Category Archives: Hoods: Laminar Flow & Fume

Laminar Flow and Fume Hoods

Under the Microscope: What are Hoods?

Whether you’re using a laboratory hood to limit exposure to chemical fumes or you require a particle-free work environment, all components of your hood work in tandem to attain optimal functionality. Hoods are enclosures, sometimes called work benches, work stations, or cabinets, that either blow filtered air down onto the work surface (positive pressure) or exhaust filtered air to the outside (negative pressure), based on the nature of the application. Below is a guide to understanding the different components of your cleanroom hood.

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Horizontal vs Vertical Laminar Flow Hoods

A laminar flow system is a vital concept in the control of particulate contamination. Laminar airflow is defined as air moving at the same speed and in the same direction, with no or minimal cross-over of air streams (or “lamina”). By contrast, turbulent flow creates swirls and eddies that deposit particles on surfaces randomly and unpredictably. Most contamination-sensitive environments such as dust-free hoods require laminar flow because it predictably sweeps particles in a uniform direction, from the cleanest area under the hood (the filter face) to the exit area, which is generally the sash opening or vents along the back or bottom of the hood. This design ensures that the cleanest (and most germ-free) area will always be the upstream area closest to the filter face. Work is generally done in that clean zone, as far as possible from obstructions that create turbulence.

Vertical laminar flow hood with top-mounted fan/filter unit.
Model shown includes rear exhaust baffles and a perforated base to optimize laminar flow, and an overhead duct to feed exhaust air to an in-house ventilation system.

But which direction should the laminar flow move: vertically (downward from a filter positioned above the work surface) or horizontally (forward from a filter positioned behind the work surface)? There are compelling reasons to choose either airflow system, depending on clearance requirements, process location, work surface design, and operator safety.

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