How do Pressure Washers Work

Why does blasting grime with pressurised water get rid of the dirt that soap alone can’t shift?

The basic mechanics of a pressure washer involve squeezing pressurised water through a series of small openings to create a powerful jet of liquid that’s capable of dislodging dirt and grime from hard surfaces such as patios, walls and vehicles.

Domestic pressure washers are connected to the mains water supply by a hose attached to an intake valve on the outside of the washer. When the unit is switched on, the motor spins an angled swash plate at over 3,000 revolutions per minute. This swash plate is used to transform circular motion into reciprocating (up and down) motion to drive spring-loaded pistons up and down; it is these pistons that are responsible for drawing water into the contraption.

Every time the wide end of the angled swash plate passes, it pushes down on the top of the pistons, causing them to descend. Conversely, when the thin end of the plate passes, the pistons then rise back up to their original positions. The rising of the pistons draws water into the pump through a one-way valve. Because water cannot be squeezed, when pressure is applied it will always naturally proceed to a zone of lower pressure. Therefore when the piston falls and starts to press down on the water that’s just entered the pump, the liquid must escape through another one-way valve – this time exiting into the outlet hose under a great deal of force.

Pressure WashersThe high-pressure water travels at speed along the outlet hose until it hits the closed trigger in the gun. Here the water is blocked – and the motor is temporarily interrupted. When the trigger is pulled by the operator, the motor starts up again, channeling a steady jet of high-pressure, high-speed water out through the tiny hole in the nozzle.

This model from Karcher’s wide range of domestic pressure washers features a reservoir for detergents, which have the added benefit of breaking down the dirt further. It also boasts a water-cooled motor, which helps it perform more efficiently.

Under pressure

As the name suggests, pressure is what drives these machines, but what is it? Pressure is the force applied to a solid, liquid or gas, which causes a substance’s molecules to be squashed. The pump in a pressure washer increases the pressure on the liquid, causing the water molecules to bunch together. Unlike air, however, water cannot be compressed, so the crowding water molecules push against the walls of the container. If the fluid can move, when the pressure increases, the molecules have to move towards any point where the pressure is lower; in the case of the pressure washer that means through the outlet valve. Power washers can bump up the pressure of a typical garden hose by 15-50 times, or even more.

Inside the pressure washer’s pump

Inside the pressure washer's pumpSpring-loaded pistons – Powered by the motor, the pump is driven at high speed to draw water in. This one features a three-piston axial flowpump, which works like a suction pump that draws water in through one end and pushes it out of the pump towards the hose.

Electric motor – The pump inside the pressure washer is driven by a motor that is fed by the mains electricity supply. This model from Karcher uses a water-cooled induction motor for extra efficiency.

Outlet hose – Due to the high pressure of water passing out of the machine, this hose must be reinforced with wire mesh and extra layers of plastic. It is fitted with a handheld gun with a trigger (which also activates and deactivates the motor) through which the water can be directed by the user. The jet of high-pressure water shooting out the nozzle is then used to tackle caked-on grime on hard surfaces.

Intake valve – The unit’s intake valve is connected to the mains water supply by a hose. Before it enters the pump, the water supply passes through a fine mesh filter to extract any small particles that could potentially damage the pump.

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