Variable Geometry Variable Air Volume (VGVAV) diffusers are an effective method of controlling the thermal comfort of a room whilst reducing energy consumption.

VGVAV diffusers are pressure dependent devices and to operate at optimal performance a low pressure duct design is needed. The pressure in a branch with VGVAV diffusers is normally set between 20-60Pa depending on the room’s air volume requirements. This pressure is typically monitored with a pressure sensor that is placed two-thirds of the way down the duct work and maintained via a variable speed fan, pressure control damper or bypass damper.

In this article we will look at good duct design principles and the use of equal friction and static regain duct design with VGVAV diffusers.

Good duct design considerations

Good duct design is important for all HVAC systems as the design influences the amount of pressure loss across a system. Poor duct design requires the fan to work harder and compensate for the loss of pressure, resulting in higher energy usage and running costs.

When designing a duct system, consider how the duct will change direction, the length of flexible duct used, and how this affects the system pressure.

Flexible duct length

The most common problem with duct design is the use of long flexible duct lengths with diffusers. A big difference in pressure can be found between minimum and maximum air flow requirements, causing noise and energy losses. It is important to keep the length of flexible duct taut and short, with a maximum of 1.5m recommended.

Flexible duct bends

It is also important that flexible duct bends have an even curvature as a smoother radius improves air flow and helps to minimise load on ducted heating and cooling units.

T-Junction

The right angle of a T-junction causes air resistance and pressure loss. Pressure losses between high and low air flow branches can be minimised by having a curved entry point. This has the additional benefit of reducing noise.

Equal friction duct design

Most engineers use the equal friction method for duct design where ducts are sized for a constant static pressure loss due to friction per unit length.

This method works satisfactory for low pressure systems with VGVAV diffusers, but the following must be considered, especially if retro-fitting VGVAV diffusers to existing duct work:

• The branch velocities should be between 4 to 6 m/s
• The duct run should be short enough so that the pressure drop from the beginning of the run to the end falls within 10% to 20% of the range that the VAV diffusers can accommodate
• Zones need to be downstream of the Pressure Control Damper (PCD) to ensure proper temperature control
• Good duct design principles should be used

By using the equal friction duct design, energy efficiency and thermal comfort can be improved

Static regain duct design

Static regain duct designs use the reduction in air velocity and therefore the reduction in air pressure after each duct section to regain the static pressure loss due to friction in the preceding duct length. The objective is to maintain the same static pressure throughout the duct run.

The more energy efficient static regain duct method is suitable for longer and larger duct systems, whereas the equal friction method is only suitable for short duct runs. On high rise buildings the static regain “ring-duct design” is often used

VGVAV diffusers are designed to work at their best in a low pressure system using static regain duct design. They bring key advantages to a HVAC system including:

• Excellent energy efficiency as they operate from low pressure duct work where smaller variable speed Fan Coil Units and Air Handling Units can be installed.
• Individual thermal comfort control as the VGVAV diffuser can manage individual spaces within a building.

Other benefits of electronic VGVAV diffusers

• Ease of Installation. All VGVAV diffusers are installed in groups of up to 15 and then repeated throughout the building
• Ease of commissioning. Using a Rickard air flow sensor allows electronic air flow control, therefore no air flow hood measurements are required.
• Flexible control. Remote monitoring enables easy tenant layout changes and building fine tuning.
• Built-in intelligence. For room comfort control operation
• Occupancy sensors. For automatic back-off and light control
• Range. Models are available for ceiling (standard and swirl), linear and bulkhead installations

With modern electronic VGVAV diffusers matched with low pressure duct designs, buildings can lower their total system pressure and hence yield higher system efficiency. When the holistic HVAC system is considered and the benefits that low pressure diffusion designs can yield over the life of the building, it makes sense why these modern trends are so attractive. For these reasons these systems are being widely used around the world.