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Waterloo Air Products - Air Terminal Device Selection

Linear Grilles - Terminal Velocity - Air Diffusion - Free Jet Applications - Vertical Projection

Linear Grilles

Continuous grilles mounted at high level on a sidewall or bulkhead may be treated in a similar manner to linear slot diffusers. If the grille is mounted to take advantage of the ceiling effect, this is always beneficial with cooling differentials as the risk of dumping is minimised.

To avoid draughts at head level, the maximum throw should be limited to the figures shown in table 5.4, based on ceiling height.

If the required throw exceeds the maximum throw shown above, consider supplying air from both sides of the room or using an alternative terminal device.

Calculate the grille duty by dividing the maximum available active length into the total flow rate.

Determine the most suitable terminal velocity for the application; for year-round air conditioning with linear grilles, a terminal velocity of 0.3 – 0.4 m/s is satisfactory.

Using selection nomograms, determine the most suitable grille height based on the required duty and throw.

Linear Slot Diffusers

These diffusers can be selected or set to provide horizontal diffusion in one or two directions across a flat ceiling surface drawing up on the ceiling effect. As the supply jet entrains room air, it expands its depth and must be prevented from prematurely entering the occupied zone.

One Way Throw

For continuous slot diffuser arrangements, divide the ceiling area into convenient strips, based on the maximum throw.

Determine the available active lengths of diffuser sections, then calculate the diffuser duty by dividing the active length into the total air flow rate to be supplied.

With the available information of maximum throw and diffuser duty, draw two lines on the selection nomogram; one passing through the minimum throw and the other passing through the maximum throw. This produces a band of possible selections.

It is now necessary to find the optimum selection for comfort (maximum number of slots to produce the ideal room air movement for the application).

If the optimum selection falls below a one-slot diffuser, then the active length can be reduced as necessary.

If the optimum selection is greater than eight slots, it is possible that a slot diffuser arrangement is not practical and further advice should be obtained.

Two Way Throw

If selecting a linear diffuser with a two-way throw, the two throws should be selected on a nomogram as though they are separate diffusers.

After working out the Qm, divide that air flow rate by the number of Slots on the Linear Diffuser.

Example: A 3-Slot Diffuser with an air flow rate of 180l/s/m would give 60l/s/m through each slot.

If the slot is to be configured with two slots diffusing one way, but the third to diffuse in the opposite direction, then selections would be for a two-slot configuration of 120l/s/m and a one-slot configuration of 60l/s/m.

Circular, Square & Swirl Diffusers

Circular diffusers produce a radial air diffusion pattern while square and rectangular devices can be selected or adjusted to produce 4, 3, 2 or 1-way directional air patterns and drawing up on the ceiling effect. Wherever possible, select a 4-way or radial pattern as this results in the most efficient air diffusion. Using table 5.6, determine the maximum recommended radial throw based on the zone ceiling height. This will prevent the supply jet from entering the occupied zone prematurely, as it expands in the vertical plane.

Standard Selection

Using a scaled ceiling plan, divide the area into convenient squares twice the size of the derived maximum radial throw. A circular or square diffuser at the centre of each area can now be selected to handle its proportion of the total airflow rate.

Using selection tables or nomograms, determine the diffuser sizes which satisfy the throw parameter. The most economical selection will produce a minimum radial throw very close to the required throw. However, the optimum selection will probably be a compromise between the most economical selection and that which will produce the most comfortable room air movement.

If the maximum radial throw produced by the smallest available diffuser is less than the required throw, then insufficient room air movement and high-level stratification will result. An alternative air terminal device should be considered.

Similarly, if the minimum radial throw produced by the largest available diffuser is greater than the required throw, it is probable that the air terminal device is unsuitable.

Wherever possible, diffuser selections should be within the limits given in the tables; extrapolating data down to very low-neck velocities will usually result in poor air diffusion – for example, high-level stratification with heating cycles and draughts due to dumping with cooling cycles.

Swirl diffuser units usually have a circular pattern of radial vanes which generate a swirling air motion when used in supply mode. This highly turbulent swirl effect allows the unit to introduce high volumes of air into the conditioned space, taking advantage of the rapid entrainment and intermixing characteristics. As a result, the unit can deliver high room air change rates as compared to conventional diffusers.

Air Terminal Device Selection

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Waterloo Air Products Plc
Quarrywood Industrial Estate
Mills Road
ME20 7NB

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Tel: 01622 711500
Fax: 01622 710648

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