A pressure washer is a high pressure mechanical sprayer used to remove loose paint, mold, grime, dust, mud, and dirt from surfaces and objects such as buildings, vehicles and concrete surfaces. The volume of a pressure washer is expressed in gallons or litres per minute, often designed into the pump and not variable. The pressure, expressed in pounds per square inch, pascals, or bar (deprecated but in common usage), is designed into the pump but can be varied by adjusting the unloader valve. Machines that produce pressures from 750 to 30,000 psi (5 to 200 MPa) or more are available.
The basic pressure washer consists of:
- A motor, such as electric, internal combustion, pneumatic or hydraulic, that drives a high pressure water pump
- A high-pressure hose
- Trigger gun-style switch
Just as a garden hose nozzle is used to increase the velocity of water, a pressure washer creates high pressure and velocity. The pump cannot draw more water from the pipe to which the washer is connected than that source can provide: the water supply must be adequate for the machine connected to it, as water starvation leads to cavitation damage of the pump elements.
Different types of nozzle are available for different application. Some nozzles create a water jet that is in a triangular plane (fan pattern), others emit a thin jet of water that spirals around rapidly (cone pattern). Nozzles that deliver a higher flow rate lower the output pressure. Most nozzles attach directly to the trigger gun.
Some washers, with an appropriate nozzle, allow detergent to be introduced into the water stream, assisting in the cleaning process. Two types of chemical injectors are available — a high-pressure injector that introduces the chemical after the water leaves the pump (a downstream injector) and a low-pressure injector that introduces the chemical before water enters the pump (an upstream injector). The type of injector used is related to the type of detergent used, as there are many chemicals that will damage a pump if an upstream injector is used.
Washers are dangerous tools and should be operated with due regard to safety instructions. The water pressure near the nozzle is powerful enough to strip flesh from bone. Particles in the water supply are ejected from the nozzle at great velocities. The cleaning process can propel objects dislodged from the surface being cleaned, also at great velocities. Pressure washers have a tendency to break up tarmac if aimed directly at it, due to high pressure water entering cracks and voids in the surface.
Most consumer washers are electric- or petrol-powered. The electric ones plug into a normal outlet, use cold tap water and typically deliver pressure up to about 2,000 psi (140 bar). Petrol washers can deliver twice that pressure, but due to the hazardous nature of the engine exhaust they are unsuitable for enclosed or indoor areas. Some models can generate hot water, which can be ideal for loosening and removing oil and grease.
The hot-water high-pressure washer was invented by Alfred Kärcher in 1950, but Frank Ofeldt in the United States claimed to have invented the steam pressure washer or "high-pressure Jenny" in 1927.
At extreme high pressure, water is used in many industrial cleaning applications requiring the removal of surface layers and for dust-free cutting of some metals and concrete. For exterior applications, gas or propane powered washers provide greater mobility than electric ones, as they do not require use in proximity to an electrical outlet, but for indoor applications, electric washers produce no exhaust and are much quieter than gas or propane washers.
The majority of pressure washers nowadays connect to an existing water supply, like a garden hose, but some models store water in an attached tank. Usually there is an on/off button that controls the water stream and many models allow you to adjust the water pressure.
High-pressure water, in combination with special chemicals, aids in the removal of graffiti, especially when the water is hot, as a quick rinser of the softened graffiti. Sometimes a pressurized mixture of air/sand or water/sand is used to blast off the surface of the vandalized area, etching the surface and making it extremely difficult to use high-pressure cleaning as a follow-up process. Sandblasting as graffiti removal often over cleans a surface and is capable of leaving a permanent scar on the surface.
Washers can damage surfaces: water can be forced deep into bare wood and masonry, leading to an extended drying period. Such surfaces can appear dry after a short period, but still contain significant amounts of moisture that can hinder painting or sealing efforts.
Washers are classified into following groups based on the type of fuel/energy they consume.
- Ultra high pressure
- Hydraulic high pressure
- High pressure steam cleaner
An insulator pressure washer is a mechanical high pressure washer designed to remove contamination/pollution from overhead power line insulators with the power on (energized) using low conductivity water. Cleaning is necessary to prevent flashovers (high voltage shorts to earth across the insulators), which can damage power line equipment.
The basic design consists of the following components:
- Power Source (a diesel engine or a power take-off from a truck chassis)
- centrifugal water pump
- stainless steel water storage tank
- high pressure water hose
- high pressure dead-man type water wash gun
- Electrical grounds for the wash gun and washing system
Insulator washers typically have a pump pressure of about 1000 psi and a nozzle pressure between 500 - 750 psi. They have a very high flow rate (about 60 gallons per minute), necessary to provide consistent and fast cleaning.
Insulator washers have several basic design layouts  as follows:
- Trailer mount for use in electricity substations 
- Chassis mount for mounting on a truck bed 
- Aerial platform mount for mounting on trucks that have aerial platforms
- Helicopter mounted units 
The washer must be able to access the towers to be washed. If the towers are off road in rough terrain, a 4x4, 6x4 or 6x6 truck chassis may be required.
The systems must have a robust design as they are used outdoors, often in adverse climates.
- "Kärcher History". Kärcher. Retrieved April 22, 2012.
- ManVentions: From Cruise Control to Cordless Drills - Inventions Men Can't Live Without. Adams Media. pp. 175–176. Retrieved April 22, 2012.
- "Frank Ofeldt". Jenny Products, Inc. Retrieved April 22, 2012.14.
- "All You Need to Know About Pressure Washers". HSS Hire. Retrieved 12 February 2014.
- "Hot Tips: Removal of Graffiti". Ace of Spray. Retrieved 24 March 2014.
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