All pressure washers produce pressure and flow. Both are important to the cleaning process. Most pressure washers also provide a method for dispensing soap or detergents, either under low or high pressure. The proper use of soap is often key to the cleaning process. Hot water pressure washer are equipped a burner system to continuously heat the stream of water, further aiding the aiding the cleaning process. Most hot water pressure washer can generate wet steam of approximately 240 degrees F, by turning down the pressure to around 150 PSI. There also true steam cleaners that generate a dry steam of approximately 320 degrees F at 150 PSI. Understanding the basic concepts of pressure flow and cleaning units will help you pick the right pressure washer for the cleaning job at hand.
Pressure and flow
Pressure washers use the impact of the water to break the bond of the dirt to the surface you are trying to clean; the higher the pressure, the greater the impact. The type of nozzle you use makes a huge difference in how much impact your pressure washer creates. A zero degree tip gives you the full impact of your pressure washer but is almost useless for cleaning because it hits such a small area. A 15 degree nozzle increases the area you can clean at one time, but reduces impact by 70%. A 40 degree increases the surface area cleaned further, but has 88% less impact than a zero degree nozzle. This is why a turbo nozzle is so effective; for it is a 0 degree nozzle rotating thousands of times a minute creating a 25 degree cone of full impact.
The amount of water, or flow, a pressure washer puts out makes a big difference in cleaning. The flow is what carries the impact to the surface. The more flow, the more impact; there is a direct relationship. A 13 HP pressure washer putting out 4000 PSI at 3.5 gallons a minutes flow has one third less impact than a 20 HP unit putting out 3,500 PSI at 5.0 gallons per minute. Moreover, flow is what is needed for effective rinsing. This is especially true when cleaning large open surfaces, like driveways; or quickly rinsing vehicles in truck washing operation.
A pressure washer pump creates a positive water flow, and by restricting that flow at the nozzle creates pressure. The more horse power driving the pump allows more flow to be pushed through the pump and the more pressure to be created at the nozzle. For a given amount of horsepower you can have only so much pressure and flow; there is a tradeoff. Most people focus on pressure (PSI), but you really need to consider both pressure and flow. One way of comparing pressure washers is to use the measure of “cleaning units”. To calculate cleaning units, multiply the pressure times the flow of the pressure washer. All other things being equal (i.e. chemical use and heat) a unit with higher cleaning units will clean better than a unit with lower cleaning units.
Cleaning unit application guidelines
Home use, light duty: 2,000 to 3,750 cleaning units.
Medium duty: 3,800 to 7,500 cleaning units
Heavy duty: 7,600 to 15,000 cleaning units'
The measure of cleaning units is a good way to compare the power of one pressure washer relative to another, with the following proviso. When two pressure washers have different pressure and flow specifications, but the cleaning units are equal, remember the unit with the higher pressure unit will make a rotary nozzle work better, while the higher flow unit will rinse better and make flat tip nozzles work more effectively. With that in mind, the following guide can be used on how much pressure and flow is needed to perform certain tasks. Some of these tasks may require a rotary Dirt Killer nozzle as well if you are using the minimum PSI. The minimum specifications are what are required to get the job done. Having more pressure and flow will make the job go faster. The column marked preferred CU shows how much cleaning power is required to get the job done more quickly. (CU means Cleaning Units: cleaning units = Pounds per Square Inch of pressure (PSI) times Gallons Per Minute of flow (GPM).