Design and evaluation of industrial ventilation systems, particularly the more difficult hoods
- Indoor Air Quality investigations
- Training courses on industrial ventilation
- Hoods
In designing a local exhaust ventilation system, one of the first elements to design is the hood – to efficiently shape the airflow and capture most of the contaminated air.
This element of industrial ventilation design is usually the worst bit.
Most industrial ventilation systems are like this:
Without the hood (wheels) properly designed the system will not work properly, not matter how flash the rest is. Unfortunately, hood design is often an afterthought.
Bell-mouthed hood
This hood was “spun” from sheet aluminium and relatively expensive. They are rarely seen in workplaces other than in Nederman-type hoods.
It is a very efficient hood, as almost all the energy provided by the suction is used to accelerate air into the hood. The hood is shown intersecting the Wind Tunnel in studies examining the effect of cross-drafts on hood efficiency.
Right: A bell-mouthed hood in 1903, from HSE archives, UK
Circular-flanged hood
A reasonably efficient hood, but losses occur at the mouth where the air tries to navigate a 90 degree corner. Often impracticable as the flange gets in the way.
Here the capture efficiency is visualised with smoke in a wind tunnel. Sulphur hexafluoride (SF6) tracer gas is used to measure the capture efficiency with cross drafts down the glass wind tunnel
Downdraft hood
Downdraft hoods can be very efficient at controlling toxic exposures from radioisotopes and pharmaceuticals, but the air velocity field has to be well designed to ensure the designed, uniform down-flows are achieved.
Here, slots in the hood are adjusted to ensure the velocity at the working surface of the hood (covered with a wire mesh) are uniform.
Tank hood
A smoke generator under the black perforated (pegboard) cover simulates the emissions from a tank. A flanged slot hood at the end of the tank attempts to capture the emissions. Push-pull ventilation is added to the tank. A Fog Machine is underneath the perforated tank “surface”, and when the hood is not operating, the surface of the tank looks like a field of miniature volcanoes.
Note the entrained smoke near the orange push-pull bar (compressed air jets though 1 mm holes at 50 mm intervals to produce an air knife over the surface of the “tank”)
The same tank hood, but with a vertical, slotted hood.