A canopy that looks correctly sized can still allow heat, smoke, steam or process fumes to escape into the room. The difference is often airflow at the point of release. To calculate canopy capture velocity properly, you must assess more than the duct diameter or fan duty: the canopy position, capture area, heat load, cross-draughts and working pattern all affect the result.
For commercial kitchens, factories and workshops, capture velocity is a practical design check. It helps establish whether contaminated air is being drawn into the canopy before it spreads into the occupied space. Used correctly, it supports cleaner working conditions, better odour control and a more efficient extraction specification.
What canopy capture velocity means
Capture velocity is the air speed needed at, or close to, the source of heat, vapour, smoke, dust or fumes to pull that contaminant towards the extraction canopy. It is measured in metres per second (m/s).
It is not the same as duct velocity. Duct velocity describes how quickly air travels inside the ductwork. Face velocity describes air speed across an opening or filter face. Capture velocity concerns the air movement in front of and beneath the canopy, where the contaminant must be contained.
That distinction matters. A system can have high duct velocity and still perform poorly if the canopy is too high, too shallow, poorly positioned or exposed to strong room air movement. Equally, simply fitting a larger fan can increase noise, energy use and make-up air problems without solving the underlying capture issue.
The basic formula to calculate canopy capture velocity
The starting point is the standard airflow equation:
Q = V × A
Where Q is the required airflow in cubic metres per second (m³/s), V is the target capture velocity in metres per second (m/s), and A is the effective capture area in square metres (m²).
To convert the answer into cubic metres per hour, multiply by 3,600:
Airflow (m³/h) = V × A × 3,600
This calculation is useful for early sizing, comparison and checking an existing proposal. It is not a substitute for a full system design. The difficult part is selecting a realistic capture area and velocity for the actual process.
Worked example
Assume a canopy has an effective capture plane measuring 2.4 metres wide by 0.45 metres deep. The capture area is:
2.4 × 0.45 = 1.08 m²
If the design target is 0.35 m/s, the required airflow is:
0.35 × 1.08 = 0.378 m³/s
Converted to hourly airflow:
0.378 × 3,600 = 1,361 m³/h
This gives a preliminary fan duty of around 1,360 m³/h before allowing for duct resistance, filters, grease management equipment, bends, discharge requirements and the performance losses created by real installation conditions.
The same canopy may need a different duty if the appliances are more powerful, the cooking equipment sits proud of the canopy, doors are regularly opened nearby or a supply-air grille blows across the capture zone. A calculation is only as sound as the assumptions behind it.
Choosing the right capture area
For an extraction canopy, the area is not automatically the full stainless steel footprint. It is the effective plane through which contaminated air must travel to reach the canopy.
With a wall-mounted commercial kitchen canopy, this will usually relate to the open face beneath the front edge and the working depth of the appliances. With an island canopy, air can enter from all sides, so containment is more demanding. A canopy with generous overhang and sensible mounting height needs less aggressive airflow than a narrow unit fitted too high above the source.
For industrial applications, consider the area between the source and the canopy opening. A process that releases fumes upwards may work well with an overhead canopy positioned close to the work. A process that creates dust, solvent vapour or fast-moving contaminants may need a closer local exhaust hood, side panels or a different capture arrangement altogether.
Do not use the duct cross-sectional area as the capture area. That would calculate duct velocity, not capture performance.
Selecting a realistic target velocity
There is no single capture velocity that suits every canopy. A low-energy thermal plume from cooking appliances, a welding fume source and a dusty fabrication process behave differently. The required air speed depends on the contaminant, how it is released and whether it naturally rises towards the canopy.
As a broad design principle, gentle rising heat and steam can often be captured with lower local velocities when the canopy is positioned correctly. Smoke, grease-laden vapour, dust and fumes released away from the hood generally require stronger capture. Cross-draughts from open doors, pass-through hatches, air-conditioning outlets and staff movement can also increase the velocity needed at the canopy edge.
Rather than selecting the highest possible figure, design for effective containment. Excessive airflow can pull conditioned air out of the building, increase fan running costs and create uncomfortable draughts. Insufficient airflow allows contamination to spread, leading to poor indoor air quality, cleaning issues and complaints from staff or neighbouring premises.
Factors that change the final extraction duty
The Q = V × A formula gives the airflow at the capture plane. The fan must then overcome the resistance of the entire system while delivering that airflow. This is where many underperforming installations fall short.
Duct length and diameter affect pressure loss, but so do elbows, branches, transitions, dampers, louvres and discharge terminals. Grease filters, baffle filters, ESP units and other filtration stages add resistance as well, particularly when maintenance is overdue. A fan selected only on free-air volume may fail to achieve the required duty once installed.
Canopy geometry is equally significant. A canopy that extends beyond the equipment line, is mounted at a practical height and has side panels where appropriate will contain the plume more effectively. A shallow canopy over a wide cooking suite may need substantially more airflow to produce the same result.
Replacement air deserves the same attention. Every litre extracted must be replaced. If the room is starved of supply air, the system can struggle, doors may become difficult to open and air may be pulled in from unsuitable areas. Well-planned make-up air helps maintain the pressure balance without blowing directly across the canopy face.
A practical method for site assessment
Start by measuring the equipment line, canopy dimensions and mounting height. Identify where heat, steam or fumes are released, not merely where the appliances are located. Observe the site during normal operation, including busy service periods or active production cycles.
Next, note sources of disturbance. Open doors, nearby roller shutters, air curtains, supply diffusers and pedestrian routes can all disrupt the rising plume. Check whether the canopy has adequate overhang and whether the rear and side boundaries assist containment.
Calculate a preliminary airflow using the intended capture area and target velocity. Then allow for the system's static pressure, including dirty-filter conditions where relevant. Specify a fan that can deliver the required airflow at that pressure, not simply a fan with a matching headline m³/h rating.
Finally, commission the installed system. Airflow measurement, smoke visualisation and observation under load confirm whether the design is containing the plume in real conditions. If performance is weak, the fix may be a canopy adjustment, a supply-air alteration, a ductwork improvement or fan balancing rather than a larger motor alone.
Common errors when calculating capture velocity
The most common error is treating airflow as a single number. A quoted extract rate means little without knowing the canopy size, the capture distance and the resistance the fan must overcome. Another is measuring or specifying velocity inside the duct and assuming it proves effective canopy capture.
It is also easy to overlook appliance position. If cooking equipment or industrial machinery projects beyond the canopy edge, the plume may escape before extraction can draw it in. Filters must be kept clean, too. A well-designed system loses performance when grease, dust or debris restricts airflow.
For bespoke commercial extraction, a site-specific approach delivers better value than oversizing standard equipment. CanopyMan designs and manufactures stainless steel extraction canopies around the equipment, room layout and extraction route, helping customers specify airflow that works reliably in practice.
A good capture velocity calculation should lead to a better question: can the canopy contain the contaminant at the source throughout a normal working shift? When that answer is tested on site, the result is an extraction system built for performance rather than just a figure on a quotation.