Slot hoods

Cross-draft velocity was normalized by dividing the measured cross-draft ve-locit by the capture velocity calculated at the tatik centerline. Capture velocity at the tank centerline was calculated using Silverman s - centerline velocity (Eq. (JO.l)) for unflanged slot hoods. There was considerable scatter in the data, show ing chat cross-draft velocity alone is not responsible for low capture efficiency. 

Rim exhausts are a specific application of slot hoods, which in turn are a type of exterior or capture hood. Rim exhausts are slot hoods placed along the rim or edge of an area source such as an open surface tank or vessel opening. Open surface tanks are widely used in indu.strial settings for cleaning, stripping. 

Rim exhausts are suitable for area sources of contaminant. They are limited in the area over which they can draw with adequate velocity. In practice, the slot hood should be within 0.6 m of the far edge of the source. For an open surface tank this means that a slot hood on one long side is necessary for tanks up to 0.6 m in width hoods on both long sides are necessary for tanks up to 1.2 m in width and rim exhaust is not practical for tanks wider than 1.2 m. For those situations, push-pull ventilation or enclosure type hoods are recommended.- ... 

Rim exhausts are slot hoods located on or around the edge of a source such as an open surface tank. Flanges may be added to decrease the airflow from behind the slot (uncontaminated air) and therefore increase the airflow from in front of the slot (contaminated air). The plenum downstream of the slot, if located above the slot, may act as a flange. Flanges may also be added to the sides of the source (tank) away from the slot hood. These flanges also act to increase the flow of contaminated air into the tank. Tank flanges, however, may interfere with process activities by limiting access to the tank. 

The pressure loss associated with this type of exhaust opening is the sum of two pressure losses. The slot hood is usually thought of as a sharp-edged orifice and the duct entry (from the slot plenum) is a flanged opening. The rec ommended hood entry loss is given by Eq. (10.56) - ... 

Another design method uses capture efficiency. There are fewer models for capture efficiency available and none that have been validated over a wide range of conditions. Conroy and Ellenbecker - developed a semi-empirical capture efficiency for flanged slot hoods and point and area sources of contaminant. The point source model uses potential flow theory to describe the flow field in front of a flanged elliptical opening and an empirical factor to describe the turbulent diffusion of contaminant around streamlines. 

FIGURE 10.36 It Is better to use a slot hood when a worker needs to lean over a bath. 

Figure 1 -3 A possible darkroom configuration. The enlarger may be placed in the middle or at either end and the trays arranged accordingly. An active slot hood would be well placed above the tray, in which case fresh air should come in from the wall opposite or the door. (Thanks to Brian MacNeil of Canada.)...

Examples of local exhaust ventilation system are drawdown exhaust tables, slot hoods, dust extraction systems, and portable vapor and dust collectors. Care must be taken that the ventilation systems are utilized and maintained as designed. The condition of the filters and air flow rate should be checked periodically as well as the condition of the duct work, motors, belts, etc. 

A flanged slot hood exhausts particulates produced in an operation. The flow volume for the exhaust system is 10,000 ft /min. The hood entry is 6 ft X 1 ft. 

Some slot hoods are mounted to be movable and may also be attached to a portable suction unit (fan) and appropriate filter. 

The geometry of the contaminant source can be compact or linear. The source geometry affects the hood geometry round, rectangular, or slot. 

Fletchep B. 1977. Center line velocity characteristics of rectangular unflanged hoods and slots undersuction. Ann. Occup. Hyg. 20 141-46. 

Systems with extraction only The most common of this type use slot exhaust devices that are more or less uniformly distributed across the exhaust wall of the booth (e.g., the back Wall). Some attention has to be paid to maintaining a uniform distribution of the exhaust volume flow across the slit openings. Rows of bellmouth inlets or even vortex hoods are more advantageous because they provide a more uniform flow field. 

Plenum chamber Any air compartment connected to one or more ducts or to a slot in an air distribution hood. 

Conduct all alkyladon reacdons in a properly functioning chemical fume hood. For safety precautions and decontaminadon procedures, see ref. 13. As the antisera to modified DNA bases often exhibit significant crossreacdvity to unmodified DNA, all slots must contain the same amount of DNA, e.g., 3 pg. Highly modified DNA should be diluted with unmodiBed DNA. 

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