Masonry Magazine June 2012 Page. 40
CLEANING
The nozzle at the end of a pressure washer's wand is the narrowest part of the water path. This restriction causes the water to back up and build pressure behind it, until the pressure is great enough to push the water through the small orifice in the nozzle. Along the way to the nozzle, there are friction losses in the coil and hose, which reduce the pressure. More specifically, for a 4-gpm water flow, 200 feet of ½-inch coil reduces pressure by about 50 psi, and 200 feet of 3/8-inch hose reduce the pressure by an additional 180 psi. The most dramatic change, however, occurs after the nozzle.
For those of you who like math, the formula for impact, the pressure exerted against the wall at 12 inches from the wand, with a 0-degree nozzle, is: I K x Q x VP, where K=0.0526, Q gpm, and P psi. For those of you who don't like math, the answer is 11 psi. To be clear, a 3,000 psi/4 gpm pressure washer with a 0-degree nozzle, generates 11 psi, 12 inches away from the nozzle. A fan spray pattern diffuses the impact that a 0-degree nozzle would have. Because we were using a 15-degree flat fan nozzle, the impact is reduced to 30 percent of the 0-degree impact, or 3.3 psi. So, 3,000 psi at the pump actually is 3.3 psi on the wall at 12 inches away from the nozzle.
Twelve inches is a little closer than most people position the nozzle, relative to the wall. Of course, moving the wand further away from the wall would continue the precipitous drop in impact pressure.
The architect still had some discomfort. So, we adjusted the pressure downward on the machine, all Resistance causes the water to back up and build pressure behind it, until the pressure is great enough to push the water through the small orifice in the nozzle.