Sizing of grilles on blowing systems

High level (above 3 m minimum height): The basis of selection is normally to obviate the noise caused by air impingement on the vanes. The maximum velocity on the entry side of the grille will depend on the application and the following maximum val­ues are suggested:

• Board rooms, private offices etc 3.5 m/s

• General offices 5.0 m/s

• Industrial applications 7.5 m/s

Low level: The basis of selection is to achieve good comfort conditions for the occupants without noticeable draughts. If the entering air is at a temperature below that in the room i. e. a cooling application, then a minimum height of 2 m is suggested. The maximum velocity on the outlet side of the grille should be:

• Occupant very near the grille 0.5 m/s

• Generally with private offices 2.5 m/s

18* Inclination

Figure 3.31 Tapered outlets for factory general ventilation

Cold air douche plants are often supplied for applications such as steel rolling mills, tin-plate rolling and glass furnaces. Here the operators are subjected to high radiant heat. As well as the copious quantities of beer which some plants allow, drop ducts from the main duct are positioned to blow cool air onto the work­ers! The air does not have to be cooled artificially, but is merely external atmospheric air. The outlet velocity is usually about 3.75 m/s (see Figure 3.32).

Textile conditioning plants have outlets on drop pipes from the main duct and are fitted with special diffusing outlets. These are spaced at intervals to cover the area of the room to be condi­tioned. Many different designs of outlet are available. Two of the

Xv

The extract volumetric flowrate Q m3/s = A

Q 12.57r2

So v m/s at any radius r =

Figure 3.36 Extract from a point source

But A = 4Ttr2 =12.57r2

Figure 3.32 Outlets for cold air douche plants

TypeC

250 mm to 350 mm diameter drop Max 7 5 m/s m this bat 6 m/s safer Allow 25 Pa static pressure Faced about 2 5 m to underside

In actual practice the extract is not from a point source and the flow is not completely the same from all directions. In 1932 Dalla Valle investigated an open ended duct freely suspended in space, and found that the centre line airflow relationship was:

Equ 3.34

Perforated cone

Q = v(l0r2 + A) where:

300 mm to 375 mm square neck Max 5 rrVs in the neck Allow 38 Pa static pressure Fixed about 3 4 to underside

V R A Hence v =

Figure 3.33 Type “C” outlet for textile air conditioning

Type M

A

//I

Perforated

Figure 3.34 Type “M" outlet for textile air conditioning

Simplest, which have proved satisfactory, are shown in Figure 3.33 and 3.34.

In factories with very high ceilings, the plenum warm air system is often fitted with drop pipes, or down corners, fixed adjacent to stanchions. These discharge the warm air nearer to floor level (see Figure 3.35). The drops may be from 200 mm to 280 mm diameter splitting into two outlets about 750 mm above the floor. Velocity should be 3.75 m/s to 5 m/s.

7777777777777777777777 Figure 3.35 Drop pipes for warm air

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