If the water in your water table is directly under the plate you are planning to cut....you will get the best fume control.....virtually no smoke, but maybe a little steam. The drawback......everything will get wet, you will have water on the surface of the plate, on the moving parts of your machine, and on the plasma torch. When water comes in contact with the cut edge, a rougher cut edge will also be the result.
You have to find a happy medium where the balance of fumes, and the effects of water splash are both in the bearable range. Likely this will vary a bit from plasma systems to plasma system (different plasma jet velocities from different torch and consumable designs will have an effect on splash) and at different power levels (different nozzle orifice sizes.
A good rule for an air plasma of about 60 amps is to probably have the water 4 to 6 inches below the bottom of the plate. In some cases you may want the water at 3 inches, and some cases 10 inches so build your water table so that you can adjust the water depth a bit. Many water table designs have pnuematic raise/lower water level systems for these reasons.
The advantages of downdraft (no water) tables:
1. No water on plate or moving parts.
2. No effect on cut quality.
3. No freeze issues
4. No lost small parts.
1. where do you send the smoke!
2. If it is blown outside during shop heating season, lost heat is costly.
Jim Colt Hypertherm