Service; UL Listing Declaration, for Laminar Flow and Fume Hoods

Laminar is derived from the Latin lamina, a thin sheet of material. When used to describe a laboratory hood, the term "laminar flow" refers to air exiting a filter in parallel layers that move at the same speed and in the same direction, vertically or horizontally across the enclosure. Any particles introduced in a laminar flow of air during a work process are predictably swept in the direction of the flow and removed from the enclosure, downstream of critical processes.

Disruption to laminarity results in turbulence, a randomized flow that suspends particles in eddies rather than removing them. Because they can remain in the system, carried by “second-pass” air that swirls around the critical work area, contaminants are deposited unpredictably, often with disastrous results.

Vertical laminar flow (VLF) hoods position the fan filter unit on top of the hood, a space-efficient design that accommodates most facilities as long as overhead clearance isn’t a problem. Horizontal laminar flow (HLF) hoods position the fan on the back of the hood, necessitating as much as 18” or more additional floor or bench space for fan, filter and make-up air flow. In a cleanroom or lab where space is at a premium, that difference alone could determine your choice. Footprint Advantage: VLF Hood

Consideration #2: Air Sash

VLF hoods often have an adjustable sash at the front of the hood to maintain the downward laminar air flow. HLF hoods are generally open at the front of the hood to ensure unrestricted horizontal air flow. The larger opening of an HLF hood makes placement of large objects and instruments easier when compared to a VLF hood although, without a sash to serve as a shield, chemical fumes or powders inside the work area may blow into the operator's face. Sash Advantage: VLF Hood (operator safety); HLF Hood (transferring large materials)

Consideration #3: Laminar Flow

Although both hoods are effective at sweeping particles from the hood, air eventually encounters obstacles that disrupt laminar flow. In most VLF hoods, the work surface creates turbulence, so critical processes must be performed above the surface to avoid contact with “second-pass” air that could contaminate a sensitive operation. Some VLF designs include a perforated floor with sub-surface plenum to reduce this obstruction, but these measures add cost. HLF hoods generally minimize this airflow disruption. However, large objects placed on the work surface can cause swirling or contaminate downstream materials, so critical operations should take place adjacent to the filter face. Laminar Flow Advantage: HLF Hood

We end with a tie; your working conditions will help you determine which of these advantages matter most in your operation.

Safety Alert: Under no circumstances should laminar flow hoods be used in the presence of hazardous materials. Use a fume hood or biological safety cabinet for safe biohazard containment.