A pressure washer is a high pressure mechanical sprayer that can be used to remove loose paint, mold, grime, dust, mud, and dirt from surfaces and objects such as buildings, vehicles, concrete surfaces, etc. The volume of a pressure washer is expressed in either gallons or litres per minute, often designed into the pump and not variable. A pump's pressure, expressed in pounds per square inch, pascals, or bar (deprecated but in common usage), is also 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:
- An electric motor or gasoline (petrol) engine that directly drives a water pump
- A high-pressure hose
- Trigger gun-style switch
Similarly as a garden hose nozzle is used to increase the velocity of the liquid, a pressure washer adds its own power to create high pressure and velocity. The pump cannot draw more water from the pipe to which the washer is connected than that source can provide. Moreover, the water supply must be adequate for a given machine connected to it, as water starvation leads to cavitation damage of the pump elements.
Several different types of nozzles are available, each useful for a particular application. Some nozzles cause the water jet to be ejected in a triangular plane (fan pattern), while others emit a thin jet of water, which 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 pressure washers, in combination with a particular 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.
Pressure 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 readily available consumer units, commonly found online or at hardware stores, 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 powered units can often 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 first hot-water high-pressure washer was invented by Alfred Kärcher in 1950. However, Frank Ofeldt in the United States claimed to have invented the steam pressure washer or "high-pressure Jenny" much earlier, in 1927.
In its extreme high pressure form, water is used in many industrial cleaning applications requiring the removal of surface layers as well as dust-free cutting of some metals and concrete. For exterior applications, gas or propane powered pressure washers provide enhanced mobility not available in electric models, as they do not require use in proximity to an electrical outlet. However, for indoor applications, electric pressure washers produce no exhaust and are much quieter than their gas or propane powered counterparts.
High-pressure water at medium pressure, in combination with special chemicals, aids in the removal of graffiti. This process (especially when the water is hot) is used as a quick rinser of the softened graffiti. Sometimes people use a pressurized mixture of air/sand or water/sand to blast off the surface of the vandalized area, thus etching the surface and therefore making it extremely difficult for anyone to ever use high-pressure cleaning as a follow-up process. Sandblasting, when used in graffiti removals, often overcleans a surface and is capable of leaving a permanent scar on the building surface
Anecdotal evidence reveals that use of pressure washers to clean fences in preparation for painting/staining fences and or decks injects sufficient quantities of moisture into the wood which is seldom left for enough time to dry. The paint/stain is applied, trapping the moisture beneath, and when the sun hits the deck/fence, simply boils it off, blistering in some cases, but generally forcing the applied coating free. While moisture meters are reasonably inexpensive, most pressure washer users do not have one or not aware that they indicate when it is prudent to paint or stain.
Personal experience has seen myself being called to "fix" leaky entrance doors....why, because when hubby uses his pressure washer to clean the front door, the water comes inside. No door weatherstripping that I know of can withstand that high pressure onslaught.
Latest I've heard of is folks using pressure washers to clean moss off their roof. Again, roofing materials are not designed to sustain high pressure water. Not only are folks who do this probably voiding any warranty their roofing material supplier may have, but they are also risking interior leaks and costs of repairing them.
Citation....Common sense, school of hard knocks, a web search reveals no apparent drawbacks to pressure washing, but my experience says there are some that a person has to be aware of.
Pressure washers are classified into following groups based on the type of fuel/energy they consume.
- Electric pressure washer
- Diesel pressure washer
- Petrol pressure washer
- Gas pressure washer
- Ultra high pressure washer
- Hydraulic high pressure washer
- High pressure steam cleaner
Specialty washers 
An insulator pressure washer is a mechanical high pressure washer designed to remove contamination/pollution from overhead power line insulators using low conductivity water with the power on (energized). 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:
- A Power Source (a diesel engine or a Power Power Take Off from a truck chassis)
- A centrifugal water pump
- A stainless steel water storage tank
- A high pressure water hose
- A 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) The high flow rate is necessary to provide consistent and fast cleaning of the insulators.
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 washing unit must be able to access the electric towers to be washed. If the towers are off road in rough terrain, a 4x4, 6x4 or even 6x6 truck chassis may be required.
The systems must have a robust design as they are used outdoors, often in adverse climates.
See also 
- "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.
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