disturbance of the velocity profile and leakage of fumes from the hood into the worker's breathing zone.
The auxiliary air fume hood was developed in the 1970s primarily to reduce laboratory energy consumption. It is a combination of a bypass fume hood and a supply air diffuser located at the top of the sash. These hoods were intended to introduce unconditioned or tempered air, as much as 70% of the air exhausted from the hood, directly to the front of the hood. Ideally, this unconditioned air bypasses the laboratory and significantly reduces air conditioning and heating costs in the laboratory. In practice, however, many problems are caused by introducing unconditioned or slightly conditioned air above the sash, all of which may produce a loss of containment.
Ductless fume hoods are ventilated enclosures that have their own fan, which draws air out of the hood and through filters and ultimately recirculates it into the laboratory. The filters are designed to trap vapors generated in the hood and exhaust "clean" air back into the laboratory. These hoods usually employ activated carbon filters. The collection efficiency of the filters decreases over time. Ductless fume hoods have extremely limited applications and should be used only where the hazard is very low, where the access to the hood and the chemicals used in the hood are carefully controlled, and under the supervision of a laboratory supervisor who is familiar with the serious limitations and potentially hazardous characteristics of these devices. If these limitations cannot be accommodated, then this type of device should not be used.
As the name implies, a benchtop fume hood sits on a laboratory bench with the work surface at bench height. These hoods can be of the CAV or VAV variety and can be a bypass or non-bypass design. The sash can be a vertical rising or a horizontal sliding type, or a combination of the two. Normally, the work surface is dished or has a raised lip around the periphery to contain spills in the hood. Sinks in hoods are not recommended because they encourage laboratory workers to dispose of chemicals in them. If they must be used, to drain cooling water from a condenser, for instance, then they should be fitted with a standpipe to prevent chemical spills from entering the drain. The condenser water drain can be run into the standpipe. Spills will be caught in the cupsink by the standpipe for later cleanup and disposal. A typical benchtop fume hood is shown in Figure 8.4.
The distillation hood is similar to the benchtop hood except that the work surface is closer to the floor to allow more vertical space inside the hood for tall apparatus such as distillation columns. A typical distillation hood is shown in Figure 8.5.
A walk-in hood stands on the floor of the laboratory and is used for very tall or large apparatus. The sash type can be either horizontal or double- or triple-hung vertical. These hoods are also usually of the non-bypass type. The word "walk-in" is really a misnomer. One should never actually walk into a fume hood when it is operating and contains hazardous chemicals. Once past the plane of the sash, the worker is inside the hood with the chemicals. If the worker is required to enter the hood during operations where hazardous chemicals are present, personal protective equipment appropriate for the hazard should be worn. This may include respirators, goggles, rubber gloves, boots, suits, and self-contained breathing apparatus. A typical walk-in hood is shown in Figure 8.6.
The California hood is a ventilated enclosure with a movable sash on more than one side. These hoods can usually be accessed through a horizontal sliding sash from the front and rear. They may also have a sash on the ends. Their configuration precludes the use of baffles and airfoils and therefore may not provide a suitable face velocity distribution across their many openings.
A ventilated enclosure is any site-fabricated hood designed primarily for containing processes such as scale-up or pilot plant equipment. Most do not have baffles or airfoils, and most designs have not had the rigorous testing and design refinement that conventional mass-produced fume hoods enjoy. Both the California hood and the ventilated enclosure are designed primarily to contain, but not capture, fumes like a conventional fume hood. Working at the opening of the devices, even when the plane of the opening has not been broken, may expose the worker to higher concentrations of hazardous materials than if a conventional hood were used.