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The different parameters for sizing a suction table
Industrial downdraft tables are essential equipment for businesses that wish to maintain clean and safe work environments. They are used to extract smoke, dust, chips, and other manufacturing debris, solvents, fumes, and odors, preventing them from accumulating on the floor or in the air.
To ensure optimal efficiency, it is important to correctly calculate the airflow and suction velocity of an industrial downdraft table. In this article, we explain the formula for calculating airflow and suction velocity, following INRS standards and guidelines.
Air flow and velocity at table surface
Airflow is the quantity of air that is extracted through the downdraft table per unit of time, generally measured in cubic meters per hour (m³/h). The required airflow corresponds to the air velocity applied to the table surface multiplied by the table’s suction surface area, according to the following formula:
Qt = S x v
With :
- Qt = suction table air flow in m3/h
- S = table suction area in m² (width x depth of suction area)
- V = desired table surface speed in m/s
For example, a vacuum table 2 m wide and 0.8 m deep requires a minimum flow rate of 2,880 m3/h to achieve a suction speed of 0.5 m/s at the table surface.
The air flow rate of a table therefore generally depends on the desired air velocity at the table surface, which in turn depends on the process to be vacuumed and the dangerous nature of the dust.
The higher the initial velocity of the pollutants to be captured, the higher the suction speed at the surface of the table must be to ensure effective and complete capture of the pollutants. Take, for example, a cutting process using a grinding wheel. The dust is ejected at the wheel’s peripheral rotation speed, so the operator must guide the flow towards the table backsplash or side cheeks, and use a high suction speed to change the direction of the finest dust and ensure that it is sucked up by the table.
The same applies to a process generating heat flow (welding, brazing, etc.): the table surface speed must be higher than the heat flow speed for the fumes to be drawn into the table, whereas they naturally tend to migrate upwards as a result of the heat flow.
For more information on the air velocities to be applied, we recommend the following sources of information:
- General Ventilation Principles – INRS Guide
- CARSAT Memo – Air Velocity Guide for Pollutant Extraction
What’s more, when CMR hazardous dusts (Chromium 6, silica, etc.) are present, it’s important to increase the suction speed at the table surface, so that the operator is well protected.
In any case, the minimum suction speed at the table surface must be greater than or equal to 0.5 m/s.
Calculation of the air flow required by a suction table in m3/h
The formula for calculating air flow is as follows: Airflow (in m3/h) = Surface area of suction table (in m²) x Air speed (in m/s) x 3600.
Calculation of the air flow required by a suction table in CFM
To calculate air flow in cubic feet per minute (CFM), use the following formula: Flow rate (in CFM) = Vacuum table surface area (in square feet) x Air velocity (in feet per minute).
The downdraft table surface area is the total surface area of the suction zone, which must be measured precisely. Air velocity is the speed at which air is extracted by the downdraft table, measured in meters per second.
Conclusion
It is important to choose the right downdraft table based on the company’s needs, taking into account the characteristics of the materials to be extracted, the process, the size of the suction zone, and the acceptable noise level. Manufacturers of industrial downdraft tables generally provide performance data for their equipment, which facilitates the calculation of airflow and suction velocity.
