MIL-HDBK-1011/2

APPENDIX C (continued)

EQUATION:

Q = 0.02 CAVref

(8)

the volumetric flow rate, cfm (m3/sec)

Where

Q

=

C

=

unit conversion factor, 88.0 for Q in cfm and 1.0 for Q in

m3/sec

the area of opening, ft2 (m2)

A

=

Vref

=

the mean velocity at a reference point in the free wind

at a height equal to that of the building, mph (m/sec).

1.2.1.1

Estimating Quantity of Inlet Air. The quantity of air forced

through ventilation inlet openings, assuming inlet and outlet areas are equal,

can be estimated by the Equation (9).

EQUATION:

Q = CKAV

(9)

airflow, cfm (m3/sec)

where

Q

=

C

=

unit conversion factor, 88.0 for Q in cfm and 1.0 for Q in

m3/sec

K

=

effectiveness of openings, 0.50 to 0.60 for perpendicular

winds and 0.25 to 0.35 for diagonal winds

free area of inlet openings, ft2(m)2

A

=

V

=

mean external wind velocity, mph (m/sec)

Equation 9 does not take into account the air damming action of the wall. For

a more precise estimation of airflow due to wind which does not require wind

tunnel testing for each building, but uses discharge and pressure coefficient

data from previous wind tunnel tests, use Equation 10.

Q = CdA [(Cp1 - Cp2) * Vref2]1/2

EQUATION:

(10)

where

Q

= volumetric flow rate

Cd

= discharge coefficient, commonly 0.65, appropriate for

small openings near the center of walls.

When openings are near the edge of a wall in the downwind space, the discharge

coefficients increase to 0.7 and 0.8, with larger values for bigger openings

(10-20 percent of the wall area.) For openings similar in size to the

cross-section of the downstream space, discharge coefficients of 0.8 to 0.9

are possible.

A

=

area of opening

Cp1

=

windward pressure coefficient

Cp2

=

leeward pressure coefficient

Vref =

velocity at reference height (pressure coefficient

measurement)

114

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