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The laboratory chemical fume hood is the most common local exhaust ventilation system used in laboratories and is the primary method used to control inhalation exposures to hazardous substances. When used properly, fume hoods offer a significant degree of protection for the user. Understanding the limitations, the appropriate maintenance techniques, and overall design of the fume hood will ensure your safety while using hazardous materials. The purpose of a chemical fume hood is to prevent the release of hazardous substances into the general laboratory space by controlling and then exhausting hazardous and/or odorous chemicals. In the event of an accidental spill, the fume hood will contain the spilled chemicals and exhaust the fumes away from the user and laboratory zone.
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A chemical fume hood is a ventilated enclosure used to trap and exhaust vapors, gases, and nanoparticles. The exhaust fan is typically stationed at the top of the building and pulls air through the duct work connected to the hood and exhausts it into the atmosphere.
Baffles: Located at the rear of the hood, they direct air in the appropriate direction. Baffles may be adjustable and the position is dependent on the vapor density of the chemicals used.
Face: The opening to which air is drawn into the fume hood. The face of the hood is parallel to the vertical sash and is the plane where face velocity measurements are taken.
Air Foil: The foil is positioned at the front edge of the hood and is designed aerodynamically to direct air into the hood to allow for less turbulence and more efficient containment of chemicals.
Horizontal: The sash moves side to side and is comprised of multiple window panes. This gives the user more freedom to work in a certain area of the fume hood while reducing costs because the sash is never completely open. REM will post an approval sticker specific to the horizontal sash openings, affirming the horizontal sash opening in inches. When not in use, close the horizontal fume hood panels for the greatest level of safety and containment. This practice will improve air quality in laboratories and reduce energy use.
Combination: This is a sash designed with a horizontal pane system built into the vertical sash. REM will measure the air velocities of each sash position and post the working height along with the horizontal sash sticker to the fume hood. Only use sash in one direction at a time. When not in use, close both the sash and horizontal panels for the greatest level of safety and containment. This practice will improve air quality in laboratories and reduce energy use.
Constant Air Volume (CAV): This type of fume hood exhausts the same amount of air at all times, regardless of the horizontal or vertical sash position. As the sash is opened and closed, the air velocity at the face of the hood will change.
Variable Air Volume (VAV): This type of fume hood contains a face velocity control, which controls the fan speed to maintain a constant air velocity at the face of the hood. This type of exhaust allows for optimal hood performance regardless of the sash position, and provides significant energy savings by reducing the air flow rate when the sash is closed. When you are not working in the VAV fume hood, CLOSE THE SASH, turn off the lights, and conserve energy.
Any fume hood not currently in use may be evaluated to determine whether it may be temporarily turned off (hibernated) by contacting Physical Facilities Engineering at PFengineering@purdue.edu. Calling PF engineering to have your fume hood shut off can reduce the electricity used to power the fan, thus conserving energy, reducing greenhouse gas emissions, and saving your University thousands of dollars per year.
High Performance: These fume hoods have containment-enhancing features allowing them to operate at lower face velocities while protecting the operator. Since less room air is exhausted, energy is conserved. REM will tag high performance hoods with a special standard operating procedure sticker, informing users of the appropriate air velocity range determined by the University.
Perchloric Acid: Special hoods equipped with a stainless steel or PVC duct and properly timed water wash down system. The wash down system must be used following each use of the acid hood. Using perchloric acid in a general lab fume hood may cause the acid vapors to settle onto the ductwork and create explosive perchlorate crystals. Serious injury or fatality may result to hood users or maintenance staff if the acid crystals are exposed to vibration and detonate.
Polypropylene (Acid Resistant): Dilute acids may be used at room temperature in most fume hoods, but if you are performing acid digestion, heating, or working with concentrated acids such as: HF, Aqua Regia, Nitric Acid, Piranha Solutions, etc., acid resistant hood and ductwork is required. Strong acids are corrosive to the duct work found in general lab fume hoods. Fume hoods constructed from polypropylene material are long-lasting and designed to resist harsh chemicals for years.
Ductless Filtered: Designed to remove potential hazardous fumes and vapors from the work area as the exhausted air passes through absorbent material, such as activated charcoal. Occasionally, the REM department is asked to approve purchases of ductless, filtered fume hoods for use in research labs. We do not recommend ductless fume hoods. We do not believe ductless fume hoods provide reliable protection against chemical exposure, and think they may, in fact, give workers a false sense of security.
Magnehelic Differential Pressure Gauge: Monitoring device that measures the difference in differential pressure across an orifice in the duct or between the laboratory and the fume hood exhaust duct. They are mounted on the outside of the hood and detect pressure differences from atmospheric, operating with an aneroid pressure gauge. REM will calibrate the Magnehelic at the annual inspection of the fume hood and record the location of the pointer for the official reading along with the date. A difference in 0.05 inches of water column of the recorded reading may indicate air velocity changes to your fume hood. Continuing to use a fume hood with high or low flow poses a risk to the user and lab occupants if the chemicals are not adequately contained and exhausted.
Digital Monitoring Device: This device measures air velocity with a sensor and either has a display in feet per minute (fpm) and/or an alarm system, alerting users if the air flow is out of range. Electronic fume hood monitors/controllers measure either the air velocity or the sash position. Sash position is correlated to the air velocity at that sash opening area to determine face velocity. Digital monitors will display velocity on a screen, while some only contain an alarm system and is color coded accordingly:
This information will be labeled on a yellow sticker and placed at the indicated approved working height related to that face velocity measurement. Users should raise the sash to the working height indicated by the black arrow on the label when operating the hood. Hood operators may raise the sash above this point for equipment set-up only. Do not manipulate chemicals or run experiments with the sash open above the designated working height. This sash height is specifically determined to contain and exhaust chemicals, and is necessary for your overall safety. Additionally, a horizontal sash sticker will be placed on the front of the hood if horizontal panels are utilized. All chemical fume hoods should also be labeled with a yellow lab hood operating procedures sticker specific to general and high performance fume hoods.
Before using a fume hood: Verify it has been inspected within the last 12 months, airflow is sufficient, the lights are working, side panels are intact, and the hood performance indicator is in good standing.
Never use a fume hood while in alarm mode. By muting the hood alarm you may be ignoring a possible malfunction with either the air flow or unsafe sash height. When a hood is alarming, lower the sash, and allow the fume hood to properly contain and balance the air velocity. Report continuing malfunctions or power failures to your designated Zone Maintenance crew or the Industrial Hygiene section of REM.
Do NOT lean into the fume hood. It is unsafe for lab users to insert their body or head inside the hood, beyond the front sash. Leaning inside the hood positions contaminants in the breathing zone of the user ad disrupts air flow. This also increases the risk for chemicals spills and accidents. Only extend properly protected hands and arms into the hood and keep sash heights as low as possible, or even completely closed, always making sure the sash is between you and your work.
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