Views: 0 Author: Site Editor Publish Time: 2022-06-27 Origin: Site
1.Fume Hood Function
The fume hood is the primary control device in most laboratories for protecting employees and students from exposure to hazardous chemicals. It is also an integral part of the building air handling system. The efficiency of operation is essential in maintaining good air quality in laboratories. It is therefore imperative that it function properly and that it be designed appropriately.
2. Definitions
ASHRAE: American Society of Heating, Refrigerating and Air Conditioning Engineers. Bypass Fume Hood: A fume hood that contains an open grill/louver above the front face of the working chamber. This grill compensates for the reduction in face area, and thus the significance of face velocity changes, when the sash is operated.
Working Chamber: The section of the fume hood system that includes: · The fume hood sash; The fume hood work space; The structural frame of the fume hood.
3. Design Standard
The fume hood is one component of the exposure control system; its performance therefore cannot be judged in isolation from the rest of the system. The total system consists of: ·
The working chamber; ·
The exhaust system; ·
Fume hood location; ·
Make-up air to the fume hood; ·
System indicators; ·
Operational parameters;
And System maintenance.
The following specifications provide the standard for acceptable performance and efficiency of fume hood systems; all new installations shall conform to these specifications and existing installations should be brought to this standard as funding permits.
The working chamber should: ·
Be by-pass type when constant volume systems are required; ·
Be constructed from non-flammable, acid/corrosion resistant materials; ·
Include a back baffle system to more evenly distribute the air across the face of the chamber such that uniform air flow through the face of the fume hood will result; · Include a stainless steel air foil along the lower, front edge at the face of the fume hood workbench which provides a streamlined and aerodynamic entrance profile; · Include vertical and/or horizontal sliding sash to minimize the size of the working aperture and to act as a safety screen. The sash should be constructed of transparent glazing that is resistant to discoloration and crazing by age or exposure to chemicals; And Contain a recessed work surface to retain spilled liquids. The work surface must be able to bear the weight of any equipment or radiological shielding material that is required. ·
Be labelled with a unique identifier. This identifier should also be labelled on: The fume hood fan; and The exhaust stack.
4. Fume Hood Ducting
The fume hood ducting shall: ·
Be designed to provide optimum air flows in the working chamber; ·
Be designed and sized to minimize noise levels from fume hood fan operation; ·
For VAV systems be designed along with the fan system to provide a buffer in addition to the design face velocity. E.g. if a 60fpm face velocity low flow hood is specified the fan and ducting should allow for 80fpm. ·
Avoid horizontal runs as much as feasible to minimize collection of corrosive condensates; Where horizontal runs do exist, the duct must slope at least 2.5 cm downward to a suitable drain, in the direction of airflow, per 3 meters of horizontal run; ·
Contain circular cross-section to reduce the number of corners and crevices where corrosion might occur; ·
Be constructed with type 316 stainless steel welded construction, except where the chemicals used require compatible materials.
