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As part of the recent renovation of our undergraduate chemistry laboratories, we were faced with the problem of what to do about hood facilities for the general chemistry laboratories. We wished to have hoods available for all students, but because these would be used only occasionally, installation of full-size hoods was not considered either cost or space effective. The alternative was small bench-top hoods, but other criteria, including ease of storage, flexibility of location, and visibility throughout the laboratory made the commercially available units that we could find unsatisfactory1. None of the commercial designs offered easy storage (unless they were very small to begin with) or the ability to take up minimal bench space while still being functional. In the end, we designed our own and will describe it here for anyone who has similar requirements.
Top view of hood.
Front view of hood.
Hood in open position,
outlined for clarity.
Hood in folded position,
occupying minimal space.
The design is illustrated in Figures 1–4. The hood has a truncated triangular footprint with the duct attached to the center of the truncation. This duct is a removable flexible hose2 4 in. in diameter and about 3 ft long. It fits into a connection (in our case, a plastic gate valve3 obtainable from building suppliers, but any slip-fit connection would work) on the bench surface for connection to an appropriate exhaust duct below the bench. This eliminates unsightly overhead hose ducts, although this hood design could be used with an overhead exhaust in a facility without under-bench exhausts. The ends of the duct are attached to metal duct fittings that mate via a slot-and-pin arrangement with another fitting on the hood in the one case, or fit into the connection to the exhaust system in the other. The hood was designed for an exhaust system that would provide a face velocity of 80 ft/min at the larger opening. This value was considered by our risk management and HVAC professionals to be an appropriate level for safety4. (It is important to have input from qualified HVAC personnel to establish appropriate design of the exhaust system, since this determines the hood performance.)
Folding sides and top (there is no bottom) permit the hood and its duct hose to be stored in a standard 18-inch-wide laboratory cabinet. Continuous (piano-style) hinges are used for rigidity. The hood can be used with sides and top folded back, providing excellent draw (in excess of 100 ft/min) for small apparatus, or with the top and sides opened to provide a comfortable working area at the design air flow. With the sides and top folded, the unit occupies only slightly more than one square foot of bench surface. In the open position, stops on the top prevent the sides from being inadvertently moved. Small slots are provided in the bottom of the movable sides to bring in power cords or hoses without cluttering the front opening.
Even with the sides open but the top folded back to accommodate tall apparatus, good air flow can be achieved. As an added safety feature, in this configuration the top flap can be further folded down to cover the top half of the face. For visibility, the construction was of clear, 3/16-inch-thick acrylic, similar to that found in some commercial hoods. Students working in pairs or small groups can easily see what is taking place in the hood. The rear baffle is made of white polyethylene to help visibility. It is mounted to the back with 1-inch plastic spacer strips at the sides. Spaces one-and-a-half inches wide at the top and bottom and two one-inch slots in the middle of the baffle provide an air flow with little turbulence, as indicated by smoke tests. Because construction of our hoods (a total of 64) was subcontracted as part of an overall renovation project, we cannot give an accurate estimate of costs for individual units, which will be very much dependent on labor. Only ability to work with plastic and with sheet metal to fabricate the duct connections is required. Our shop roughly estimates $400 for a unit.
Some applications of these hoods in studio-type classes with students working in teams of four will be described by T. Apple and A. Cutler in a forthcoming publication. In use, they have been quite effective with organic vapors and acid fumes. Loss of transparency by reaction with some organic solvents may be possible, although in our applications we have not encountered this.