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Homework Help: Pressure washer

  1. Jun 15, 2009 #1
    I'm having trouble with this question. Any guidance would be very welcome. I have left the values out because i want to do the maths myself.

    Pressure washer delivers xxx litres per hour of water
    Pressure xx bar
    Draws xxxx Watts of power

    a.) Show power in water flow is xxx watts
    b.) Calculate efficiency between electrical input energy to hydraulic energy in water
    c.) Calculate diameter of output nozzle of the washer
    d.) Use the cross sectional area of the nozzle to calculate the reaction force exerted on the user

    Attempt thus far:

    P = Power (Watts)
    p = Pressure (N/mm2)
    Q = Flow rate m3/s
    n = Efficiency
    v = velocity (m/s)
    A = Area
    Vjet = velocity of jet
    g = 9.81

    For a.) I used

    P = p x Q
    Converting xxx litres per hour to litres per second and xx bar to N/mm2

    For b.) I used

    n = output / input = answer x 100%

    For c.)

    Initially I found the velocity of the output jet in meters per second

    Vjet = sq root 2 x g x p = m/s

    Then I rearranged

    Q = v x A
    A = Q/v

    Which gave me the cross sectional area of the nozzle

    For d.) I’m have a little trouble with the final part
  2. jcsd
  3. Jun 15, 2009 #2
    The reaction force is the force required to hold the power washer nozzle steady. Use Linear momentum, F= ma=mass flow rate X Velocity. You know Q in l/hr, the density of water and velocity of the water.
  4. Jun 18, 2009 #3
    Many thanks this helped at lot!
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