Water Tables vs. Downdraft Tables
The water table is very well suited to capture 98% to 100% of the smoke and dust created by the plasma cutting process depending on the water level selected when making a cut.
A down draft is less efficient, capturing about 75% to 80% of the smoke and dust and a large part of your building’s space heating in the winter. Often the contaminants are simply pushed outside. We suggest a smoke and dust collector be installed to clean the exhaust and return the cleansed air and heat to the work area. A 14” or so piece of ductwork is required to connect the table intake system to the exhaust blower or dust collector. It is preferable to install this below floor level or it must be climbed over or walked around if it exits the table vertically or horizontally above the floor. A vertical exit can impede loading and unloading the table. A simple exhaust blower with ductwork exiting the building with no filtration can cost $5,000 or so. Filtrated dust collectors with built in blowers and collection bags cost in the neighborhood of $20,000.
When cutting with the water level about 1/2” below the bottom of the material to be cut, about 98% of the smoke and dust are contained.
Warpage is also eliminated when cutting away most of the strength from a piece of material particularly when the finished part has a narrow spine. On one occasion we cut a number of 1” thick stiffeners for a high temperature oven about 8′ wide and 12’ long. The design called for a notched edge on the heat face to eliminate some of the surface area available to conduct heat in the oven. The 1/2” notches were 4” apart and 6” deep. We made the cuts on a downdraft table and spent some time working the 1” center warp (bow) out of them in the welding process. The same cut made on a water table would render a straight finished product ready for welding without the use of wedges and stiff-backs. The same is true of the slat holders we use in every Victory table, water or down draft. These are typically cut from large sheets of 3/16” plate or 3/16” drop when available. The design leaves a 1” spine with deep slots on 3” centers. These would be almost unusable if cut on a down draft whereas the water table cuts fit perfectly.
Cool to the touch parts a very easy to remove from the water table and use immediately. The water above the air chamber is usually 6” deep when raised to the bottom of the material. This 6” of water can be raised or lowered in 3 minutes. Simply exhaust the air to expose a dry bottom where the small parts accumulate. It certainly beats burning hands and gloves.
On a down draft, parts must be cooled enough to handle with gloves or tongs. In the case of thick plate this can take a while. With the standard design of a down draft the small cut parts and the slag fall several inches to the floor.
Water levels are controlled by the introduction or elimination of compressed air; there are no pumps involved. Plant air, usually around 120psi, is applied through the opening of a small hand-operated valve forcing the water out of the air chamber and onto the upper deck. Once the water is forced out of the air chamber the pressure required to hold it up is only 1.5 psi. The air chamber is built in such a way as to always be open at the proper places to avoid over-pressurization. Lowering the water level is accomplished by opening a larger hand operated valve that exhausts the trapped air from the chamber and allows water to replace the air. Both the raising and lowering of the water may be accomplished through the computer program with the installation of optional electric solenoids.
Once a year or depending on use, the water should be lowered, the slats removed and the slag removed by shovel or vacuum truck. This is also a good time to turn the slats over and begin using the new “bottom side.” Sometimes the relocation of slats from a little used area of a table to the most frequently used area is beneficial as well. We supply short slats for this purpose. Some of our competitors supply full width slats that can’t be flipped easily or will leave you with one cut-up section while most of the slat is undamaged. The Victory slats are always installed at an angle to the gantry to avoid cutting right down the top of a slat. Slat holders are cut for 3” spacing but supplied with slats on 6” centers. I have never seen anyone use slats on 3” centers, but if someone was cutting many small parts, the slats could be concentrated under such a cut and keep these small parts on top of the table. These parts falling 6” in a water table are not a big issue where falling 12-14” in a downdraft might be.
The same cleaning procedure should be done with a down draft although perhaps less often. However, the operator will be removing the slats frequently to retrieve small parts that have fallen through.
Cutting aluminum is done by setting the water level 2” below the bottom of the plate to be cut to allow escape of all hydrogen gas generated in the cutting process. This does allow some of the smoke to escape but eliminates possible explosion of the hydrogen. A hydrogen explosion is not a good thing! As soon as the cut is complete the water can be raised to cool the parts.
Cutting aluminum on a down draft is generally considered safe, as hydrogen is less likely to be trapped. However, action should be taken to keep the area under the table where aluminum fines can accumulate clean and free of concentration. These fines are extremely flammable when exposed to an open flame.
The green color in the water is not antifreeze. It is Plasma Green®, a chemical designed to keep rust and odors in check. Using a table with plain water will result in rusted slats and rust on the material being cut if left on the table over the weekend. Plasma Green® in its concentrated form is considered a Class III hazardous chemical and must be shipped under Hazmat rules. It is reduced to 4% of the water volume in the water table and, according to the MSDS sheets supplied by the manufacturer, it is legal for disposal in any public drain or even on the ground.
Freezing is generally not a problem as long as the water table is inside a building, even an unheated building. A heating or circulation system can be added for extreme conditions.
We see water tables generally sell at the rate of 5:1 to down drafts. Some of our competitors sell only down drafts.
A Victory water table or downdraft table is fitted with an extremely efficient electrical grounding system to avoid electrical shock and high frequency noise transmission. Not only are all components grounded/bonded to each other and the table frame, the whole assembly is grounded to the plasma cutter, the building steel and one or more driven ground rods.
Delivered “turn key” ready to operate: simply supply compressed air, water and electricity and you are ready to start producing in a relatively clean environment.
Delivered “almost turn key” ready to operate: simply supply compressed air, electricity and a duct to connect the table to the blower and the outside discharge. Now you are ready to start producing in a less clean environment. Your winter heating bill, your glove bill and perhaps your first aid bills will increase. Your operator will have to step/climb over/work around the discharge duct forever. Your people will spend some time dealing with warped parts.