worker and the environment by using a special plastic barrier "bag" and special procedures to prevent exposure to or release of the hazardous agent.
Thermal oxidizers and incinerators are extremely expensive to purchase, install, operate, and maintain. However, they are one of the most effective methods of handling toxic and etiologic agents. The operational aspects of these devices are beyond the scope of this book. Also, their application to fume hoods has historically been rare. When considering this method of pollution control, an expert should be called to assist.
There are many types of laboratory equipment and apparatus that generate vapors and gases but should not be used in a conventional fume hood. Some examples are gas chromatographs, atomic absorption spectrophotometers, mixers, vacuum pumps, and ovens. If the vapors or gases emitted by this type of equipment are hazardous or noxious, or if it is undesirable to release them into the laboratory because of odor or heat, then they should be contained and removed using local exhaust equipment. Local capture equipment and systems should be designed only by an experienced engineer or industrial hygienist. Also, users of these devices must have proper training, or they may be ineffectively used.
Whether the emission source is a vacuum-pump discharge vent, a gas chromatograph exit port, or the top of a fractional distillation column, the local exhaust requirements are similar. The total airflow should be high enough to transport the volume of gases or vapors being emitted, and the capture velocity should be sufficient to collect the gases or vapors.
Despite limitations, specific ventilation capture systems provide effective control of emissions of toxic vapors or dusts if they are installed and used correctly. A separate, dedicated exhaust system is recommended. The capture system should not be attached to an existing hood duct unless fan capacity is increased and airflow to both hoods is properly balanced. One important consideration is the effect that such added local exhaust systems will have on the ventilation for the rest of the laboratory. Each additional capture hood will be a new exhaust port in the laboratory and will compete with the existing exhaust sources for supply air.
Downdraft ventilation has been used effectively to contain dusts and other dense particulates and high concentrations of heavy vapors that, because of their density, tend to fall. Such systems require special engineering considerations to ensure that the particulates are transported in the airstream. Here again, a ventilation engineer or industrial hygienist should be consulted if this type of system is deemed suitable for a particular laboratory operation.
An elephant trunk, or snorkel, is a piece of flexible duct or hose connected to an exhaust system. It cannot effectively capture contaminants that are farther than about one-half a diameter from the end of the hose. Elephant trunks are particularly effective for capturing discharges from gas chromatographs, pipe nipples, and pieces of tubing if the hose is placed directly on top of the discharge with the end of the discharge protruding to the hose. In this case, the volume flow rate of the hose must be at least 110 to 150% of the flow rate of the discharge.
The capture velocity is approximately 8.5% of the face velocity at a distance equal to the diameter of the local exhaust opening. Thus, a 3-inch-diameter snorkel or elephant trunk having a face velocity of 150 fpm will have a capture velocity of only 1 fpm at a distance of 3 inches from the opening. Because the air movement velocity is typically at least 20 fpm, capture of vapors emitted at 3 inches from the snorkel will be incomplete. However, vapors emitted at distances of 2 inches or less from the snorkel opening may be captured completely under these conditions.
Slot hoods are specially designed industrial ventilation hoods intended to capture contaminants generated according to a specific rate, distance in front of the hood, and release velocity for specific ambient airflow. In general, if designed properly, these hoods are more effective and operate using much less air than either elephant trunks or canopy hoods. In order to be effective, however, the geometry, flow rate, and static pressure must all be correct. Typical slot hoods are shown in Figure 8.8. Each type has different capture characteristics and applications. If the laboratory worker believes that one of these devices is necessary, then a qualified ventilation engineer should be called to design the hood and exhaust system.
The canopy hood is not only the most common local exhaust system but also probably the most misunderstood piece of industrial ventilation equipment. It is estimated by industrial ventilation experts that as many as