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Air Flow in Laminar Flow Hoods

Ossila Laminar Flow Hood

Laminar flow hoods (LFH) are critical pieces of equipment in a wide range of applications where cleanliness is of high importance. The hood is designed to create an environment free from physical contamination. The basic operating principle is that everything within the hood is continuously ‘washed’ with a stream of clean air. The clean air flow has several functions:

  • It sweeps away contaminants initially present within the laminar flow hood.
  • It maintains a clean environment by carrying away contaminants that are generated in the workspace.
  • It stops external contaminants from entering, or re-entering, the system.

Overall, the clean air provides the protection needed to perform critical experiments without contaminants impacting the results.

The Importance of Air Flow

Although the idea behind a laminar flow hood is simple, the implementation of a clean air flow is quite complex. The formation and preservation of optimal air flow characteristics within the hood are essential to maintain a contaminant-free workspace.

Air Flow

Laminar flow must be achieved to guarantee air flow will move in a single direction and ensure optimal performance of the hood. This is needed to achieve the clean air functions listed above.

In turbulent conditions, the air flow is not guaranteed to always proceed in one direction. This means that air flow can loop back upon itself.

If you are working downstream of your samples, your initial assumption is that the air flow will move any contaminants generated downstream, leaving your samples protected. Yet, the introduction of turbulent airflow inside the LFH can result in contaminants being deposited upstream.

Left: The irregular, unstable air flow direction of turubulent flow. Right: The parallel, stable air flow direction of laminar flow.
The movement of air flow in turbulent and laminar flow

Turbulent airflow can result in the formation of unstable vortices. A vortex involves an air flow that loops in on itself trapping the contaminants caught within it. Typically, these appear after obstructions or at the edges of air flow.

Air Speed

The laminar flow is directly impacted by air speed within the hood. Alongside obstructions and edges, air speed can cause turbulent air flow. When the air speed is too high, turbulence can form at any point in the hood.

Insufficient air speed is also an issue in the maintenance of laminar air flow. Sufficient air speed is necessary to ensure laminarity of air flow across the whole length of the hood. If the air speed is too slow, the laminar air flow will break down inside the workspace, rather than outside.

The uniformity of the air flow must be optimised to maximize the distance across which the air remains laminar. Poor uniformity results in differences in air speed across the laminar flow hood. When differing air speeds occur, air flow is unable to remain perpendicular to the filter. While these air streams remain unidirectional and do not overlap, the air flow pathway is extended. The resulting air flow is unable to remain laminar for the entire length of the hood.

Diagram of how air flow is affected by different air speeds. Left: Optimal air speed. Centre: Too slow. Right: Too fast
The air speed determines the behaviour of the air across the LFH

With design of the geometry and air flow properties, it is possible to ensure the correct air flow speed for laminarity. These efforts to guarantee laminar air flow also limit regions in which instabilities or turbulence can occur. A well-designed LFH will also quickly resolve any instabilities in air flow.

Air Direction

Depending on the configuration of the laminar flow hood, air can either move vertically or horizontally. The direction of air flow impacts how contaminants within the workspace behave.

Horizontal and vertical air flow direction in a laminar flow hood
Horizontal and vertical air flow direction in a laminar flow hood

Horizontal air flow pushes air from the back of the hood towards the front, sweeping parallel to the work surface. Contaminants are carried directly out of the LFH. Yet, the direction of movement can risk samples towards the back of the workspace contaminating those at the front. On the other hand, vertical air flow pushes particles down towards the workbench before exiting the hood. This avoids contaminated air moving across samples but instead, can generate turbulence at the workbench.


The laminar flow hood aims to create a clean workspace using controlled laminar air flow. Achieving laminar flow is complex and crucial for effective contaminant removal. Air speed and direction play key roles. Turbulence can result from incorrect air speed, both too high and too low, reducing the effectiveness of the LFH. In addition, air can flow either horizontally or vertically, and each direction has contamination challenges that need to be considered.

Overall, the system requires a balance of consistent air speed and stable, uniform air flow. Without this control, turbulence and instability in the air flow prevent the hood from effectively removing contaminants.

Laminar Flow Hood

Laminar Flow Hood

Contributing Authors

Written by

Dr. Jon Griffin

Product Developer

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