corners of the workbench to place large pieces of lagging in double bagged containment。

b。 The top baffle swings up to allow access to overhead cranes。

Figure 3-2。  Exhaust hood for high profile work pieces。

3-5。1。2 Typical Hood Design for Low Profile Work pieces。  Figure 3-3 shows a hood design consisting of a workbench with a grating strong enough to support the heaviest expected work piece。 This is a downdraft hood that draws small pieces of lagging through the grating。 The perforated plate below the grating creates even airflow over the grating。 This design is best for low profile work pieces such as piping。 Design each hood with stands and swinging baffles on each end to accommodate long work pieces (e。g。, pipes)。

3-5。3 Ductwork。  Size the exhaust ductwork to provide a minimum transport velocity of 25。4 m/s (5,000 fpm)。 The high velocity is necessary because the practice of wetting the fibers makes them heavier and more difficult to transport。  See paragraph 2-

4。1 for general duct considerations。

Figure 3-3。  Exhaust hood for low profile work pieces。

3-5。4 Fans。  See paragraph 2-4。2 for general fan considerations。

3-5。5 Weather Stack Design and Location。  See paragraph 2-4。3。

3-5。6 Air Cleaning Devices。  A delagging facility requires multistage filtering, which consists of a fabric filter collector, prefilters, a mist eliminator, and high efficiency particulate air (HEPA) filters。 Prefilters extend the life of the HEPA filters。 Use "bag in, bag out" styles of HEPA filters, which allow for safe replacement of the filter element without exposure to asbestos。 A mist eliminator before the HEPA filter protects it from the moisture generated during asbestos removal。

a。 Have all collectors deliver the collected asbestos to a common pickup point to minimize the risk of exposure。 Provide a double acting valve at each collector hopper throat, in accordance with the ACGIH IV Manual, Chapter 4。

b。 Use a single chamber, shaker type collector to minimize the number of collection points。

3-5。6。1 Filter Efficiency。  The fabric filter collector requires a minimum efficiency reporting value (MERV) of not less than 15 in accordance with ASHRAE 52。2, Method

of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size。

3-5。6。2 Sequencing。  Figure 3-4 illustrates the required sequence of air cleaning devices。

Figure 3-4。  Sequence of air cleaning devices for asbestos delagging。

3-5。7 Industrial Vacuum System。  Provide a low volume, high velocity (LVHV) central vacuum system at delagging shops to exhaust fibers and dust from power tools (e。g。, grinders and saws) when they are used, as specified in 29 CFR 1910。1001。

3-5。7。1 Design a central vacuum cleaning system, which consists of a motor driven exhauster interconnected with bag type separators。

3-5。7。2 Connect the separator to rigid tubing, which extends throughout the plant。 Terminate the rigid tubing with inlet valves at the various workstations。 Provide flexible hose connections to allow workers to do shop cleanup and to decontaminate their protective outerwear。

3-5。7。3