ASHRAE Journal - June 2011 - 87

contained in the body’s thermal convective boundary layer (if it is not disturbed). This means that even though the thermal stratification zone is lower than the occupant height, the inhalation air quality may also be better than in mixing ventilation.1 The upper mixing zone is warmer than the lower zone. This suggests that a higher exhaust air temperature is possible than that in mixing ventilation. A higher supply air temperature than in mixing ventilation is also used. The clean thermal stratification zone also suggests the possibility of using less ventilation air for acceptable air quality in this zone. Therefore, displacement ventilation can be a more energy-efficient ventilation solution. The only way for air and pollutants to transport from the lower to upper zone is via the plumes that penetrate the interface (barrier). Displacement ventilation creates a flow pattern that makes it difficult for a pollutant released in the upper zone to spread to the lower one. But it is also Figure 3: Displacement ventilation performs best when the exhalation is updifficult for a pollutant released in the lower zone ward (top photo) and worst when the exhalation is sideways (bottom photo).7 to move to the upper one and then be extracted from the room. This is because only one “road” is available thermal breathing manikins in their study was 10 min–1. Care (the plumes). was also taken to “ensure a realistic momentum and direction The stable thermal stratification zone plays a dual role for of the flow exhaled from the nose.” displacement ventilation. The lower zone is well ventilated with Qian, et al.,7 studied the exhaled flows of two manikins in the direct supply air. It is difficult for air pollutants to escape if a full-scale experimental hospital ward with three ventilation they are not caught by the thermal plume penetrating into the systems. For displacement ventilation, the exhaled jet was upper zone, so the residence time of the pollutants is high in found to penetrate a long distance (Figure 3). The exposure of this zone. If heavier particles enter this zone, more deposition the receiving manikin was high when the receiving and source occurs. Displacement ventilation creates what might be referred manikins were placed sideways facing each other. Their data to as inversion “clouds” in rooms (Figure 1). However, this po- showed that in some conditions displacement ventilation with tential weakness of displacement ventilation has not always been 6 ach can only achieve a comparable personal exposure to that fully recognized in the field, including our own early studies.2 of mixing ventilation with 4 ach or less. In other favorable conditions, displacement ventilation can achieve equivalent studies on Displacement Ventilation in Hospitals performance to that of mixing ventilation with a much lower air Displacement ventilation has been rarely applied in hospitals. change rate. Mundt8 also found that “the particle concentration Friberg, et al.,4 may be the first to caution against its use in operat- in the convection flows was much higher with displacement ing rooms. They found two to three times higher air and surface than with mixing ventilation indicating a higher exposure risk,” bacterial counts in the areas important for surgical asepsis in an though the decay rate after a disturbance is much faster with operating room with displacement ventilation at 17 ach. displacement ventilation. The authors wrote: “The possible explanation of the inferiorThe following three unfavorable conditions are the most ity of the upward displacement system in eliminating bacteria- problematic in hospitals. The first is when the pollutants are carrying particles is that the airflow in the thermal ventilation denser than the room air. For example, large droplets are relawas insufficient to transport larger particles up to the ceiling for tively heavy. evacuation.” Lai and Cheng5 found that droplets of 10 microns The second is when the pollutants are injected into the in diameter begin to settle in the lower region of the room under thermally stratified zone without any positive buoyancy force. displacement ventilation. An exhalation puff continuously entrains the surrounding air, Stymne, et al.,1 might be the first in finding the lock-up though the entrainment is limited due to thermal stratification phenomenon in displacement ventilation. Brohus and Nielsen3 (note: some turbulence models do not consider this effect). The and Bjørn and Nielsen6 found that air exhaled through the puff travels further than it does in mixing air. The entrainment mouth could be locked-up in a thermally stratified layer, if the also cools the exhaled air puff along its way. When the puff loses vertical temperature gradient in the breathing zone height is its momentum, the puff with the infectious droplets may float sufficiently large (Figure 2). The respiration frequency of the in the thermal stratification zone for a long time (Figure 4). In
June 2011 ASHRAE Journal 87

ASHRAE Journal - June 2011

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