Hood testing

Fume hood testing can be performed under three sets of criteria with a flow rate stipulated and the resulting control level attained as presented in Saunders (1993) which references ASHRAE's 110 testing protocol:

xxAMyyy or

xxAIyyy or

xxAUyyy

where:

• xx is the flow rate in liters per minute (lpm) of a tracer gas [usually SF6} ejected inside of the hood being tested.
• AM is "AS MANUFACTURED" and is usually performed by a manufacturer in a test room with no apparatus located inside the hood.
• AI, "AS INSTALLED," involves testing after construction but before occupancy. The laboratory ventilation system strongly influences the outcome of this test procedure.

When a project can afford to test a fume hood's performance "AS INSTALLED," the face velocity chosen can be checked for effectiveness and possibly reduced even further to save energy.

A third category is "AS USED," which is the safest way to test a hood because it reflects actual operating conditions.

• AU is "AS USED" and is done in the resident laboratory with the equipment inside the hood as it [will be] used. The laboratory's ventilation and control systems are also in operation. Challenges from room "cross drafts" are considered in evaluating a hood's ability to contain during this level of testing.
• yyy [is] the sampled test gas concentration, in ppm, as determined by the test protocol.

Saunders explains:

To understand whether a hood performs well, the following ratings are reviewed to give the energy engineer a point of reference.

The American Conference of Governmental Industrial Hygienists and the American National Standards Institute have suggested that a hood tested at 4 lpm under AM conditions should have a control level not to exceed 0.05 ppm. New fume hoods should also be able to achieve this performance rating of 4.0-AM-0.05 when measured by ASHRAE 110-1995. For hoods tested under AU conditions the control level of 0.10 ppm is desired. [Saunders, 1993, ANSI…Z9.5: p.13, 1992]

According to Knutson (1987),

When an adequately designed and manufactured fume hood is installed in a laboratory where most of the variables that affect the hood performance have been controlled, the [as-installed] hood should be able to perform at a 4.0-AI-0.10 level. In order to achieve this level, the hood [installation] must be well designed and properly balanced. …supply air systems, location of the laboratory hood, pedestrian traffic, and operations within and around the hood, must be considered in the design. [Knutson, 1987]