Understanding air distribution
Selecting the right air-distribution products can have a major impact on the efficiency of HVAC systems, explains Rick Edmondson.
Building owners are increasingly circumspect when it comes to the performance of their buildings and the efficiency of their air-handling plant. The requirement for them to provide EPCs (Energy Performance Certificates), combined with the demands for natural ventilation, reduced emissions, energy efficiencies and cost savings has created fresh opportunities for the air-terminal industry.
There are three major cost components of any HVAC application — labour, the HVAC system and air-distribution systems. Choosing the right air-distribution products for the application can have a dramatic impact on the performance of an HVAC system.
If the air distribution is improperly applied or selected, it could have an adverse effect on the occupants’ comfort, air quality, noise, and energy conservation. Although the selection of air diffusers and terminal devices has a lot to do with aesthetics, selection must combine a proper choice of engineered products efficiently delivering conditioned air to the space, while adding architectural features which complement the interior design.
There is a variety of products on the market, all precisely designed for specific applications and optimum results, but selecting the right air-distribution devices can be confusing.
Much thought is given to the performance of air-conditioning systems in terms of performance, but all too often air-terminal devices are overlooked when deadlines are tight.
That is changing now that building owners are increasingly becoming total-ownership cost savvy. They and their financial directors understand the bottom-line impact of energy budgets and energy consumption profiles and use scientific and analytical tools such as energy modelling to provide them with accurate energy-consumption patterns.
This is leading to a more integrated approach to building and systems design, with air-distribution devices being recognised as more than ancillary products but key components.
Manufacturers are responding by helping building owners, consultants and specifiers to take a greater role by offering a variety of tools such as selection programs and reference manuals detailing the different products and applications to help simplify selection and put customers in greater control of their selections.
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| How far will the air go? When the terminal device is mounted away from a surface, the velocity decay is more rapid, and the throw is reduced. |
Air-terminal devices include grilles for wall, ceiling and floor applications, diffusers for various ceiling systems and louvre products for mainly external wall applications — in addition to displacement solutions for to provide high ventilation efficiency with excellent comfort characteristics.
The location, type, size and shape of an air-terminal device will determine how supply air and the resultant room air motion will behave. When used with full-air conditioning schemes they will determine changes in supply-air temperature from a cooling to a heating cycle and also modify the jet trajectory and room air-movement patterns.
Grilles are typically found in side walls, but can be fixed into the floor (linear bar grilles) or used as ceiling-mounted extract terminals (usually square or rectangular). Grilles are essentially decorative covers for return air inlets and frequently used to block sightlines and prevent occupants seeing directly into return-air openings. Installations are governed by the throw, spread and drop of the resulting air stream.
Diffusers are normally installed in ceilings to supply air, but they can also be used for extracting air.
Linear slot diffusers have either single or multiple slots. Each slot may or may not have adjustable control elements to vary the direction of the air or the rate at which it is delivered to a space.
Circular diffusers, on the other hand, produce radial horizontal or vertical air diffused patterns — as do square and rectangular diffusers. Swirl diffusers cause a swirling of the air or create a high-turbulence discharge which accelerates the mixing of supply air into a surrounding space; this is greater than the mixing effect generated by conventional square or circular diffusers. Designs can vary from radial face to multiple slot or perforated face units.
Floor-mounted diffusers are specially designed to generate a swirl motion of the discharge air which can be vertical or horizontal.
Grilles and diffusers need to be selected and sized to provide the correct performance, and consideration needs to be given to prevent draughts, excessive noise and pressure losses.
Louvre systems are typically used as airflow terminations for the exterior of buildings or purely to provide an aesthetic solution and disguise and/or protect roof-top plant and equipment. Such systems comprise a series of horizontal or vertically mounted angled blades assembled at a fixed pitch. Depending on the application, weather, cladding, louvred screens, penthouse and acoustic louvres will be used.
Louvres can also be provided with bird and insect guards and insulated or un-insulated blanking panels. They need to be securely fixed to structures. Climatic conditions and wind loads need to be considered to prevent any part of the system becoming detached or compromise its performance.
There are numerous specialist air-terminal devices — such as nozzles designed for maximum throw with minimum entrainment and displacement-ventilation panels to provide uniform air discharge at low velocity over the face area and supply air directly into the occupied zone of a conditioned space.
A good understanding of the various types of air-distribution system will result in a well designed HVAC system. Good reference tools that provide essential information on products as well as detailed comparative selections, comfort criteria, acoustic information and performance data enable designers, specifiers and contractors to make informed selections which will ensure that the building gets the environmental system that will best fit its needs.
Rick Edmondson is chairman of Waterloo Air Products.
