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Technique

Technique

The following basic terms and definitions will be used in this and the forthcoming discussions on air diffusion.

Air terminal Device (ATD)
A general term used to describe supply, exhaust or transfer diffusers and grilles.

Diffuser
Generally refers to ceiling mounted air terminals which ‘diffuse’ the supply air along the ceiling and induce the room air from below. There are however certain types of terminal such as the jet nozzle and laminar flow panel which are termed diffusers but actually have the aerodynamic characteristics of grilles. To maintain architectural continuity diffusers can also be used as extract terminals.

Grille
Originally defined as a lattice or screen and in the early days of ‘forced ventilation’ systems was used as a decorative covering rather than a functional air terminal. Modern types of grille are in general used as sidewall, cill or floor mounted devices, and, depending on the type, they may incorporate sets of adjustable vanes or have various fixed blade profiles Grilles can in general be used for supply or exhaust applications, although certain types of grille are specifically designed for exhaust or transfer use.

Damper
Air dampers are used in conjunction with grilles and diffusers to provide a means of balancing or regulating the air flowing through the terminal. Various types are available to suit round, square, or rectangular terminal necks.

Plenums and Neck reducers
These are used as a means of conveying air from the system ducting to the grille or diffuser and are essentially sheet steel shrouds attached to the neck of the terminal.

Air diffusion terminology

Air flow rate: Is the quantity or volumetric flow rate of air passing through the air terminal.
Air velocity: Is an engineering term used to describe the ‘speed’ of an air current in a room or air flowing through a duct. The actual definition of velocity is ‘constant speed in a straight line’.
Jet throw and projection: The distance that an air jet will travel from the diffuser before its velocity (speed) becomes insignificant. This is used as the basic method of selecting terminal type and size. The term ‘projection’ is used specifically where the air jet is discharged vertically.

The basic principles of air diffusion schemes
It has already been noted that the principle requirement of any air terminal device is to introduce the air without causing draughts. In most situations, this means positioning the terminal above what is termed the ‘occupied zone’, which is generally taken as being any part of the room above standing head level. (1.8m from floor level).

However, this alone is no guarantee of producing a satisfactory room environment and many other factors may have to be taken into consideration when selecting an appropriate scheme. Fortunately, in most cases we are able to make use of a physical principle known as the ‘Coanda effect’ (sometimes known as the ceiling effect), which in basic terms states that a moving airstream will have a tendency to be drawn towards and adhere to a smooth surface.

In this way the ceiling surface can be used to ensure that the relatively high velocity (speed) airstream is directed away from the occupants.

As the air leaves the terminal it will start to drag the stationary room air with it,the net result being that in moving across the ceiling the terminal airstream velocity (speed) will gradually be reduced, at the same time mixing with the room air to either heat or cool it.

This process is commonly termed entrainment and is illustrated for sidewall mounted grilles and ceiling mounted diffusers in the following sketches.

Methods of supplying and extracting air from buildings
Air terminals can be used to supply or exhaust air in a number of ways, but more often than not the chosen method and the type of terminal will be dictated by physical or architectural constraints. The following sketches will be used in the product descriptions to indicate their principle uses.

Manufacturing and finishing methods
The majority of products within the grille, diffuser and damper ranges are manufactured from aluminium extrusions or spinnings. These offer many advantages over other materials and manufacturing processes, most notably, lightness, strength, and the ability to design features into the profile to simplify assembly or provide a particular performance characteristic.

Jointing techniques mostly rely on the corner cleating process whereby a feature in the extrusion is deformed around a corner insert. Welding or soldering is also used on joints where strength is an important factor.

Grilles which feature non adjustable blades are assembled using a technique known as tube expansion which provides a quick and relatively cheap assembly method. With this system, the individual blades are cut to length and pre-punched to accept the assembly tube which is then expanded internally to lock the blades in place.

The standard form of paint applied to all products is a polyester powder coating which produces a high quality, long lasting finish suitable for most applications.

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The following basic terms and definitions will be used in this and the forthcoming discussions on air diffusion.

Air terminal Device (ATD)
A general term used to describe supply, exhaust or transfer diffusers and grilles.

Diffuser
Generally refers to ceiling mounted air terminals which ‘diffuse’ the supply air along the ceiling and induce the room air from below. There are however certain types of terminal such as the jet nozzle and laminar flow panel which are termed diffusers but actually have the aerodynamic characteristics of grilles. To maintain architectural continuity diffusers can also be used as extract terminals.

Grille
Originally defined as a lattice or screen and in the early days of ‘forced ventilation’ systems was used as a decorative covering rather than a functional air terminal. Modern types of grille are in general used as sidewall, cill or floor mounted devices, and, depending on the type, they may incorporate sets of adjustable vanes or have various fixed blade profiles Grilles can in general be used for supply or exhaust applications, although certain types of grille are specifically designed for exhaust or transfer use.

Damper
Air dampers are used in conjunction with grilles and diffusers to provide a means of balancing or regulating the air flowing through the terminal. Various types are available to suit round, square, or rectangular terminal necks.

Plenums and Neck reducers
These are used as a means of conveying air from the system ducting to the grille or diffuser and are essentially sheet steel shrouds attached to the neck of the terminal.

Air diffusion terminology

Air flow rate: Is the quantity or volumetric flow rate of air passing through the air terminal.
Air velocity: Is an engineering term used to describe the ‘speed’ of an air current in a room or air flowing through a duct. The actual definition of velocity is ‘constant speed in a straight line’.
Jet throw and projection: The distance that an air jet will travel from the diffuser before its velocity (speed) becomes insignificant. This is used as the basic method of selecting terminal type and size. The term ‘projection’ is used specifically where the air jet is discharged vertically.

The basic principles of air diffusion schemes
It has already been noted that the principle requirement of any air terminal device is to introduce the air without causing draughts. In most situations, this means positioning the terminal above what is termed the ‘occupied zone’, which is generally taken as being any part of the room above standing head level. (1.8m from floor level).

However, this alone is no guarantee of producing a satisfactory room environment and many other factors may have to be taken into consideration when selecting an appropriate scheme. Fortunately, in most cases we are able to make use of a physical principle known as the ‘Coanda effect’ (sometimes known as the ceiling effect), which in basic terms states that a moving airstream will have a tendency to be drawn towards and adhere to a smooth surface.

In this way the ceiling surface can be used to ensure that the relatively high velocity (speed) airstream is directed away from the occupants.

As the air leaves the terminal it will start to drag the stationary room air with it,the net result being that in moving across the ceiling the terminal airstream velocity (speed) will gradually be reduced, at the same time mixing with the room air to either heat or cool it.

This process is commonly termed entrainment and is illustrated for sidewall mounted grilles and ceiling mounted diffusers in the following sketches.

Grille

Methods of supplying and extracting air from buildings
Air terminals can be used to supply or exhaust air in a number of ways, but more often than not the chosen method and the type of terminal will be dictated by physical or architectural constraints. The following sketches will be used in the product descriptions to indicate their principle uses.

Airgrilles

Air transfer grilles
Air diffusers

Manufacturing and finishing methods
The majority of products within the grille, diffuser and damper ranges are manufactured from aluminium extrusions or spinnings. These offer many advantages over other materials and manufacturing processes, most notably, lightness, strength, and the ability to design features into the profile to simplify assembly or provide a particular performance characteristic.

Jointing techniques mostly rely on the corner cleating process whereby a feature in the extrusion is deformed around a corner insert. Welding or soldering is also used on joints where strength is an important factor.

Grilles which feature non adjustable blades are assembled using a technique known as tube expansion which provides a quick and relatively cheap assembly method. With this system, the individual blades are cut to length and pre-punched to accept the assembly tube which is then expanded internally to lock the blades in place.

The standard form of paint applied to all products is a polyester powder coating which produces a high quality, long lasting finish suitable for most applications.