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Homework Help: Problem based on the Energy equation (thermodynamics)

  1. Jun 7, 2013 #1
    1. The problem statement, all variables and given/known data
    We have compressed air @ 6 bar and 298 K. This is input to a pneumatic motor (in which pressure energy is converted into kinetic energy) flow rate is 22 L/sec and we get shaft power (1.06 KW) from pneumatic motor as output. And air leaves pneumatic motor @1 bar. Density of air @ 6 bar is 7 Kg/m^3. And we need to find the efficiency of motor.

    2. Relevant equations

    I'm using the Steady Flow Energy Equation (SFEE).
    So h1 +KE1 +PE1 +Q=h2 +KE2+PE2+W (shaft work)

    and h= u +pv

    3. The attempt at a solution

    Consider the motor as the control volume
    Inlet is state 1 and outlet is state 2
    State 1-
    P1=6 bar =600 KPa
    T1= 298K
    density= 7 kg/m^3
    (all above are given)
    From calculation, mass= Qxdensity=0.154kg

    State 2-
    P2=1 bar=100Kpa
    V2= 0.08 m^3 (for the 0.154kg at the outlet)- after using the adiabatic law i.e. P1(V1^γ)=P2(V2^γ)
    T2= 178.6 K (from ideal gas equation, after putting (P1xV1)/T1 = (P2xV2)/T2

    We neglect the change in KE and PE

    But in a pneumatic motor, Q=0 and W is given....so how do we find the efficiency???
  2. jcsd
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