ESAB Knowledge center.

How Does a Plasma Down Draft Table Work

June 30, 2013

Why down draft?

Down draft tables are commonly used under plasma and laser cutting machines as an effective method for smoke collection and removal. Small down draft tables are common in shops where a work cell includes a hand-held plasma cutter. But larger CNC plasma machines and large plate lasers also use down draft tables on a much larger scale.

Down draft table basics

The concept of a downdraft table is simple, a plate is plasma cut while sitting on top of the table, and a blower pulls air out of the enclosed table below the plate. The reason this works so well is because almost all plasma smoke is ejected below the plate. The smoke is ejected out of the bottom of the kerf at a pretty high velocity, and tends to billow outward. As long as enough air is being pulled out of the table by the blower, none of the plasma smoke will escape.

How a simple down draft table works, pulling plasma smoke down into the table and out through a duct - Click for larger view

For small CNC machines, a downdraft table might be a single zone, meaning the table is all one big open area underneath the burning bars. Construction of this type of table is simple, and a single duct connection to the side or bottom of the table is all that is required. This can be effectively done up to about 5’ x 10’, or maybe 6’ x 12’. But at that point the required blower size gets pretty big, and expensive. That is why almost all down draft tables under CNC plasma cutting machines are “zoned”.

Zoned down draft tables

A zoned down draft table is divided into sections, or zones down the length of the table by walls that separate each section. Then a duct is built into the table down the length such that it goes through each zone. The duct may be built down the center of the table, or may be built into the side or sides. The duct has an opening in each zone where it pulls in the smoke from that zone. Each opening has a door, or damper, that opens and closes, so that the table only pulls smoke from one or two zones at any one time. This reduces the size of the blower & dust collector that is required, since it effectively reduces the size of the table area from which it is pulling smoke. Because the cost of a dust collector goes up significantly as the size increases, reducing the air flow requirement can save big money on the initial purchase, as well as the operating cost of the blower motor.

How A Zoned Down Draft Table Works - Click for larger view

Design considerations

Beyond the basics described above, down draft table designs may vary significantly. Some have tubes or covers that distribute air flow across the width of the table. Some have multiple ducts, some only one. Some are self-cleaning, others have slag buckets that have to be lifted out and emptied. Zone dampers may be activated mechanically, pneumatically, or electrically.

A plasma down draft table needs to be designed properly to ensure complete capture of the plasma smoke. Of course the plate size and thickness are important details to consider. But the volume and velocity of plasma smoke depends entirely on the application.

For example, making a vertical cut on 1/8” stainless steel with 60 Amp Nitrogen plasma is entirely different than making a 45 degree bevel cut on the edge of a 1” carbon steel plate with 450 Amp Oxygen plasma. The volume, velocity, and direction of smoke have to be considered and compensated, otherwise a significant amount of smoke will escape from the table while cutting, causing your shop to fill up with hazardous pollutants.

The details of those design criteria are the subject for a future post. But in the mean time, feel free to contact us if you need help finding the best table design solution for your application. 

 

Posted in Cutting Systems , Tagged with Plasma, Safety, Tables

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