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Waterloo Air Products - Air Terminal Device Product Selection

Linear Slot Diffusers - Nomogram - Airflow Rate - Grille - Air Terminal Device

Introduction

Frequently the type and location of a grille or diffuser will be determined by architectural or other requirements. If this is the case, performance data can be applied directly to determine whether the resulting performance is acceptable. If an entirely free choice is available, refer to Sections 2 and 8 where comparative selection data can be used to determine the most suitable air terminal device. Usually, the sizing and design of a particular terminal device is based on the throw, but at each stage, it is necessary to check that any acoustic or pressure drop specification is satisfied.

Having decided which type of grille or diffuser is required, apply the following techniques for selection in conjunction with necessary information, such as total airflow rate and room size. It is helpful to have scale drawings of air terminal device layouts.

Mixed Flow Applications

Consideration must be given to the following which will determine the final selection.

  • Mounting position
  • Size of ATDs
  • Blade divergence
  • Acceptable local velocities
  • Noise criteria

Sidewall Grilles

Sidewall supply grilles may be selected or adjusted to discharge air to take advantage of the ceiling effect with or without spread in the horizontal plane. The blade angle can be used to balance the spread and throw for the required air flow rate. The maximum available throw should be based on the recommended ceiling height and throw.

If the required throw exceeds the maximum throw, consider an alternative scheme. Choose suitable grille locations and calculate the individual grille duty.

Determine the most suitable terminal velocity on which the throw is to be based; in general, a jet terminal velocity of 0.25 – 0.4 m/s will be satisfactory for year round air conditioning schemes using sidewall grilles.

Using selection tables or nomograms, select the most suitable grille size.

Grilles with an aspect ratio (width to height ratio) between 2:1 and 5:1 produce a better air diffusion pattern than square grilles and are less likely to cause draft problems due to excessive drop of the supply jet.

When the terminal device is mounted away from a surface (a free jet) the velocity decay is more rapid and throw is less than surface clinging jets.

Approach 1

It is recommended that the space between ATDs should be equal to 1/3 of the particular ATD’s throw.

Be aware that an ATD installed close to the ceiling may cause a Ceiling Effect, as described in Section 1. This may cause the Throw to be extended beyond the data in a manufacturers’ guidelines.

Taking this into account divide the room into equal sections along the length (L) ensuring that each individual section has a width:length aspect ratio of at least 1:3 or more.

The example shows 4 ATDs, for which the air flow rate for each can be acquired by dividing the total air flow rate (Qt) by the number of total amount of ATDs as represented in the following expression = Qt/4.

Upon obtaining the air flow rate for individual ATDs, the exact throw can be ascertained relative to the segment length (W) from the manufacturer’s technical data.

To prevent causing draughts, identify an acceptable air flow drop (based on velocity) which ensures the air flow envelope does not breach the occupied zone, normally a height of 1.8m.

Select an ATD size relative to the specific data of the ATD. Remember to consider the above along with temperature differential and position of the ATD in relation to the ceiling.

Approach 2

By dividing the room area lengthwise, it increases the amount of ATDs. This in turn lowers the air flow rate per ATD.

Proceed as per Approach 1.

Approach 3

Begin by dividing the room by the same aspect ratio (1:3) as seen in Approach 1. Then, maintaining the widths of the segments divide the room area lengthwise.

Set the blades to 22° and proceed as before.

Air Terminal Device Selection

Request Waterloo Air Products Info

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

Email Waterloo Air Products

Tel: 01622 711500
Fax: 01622 710648

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