Variable Geometry Variable Air Volume (VGVAV) diffusers have been used for more than 40 years to effectively manage the air distribution system in a building. The functionality of early diffusers was limited in their capabilities, but in the past 10 years VGVAV diffusers have been transformed by the addition of advanced, modern electronics to help optimise indoor comfort.

When designed into a HVAC system correctly, electronic VGVAV diffusers offer the building occupant:

• Excellent energy efficiency as they operate from low pressure duct work where smaller variable speed Fan Coil Units can be selected and therefore running costs significantly lowered;
• Individual thermal comfort control as the VGVAV diffuser can manage individual spaces within a building; and
• Flexible control through remote monitoring and building fine tuning.

This article examines the changes in VGVAV diffusers over the years and compares a pressure independent air distribution system to a pressure dependent air distribution system design.

Pressure Independent System – VAV Box design

VAV Boxes and constant grilles became popular in the 1970s as an economical way to manage the air distribution system in a building. These designs are used to control large areas or zones, but are limited in their ability to manage individual thermal requirements of a building.

Advantages

• The relatively high duct pressures and velocities allow the duct size and initial cost to be reduced.
• Cost savings are realised when a large number of outlets are used.
• The system is tolerant of inadequacies in duct design and installation because it is not dependent on accurate control of duct pressure.

Disadvantages

• A VAV Box can only control zones, hence individual room comfort is compromised.
• Excessive fan power is required to maintain the high pressure system.
• Associated with excessive fan power is a higher noise level which often needs attenuation and therefore added cost.
• High risk of cold air dumping and hot air stratification.
• Inflexible as room layout is not easily changed.
• Limited controllability.

Pressure Dependent System – VGVAV Diffuser design

Rather than controlling a particular zone in a building, electronic VGVAV diffusers are networked together and can manage thermal comfort down to the individual room level. These diffusers are perfectly suited to low pressure systems and hence bring key advantages to the HVAC market.

Advantages

• Lower fan power is required, significantly reducing energy consumption.
• Commonly used in Green Building and LEED - GOLD rated buildings.
• Lower fan noise levels, less attenuation and lower initial capital cost.
• Flexibility, each outlet can be individually controlled so office layout changes are easy.
• Because the duct static pressure is controlled and kept constant, constant volume and variable volume diffusers can be operated on the same duct system.
• No risk of cold air dumping or hot air stratification, because of the variable geometry nature of the outlet.
• More cost effective where each outlet can be individually controlled. Constant grilles can also be used in open plan areas.
• Every space in a building can have its room temperature, supply air temperature, mode of operation, occupancy, pressure and air flow monitored and managed from the Building Management System. The system is typically managed from a remote central computer, making building tuning convenient and easy.

Disadvantages

• Duct size needs to be larger to compensate for air velocities, although the system can manage branch pressure drops up to 70Pa.
• For optimum performance, it is important a VGVAV system is considered early in the building design stage.

The next issue of Tech Talk will take a step further and talk about good duct design and commonly used duct designs of equal friction and static regain with VGVAV diffuser

For further information on VGVAV diffusers please refer to the Fantech website www.fantech.com.au under Diffusion.