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A man pressure washing a wall with a high-pressure hose

Pressure washers

by Chris Woodford. Last updated: November 25, 2014.

You've tried water. You've tried soap. You've tried scrubbing and scouring. You've tried nasty chemicals that don't do what they say. So what do you do when it won't come clean? Roll out the pressure washer! Many people now routinely use these super-powerful hoses to blast things clean with water jets pressurized at about 100–200 times the pressure of the air around us (that's 1500–3000 pounds per square inch or psi). They're brilliant on patios, drives, lawn furniture, barbecue grills, and all kinds of other outside grime. Let's take a closer look at how they work!

Photo: Left: Pressure washing bricks. Note the extremely thin metal pipe that directs the high-pressure jet downward. Photo by Senior Airman Ciara Wymbs courtesy of US Air Force and Defense Imagery. Right: A small Kärcher pressure washer suitable for home use. Photo by Explain that stuff.

Kärcher pressure washer

Why pressure jets get things cleaner

There's a good scientific reason why water gets things so clean: its molecules have a slight electrical polarity (one end is positively charged and the other is negatively charged), so they tend to stick to things all by themselves. Detergents (soap chemicals) help water to do its job even better by breaking down gunge and grease and making it easier for water to flush away. But some kinds of ground-on dirt just won't budge, no matter how hard you try. That's when a pressure washer comes in really handy. It uses a narrow, high-pressure jet of hot or cold water to blast dirt free. Because the water is traveling fast, it hits the dirty surface with high kinetic energy, knocking dirt and dust away like a constant rain of tiny hammer blows. It's only water, though, so it doesn't damage most hard surfaces. Having said that, it's a good idea to test a pressure washer on an inconspicuous area before you start work to make sure it doesn't harm the surface you're cleaning. Always read the instructions before you use a pressure washer!

Parts of a pressure water

A pressure washer is less sophisticated than it sounds. It's really just a water pump powered by an electric motor. The washer takes in ordinary water from a faucet (that's a tap to you folks in the UK), the pump accelerates the water to high pressure, and then squirts it from a hose at speed through a trigger gun. You can fit various other attachments to the end of the hose for cleaning different things.

Filling up a gasoline-powered pressure washer

Photo: Although domestic pressure washers are typically powered by electricity, bigger ones are often driven by small gasoline engines. Here's the engine inside a typical gas-powered washer used by the US Marine Corps being refilled ready for cleaning helicopters. This one's made by Jenny and rated at a pressure of 3400 psi (roughly 230 times normal atmospheric pressure). It's just about the most powerful washer you'll come across! Photo by LCpl. Jesse D. Leger courtesy of US Marine Corps and Defense Imagery.

These, then, are the main parts you'll find inside a pressure washer:

Kärcher pressure washer gun

Photo: The trigger gun from a Kärcher pressure washer. The reinforced, high-pressure hose runs up inside the plastic casing, through a valve, and out of the open end on the right.

Some pressure washers have additional features. Water and electricity are not a good mix, so many power washers have ground-fault circuit breakers, also known as residual current devices (RCDs), built into the power supply to protect you in case of an electrical fault. Most washers work in the same way and do exactly the same kind of thing, but the more expensive ones tend to operate at higher water pressures (and have better cleaning power).

How a pressure washer works

Diagram showing the basic parts of a high-pressure power washer and what they do

Here's a quick summary of the basic principle:

  1. Detergent flows in from a bottle or container through one hose.
  2. Cold water flows in from a faucet (tap) through another hose and is filtered on the way in.
  3. An electric motor or diesel engine powers the washer.
  4. Powered by the engine or motor, a water pump (impeller) draws in the detergent and water and mixes them together. Most washers also heat the water to a temperature of 50–70°C (125–155°F).
  5. The pump squirts out the hot, soapy water through the reinforced, high-pressure exit hose (and whatever attachment is fixed onto it). The narrow nozzle on these attachments helps to increase the pressure of the water jet even more. The high-pressure of the jet not only cleans more effectively but means you're wasting around 80 percent less water than if you used an ordinary low-pressure hosepipe (which is more economical if your water is metered).

What's a real washer like inside?

Technical patent drawing of the parts inside a typical Kärcher pressure washer

That's a hugely simplified version; in reality, a pressure washer is quite a bit more complex inside. There are several pumps, for a start, and for safety reasons quite a lot of attention is paid to keeping the wet parts of the machine completely insulated from all the electrical parts. I can't take a washer apart for you, but I can do the next best thing: show you a technical diagram of the inside of a typical Kärcher pressure washer from one of the US patents the company has filed. I've colored it and greatly simplified the numbering so it's easier to follow:

  1. Main outer plastic housing (yellow).
  2. Electric motor (red).
  3. Insulating plastic foil (blue): This ensures no water penetrates inside the motor.
  4. Central shaft of the motor spins around at high speed, powering the water pump.
  5. Reciprocating water pump (gray): The motor turns around (rotates), but the pump moves back and forth (reciprocates) to convert the water to a high-pressure jet. There are several pump units inside a pressure washer and (for simplicity) only one is shown here.
  6. Pump piston (orange): This is the chamber where water is pumped to high pressure.
  7. Water pipe (blue): This is where water is sucked in and pumped out.

You can find much more detail about all the parts and how they work by looking at US Patent #5,886,436: High-pressure cleaning apparatus (via Google Patents) by Josef Schneider et al, Alfred Kärcher GmbH & Co., granted March 23, 1999.

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Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Pressure washers. Retrieved from http://www.explainthatstuff.com/pressurewashers.html. [Accessed (Insert date here)]

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