Downdraft table basics
Let’s start the discussion by explaining how a downdraft table works. A downdraft table is a simple concept, a blower pulls air out of an enclosed table while a plate is being plasma cut. This works because most of the plasma smoke is pushed below the plate. The smoke flows out of the bottom of the cut and billows outward. As long as enough air is being sucked by the blower, very little of the smoke will escape. A single zone, downdraft table, is usually used for smaller tables. This means that there is one open area under the slats. These are the simplest to construct and usually incorporates a single duct connection. A single zone table is usually only effective for tables up to 5’x10’. For larger tables a zoned system is used.
Zoned downdraft tables
Zoned downdraft tables are divided into sections, or zones. A duct is built down the length of the table either in the center or into the sides. This duct has openings in each zone with doors or dampeners which open and close according to the position of the gantry. When the gantry is above a zone the door opens in the zone while the remainder stay closed. This allows the blower to pull smoke only from over the zone that is being cut. This reduces the size of the blower needed and greatly increases the efficiency of the zoned table.
Water table basics
Many water tables are simply a tank filled with water. This type of design, though effective, will not allow you to fully utilize the advantages of a water table. By using a table that has an adjustable water level, a better level of smoke extraction and noise reduction can be achieved.
The basic construction of a water table is simple; it consists of a large chamber built into the table. The chamber has openings inside the table along the bottom edge, this allows air to be trapped inside. By putting air into the chamber, the water level is forced up, by allowing air out of the chamber the water level is forced down.
At minimum, the water level controls have an air supply valve and an air release valve, which control the compressed air going into and coming out of the chamber. These valves can be simple ball valves operated by hand, or can be solenoid valves controlled by the CNC.
To raise the water in the table, the release valve is closed, and then the supply valve is opened. Compressed air pushes into the chamber, forcing the water out through openings along the bottom of the chamber, which results in the water level rising. Once the water has reached the desired level, the air supply valve is closed, and the air release valve remains closed, holding the air in the chamber so the water level stays up.
Underwater plasma cutting
One of the main reasons for using a water table is for underwater plasma cutting. Some plasma torches are not designed for underwater use. Before submerging any torch verify with the manufacturer, as additional hardware may be needed to be successful. The most common device used is an air-curtain. The air curtain uses compressed air to form a bubble around the front end of the plasma torch protecting the plasma arc.
For larger tables, even if you don’t ever plan to cut underwater, a water table with water level control is recommended. This allows the water level to be lowered for easier access to clean out small parts that drop between the cutting slats.
Both tables offer advantages and disadvantages. Downdraft tables are more expensive and louder than water tables, but are easier to clean and your only option for cutting aluminum. Water tables are less expensive and quieter, but more difficult to clean and not recommended to cut certain materials. Ultimately, you will have to weigh the pros and cons for the applications you will be cutting and decide which fits your needs the best.