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Water Tables vs. Downdraft Tables

Filtering cutting fumes with water rather than air can save money.

While only suitable for cutting of mild steel, water tables are up to 40 percent cheaper to operate because they require no expensive filters.
The current lack of manufacturing activity has metal centers everywhere looking for ways to trim expenses. One option for metal cutting operations is the use of a water table for workspace ventilation instead of a downdraft table, which might have been the automatic choice a few years ago. Depending on the uses, a water table can cost substantially less to install and operate than a downdraft table.

Water tables are relatively limited in their applications—just plasma and oxy-fuel thermal cutting. To narrow their use even further, water tables are best used when cutting mild steel. But that still accounts for a big chunk of metal processing activity, which makes water tables an option for many shops.

By comparison, downdraft tables are much more versatile. They can be used when cutting a wide range of metals, including galvanized, stainless steel, aluminum and other metals where toxic fumes might occur. Downdraft tables work by drawing particles and fumes through a slatted or perforated table and sending them through a filtering system. When the type of metal being cut is changed, the filter must be changed since different applications require different filter arrangements. For example, if the operator were cutting stainless, he’d insert a filter that would capture hexavalent chromium. HEPA filters are also available if the application requires it.

Water tables use no filters or fume collectors. Water tables remove particles and fumes by scrubbing, which makes use of the kinetic energy of the stream of molten metal and gases from the cut in the metal plate. In the case of plasma cutting, for instance, the water acts like a filter. All of the molten metal, gases and fumes are pushed into the water resulting in no fumes to the atmosphere. Water tables also reduce noise and trap sparks, but they aren’t recommended for most cutting processes. Water on the table will affect the bottom side of the cut surface on some high-definition plasma systems, for instance. Aluminum and non-ferrous metals are also problematic because gas can build up under aluminum plate and between slats on a water table, which can cause an explosion.

So, given downdraft tables’ greater versatility and familiarity in the marketplace, what’s the attraction of water tables? The answer is simple: money. A water table can cost up to 40 percent less to install and operate than a downdraft table with a fume collector, according to Brad Williams, national sales manager at Koike Aronson/Ransome, Inc., which manufactures both downdraft and water tables. The tables themselves cost essentially the same. The big extra expense of a downdraft table comes up front because of the fume collector, ducting and filters.

There are greater ongoing equipment costs, too. Downdraft tables require regular filter changes—after all, they’re basically a giant version of a home vacuum cleaner. For example, if a metal center has a downdraft table system with 16 filters, changing the filters once a year would cost about $1,900 at $120 per filter. If the center is running more than one shift and has to change filters more frequently, that cost goes up.

Electrical usage should also be considered. Most water tables are manually operated, but a downdraft system’s blowers are going to be drawing on a shop’s 240 volt or 480 volt electricity whenever the table is in use. Labor costs would be the same since water and downdraft tables don’t require any extra skills on the part of the person doing the cutting. There is also no difference in placing work pieces—either type of table can handle pieces brought in by an overhead crane, jib crane or lift truck.

Filter costs aside, water tables are slightly more expensive to maintain than downdraft tables. Water must be regularly added to the table due to evaporation, and chemicals also have to be added to inhibit the growth of algae. The water must be periodically drained, requiring a place to dump the liquid legally. (Water table users should check with local authorities on how to handle such wastewater.)

In a downdraft table system, the fine particulates that are trapped by a filter are periodically knocked off the filter and collected in a drum, which can then be disposed of like any solid metal waste. All in all, filter changing and dry particulate disposal are easier than water changing and disposal.

Downdraft tables are easier to clean. Both water tables and downdraft tables are available with removable slats and slat frames, which makes cleaning simpler. But water table slats are more prone to slag buildup because the slag pans in a pneumatic water table are not as deep as in a downdraft table. As the molten metal drops into the water, it cools very quickly forming a slug of metal. These slugs can build up in the pan and on the slats. The slag pans beneath downdraft tables also are easier to clean because they’re dry.

Water tables reduce the noise of thermal cutting. Downdraft tables, by contrast, are net contributors to shop noise because of air movement at the table and, sometimes, motor noise. In most installations, though, fume collectors are located outside of the building that houses the table. If the collector is inside, it can be covered with a motor silencer, in effect an insulated steel box.

All downdraft table systems need a spark arrestor when used in cutting applications to protect against filter fire. If a spark is thrown from the cut, it enters a “mouse maze” that captures the spark before it hits the cloth filter. This maze is usually some combination of grates, baffles and water trays. Sparks produced when cutting on a water table are doused in the water, so no extra arrestor is needed.

Downdraft tables offer more flexibility than a water table, should a shop want to expand the cutting table for more production. Some downdraft table designs are modular and can be expanded easily by adding more tables and rails. A water table, on the other hand, is welded 100 percent tight. Expansion would require a lot of expensive, on-site prep work.

To sum up, if your operation processes a variety of metals and does welding, a downdraft table is your best option for a ventilated table system. But if your business is primarily cutting mild steel, a water table can deliver considerable savings.

Koike Aronson/Ransome, Inc., Arcade, N.Y., supplies advanced cutting machines, welding positioning equipment, portable welding and cutting machines, and gas apparatus. For more information, visit www.koike.com.
  
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Friday, November 28, 2014