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Fume Hood Operation

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Fume Hood Operation

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    1. Fume Hood Operation Marcy Huey August, 2009

    3. L a b o r a t o r y V e n t i l a t i o n Key Concepts • Be Familiar with the Three (3) Common Types of Fume Hood – Constant air volume (CAV) – Variable air volume (VAV) – Specialty laboratory exhaust systems • Minimize potentially harmful chemical exposure to laboratory workers, by conducting ALL chemical work inside a properly functioning fume hood. • Never use the fume hood for storage!

    4. Fume Hoods • The first defense to minimize chemical exposure to laboratory workers. • The primary means of protection from inhalation of hazardous vapors. • All potentially harmful chemical work must be conducted inside a properly functioning fume hood. • To ensure safety, all fume hoods should be inspected annually. • There are many types of hoods, each with its own design and function. • Common Types of Fume Hood – Constant air volume (CAV) – Variable air volume (VAV) – Specialty lab exhaust systems

    5. Conventional Hoods • A properly functioning fume hood is one of the most important pieces of laboratory safety equipment. A chemical fume hood can protect workers from inhaling chemical fumes by constantly pulling contaminated air into the hood and exhausting it out of the building. It can also protect users in case of a fire or explosion by helping to physically contain the event. • A conventional fume hood is designed with an adjustable sash which can be raised and lowered in front of the user's face. These traditional fume hoods use constant air volume (CAV) exhaust fans, which exhaust air from the hood at a constant rate, regardless of sash height. This simple design can result in unacceptably high air velocities at the face of the fume hood when the front sash is lowered and nearly closed. • The most common fume hood is the constant volume conventional hood. This hood is enclosed on three sides, and has a sash that slides in the front. The sash can go up or down, which determines the hood's performance and face velocity.

    6. Variable Volume Air Principle • Variable air volume (VAV) hoods differ from constant air volume (CAV) hoods because of their ability to vary air volume exhausted through the hood depending on the hood sash position. • VAV hoods are becoming the preferred hood due to the elimination of excess face velocity that can generate turbulence leading to contaminated air spillage, endangering the worker. • They also reduce the total quantity of supply and exhaust air to when not needed, thereby reducing total operating costs.

    7. Variable Volume Hood • A VAV hood maintains a constant face velocity regardless of sash position. • To ensure accurate control of the average face velocity, VAV hoods incorporate a closed loop control system. • The system continuously measures and adjusts the amount of air being exhausted to maintain the required average face velocity.

    8. Specialty Lab Exhuast • Walk-in Hood • Fume Exhaust Connections: “Snorkels” • Canopy Hoods • Glove Boxes • Perchloric Acid Hoods • Radioisotope Fume Hoods • Distillation Hoods • Others …

    9. Walk In Hoods • A walk-in hood is a hood which sits directly on the floor and is characterized by a very tall and deep chamber that can accommodate large pieces of equipment. • Walk-in hoods may be designed as conventional, bypass, auxiliary air, or VAV. • If you have a walk-in hood, contact your safety officer for operating protocol and inspection procedures. • Walk in fume hoods are basically a ventilated room with an air baffle at the back and adjustable slots to insure laminar flow. • Access is normally gained by sliding or folding doors. • Walk in fume hoods are mainly used to set up large scale experiments or processes, experiments are erected in the walk in fume hood, the doors are closed and the experiment is monitored or controlled remotely. • No protection is provided to persons while inside the walk-in fume hood.

    10. Fume Exhaust Duct Connections Commonly called “snorkels”, “elephant trunks,” or “flex ducts” Designed to be somewhat mobile allowing the user to place it over the area needing ventilation. However for optimal efficiency, these connections must be placed within six (6) inches of an experiment, process, or equipment. These funnel-shaped exhausts aid in the removal of contaminated or irritating air from the lab area to the outside.

    11. Glove Boxes • Glove boxes are used when the toxicity, radioactivity level, oroxygen reactivity of the substances under study pose too great a hazard for use within a fume hood. • The major advantage is protection for the worker and the product.

    12. Canopy Fume Hoods • Canopy hoods are horizontal enclosures having an open central duct suspended above a work bench or other area. • Most often used to exhaust areas that are too large to be enclosed within a fume hood. • The major disadvantage is that the contaminants are drawn directly past the user's breathing zone. • Designed to vent non-toxic materials such as heat, steam and odors from large or bulky apparatus such as ovens, steam baths and autoclaves. • May be wall-mounted or suspended from the ceiling. • Built-in baffles to increase air velocities and enhance overall capture efficiency. • Similar to a kitchen stove exhaust hood.

    13. Perchloric Acid Fume Hoods • Perchloric acid fume hoods are to be installed and used whenever process involving the production of perchloric acid fumes. This type of hood is designed to prevent the deposition and build up of perchloric salts on the hood or duct surfaces. • Perchloric acid hoods are designed with wash down devices which periodically (after each use) rinse the fans ducts and fume hoods surfaces with water. The fume hood and duct work is made of stainless steel and the duct work is kept straight, vertical and seamless as possible to aid in washing away perchloric salts. • Heating perchloric acid should be undertaken with extreme caution. • Do not use oil baths or open flames to heat perchloric acid. • Do not dry filter paper used to collect perchloric acid precipitates. • Keep perchloric acid away from organic chemicals, especially alcohols and glycerol. • Store perchloric acid in ceramic trays.

    14. Perchloric Acid & Radioisotope Fume Hoods • Both perchloric acid and volatile radioisotope work require specific fume hood use protocols. • If you have questions or concerns about working with perchloric acid or volatile radioisotopes within a fume hood, immediately contact your Safety Officer for further guidance.

    15. Sash • The movable glass panel that covers the face area of a fume hood. • Sashes can be vertical, horizontal, or a combination of the two. • Many hoods are installed with a sash stop, which stops the sash at approximately a 14 inch work level. • Sash stops should never be removed, overridden, or modified. • All lab work in a properly functioning fume hood be performed at the sash stop level or lower, whenever possible.

    16. Alarms, Sensors, Controls, Gauges • New hoods are installed with alarms, sensors, air flow controls, and gauges. • These features are included to provide lab personnel with a constant reading of fume hood performance. • If the face velocity falls below an acceptable work range the hood sensors will trigger an alarm to notify lab personnel.

    17. Alarms, Sensors, Controls, and Gauges • Hoods usually go into alarm mode either because: – the sash has been raised to a height at which the hood can no longer exhaust a sufficient amount of air, – the building air exhaust system is not working properly, or – there has been a power outage.

    18. Alarms, Sensors, Controls, Gauges • When a hood alarms, no chemical work may be performed until the exhaust volume is increased. • Additionally, lab workers should not attempt to stop or disable hood alarms. • The university Physical Plant office should be notified for adjustment of air handling system exhaust and fume hood maintenance.

    19. Air Foil (Sill) • The air foil or sill, located at the front of the hood beneath the sash, creates a smooth air flow, minimizing turbulence of the air entering the hood. • The recessed work area is directly behind the sill. All work should be done at least six (6) inches into the recessed area.

    20. Face Velocity • The average velocity at which air is drawn through the face opening to the hood exhaust. • The acceptable range of the average face velocity is 60-100 feet per minute (fpm). • The current NFPA-45 specifies hood face velocity at 100 fpm with a low flow alarm. • The face velocity measurement should be: – 100 fpm ± 10 linear feet per minute of air flow. • If non-carcinogenic materials are being used the acceptable face velocity for minimally hazardous materials is 50 fpm.

    21. Face Velocity • The ideal average face velocity is 100 fpm for most operations. Hoods installed today are at 100 fpm as the industry norm and for Fire Code compliance. • If using a carcinogen, reproductive toxin or acutely toxic material, it is recommended that the face velocity range from 60 to 125 fpm. • At velocities greater than 125 fpm, the creation of turbulence causes contaminants to flow out of the hood and into the user's breathing zone.

    22. Safe Work Practices • Do not use the fume hood as a storage area. • Do not keep equipment and chemicals unnecessarily in the hood – may cause airflow blockage. • Do not use the hood as a waste disposal mechanism (e.g., for evaporation of chemicals) • Do not override or disable mechanical stops on the sash. • Do not place your head inside the hood. • Do not make rapid movements in front of the hood including opening and closing the fume hood sash rapidly and/or swift arm and body movements in front of or inside the hood – Such movements may increase turbulence and reduce the effectiveness of fume hood containment.

    23. Safe Work Practices • Do not use the fume hood as a storage area. • Do not keep equipment and chemicals unnecessarily in the hood – may cause airflow blockage. • Do not use the hood as a waste disposal mechanism (e.g., for evaporation of chemicals) • Do not override or disable mechanical stops on the sash. • Do not place your head inside the hood. • Do not make rapid movements in front of the hood including opening and closing the fume hood sash rapidly and/or swift arm and body movements in front of or inside the hood – Such movements may increase turbulence and reduce the effectiveness of fume hood containment.

    24. Safe Work Practices: Rules • The health & safety of laboratory personnel and building occupants must be the primary goal of laboratory management. • Properly functioning fume hoods provide protection from the hazards of chemical vapors and other harmful airborne substances. • Substitute toxic chemicals with less hazardous materials whenever possible. • Train and educate employees regarding specific hazards and include work methods that help reduce contaminant exposure. • Have a general awareness of the operation of your hood and be aware of any differences in visual or audible cues that may imply a change in function.

    25. Contact your safety officer immediately, if the fume hood is not functioning properly.

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