I do not think the following work is scientifically true.
In this work, the case where it says (to remove loosely adhering dirt) favors more GPM than PSI,
while in the case of (the most demanding tasks …), the velocity squared (and therefore the kinetic energy) of machine A is 33% more than machine B, with both having the same cleaning units, and so pressure (PSI) is the determining property in cleaning.
Another source states:
GPM is usually more important to contractors than PSI. Since most contractors use cleaning chemicals to do all of the cleaning, their job becomes one primarily of rinsing the dirt away.
That first article is TLDR.
It seems that you are trying to do some research so i will help.
Pressure vs GPM depends mainly on what you will be cleaning. Overall gpm is prefered over psi.
The reason:
1: If your “soft washing” your not using high pressure anyways so having the extra gpm helps with rinsing of the walls
2: it really does not take that much psi to get flat work clean. Its said that most residential concrete can only safely handle about 2500 psi before you start taking the cream off (aka damaging it).
3: rinsing flat work will also be faster with higher flow
4: more flow “can” reach father than more psi. A lot of factors can play into this one
5: bragging rights
A very good way to explain this is to compare the 4gpm @ 4k psi VS the 5.6 @ 2500 psi machine both with honda 390s. If you go based of cleaning units (which all the professionals i know dont) the 4k machine is better and in theory will clean faster. The reality of it is the 5.6 machine will actually clean faster in every way. I have both and always choose the 5.6 for speed, distance, larger surface cleaner.
The main time higher pressure is preferred by most is if you plan on running long lengths of hose often (400+ feet) because friction loss plays a huge factor in your psi at the wand. There are also some applications such a industrial type work where you will need the higher pressure.
There are draw backs of running higher gpm machine as well, like:
1 water supply issues
2 may need larger high pressure hose like 1/2 in
3 have to have a buffer tank, generally a larger one which means having a really heavy duty truck or pulling a trailer.
4 Higher over all cost: but most would say the speed out ways this one
I do not know the work I posted is scientifically true, because it sets pressure as a factor in determining cleaning ability. I do not think it is, and so using cleaning units is wrong.
I think the determining factor, assuming constant nozzle/tip diameter for each machine we are comparing, is GPM squared, because kinetic energy (KE= 1/2 mass velocity^2) is proportional to velocity^2, and velocity is directly proportional to GPM.
