Optimising Indoor Air Quality
via
Thermal Displacement
Air-Conditioning
and
Computer Modelling (CFD)

CONVENTIONAL AIR-CONDITIONING

In conventional air-conditioning, supply and return air grilles are usually located at the ceiling. The supply air is blown into the space with sufficient velocity to cause churning or mixing of the air throughout the whole space. The result is that all air pollutants are mixed and distributed to all areas. Each occupant shares pollutants with all others in the space. The supply air temperature is typically 11-13ºC and occupants in close proximity to the supply air may experience drafts. To minimize these the air is directed across the ceiling rather than straight down. Thus some supply air "short-circuits" to the exhausts creating sizeable energy and efficiency losses. Ventilation Efficiency (Ve) with respect to removing pollutants can be defined as:

In a perfectly mixed room the concentrations would be equal and Ve = 1.However, any short-circuiting dilutes the pollutants at the exhaust and the efficiency is compromised. Even well designed systems have a Ventilation Efficiency as low as 0.7.

THERMAL DISPLACEMENT AIR-CONDITIONING

With displacement air-conditioning, cool air is supplied at a low level and at a low velocity into the space. Gravity holds the cooler air down and it spreads across the floor in a "pool". At heat sources (people, computers, equipment, etc) the heat warms the surrounding air which rises in a plume and "draws" the cool air at floor level to the heat source. Because the heat sources in a space are usually the contaminant sources, the vertical air flow conveys those contaminants above the sources directly to the exhausts. The objective is to separate the room into a stratified micro-environment, with a warm polluted region above the occupants and a cooler, cleaner region around the occupants. Pictorially, the difference between mixing and thermal displacement ventilation is shown here:

HBI'S STRATEGY

To analyse and model HVAC system operations and propose innovative design solutions.

PROCEDURES

Computational Fluid Dynamic (CFD) Modelling: the ability to graphically show (in 3-D) air and particle movement, velocity vectors, and temperature gradients within a space.

Thermal Displacement Air-conditioning: an innovative air-conditioning system design offering improved ventilation efficiency, enhanced comfort and superior air quality.

BENEFITS

• Improved indoor air quality (IAQ)
• Reduced employee absenteeism
• Enhanced manager/tenant relations
• Lower operating costs
• Increased productivity and comfort
• Better overall building operations

METHODOLOGY

Healthy Buildings International Pty Ltd has formed a partnership with Stevenson & Associates, a consulting engineering firm, to provide the following services:

Data Gathering: Input data for the model: space configuration, people and equipment layout and thermal loads, air-conditioning operating characteristics.

CFD Model: Model design and execution yielding multiple graphic images identifying air-conditioning system performance. 3-D animation of preferred designs as a communications tool.

Presentation of Results: Analysis of modelling and evaluation of options towards optimisation of final design.

Report: Preparation of written report including full colour graphs summarising results with recommendations.

Design Contract: Design of selected air-conditioning system; preparation of contract documents for bidding and award to air-conditioning Contractor.

Construction Administration: Technical assistance throughout construction to assure contractor performance.

Commissioning and Testing: Air-conditioning system start-up, operational intent and training of maintenance personnel. Comprehensive IAQ testing to confirm design goals are attained.

COMPUTATIONAL FLUID DYNAMICS

This refers to the mathematical modelling and analysis of the movements of fluids (liquids or gases) under specified conditions to predict behaviour. Because the calculations are so extensive and repetitive, CFD analysis requires the use of a powerful computer. Graphical output shows the results of the analysis.

CFD analysis applied to air-conditioning systems has been commonplace in Europe for some time, and many of the earlier commercial software systems were developed there. Colourful graphical images which show the operation of the air-conditioning system within the space can be examined, varied and optimised. Additional refinements include three-dimensional animation allowing visualisation of the spaces under typical working conditions.

In the design of air-conditioning systems, CFD modelling is a very useful tool. In the case of thermal displacement designs, CFD is strongly recommended and enables us to:

• See temperature stratifications
• Confirm heat loads are offset
• Optimise diffuser locations
• Prove contaminant removal
• Ensure absence of drafts
• Present a visual picture of the air-conditioning to the client.

The modelling of the air-conditioning system, prior to committing to building the space, is an invaluable tool in the overall design and construction process.

CFD EXAMPLE - CLASSROOM

3-D Animation:
Video clips allow visualisation of airflows, pollutant concentrations or temperature gradients.

THERMAL DISPLACEMENT (ENERGY SAVINGS)

When we consider switching from conventional mixing air-conditioning to thermal displacement air-conditioning we must accept a whole new thought process. In a mixing system all heat loads within the occupied area have to be dealt with in that space. Heat loads from lights, equipment and people are all additive and of equal importance.

In the case of thermal displacement air-conditioning, heats loads are not all equal. The loads must be divided into source and location within the space. With some exceptions, any load above 2 metres can be discarded. For example, this eliminates most of the lighting load. Heat loads below 2 m must be considered but they may be discounted because the actual load varies with the height of that load.

For example, a typical busy casino could have heat loads of 140-200 watts/m2. However, the actual loads to be handled with thermal displacement would be much lower, say in the range of 60-100 watts/m2. These loads can be offset in a casino environment with high ceilings without exceeding guidelines for temperature gradients in the occupied zone.

In the mixing air-conditioning design, to offset the above loads, air supply volumes of 15-20 L/sec.m2 would be required. The displacement air-conditioning design achieves the same goal with supply volumes of only 10-15 L/sec.m2. This fact manifests itself in the typical energy savings of 25-30 percent from displacement ventilation versus mixing ventilation designs.

DISPLACEMENT AIR-CONDITIONING APPLICATIONS

Commercial offices, healthcare facilities, clean rooms, casinos, restaurants, conference centres, theatres, school classrooms and diverse industrial applications.

This USA patent, filed in October 1995, titled Demand Ventilation System is owned by HBI Inc. It describes a thermal displacement ventilation system using variable air volumes controlled by air quality sensors. The specific claims of the patent include:

• Low temperature and low velocity air, supplied at floor level with exhausts located high in the space;
• Varying the supply air flow rate using air quality sensors;
• Use of temperature sensors to control supply air temperatures below room temperature to aid thermal displacement.

Back to Top of Page