9.C.2.1 1.5 Thermal Oxidizers and Incinerators
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 chemical hoods has historically been rare. When considering this method of pollution control, call an expert for assistance.
9.C.3 Other Local Exhaust Systems
Many types of laboratory equipment and apparatus that generate vapors and gases should not be used in a conventional laboratory chemical hood. Some examples are gas chromatographs, atomic absorption spectrophotometers, mixers, vacuum pumps, and ovens. If the vapors or gases emitted by these types of equipment are hazardous or noxious, or if it is undesirable to release them into the laboratory because of odor or heat, contain and remove them 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 appropriate training.
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 installed and used correctly and, in some cases, can result in energy savings. A separate dedicated exhaust system is recommended. Do not attach the capture system to an existing laboratory chemical 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 air supply.
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, consult a ventilation engineer or industrial hygienist if this type of system is deemed suitable for a particular laboratory operation.
9.C.3.1 Elephant Trunks, Snorkels, or Extractors
An elephant trunk, or snorkel, is a piece of flexible duct or hose connected to an exhaust system. To capture contaminants effectively, it must be closer than approximately one-half a diameter of the hood from the end of the hose. An elephant trunk is 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. Note that unless the intake for the snorkel is placed very close to the point source, it will be susceptible to inefficient capture. Newer designs mount the intake on an articulated arm, which tends to make the systems more effective and convenient to use. (See Figure 9.11.) The volume flow rate of the hose must be at least 110 to 150% of the flow rate of the discharge.
The face velocity for a snorkel or elephant trunk is usually 150–200 fpm. The velocity and the capture efficiency drop sharply with distance from the intake. As a result, efficient capture of contaminants is generally adequate when the discharge source is 2 in. away, but inadequate if it is 3 in. away. In cases where there is a question about efficacy of capture, perform a smoke test to determine if the flow rate is adequate (ACGIH, 2004).
9.C.3.2 Slot Hoods
Slot hoods are local exhaust ventilation hoods specially designed 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. To be effective, however, the geometry, flow rate, and static pressure must all be correct.
Typical slot hoods are shown in Figure 9.12. Each type has different capture characteristics and applications. If laboratory personnel believe that one of these devices is necessary, a qualified ventilation engineer should design the hood and exhaust system.
9.C.3.3 Canopy Hoods
The canopy hood is not only the most common local exhaust system but also probably the most misunderstood piece of industrial ventilation equipment. Industrial ventilation experts estimate that as many as 95% of the canopy hoods in use (other than in homes and restaurants) are misapplied and ineffective. The capture range of a canopy hood is extremely limited, and a large volume of air is needed for it to operate