The Physics of Pressure Washers

A popular brand of pressure washer claims to deliver 330 litres per hour of water at a pressure of 80 bar, drawing 1,400 W of electrical power in the process. Pressure washers of this type are made up of an electric motor that drives a small compressor to pressurize water fed into it through a garden hose.
a.Show that the amount of power in the water flow when the washer is in operation is approximately 736 W.

b.Calculate the efficiency with which the electrical input energy is converted to hydraulic energy in the water.

c.List three significant contributory factors to this energy loss, and indicate which is likely to be the most important.

d.Calculate the diameter of the output nozzle of the washer.

e.Assuming all the pressure is dropped at the output nozzle, use its cross-sectional area to calculate the reaction force exerted on the user when the washer is in operation.

f.Explain in a few words why, when operating a garden hose fitted with a trigger-operated nozzle, the water jet is momentarily more powerful at the instant the trigger is squeezed than later, when there is a continuous stream of water.

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