hood discharge to another is eliminated. Providing redundancy and emergency power for this arrangement is difficult and expensive; however, a fan failure will affect only one fume hood. The potential to use diversity is limited, as is the potential to use VAV controls. The potential to treat individual fume hood exhaust (as opposed to treating all fume hood exhaust) is excellent. For the same reason, the potential to recover heat from individual fans is almost nonexistent. The maintenance requirement for these systems is considerable, because they contain many pieces of equipment and have many roof penetrations, which can cause leakage problems. The mechanical (shaft) space requirements, initial cost, and operating cost are higher than for alternative systems, such as manifolded systems.
For compatible exhaust streams, providing a common, manifolded exhaust system is an attractive design alternative to individual hood fans. This design is chosen increasingly for new laboratory buildings and is compatible with VAV systems. Manifolded systems have the following advantages and disadvantages: The potential for mixing and dilution of high concentrations of contaminants from a single fume hood by the air exhaust from all the other fume hoods on the system is excellent. The cross-contamination potential from one hood to another is minimal. The potential to provide redundancy of exhaust fans and/or provide emergency power to these systems is excellent. Conversely, the effects of a fan failure are widespread and serious; hence, redundancy is required in most cases. The potential to take advantage of VAV diversity and flow variation is also excellent, as is the ability to oversize the system for future expansion and flexibility. The ability to treat individual exhausts is retained by using new in-line liquid scrubber technologies. The maintenance, operating, and initial costs of these systems are all lower than for individual hood fan systems, and these systems require fewer roof penetrations. The heat recovery potential for these systems is maximized by collecting all the exhaust sources into a common duct.
Certain types of fume hoods and exhaust sources, such as perchloric acid hoods, should not be manifolded with other types of fume hood exhausts. In large buildings where the designer wishes to take advantage of the benefits of manifolded exhaust systems but wishes to isolate a few exhaust streams, a combination, or hybrid, of these two types of systems is usually the most prudent and cost-effective alternative.
Proper stack design and placement are an extremely important aspect of good exhaust system design. Recirculation of contaminated air from the fume hood exhaust system into the fresh air supply of the facility or adjacent facilities may occur if stacks are not provided or if they are not designed properly to force the contaminated exhaust airstream up and into the prevailing wind stream. Stack design should take into account building aerodynamics, local terrain, nearby structures, and local meteorological information. An experienced laboratory consultant or an expert in atmospheric dispersion should be consulted to design exhaust stacks for a laboratory facility.
Laboratories and clean rooms usually require that a differential pressure be maintained between them and adjoining nonlaboratory spaces. This requirement may come from code considerations or from the intended use of the space. For example, NFPA Standard 45 states that "laboratory work units and laboratory work areas in which hazardous chemicals are being used shall be maintained at an air-pressure that is negative relative to the corridors or adjacent non-laboratory areas . . ." (NFPA, 1991d). This rule helps to prevent the migration of fire, smoke, and chemical releases from the laboratory space. Laboratories containing radiation hazards or biohazards may also be required by government agencies to maintain a negative pressure in order to contain these hazards. Clean rooms, on the other hand, are normally operated at a positive static pressure to prevent infiltration of particulates. (See sections 8.E.2 and 8.E.3 below for further information.)
Glove boxes are usually small units that have multiple openings in which arm-length rubber gloves are mounted. The operator works inside the box by using these gloves. Construction materials vary widely, depending on the intended use. Clear plastic is frequently used, because it allows visibility of the work area and is easily cleaned.