Don't compromise on HVAC efficiency with poor air distribution

Poor air distribution will significantly compromise the efficiency of a building’s HVAC system. That’s according to Luke Bigg, Research & Development Engineer of Waterloo Air Products, who explains more here.

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espite the need to meet the requirements for more efficient systems and tighter building regulations, many buildings continue to rely on inefficient air distribution systems that are poorly specified or, in some cases, obsolete for this critical function.

The heating, ventilation and air-conditioning (HVAC) process is one of the most complex systems installed in a building and is responsible for a substantial component of the building’s total energy use.

A correctly sized HVAC system will provide the desired comfort and will run efficiently. The performance criteria of a well-designed air distribution system will ensure draft-free air delivery and acceptable noise levels which are dependent on the type of building.

The correct specification of a HVAC system involves the selection of equipment and the design of the air distribution system to meet the accurate predicted heating and cooling loads of the building. These loads are dependent upon a number of factors including the building’s location, construction of the building, functionality and infrastructure. The equipment selection and the air distribution design are dependent upon both the loads and each other.

The overall objective of any HVAC system is the comfort of the building’s occupants. Comfort is subjective and can vary according to individuals’ activities and their metabolic rates. The principal comfort criteria related to air distribution systems are thermal comfort performance, energy efficiency and the avoidance of hot and cold spots. Thermal comfort also depends on the following air temperature, mean radiant temperature, air velocity, relative humidity, type of clothing and degree of activity.

ISO 7730 (2005) delivers guidance on defining and setting comfort criteria for occupied areas. Air velocity is the most significant factor in the design of air diffusion schemes providing that:

  • Dry bulb air temperature is controlled according to air movement in the range 20-26°C
  • Mean radiant temperature is within 10°C of mean air temperature
  • Relative humidity is in the range 30-70%
  • Activity and clothing type is taken into account for the occupants

Many building owners are reasonably informed when it comes to the performance of their buildings and the efficiencies of their air handling plant. They are aware of the need to provide EPCs (Energy Performance Certificates), the benefits of natural ventilation, reducing emissions and the financial benefits these can bring as well as the reputation of a company.

For many projects, aesthetics is a prime factor. Increasingly air terminal selection has to combine well engineered products that efficiently deliver conditioned air to the space alongside architectural features which complement the interior design. The wide choice of available products can be daunting.

While much thought is given to the performance of the HVAC system, projects often overlook air terminal devices. This is changing as CEO, FDs, Facilities Managers, and CTOs, as well as the architects, take a holistic approach, thus leading to a more integrated approach to building and systems design.

Waterloo aims to simplify the process further and believes that collaborative relationships help customers to feel more confident in their choices.

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, displacement solutions provide high ventilation efficiency with superior comfort characteristics.

The location, type, size and shape of an air terminal device will determine the way in which supply air and the resultant room air motion will behave. When used with full air conditioning schemes they will provide changes in supply air temperature from a cooling to a heating cycle and will also modify the jet trajectory and room air movement patterns.

Air supply

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 are frequently used to block sightlines and prevent occupants from seeing directly into return air openings. Installations are governed by the thrown, spread and drop of the resulting air stream.

Diffusers are normally installed in ceiling areas providing supply air, but 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 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.

There are various specialist air terminal devices such as ‘Nozzles’, which produce a maximum throw with minimum entrainment. Displacement ventilation panels displace the air 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 better understanding of the various types of air distribution systems will produce a well-designed HVAC system. Good reference tools that provide essential information on products, detailed comparative selections, comfort criteria, acoustic information and performance data enables designers, specifiers and contractors to make informed selections, thus ensuring buildings gets the environmental system that best suits their needs.

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