Air change effectiveness contributes to Green Star

NDY sustainability consultant Kemal Gungor* (PhD) examines the factors affecting air change effectiveness and outlines solutions to assist in achieving compliance with Green Star requirements.

Green Star rated buildings are increasingly considered and demanded by both building developers and tenants. One of the Green Star rating categories is IEQ-2: Air Change Effectiveness. In simple terms, air change effectiveness (ACE) describes an air distribution system’s ability to deliver ventilation air to a building, zone or space.

Two Green Star points are awarded if ventilation systems are designed in accordance with ASHRAE 129-1997. During the design stage, this is demonstrated through computational fluid dynamics (CFD) modelling of the air-conditioning system.

In order to achieve improved ACE, and comply with GBCA requirements, air distribution systems are required to deliver the supply air to the breathing zone. Air movement within this space directly affects occupancy comfort, indoor air quality and ACE. Displacement flow and entrainment flow are the two distinct flow patterns commonly used to characterise air movements in buildings.

Displacement flow is characterised with the movement of air within a space like a piston motion in Figure 1. In an ideal displacement flow, the room air does not mix.

A typical entrainment flow is shown in Figure 2. Ceiling-based air supply and return air grilles generally exhibit an entrainment flow. Displacement flow has a higher potential to achieve a better ACE value than the entrainment flow. Poorly designed, installed or operated systems can exhibit short-circuiting, especially ceiling-based systems in heating mode.

To achieve maximum energy efficiency, high thermal comfort and optimum indoor air quality and ACE, the details of the flow pattern must be established during the design phase of a building.

Variables affecting flow patterns

The basis for air movement in buildings, both internally and externally, is temperature and pressure differences. ACE is dependent on the airflow pattern, and in a building environment, flow pattern and air change effectiveness are dependent on a number of variables.

Even small changes in these parameters can have a pronounced effect on the assessment outcome.

Anticipating the impact of any change is not always possible. Field measurement is the only objective method to determine the effect of change on the flow pattern and ACE in built environments. During the design stage, however, CFD simulation is successfully used to identify flow patterns for the building.

Selection of the HVAC system can have a profound effect on ACE results. Following is an overview of the ACE performance potential of the most common ventilation systems.

CAV+ chilled beam

Chilled beam systems (active and passive) offer energy-saving advantages and are available in a variety of configurations, from rectilinear slots to 600 mm squares and rectangles. There are also varieties of both passive and active chilled beams that incorporate other elements - such as lights, sprinklers, speakers, space occupancy sensors and smoke detectors - in a multiservice beam configuration. All chilled beam arrangements have a common feature: they create additional vertical air movements.

This additional vertical air movement provides improved air mixture within the space compared to a VAV system. CAV+ chilled beam systems usually achieve compliance with the GBCA air change effectiveness requirements.

Displacement ventilation (UFAD)

Displacement ventilation is based on the principle that cooled air is supplied with low momentum in the lower part of the room. The cold air displaces the contaminated air from the occupied zone upwards in the room. Buoyancy forces (temperature differences) control the air movement in the room as the free convection around heat sources - including occupants, equipment and lightning - creates vertical air movements in the room. In the same way, a cold window or a cold wall will result in a downward convective flow.

An integral characteristic of displacement ventilation is the formation of stratified layers of air. This characteristic provides a significant advantage in order to achieve GBCA air change effectiveness compliance. It is expected that many of the underfloor distribution systems would achieve compliance with GBCA air change effectiveness requirements.

Variable air volume systems

In variable air volume (VAV) systems, changes in the room heat load are met by controlling the volume of air supply to the room without changing the supply temperature until the minimum permissible air supply is reached. Seasonal control of the supply air temperature takes place as a function of the outdoor temperature. A VAV system can operate over a range of airflow rates which will have subsequent effects on the flow pattern within the room. GBCA air change effectiveness assessment requires that all the simulation would be performed at the lowest turndown ratios.

Figure 1: Displacement flow within a space. Figure 2: Entrainment flow within a room (ASHRAE).

Finding the correct solution

Each building has its own unique characteristics which will influence ACE outcomes. The following general design considerations provide a broad framework for evaluating ACE outcomes.

• Increasing the supply of fresh air may not necessarily improve the ACE.
• Ventilation systems that can preferentially deliver the air to the breathing zone would achieve better ACE. Personal ventilation systems, task air ventilation, displacement ventilation and underfloor ventilation will generally provide best ACE results.
• Chilled beam systems create additional vertical air circulation which improves ACE and, in most cases, complies with GBCA ACE requirements.
• The use of return air light slots significantly impairs the achievement of a good air change effectiveness outcome when the supply air terminals are also located at ceiling level.
• The use of discrete return air grilles provides a better solution compared to perforated ceiling/air handling luminaires, and this can be further improved by regulating the return airflow through each with a balancing damper.
• Location of return air grilles have greater impact on ACE than the number of return air grilles.

*Kemal Gungor (PhD), M.AIRAH, is a sustainability consultant at Norman Disney & Young’s Melbourne office. His expertise in heat transfer, thermodynamic analysis, design and computer modelling of thermal systems (energy modelling, facade analysis) is in high demand. He is also a specialist in computational fluid dynamics analysis. His skills are being utilised by NDY to develop and implement ecologically sustainable design solutions throughout the building industry.

This article first appeared in the NDY Lifecycle magazine.
For a copy of the comprehensive study analysis by Kemal, visit tinyurl.com/G2ACE.