For those of you who are thinking of making you're own jet impingement blocks, I would guess that *one* of the critical parts is sizing the jets. This is perhaps the easiest bit to design "right", and will probably also make or break the whole project.
The graph below could be used as a rough starting point. If you know how many nozzles you're going to use, then it will give you an idea of what diameter nozzle you should be aiming at.
It would appear that 2-3 m/s velocity is not an unreasonable target at 5 lpm flow rate (the green lines). Less is going to be too sedate, more will be tricky without a high pressure pump.
Dotted lines correspond to my particular design
