Canopy hood design requires different consideration for both hot and cold processes. Hot processes transfer significant heat to the air surrounding the process via conduction and convection mechanisms. In doing so, thermal drafts are present and can cause upward air currents with air velocities up to a few hundred feet per minute. The design of the canopy hood and exhaust rate must account for these thermal drafts during the sizing phase. Typical hot processes include (but aren’t limited to) MIG welding, stick welding, TIG welding, kitchen exhaust applications, tobacco smoke, etc.
Alternatively, cold processes don’t benefit from the natural upward momentum from thermal drafts like their hot process counterparts. Cold processes will require higher airflows and particular mounting locations when compared with hoods designed for hot processes. Typically, cold process hoods should be located as close to the source of contaminant generation as possible – though this practice is nearly always preferred for both hot and cold processes when applicable. Cold process examples could include metal machining, drilling, boring, milling, grinding, etc.
After establishing the type of process a hood should be sized for it’s time to properly size and place the hood.
Low Canopy Hoods – Hot Processes
The hood should be at least 1 foot larger in both axes than the source to account for the diameter of the rising hot air column.
For example, if welding a frame together that is 4′ x 5′, the corresponding canopy hood should be 5′ x 6′.
The hood should be located no more than 3 feet from the source.
The greater the distance between the source and the face of the canopy hood, the wider the rising column of contaminant will be. If it’s not possible to place the hood within 3 feet of the source, other precautions must be taken to effectively capture the contaminant including: increased, flange sizes, larger overlap, higher airflow and/or perimeter curtains.
The face velocity of the hood should be no less than 100 feet per minute.
In the example above the recommended hood size was 5′ x 6′ = 30 square feet. The required airflow for a hot process canopy hood of that size is approximately 30 square feet x 100 FPM = 3,000 CFM.
Low Canopy Hood – Cold Process (Ambient Temperature)
The face velocity of the hood should now be approximately 200 – 250 feet per minute at a minimum.
The proximity of the hood to the contaminant source is critical here since the contaminant will not move towards the hood on its own. Wherever possible, it’s recommended to utilize the contaminant’s existing momentum to bring it in contact with the air stream.
The rule of thumb for capture velocity is that one hood diameter away from the contaminant source is approximately 10% of the face velocity (for a flanged hood inlet – it is approximately 7.5% for a hood without a flange).
For example, a 12″ diameter flanged hood with a 1,000 FPM face velocity will result in a capture velocity of roughly 100 FPM at a distance one foot from the hood face. The lowest recommended capture velocity is 50 FPM. Random air movement is approximately 10 – 15 FPM in a relatively still room.
Perimeter Capture Hoods – Hot Processes Only
For large hoods, a perimeter capture hood can save money by requiring less airflow.
A block-off plate installed in the face of the hood forces the contaminants to travel to the perimeter of the hood. A two inch gap around the perimeter with an increased velocity (500 FPM is the rule of thumb) will capture this laterally flowing contaminant.
In the example above, the welding hood was sized at 5′ x 6′ and required an airflow of 3,000 CFM. Turning that hood into a perimeter capture hood would yield:
Perimeter of 5′ x 6′ = (5’x2)+(6’x2) = 22′ (lineal feet)
Total area using perimeter = 22 lineal feet x (2″/12″/ft.) = 3.7 sq. ft.
Total required airflow = 3.7 sq. ft. x 500 FPM = 1,850 CFM
There are diminishing returns on lowering the airflow when a smaller hood is used.
The approximate break-even point is a 3′ x 4′ hood.
For more information regarding AQE, Inc. equipment, canopy hood sizing and air quality applications please contact a representative by phone at 1-888-883-3273 or by email at firstname.lastname@example.org.