ADM-860C 04/14/08
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6.5.1 CHEMICAL EXHAUST HOODS - VELGRID
The VelGrid provides the average of 16 measurement points at 3.5 inch centers, and represents a 14" x 14"
area for each reading. When using the VelGrid for chemical exhaust hood readings, the sash opening must be
set at a minimum opening of 14 inches in width for horizontal sliding sash, or 14 inches in height for vertically
adjustable sash. If the opening is less than 14 inches in width or length, the AirFoil probe should be used.
The VelGrid must be carefully positioned so that the perimeter orifices of the VelGrid are at least 1.75 inches
in from the edge of the opening. The leading (side the air strikes first) surface of the VelGrid should be evenly
aligned and parallel with the plane of the sash. Correct positioning of the VelGrid is easier if equal length, stiff
wire "feelers" are taped to the leading surface of the VelGrid. Coat hanger wire taped in place with plastic
electrical tape works well for this purpose.
6.5.2 LAMINAR FLOW WORKSTATION - VELGRID
The VelGrid can be used to measure average face velocities at the work zone or plane of a laminar work station
using much the same method as described for chemical exhaust hoods. It is important to position the leading
edge of the VelGrid at 90 degrees to the direction of air flow when measuring work zone velocities. The VelGrid
may also be positioned so the 1.5 inch standoffs are placed directly against the perforated supply panel face.
The velocity average obtained in this manner can be used to calculate the volumetric air flow rate as described
in Section 6.5.3 AIR FLOW CALCULATION FROM VELGRID VELOCITY.
6.5.3 AIR FLOW CALCULATION FROM VELGRID VELOCITY
Accurate volumetric air flow calculation using the average face velocity requires careful measurement of the
active gross face area
of the filter, grille, coil, or opening. Be sure to deduct the area of all obstructions to air
passage through the device to be tested, such as: support bands; T-bars, including the perimeter glue line; and
repaired areas of HEPA filters.
Even with careful measurement of the active area, the meter and the sensing probe will be affected by different
design configurations of the outlet, inlet, filter, coil or exhaust hood. It is best to establish a procedure and
confirm the air flow by pitot tube duct traverse or some other reliable flow measurement means for a given type
of air movement device. IMPORTANT: See Section 6.1 VELOCITY CORRECTION FACTORS.
The measurement of exhaust hood intake velocity requires careful placement of the VelGrid to align the leading
edge of the grid directly in line with the plane of the sash opening. Maintain the 1.5" perimeter margin as
illustrated. The total intake area and air flow of an exhaust hood includes all areas of air entry, including the
space behind and around the sash; under the threshold; and through service openings. It is accepted practice
to assume that the velocity through these additional areas is the same as that of the sash opening area.
Summary of Contents for ADM-860C
Page 38: ...ADM 860C 04 14 08 33 FIGURE 6 3 VELGRID ASSEMBLY...
Page 47: ...ADM 860C 04 14 08 42 FIGURE 10 1 FRAME STORAGE FIGURE 10 2 FLOWHOOD IN CASE...
Page 48: ...ADM 860C 04 14 08 43 FIGURE 10 3 FLOWHOOD ASSEMBLY...
Page 50: ...ADM 860C 04 14 08 45 FIGURE 10 7 1X5 FRAME ASSEMBLY FIGURE 10 8 3X3 FRAME ASSEMBLY...
