Depth to water for plasma quench?

[jmg3234]

I'm planning on a table that has a water tray that sits below the flat stock grate, and can be removed like a shallow drawer. What are should be considered when designing the depth to the water tray? Is there a benefit to the cut quality to have the water close to the work piece?

[jimcolt]

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.

Disadvantages:

1. where do you send the smoke!
2. If it is blown outside during shop heating season, lost heat is costly.


Jim Colt Hypertherm

[jmg3234]

Thank you Jim.

[Ubatoid]

Jim, what are you doing to me ! Ive just finished the design of our water table, and the water hight goes from level with the top of the slats to minus 4 inches. I was working on the theory that the water will help stop distortion on the thinner sheet, are you saying that no water is better and I will get better cuts without ?

[jimcolt]

All plasma systems provide the best cut quality in terms of edge smoothness (on carbon steel) when the steel is not in contact with water. That will never change!

Water is the easiest method for fume removal, and water splashing on the plate will also minimize warpage and camber in your parts.

It is a balancing act, as is almost everything in life. If the water is lower, not as much of the fumes will be trapped by the water, but there will be less splash o the plate. Some plasma systems and consumable sets have a higher velocity arc and will splash the wter differently that other plasmas/consumables.

I'm not trying to ruin your day, just tell it like it is.

Jim

[Ubatoid]

Not to worry Jim, a little re-design is needed though. We plan to use a Hypertherm 1250, what would you recommend for cutting .5 to 12mm mild steel?

[jimcolt]

I'd be guessing....My home shop machine is a downdraft table. If I wanted minimal splashing and reasonable fume control with a water table for cutting at 60 Amps on carbon steel between .5mm and 12 mm.....I would guess that the water would be best at about 5 to 6 inches (125 to 175 mm) below the bottom of the plate.

Jim