September 11th, 2018 | Posted in Welding
For welding, AQE recommends utilizing a source capture hood as the primary means of capturing smoke, fumes and VOC’s. For many manual welding jobs, the welder will be welding on parts that are positioned or clamped on a table. In addition, there may be a custom fixture in use to assure repeatability. For larger weldments, the welder will be welding directly on a part that is “free-standing”.
In either scenario, the positioning of the source capture hood is critical for maximizing the performance of the collection process and the safety of the welder. Let’s discuss hood placement for larger, free-standing projects that are too large to be welded on a table, as well as for table-mounted welding projects.
It’s good to understand capture velocity and how it relates positioning the capture hood. Typically, a source capture hood should be placed no further than one diameter of the hood diameter of a circular hood. At this distance, the capture velocity is 100 feet per minute, which is what Air Quality Engineering seeks as the maximum because per the American Welding Society, air velocities above 100 feet per minute (at the arc) may draw too much of the shielding gas from the weld. If the shielding gas is drawn away, the quality of the weld will be adversely affected.
The reason the hood has to be so close is because while the air is being drawn into the hood the air will flow in from all directions. This is depicted in the airflow analysis chart shown below. Notice the airflow arrows coming from all directions around the inlet of the hood. This is because when you draw air molecules into the hood, the closest air molecules to the inlet of the hood fill the void that was left.
If you are wondering if there is a way to extend the reach of the source capture hood, there is. Before we go down that road let’s discuss proper positioning of a source capture hood when one cannot use a flat surface to their advantage.
In most cases, the hood will be in the center, left or right of where the welding is occurring. In all cases, the hood should be angled away from the welder to ensure that the smoke and fumes are being drawn away from you as you can see in the image below. The hood should be drawing from a horizontal plane or up to a 45° angle. You should be able to see the smoke moving away from the welder, and none of the fumes should be reaching the breathing zone of the welder.
One advantage afforded when welding on a table or a flat surface is that you can extend the reach of the welding hood by 1.5 to 2 times the diameter of the hood. This is because the placement of the hood flat against the surface creates a physical boundary that virtually eliminates one escape route for fugitive smoke. An example of the extended reach can be seen in the image below. Notice the differences between the above image and the one below. Above the hood is just to the right of the welding gun, and below the table is allowing the welder to place the hood to their left side and weld on their right side.
We know it can take time to position a source capture hood to ensure proper capture, however with using flat surfaces to your advantage you can reduce the amount of time spent adjusting a source capture hood. If you don’t have a surface you can use, make sure to pull the fumes away from your breathing area. The initial instinct for most people is to place an extraction hood above their head, which will actually draw the fumes across their breathing zone. The idea of positioning a hood overhead is most likely because most kitchen exhaust hoods sit overhead. When people are cooking food they don’t typically lean over the top and get as close to what they are cooking as a welder does to see the weld.
Safety Note: Depending on the application, additional protection may be required to further protect the welder and nearby workers.