The NEBB Professional 2024 - Quarter 2

sures. Figure 4 shows the loss of constancy occurring at approximately 0.012” H 2 O. This value is substan tially in agreement with the suggestions found in NSF/ ANSI 49: This pressure should typically be 0.001 to 0.01 in w.g. at the canopy’s connection to the exhaust system. This increased exhaust airflow phenomenon occurs because by reducing the downstream static pressure on the exhaust HEPA filter, the pressure drop ( Δ p) in creases on the exhaust HEPA filter and augments the resultant airflow. Based on the assumption that 100 CFM/ft. of cabinet width is an adequate exhaust quantity, Figure 5 tabu lates the calculated canopy slot height (nominal), D, based on common exhaust filter perimeters at the mini mum and maximum allowable canopy suction pressure (0.001 in H 2 O – 0.012 in H 2 0) for 8-inch and 10-inch work access opening sash heights for nominal 4-, 5- and 6-foot wide BSCs. With a nominal inflow velocity work access opening of 105 fpm, this table recommends be tween approximately 15 - 45% bypass flow beyond the work access opening inlet flow. Each installation is go ing to have a maximum available branch/valve CFM to service that IIA2 Vented BSC. The net canopy bypass CFM is described by Equation 1. The work access open ing CFM, Q I , must be maintained ±5% during operation for the BSC to remain certified. The table values are consistent with the NSF/ANSI 49 table I-1.1 guideline of: ≤ 100 CFM/ft. A coefficient of en

try loss factor, C e = 0.7 is assumed for conversion of suc tion pressure to canopy cross-section inlet velocity, V B . The range of canopy slot access openings is between 0.4 inch and 3.7 inches. This coincides with a range of between 18 - 49% above Q I . These heights are based on the nominal perimeter, P N , of the exhaust HEPA fil ter and, in practice, would need to be increased to ac commodate the physical open perimeter of the canopy transition including the support frame, spacing or any sliding panel openings to realize the actual perimeter opening, P A . Equation 5 describes the relationship between velocity and velocity pressure. Where: V is velocity (fpm). VP is velocity pressure (in H 2 O). C e is a dimensionless, empirical energy conversion loss coefficient. (0 < C e < 1.0) Equation 6 expands upon Equation 5 to incorporate volumetric flow, Q, and area A, under a static pressure potential. Equation 5: Velocity/Pressure Conversion V = 4005 * C e * √ VP

Equation 6: Effective Leak Area

Q' B

ELA =

2610 144

* √ SP * C e

Figure 5: Canopy slot elevations (D)

Work access opening area (ft 2 )

Work access opening inflow @ 105 fpm (CFM)

Maximum total

tempered air loss per NSF/ANSI Table I-1.1 (CFM)

Net bypass canopy

tempered air loss from Eq. 1 (CFM)

Percent of work access opening airflow in

canopy bypass (%)

Typical maximum exhaust HEPA

perimeter standard models (ft)

Canopy D @ 0.001”

duct static (Ce=0.7) (inches)

Canopy D @ 0.012”

duct static (Ce=0.7) (inches)

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

400 400 500 500 600 600

131

49

6.0 6.0 7.0 7.0 9.7 9.7

3.0 1.6 3.1 1.5 3.7 1.3

0.9 0.5 0.9 0.4

3.12

72

22

3.22 4.03 3.89 4.86

162

48

77

18

265

44

1.1

510

90

18

0.4

The NEBB Professional | Quarter 2 | 2024

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