HAVAC, the indoor dispersion modelling tool

Monitoring and regulating the air quality in indoor areas such as cruise ships and critical infrastructure is important. The outdoor air can be contaminated with agents such as diesel and car exhaust or Toxic Industrial Chemicals (TICs). These airborne agents can penetrate the building via windows and doors or via the ventilation system.

The main purposes of a Heating, Ventilation, and Air Conditioning (HVAC) system is to help maintain good indoor air quality in a building through adequate ventilation and also to provide thermal comfort. However, this also makes buildings vulnerable to external toxicological threats. Once toxic agents reach the air intakes of the HVAC or once these toxic agents are released somewhere inside the building (either accidentally or even intentionally), the ventilation system can transport them rapidly to various parts of the building. Within each ventilation zone, the agents can be drawn through return ducts and distributed through supply ducts at high flow rates. The indoor dispersion can also be driven by draft through the building as a result of external wind or even by the chimney effect via stairwells or elevator shafts, particularly in winter when temperature differences between inside and outside are large.

The HAVAC model

HAVAC is a computer model, describing the air flow in the building can give insights into the transportation of these agents through the building. Furthermore, once combined with a detection system, the HAVAC model can use the output of the detection system to fine-tune the prediction of how the agent will disperse into the building. That way HAVAC can indicate which rooms in the building are “hotzones” of agent and which rooms are safe. Possible further actions, such as evacuation of people or even decontamination of the specific rooms can be taken, based on the model results.

In the HAVAC model every room in a building as well as every duct in the HVAC system is defined as a zone. The outdoor environment is also defined as a zone. A zone can exchange air with a neighboring zone via flow paths, which represents doors, windows and other leakages. The air flow through these flow paths can be present due to natural causes, such as outdoor wind pressure or as a result of ventilators, which are located in these flow paths (for instance in the HVAC system). As a result, air will flow through the rooms and the ducts (the zones). As a result it is possible to calculate the contaminant concentration in every room.

Capabilities

The most important capabilities of the HAVAC indoor dispersion model are:

• The model can describe the concentration of agent in every room in the building as a function of time. That way it can identify hotspots of agent.
• The model can even work with airborne Toxic Industrial Chemicals (TICs) and Chemical or Biological warfare agents.
• The building structure (floorplan) can easily be implemented inside the model, including rooms, corridors, doors and windows and various floor levels.
• An HVAC system can also be integrated into this digital building description, with various ventilation rates per room.
• The outdoor air concentration of agent around the building can be used as input for the model, including the incorporation of the outdoor wind direction and wind speed (important for windows etc.). For ships this capability is especially important while sailing.
• Agent detector output can be integrated with the model results to generate even better dispersion predictions.
• The model can be used to indicate which locations (rooms or ventilation ducts) are best to deploy detectors.