Where fume hoods are found to have face velocities that lie outside the recommended or acceptable values for the design, steps must be taken to fix this. In special situations, the fume hood may be designed specifically to operate at low face velocities. The point in the center of the grid it is the area where the probe tip should be placedįace Velocity too fast: Velocities greater than 0.8 m/s are likely to generate eddy currents around users standing in front of the cupboard and these are then able to draw contaminants out through the aperture, particularly during movement by the operator.įace velocity too slow: In a standard fume hood it is unlikely that velocities below 0.4 m/s are able to arrest and contain contaminants within the enclosure, particularly where external air movements, due to movement of users or opening/closing of doors and windows, are likely to exceed the face velocity. However, the following are the generally accepted principles:įigure 2: Grid pattern for face velocity test. The range of acceptable values is open to debate. The same procedure should also be carried out at 25% of the specified opening and fully open for reference and safety purposes. The sash position should then be reduced to 50% of the specified opening and repeat airflow face velocity measurements/calculations as described above. The hood is set to the operational sash opening position as specified in the contract documents or as agreed to between the owner/buyer and then the velocity is measured in the same way as in CAV fume hood. Variable air volume (VAV) hoods are tested in a similar manner to CAVs for determining airflow face velocity. Table 1: Face velocity traverse grids for different size openings Variable air volume fume hood To do this, it should be mounted on a movable stand and not handheld.Ī quick way to determine the number of grids according to the dimensions of the hood opening is shown in Table 1. For consistency, the sensing element of the anemometer should be positioned in the plane of the hood sash to measure the velocity of the air perpendicular to the sash (see Figure 2).
Velocity readings should then be taken with a calibrated anemometer fixed at the centre of each grid rectangle. The grid spaces should be less than 1 sqft (0.09 sqm), and the larger side of the grid rectangle should not exceed 13 in (330 mm). The design sash position is the maximum opening or configuration allowed by user standards, SOPs, etc., whichever is applicable, and used in the design of the exhaust system to which the hood is connected.įor this test, the operator should form an imaginary grid pattern by equally dividing the design hood opening into vertical and horizontal dimensions. Constant air volume fume hoodįace velocity measurements in a constant air volume fume hood (CAV) should be taken with the sash in the design sash position. The purpose of this test is to validate that the average airflow face velocity meets the specified requirements at the required sash configuration.Īn adequate face velocity is necessary but is not the only criterion to achieve acceptable performance and should not be used as the only performance indicator. The equipment and tools remain in the hood and other activities in the laboratory continue.įigure 1: Velocity test with the hood in operational condition “as used” Airflow face velocity test For the "as used" test, the assessment is conducted after the hood has been installed and used by the chemist. When the hood is already operational in the laboratory, it is recommended to test in two operating conditions: as installed and as used.įor the "as installed" test, the laboratory hood is installed at the customer's location and tested empty, but with the ventilation system in the installation balanced and the hood in its final location. The tests are based on ANSI/ASHRAE Standard 110, Method of Testing Performance of Laboratory Fume Hoods. Performance testing is essential to confirm that the fume hood is operating to a level to provide the desired arrest, containment and removal of the fume. They are a conventional fume extraction unit designed so that airborne contaminants are contained in the enclosure prevented from entering the room or laboratory by means of a protective air barrier between the user and the materials placed within the equipment. Fume hoods are both a protective and control device fitted in laboratories where chemicals are used.
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