The NEBB Professional 2024 - Quarter 2

Where: ELA is effective leak area (in²) SP is static pressure potential (in H 2 O) Q’ B is the airflow quantity flowing out of the canopy transition into the room (CFM) Suggested Solutions The engineering design constraints for a properly de signed canopy connection is summarized below: 1. Maintain cabinet bypass velocity, V B , (fpm) under normal operating conditions according ly: 88 ≤ V B ≤ 307. This is the resultant velocity with canopy duct static pressure P C (in H 2 O), 0.001 ≤ P C ≤ 0012, with a C e = 0.7, to maintain capture phenomenon. 2 2. Provide, under normal operating conditions, a sufficiently large enough cross-sectional area (ELA), A B , such that in the event of exhaust fail ure, Q I , is maintained adequately large enough to ensure that V I is not below (92 fpm). Where V I is the velocity at the work access opening. 3. Maintain consistent work access opening ve locities, V I , ± 5 fpm under varying exhaust sys tem static pressure conditions. 4. Provide an audible and visual alarm strategy which indicates both when the canopy pres

sure is too high (more positive) to indicate loss of capture as well as too low (more negative) to indicate loss of certification tolerance at V I . 5. Enable adequate clearance space for exhaust HEPA filter integrity testing during routine certifications. 6. Enable the certification technician to reduce the exhaust air volume, Q T , both when per forming the site installation alarm verification and when performing the exhaust HEPA filter integrity testing without affecting the adjacent facility space. 7. Enable the certification technician adequate space and ability to block the exhaust suction to reliably seal the exhaust HEPA filter with tape and plastic during the decontamination process. Our internal VTG research mentioned above, also dis covered that the maximum achievable positive static pressure, downstream of the exhaust filter, in a sealed duct (simulating exhaust fan failure) was approximate ly 0.05” H 2 O produced by the internal cabinet blower. If we revisit Figure 5 with the additional condition de tailed in design constraint #2 and the application of Equation 6, we produce a modified list of canopy slot heights, D as shown in Figure 6.

Figure 6: Canopy slot elevations (D)

Canopy D from effective leak area (ELA) (Ce = 0.7) SP = +0.05” H 2 O (inches)

Typical maximum exhaust HEPA perimeter standard models (ft)

Canopy D @ 0.001” duct static from Figure 5 (Ce=0.7) (inches)

Canopy D @ 0.012” duct static from Figure 5 (Ce=0.7) (inches)

Work access opening inflow @ 105 fpm (CFM)

Work access opening inflow @ 92 fpm (CFM)

Work access opening area (ft²)

Nominal 4’ Hood 8” Sash Nominal 4’ Hood 10” Sash Nominal 5’ Hood 8” Sash Nominal 5’ Hood 10” Sash Nominal 6’ Hood 8” Sash Nominal 6’ Hood 10” Sash

2.56

269 328 338 423 335

235 287 296

6.0 6.0 7.0 7.0 9.7 9.7

3.0 1.6 3.1 1.5 3.7

0.9 0.5 0.9 0.4

1.2 1.4 1.2 1.6

3.12

3.22 4.03 3.89 4.86

371

358 447

1.1

1.1

510

1.3

0.4

1.4

2 A canopy pressure range of 0.001” – 0.010” H 2 O is assumed for this article in view of consistency with the NSF/ANSI 49 recommendations. Some BSC manufacturers recommend other canopy pressures (-0.05” H 2 O). The internal cabinet exhaust airflow damper can be adjusted to accommodate any negative canopy pressure to maintain Q I & V I . There may be a problem with cabinet blower start up in the presence of high negative canopy pressures, however.

The NEBB Professional | Quarter 2 | 2024

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