091874 kg/s)(0.287 kJ/(kg*K))(1200K)]/[(300000 N/m2)(0.025 m2)]

    I got v2= 4.2189 m/s as my velocity which is incorrect since my answer can only be 20,30,27,or 3.3333 m/s.

    I can't see what I did wrong.

    Attached Files:

    • HEx.png
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  2. jcsd
  3. Feb 9, 2016 #2


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    Staff Emeritus
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    Homework Helper

    In your calculation of the inlet mass flow, you have apparently used the density of air at normal atmospheric pressure rather than a pressure of 1.0 MPa.
  4. Feb 9, 2016 #3
    Perfect, thanks!!
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