Interzonal airflow and mixing processes

Contaminants spread through the rooms of a building by advective transport and interzonal airflow. The transport of contaminants within the building affects the spatial distribution of concentrations that may be observed in the event of a contaminant release and this has consequences for occupant exposure. Compared with the ventilation literature, there are fewer publications that address interzonal airflow processes for both commercial and residential buildings. However, exposure considerations have motivated indoor air research that seeks to understand the complicated relationships between interzonal airflow and contaminant transport (Miller and Nazaroff, 2001; Ott et al., 2003). Recent studies also consider the influence of occupant behavior (Klepeis and Nazaroff, 2006a; Klepeis and Nazaroff, 2006b).

Separate interior sections of the building are often referred to as zones. Zones may include rooms or areas that have some physical barrier to distinguish them from adjacent sections of the building. Supply ducts, stairwells, or a ceiling plenum are other examples. Airflow between rooms, vertical airflow through a stairwell, and supply airflow from a duct are relevant and important interzonal airflow processes.

Thermal differences, wind, and mechanical systems drive air exchange within the building. These are the same processes that drive ventilation airflow. Thermal differences between rooms stimulate air exchange because of the buoyancy differences between the adjacent columns of air. In many residential buildings, natural convection drives interzonal airflow due to the absence of a mechanical system. Although their work did not quantify interzonal airflow, Wilson and Kiel showed that small temperature differences (1 °C) can generate significant buoyancy-driven airflow.

Mechanical system or air handling unit (AHU) operation often determines the airflow patterns in commercial buildings. In a typical commercial building design, air is circulated by the AHU through a system of overhead ducts. Overhead systems generally meet thermal and ventilation requirements simultaneously. The airflow from the ducted system to the rooms and from the rooms to the return space (which is often the plenum space above a drop ceiling) result in significant interzonal airflows. Mechanical systems alter internal pressure distributions that can also induce airflow between rooms.

The rates at which air is exchanged among zones determines the rate at which contaminants are transported through the building by advection. Consider a scenario in which a contaminant is released as a pulse in a room. For this scenario, the interzonal transport and mixing time scale is the time required for the contaminant to be transported out of the release room and to the remainder of the building, or to the outside. The interzonal transport time scale is inversely proportional to interzonal airflow rates. Uniform concentrations may be reached among a group of zones that share airflow communication paths within the interzonal transport time, depending on the relative rates of air exchange and ventilation rates.

Systems with higher interzonal airflows may exhibit more rapid whole-building mixing. Rapid interzonal transport tends to equalize contaminant exposures. Slower interzonal transport provides more opportunity to minimize occupant exposure to an unexpected contaminant release, in the event that a greater number of occupants are not in close proximity or in a room in which a release has occurred. Klepeis and Nazaroff (2006a) found that residential exposure to secondhand smoke could be reduced by isolating the smoker in a closed room with an open window.

Mechanical air distribution systems, especially overhead systems, are effective at inducing mixing on the whole-building scale. Consider that a mechanical system recirculates a portion of the air. In the event that a contaminant is released somewhere in the building, the transport of the contaminant to other locations in the building is accelerated. Mechanical systems generally increase the risk that an occupant anywhere in the building will be exposed to some level of contaminant more quickly, irrespective of their location in the building. The opposite effect occurs if a mechanical unit delivers 100% outdoor air. Instead, the contaminant air may be quickly expelled from the release room to the outdoors, preventing contamination of other zones.

Commercial buildings may alternatively be designed with radiant thermal systems and underfloor systems. In radiant systems, heated or cooled water may be supplied to a system of tubes embedded in a concrete slab or through ceiling panels. A small overhead mechanical unit may be installed for meeting ventilation requirements. Underfloor systems are similar to a traditional overhead systems except air is supplied to rooms by pressurizing the plenum space below a raised floor. Ideally, underfloor systems condition the occupied space instead of the whole room.

Buildings with radiant systems may exhibit different interzonal airflow processes than buildings using overhead systems. In radiant system, natural convection airflow within a room and across rooms may be enhanced by large heated or cooled surfaces. An overhead mechanical unit, if included in the design, may provide minimal air to meet ventilation requirements and is unlikely to recirculate air in the building.

Interzonal airflow can be determined experimentally using two methods. Fan pressurization techniques can be applied to determine airflow between specific rooms and

across individual components. In brief, a fan is inserted in an opening and the airflow is adjusted to reach a target pressure differential. Adjacent zones are pressurized (using additional fans) to suppress any leakage into the zone of interest, thus isolating it.

The second method uses tracer gas experiments to quantify interzonal airflow. Tracer gas experimental procedures are well documented and provide a non-intrusive method of studying airflow under typical operating conditions (McWilliams, 2002). A known amount of tracer is released and the concentration in different rooms is measured. Typically, contaminant mass balance equations are derived with the unknown quantities being the interzonal airflow rates. Usually, the concentrations are assumed to be uniform throughout a room. Inaccuracies can arise from the well-mixed assumption and from measurement error as is documented by Miller et al. (1997).

There are some published data on interzonal airflows. Miller et al. (1997) used tracer gas decay methods to estimate the interzonal airflows in a two-zone residential building. They measured interzonal airflow rates of 60 ± 2 m3/h and 154 ± 17 m3/h for two sets of experiments. The first was conducted with no active ventilation and the second with 20 m3_{/h fresh air. Ott et al. (2003) also used tracer gas techniques to} estimate airflow between two zones in a residence. They measured interzonal airflow rates of 102 and 120 m3/h, and air change rates of 4 and 4.6 ACH.