November 29, 2012
by Ben Hagemann
It has been said that water without pressure is just a puddle, the other extreme is pressure with little water is just mist is also true. When looking at any application both pressure and water volume are key, it is always a balance. On any application there is a minimum pressure that will clean, and beyond that water volume and higher pressure just serve to decrease cleaning time and therefore reduce labor costs. The relationship between pressure and water volume is quadratic, meaning to double the impact or cleaning power of a machine you must either double the water volume or quadruple the pressure. Higher pressure water will also have smaller droplets, causing more atmospheric loss over the same distance and making the equipment more difficult to use. For instance a 2000 PSI washer might effectively clean a surface up to 30 inches away and start damaging the surface at 3 inches, while the same water volume at 4000 PSI might clean the surface effectively up to 25 inches away and begin damaging the surface at a distance of 12 inches. Because surfaces are generally not completely uniform it is easy to have a washer that has enough pressure so it always causes damage when cleaning, because you have to get close enough to clean and at the same time are causing damage to softer areas. This is especially true with wood or other soft surfaces.
Will it Clean or Damage? Generally speaking if you are under 1 GPM for each 1000 PSI of pressure you will cause damage and not clean that effectively. So a 2 GPM @ 3000 PSI washer would simply not produce a spray that cleans well without damage and on surfaces that could handle the spray it would be a very poor choice anyway. Usually 2 GPM for every 1000 PSI of pressure is a better choice. 4 GPM @ 2000 PSI is an effective combination to clean efficiently without damage for most applications.
Why are models available with low flow and high pressure if they don’t clean? Two reasons: It is much cheaper to produce pressure than water volume and a higher PSI is marketed as an advantage. A motorcycle, although it can go fast, is a poor choice to tow a boat or transport a family. It does provide for a high speed at a reasonable price tag.
One of the best analogies is mowing a lawn. The deck width is your volume, the drive speed is your pressure. On many lawns a larger deck will save you time in proportion to the size of the deck. Going faster will also save you time as well but will make many surfaces more difficult to deal with and cause damage more easily when turning. Running two spray wands from one washer will split the water volume in half, this can save job time on detailed work, but usually adds extra labor costs as well.
Summary: Too low of flow with high pressure will damage and not clean. Increased Water Volume more effectively uses power to clean. Find the pressure that cleans and then increase water volume to speed things up.