Water depth, slats, and underwater cutting

I have a 20'x6'(6m x 1.8m) plasma that I'd like to convert to a water table. I plan on having a reservoir underneath that gets filled/drained with air pressure.
I have seen some water tables that have the water at the bottom of the sheet, but I've yet to see one that cuts underwater. How deep underwater does the plate need to be?
I guess I'm trying to figure out how deep the pan should be and how tall the slats should be.
The way the table is, it'd be fairly easy to make a 4"-6"(10-15cm) deep pan. It wouldn't be too hard to go deeper, but we're already at 100's of gallons and thousands of pounds at that depth.

My main reason for a water table is smoke control, but minimizing warping on sheet metal would be a plus.
I'm not certain I want underwater cutting, but I like the idea of it.

Some other questions.

How to keep sediment out of the reservoir? I thinking of having the last few inches of the pan go directly to the reservoir, with a sort of lip and screen. The upside of this would be easier fabrication, but that would also make the reservoir/pan one piece. Alternatively, I could use pipes, but I still don't know how I'd deal with sediment and I'm sure that would really increase the drain/fill time.

The building the plasma is in is unheated so I need a heater to keep the reservoir from freezing. I think a screw in or tri clamp element like you'd see in a residential water heater would be the best choice. How would I go about controlling the temp? I don't wanna reinvent the wheel if there are already off the shelf solution if anyone has a suggestion.

Any insights are appreciated.
 

I figure being able to quickly drain/refill the pan to fish out small parts is best reason for a reservoir, but I also worry about the pan freezing in the winter since the glorified shed it lives in is not heated. So figure when it gets cold, the water will already be in the heated reservoir, ready to refill the pan.

I'm thinking 12-15cm(5-6") deep. How much higher should the pan sides be than the top of the slats?

I appreciate your suggestions. I wanna hear them, especially the ones pointing out my potential problems. Mostly basing this plan on a few other large tables I've seen in person and forums, not youtube videos. The only reason why I even looked into underwater cutting is cause I think you prefer to cut underwater? I've only seen ones that that are up to the plate, not submerged. But apparently some plasma's cut under several inches of water so I figured I'd ask the experts on the linuxcnc forum.

I agree with Tommy that a reservoir on a daily-use table is kinda pointless.  But for an intermittent-use table I'm in favor.

We built a 5x10 table with a 4" deep pan.  Slats are 2", giving us the ability to raise the water level ~1"-2" above thin sheets.  We don't frequently cut with the nozzle below the surface, but it makes a noticable improvement when cutting stainless.  And reduces warping on thin materials.  And makes a huge mess because the nozzle isn't deep enough, but oh well.

Our reservoir is a large polyethelyne tank under the pan.  Takes up about 1/3 the length of the table.  We have a motorized ball valve for a drain and a 2-speed pump for filling.  The pump can fill the pan in just a few minutes - it's a big hot-tub pump - so you don't get annoyed waiting to fill the table. 

The ball valve is a powered-closed version, which means we can open the drain, shut the machine/control down and walk away without having to babysit the drain valve.

Our table is not in daily use - perhaps once or twice a week.  Here's why we like the ability to drain the pan:

• The shop full of other CNC equipment isn't nearly as humid.  Yes, the dirty plasma monster is in a separate section of the shop, but the humid air can still circulate to the 'clean' parts of the shop.
• The table is on casters (with locking feet).  We only move it a couple times a year, but draining the pan in to the tank makes dragging it out of the corner way less of a splashing disaster.
• It's easier to muck out the pan
  • The pan is not sloped, nor does it have a sump. I think those are pointless unless you have a complicated automatic, underwater rake.  Which means we don't get swarf/junk in our tank - it just drops to the pan bottom and sits there waiting for us to stop ignoring it.
  • If you've ever tried to dig out a deep sump packed with plasma crap, you'll never want to do it again.  That $hit is heavy and turns to concrete.
• Our plasma can double as a router table.  We have removable aluminum-framed panels that sit on top of the pan fram and connect to a vacuum system, and the plasma floating head assembly can come off and a 3kw spindle bolts in place.
  • Being able to drain the water is a must, otherwise the MDF spoilboards would disintegrate after a few days of the humidity.  Even though the top and bottoms of the removable panels are HDPE, any MDF on top turns to oatmeal if we forget to drain the water.
• Getting water to the table is sort of a hassle as the only source is ~60 feet away.  Hauling buckets sucks, so the less we have to do it the better.
• We use borax as an anti-rust agent in the water, and not having to mix up a batch as frequently is nice.

If you have problems with freezing, maybe look for a sous-vide heater that's adjustable to a very low temp. You don't need much heat, and the circulation function of a sous-vide dingus will go a long way towards avoiding a freeze event.

Oh, and the table is not controlled by Linuxcnc, which means it's horrible in every possible way and I'm ashamed to be associated with it.