Local ventilation consists of systems designed to remove the toxicant or contaminant from the point of generation, such as a chemical fume hood. General ventilation serves an entire work area supplying and removing air through diffusers or vents strategically located throughout a room. Many standards exist for proper design, maintenance, and operation of ventilation systems.
The OSHA’s Technical Guide recommends a face velocity of 80-120 feet per minute at a sash height of 18 inches for chemical fume hoods. Face velocity indicates the speed with which air moves into the hood interior. Fume hoods should be used for one of two purposes; either procedural use or storage, not both. Hood interiors should be kept free of objects that may impede airflow. Disruption of airflow may reduce the hoods ability to protect personnel. Face velocity is measured with an instrument called an anemometer or thermal anemometer. Face velocity measurements are the responsibility of EHS and will be verified annually. In addition, smoke inducing tubes should be used to verify proper airflow. Working sash height should be as low as practical. Biological safety cabinets are the subject of specific design and operating standards (National Sanitation Foundation Standard 49).
The Guide also recommends that air be supplied to laboratory rooms at a rate of 4 to 12 room changes per hour. General ventilation is important in maintaining employee comfort in the room and for removing low levels of contaminants that would be difficult to contain within a local exhaust hood. Contact EHS if you suspect the general exhaust ventilation is insufficient.
Environmental Health and Safety can offer assistance with ventilation-related questions and concerns.
Conventional hoods represent the original and most simple of the hood design styles. With a conventional hood the volume of air exhausted is constant, regardless of sash height. As the sash is lowered the opening area decreases, resulting in an increase in face velocity. Since face velocity changes dramatically with sash position it is particularly important when working with conventional hoods to maintain the sash at its optimal height as indicated by the yellow sticker attached to the hood frame. Optimal sash height represents the point where face velocity equals 100 fpm.
Variable air volume (VAV) hoods are the most sophisticated of the hood types, requiring technically proficient design, installation and maintenance. The primary characteristic of VAV hoods is their ability to maintain a constant face velocity as sash height changes. Sash height is continuously monitored and total fan exhaust volume adjusted so that the average face velocity is maintained within acceptable parameters.
With this type of hood a dedicated duct supplies outside air to the face of a bypass hood. The main advantage of an auxiliary air hood is the energy savings realized by reducing the amount of heated or air conditioned room air exhausted by the hood. While energy savings can be substantial, the unconditioned air flow can cause discomfort for those working near the hood. It is important to keep in mind, however, that the auxiliary air supply is necessary for proper functioning of the hood.
Hood Body: The visible part of the fume hood that serves to contain hazardous gases and vapors.
Baffles: Moveable partitions used to create slotted openings along the back of the hood body. Baffles keep the airflow uniform across the hood opening, thus eliminating dead spots and optimizing capture efficiency.
Sash: By using the sash to adjust the front opening, air flow across the hood can be adjusted to the point where capture of contaminants is maximized. Each hood is marked with the optimum sash configuration. The sash should be held in this position when work involving the fume hood is being performed and closed completely when the hood is not in use.
Airfoil: Found along the bottom and side edges airfoils streamline air flow into the hood, preventing the creation of turbulent eddies that can carry vapors out of the hood. The space below the bottom airfoil provides source of room air for the hood to exhaust when the sash is fully closed.
Work surface: Generally a laboratory bench top, but also the floor of a walk-in hood, this is the area under the hood where apparatus is placed for use.
Exhaust plenum: An important engineering feature, the exhaust plenum helps to distribute air flow evenly across the hood face. Materials such as paper towels drawn into the plenum can create turbulence in this part of the hood, resulting in areas of poor air flow and uneven performance.
Face: The imaginary plane running between the bottom of the sash to the work surface. Hood face velocity is measured across this plane.