Impact and reach
Why is this important?
For applications like cleaning it is fairly obvious why the impact of a spray is an important characteristic. For other applications it may be important to ensure the spray has sufficient reach to ensure it hits its target or is sufficiently well distributed. If spraying into any kind of gas flow or in windy conditions the overall momentum of the spray will be important to know as spray drift can result in the target being missed, contamination of other areas or greatly reduced effectiveness.
What affects impact?
Pressures, flow rate and spray pattern type
Fairly obviously, the higher the flow rate the greater the impact/momentum of the spray will be. But increasing pressure to increase impact will be less effective in certain nozzles. If the nozzle is very efficient at atomising the spray (for example an impingement design misting nozzle) then increases in pressure will serve to atomise the spray into finer droplets. These inherently will have less momentum and so, even with increased flow rate, the overall impact and projection of the spray will hardly be affected. At the opposite end of the spectrum is a solid stream nozzle. Here an increase in pressure will result in increased flow rate at higher velocity and no change in atomisation. As such the impact and projection will increase in line with pressure.
Increasing the fluid pressure increases the overall internal energy of the fluid. How much of this increase in energy is used to atomise the spray and how much is used to increase momentum and impact depends very much on the nozzle being used. General rules of thumb are that solid stream nozzles are the most efficient at transferring energy into momentum, followed by flat fans, then hollow cones, then full cone nozzles. It should be noted that often nozzles will be discussed as being very energy efficient. This often means they are very efficient at using internal fluid energy to atomise the fluid and so in this respect are very inefficient at energy transfer.