John and his House


John has a power washer, and plans to clean the siding of his house this weekend. John can not find the instruction of the power washer, hence he does not know the power needs for running it.

What is known:

  • Water enters the washer at 20 oC and 1 atm with a volumetric flow rate of 0.1 L/s through a 25-cm-diameter hose
  • Water leaves the jet at 23 oC, 1 atm, and a velocity of 50 m/s at an elevation of 5 m
  • The magnitude of the heat transfer rate from the power unit to the surroundings is 10% of the power input

  • Determine the power input to the motor of the power washer.
  • The power input is used to increase the enthalpy, kinetic energy, and potential energy of the water. Which one dominates power consumption?

Control Volume

  • Consider the power unit and the delivery hose as a control volume. "1" denotes the water at inlet and "2" denotes the water at exit
  • The control volume is a one-inlet-one-exit system. So the energy balance equation is: