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Homework Help: Energy Transfer homework

  1. Sep 14, 2006 #1
    Hi guys i really need some help on this question.

    Refrigerant 134a enters the compressor of a refrigeration system as saturated vapour at 0.14MPa and leaves as superheated vapour at 0.8MPa and 50 degrees celsius as a rate of 0.04kg/s. The suction area of the compressor is 10cm^2 and the discharge area 5cm^2. Determine the rates od energy transfer by mass into and out of the compressor, neglecting potential energy. Note the order of magnitude of the enthalpy and kinetic energy terms.

    I cant seem to be able to get the value for the mass. can someone help? thanks
  2. jcsd
  3. Sep 16, 2006 #2


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    Staff Emeritus
    Science Advisor

    One does not need mass, since the problem seems to be asking for energy on a per unit mass basis.

    The kinetic energy per unit mass is simply V2/2. Thermodynamic properties are usually given on a per unit mass basis, e.g. specific enthalpy, h (J/kg), or internal energy, u (J/kg).

    Mass flow rate is given - 0.04kg/s - and assuming the continuity equation, as steady-state, mass rate (in) = mass rate (out), otherwise there would be an accumulation (if mass flow in exceeds mass flow out) of the system.
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