The length of “throw” from an air supply grille is important in de­sign to avoid draughts. Throw is usually defined as the distance from the grille to where the air velocity has fallen to 0.25 m/s. This velocity should be achieved at not less than 2 to 2.14 m above the floor. Modern grilles are manufactured to a number of proprietary designs for which it is best to consult the manufac­turers for recommendations as to the best type for a particular application.



подпись: 1/2One design has multi-deflecting vanes approximately 6 mm deep x 6 mm centres. These may be set to the required angle at the manufacturers or may be adjusted on site by bending the vanes with a special tool. Grilles with straight deflecting vanes generally produce the maximum throw for a given entering air velocity, but other types are available which produce a wide spread of the air with less risk of complaints from draughts.

With deflecting vanes, the air velocity is increased after leaving the grille. It is obvious from Figure 3.29 that width B is less than A, becoming more reduced as the angle of deflection is increased.

As stated, “angling” the vanes produces a greater outlet veloc­ity than that normally on the inlet side. Multiplying the selected velocity by the appropriate factor in Table 3.4 will provide the entering air velocity.

Vane angle degrees














Table 3.4 Factor for the entering air velocity

At the design stage it is usual to assume a mean angle and fac­tor of around 0.85.

The resistance may be determined as the leaving velocity pres­sure. Normally it is preferable not to spread the air vertically in industrial applications (and indeed in some offices with ex­posed steelwork) as there is a risk of hitting beams at ceiling height, or of blowing cooled air too rapidly down into the occupied zone.

Grilles fitted at the top of riser ducts in walls may have several horizontal deflectors behind the vanes. These may then be set to assist the air in turning.

Posted in Fans Ventilation A Practical Guide

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