WORK BENCHES AND FUME HOODS

Work benches and fume hoods constitute a major cost factor in any laboratory. A careful study of available products and good planning, however, can bring about significant savings with no loss in serviceability. 

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Distributors of general laboratory equipment also offer one or more lines of work benches and fume hoods, often shown in a separate catalog. In addition, there are several companies specializing in this field. The annual LabGuide issue of Analytical Chemistry, published by the American Chemical Society, has a good listing of suppliers. A laboratory planner should obtain catalogs from several sources and compare both features and prices. 

All bench-mounted equipment should be listed with its dimensions, which can be taken from measurements or from catalog data. It should be noted which instruments may have to be put at a certain minimum distance from other objects in order to avoid interference or allow for servicing. Work space should be allowed next to each instiiiment for samples, notebooks, etc. This space may be considerable in case of an analytical instrument on which many samples are to be tested at one time. Space sharing should be discouraged, with the required work space next to an instrument reserved for that alone. Sinks and fume hoods should also be included in this list, with an allowance of at least 18 inches of free space on each side of a sink. 

The acetylene cylinder must be chained securely to the wall or laboratory bench. Work on a vacuum system requires a preliminary review of procedures and careful execution in order to avoid damage to the apparatus and possible injury from broken glass in addition, the liquid nitrogen used for cold traps must be handled properly (see Appendix C). Safety glasses must be worn. Acetylene is flammable no flames can be permitted in the synthesis area. Take great care not to allow oxygen condensation to occur in the presence of acetylene (potentially explosive). Carry out as many operations as possible in a fume hood. Dispose properly of excess CaC2. 

Work Surfaces. All horizontal work surfaces (bench tops, containment cabinets or fume hoods) should be protected with impervious material to prevent contamination of the work surfaces with chemical carcinogens. One of the systems that has proved useful is to use the dry, absorbent polyethylene-backed paper (Benchkote, VWR Scientific Co., Cat. No. 52855 , Continuous Sheet Type, Scientific Products Co., Cat. No. P1180). Following contamination, or upon completion of an experiment, or at the end of the day, this protective cover can be rolled up, packaged for safe removal, and disposed of in an appropriate manner. 

Caltech had high standards of safety and protection of research workers and students. The new Crellin Chemical Laboratory, finished in 1939, had at least one large fiime hood in every laboratory. There were small downdraft fume hoods along the center of each work bench in the freshman chemistry laboratory, so that every two students had their own mini-hood. Before carrying out war-gas research. Professor Roscoe Dickinson asked for doubling the pumping capacity for two fume hoods in 65 Crellin and raising the... 

Never place a chemical, not even water, near the edges of a lab bench. Use a fume hood that is known to be in operating condition when working with toxic, flammable, and/or volatile substances. 

Confine your work with radioisotopes to a small area in the laboratory. A convenient plan is to use a stainless steel tray lined with absorbent blotter paper coated on the bottom side with polyethylene. The paper must be replaced every day. If the radioactive matenals are volatile, the work should be done in a fume hood. If spills occur, a small work area such as a tray is much easier to clean than a large lab bench. If or other strong j8 emitter is used, it is necessary to work at all times with shielding between yourself and the radioactive samples. The most cost-effective and convenient shielding material is Plexiglas. The thicknesses of shielding required for various materials are given in Table 6.3. 

Fume hoods are intended to be used to house activities that should not be done on an open bench because of the potential hazard which the activities represent, usually the generation of noxious fumes. The ability of fume hoods to capture and retain fumes generated within them is especially vulnerable to air movement, either due to traffic or other factors such as the location of air system ducts, windows, doors, or fans. Clearly, they should be located, as in the standard laboratory module, in a remote portion of the laboratory selected for low traffic and minimal air movement. Other fume generating apparatus, such as Kjeldahl units, should also be placed in out of the way places where errant air motion will not result in dispersion of the fumes generated into more heavily occupied areas of the room. A point that needs to be considered is the work habits of laboratory employees. Data on the possible health effects of long-term exposures to the vapors from most laboratory chemicals is relatively scant, although there are beginning to... 

Wherever possible, work with mercury should be done in a fume hood, preferably one that has a depressed surface, so that a hp wiU aid in preventing mercury spills from reaching the floor, and with a seamless interior, as recommended for radiological work and work with perchloric acid. As noted earher, heating mercury causes it to emit fumes at concentration levels two to three orders of magnitude above the PELs. Heating of mercury shorild never be done on the open bench. 

The bench-scale continuous membrane bioreactor (MBR) system consisted of three parallel bioreactors, each with 1 L working volume and designated as bioreactors 2, 3 and 4. A hollow fiber immersed ultrafiltration membrane (ZeeWeed, ZW-1 , GE Water) with 0.045 m (0.5 tf) of membrane area was installed in each bioreactor (Figures 1 2). Filtration was by siphon demanding no permeate pumping. The transmembrane pressure (TMP) necessary to produce permeation was provided by the 1.1 m difference in elevation between the bioreactors (installed in a fume hood), and the permeate collection assembly at floor level. The TMP was kept constant at 0.11 bar or 11 kPa. 

A few obvious safety measures are worth mentioning. A good fume hood or fume cupboard is essential. It should be large enough to work with comfortably, and the air flow should meet modem regulations. Perforated downdraught extraction benches have recently become available as an alternative, and they would seem ideal for derivatization chemistry and far less claustrophobic... 

Bench Hood. The most common fume hood in use, the bench hood, is best for general use for containing experiments or apparatus that produce hazardous or toxic products (Figure 12.5). The work surface is at a 91-centimeter (36-inch) height, and utilities are generally provided on the side walls. 

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