Personal exposure between people in a room ventilated by textile terminals--with and without personalized ventilation.
This paper describes an investigation made in a room ventilated by an air distribution system based on a textile terminal. The air distribution in the room is mainly controlled by buoyancy forces from the heat sources, although the flow from the textile terminal can be characterized as a displacement flow with a downward direction in areas of the room where no thermal load is present. The system was extended by a personalized ventilation system to study the improved protection of people in a room. The investigation involved full-scale experiments with two breathing thermal manikins. One manikin is the source and the other the target. In general it was found that when the air is supplied from the textile terminal alone, the flow in the room is fully mixed with limited protection of the occupants. Selected locations of supply, return, and heat sources can produce a displacement flow in the room with increased protection of the occupants. It is shown that personalized ventilation improves the protection of occupants by increasing the personal exposure index.
More and more people are spending a considerable amount of time in an indoor environment. It is important to minimize the amount of pollutants that people are exposed to indoors, to give an experience of good air quality, and to minimize the danger of, e.g., passive smoking and cross-infection. The latter problem was clearly demonstrated in the worldwide SARS outbreak in 2003 (Li et al. 2004a, 2004b).
Different air distribution systems, such as mixing ventilation and displacement ventilation, offer different possibilities for the protection of people against pollutants. The pollutants are almost fully mixed in the occupied zone in a room ventilated by mixing ventilation, and they are removed by a diluting process (Jensen et al. 2001). If the pollutant source is also a heat source, displacement ventilation offers possibilities to work with two zones, a low zone with clean air and an upper zone with pollutants. It is possible to design a system with low exposure of people under certain conditions (Brohus and Nielsen 1996; Skistad et al. 2002), but high exposure can also exist in rooms with displacement flow in certain situations, as shown by Bjoern and Nielsen (2002) and Qian et al. (2006).
An investigation was made in a room ventilated by an air distribution system based on a textile terminal. The air distribution in the room is mainly controlled by buoyancy forces from the heat sources, although the flow from the textile terminal can be characterized as a displacement flow with a downward direction in areas of a room where no thermal load is present. The displacement flow, which exists in different areas of the room, may indicate the possibility of obtaining improved protection in those areas. The air distribution system is evaluated and compared with other systems by Nielsen et al. (2005). To study the possibility of improved protection, the textile terminal is extended by a personalized ventilation system in some of the experiments. Personalized ventilation systems are intensively studied by Melikov (2004) and Melikov et al. (2002).
This paper addresses the air quality index in the occupied zone and the personal exposure index for the different systems and the different layouts in the room.
TEST ROOM AND RESEARCH METHOD
Figure 1a shows the full-scale room and location of the textile terminal symmetrically at the ceiling in the first part of the experiments. The dimensions of the room are in accordance with the requirements of the International Energy Agency Annex 20, with length, width, and height equal to 4.2, 3.6, and 2.5 m. The return openings are located in the low part of the end wall to create a vertical piston flow that will develop when the room has a small load. Figure 1b shows the layout with the textile terminal located 600 mm from the side wall in the second part of the experiments. The return openings in this case are located on the back wall, close to the ceiling, to support the displacement flow, which exists in this situation at full load.
[FIGURE 1 OMITTED]
Figure 2a shows the textile terminal, which is designed as half a cylinder (d = 315 mm) located close to the ceiling. Figure 2b shows the furnishings and the heat load of the room. The heat load consists of two PCs, two desk lamps, and two manikins producing a total heat load of 480 W in all the experiments that are related to an office.
[FIGURE 2 OMITTED]
The source manikin is located in position A in some of the experiments and standing close to position B in others. The target manikin is located in position B, which is close to the end wall with the return openings. This manikin inhales through the mouth and exhales through the nose. [N.sup.2]O is used as a tracer gas in the experiments. The concentration of the tracer gas is measured by a calibrated Bruel & Kjaer multigas monitor and a Bruel & Kjaer multipoint sampler and doser. The concentration of [N.sup.2]O is at the level of 30 ppm in the experiments, and the background concentration of [N.sup.2]O is about 0.4 ppm.
The concentration is measured at six points. One measuring point is located in the return opening and one in the occupied zone (1.7 m from the end wall with the return openings, 1.3 m from the side wall in front of the manikins, and 1.25 m above the floor). One measuring point is the inhalation of the target manikin, and three points are located in a virtual sphere around the head of the manikin. The sphere has a radius of 30 cm.
The measurements are compared with results obtained by Qian et al. (2005) for the same room. The same two layouts of the air distribution system are used (see Figure 3), but the furnishings correspond to a hospital ward with two lying manikins. The heat load is 418 W (1427 Btu/h) in the Figure 3a case and 168 W (573 Btu/h) in the Figure 3b case. Measurements are also compared with results obtained by Buus et al. (2006) in the Figure 3b case. In this case, the heat load is equal to 362 W (1235 Btu/h).
[FIGURE 3 OMITTED]
Personalized ventilation is used in some of the experiments. The personalized ventilation is of the movable panel (MP) type developed by the International Centre for Indoor Environment and Energy and described in detail by Bolashikov et al. (2003). The opening of the device is located 35 cm from the mouth of the target manikin, supplying air in a 45[degree] downward direction, as identified to be used by people in Kaczmarczyk et al. (2002); see Figure 4.
[FIGURE 4 OMITTED]
AIR DISTRIBUTION THE ROOM
The air distribution in the room is mainly controlled by buoyancy forces from the heat sources, with an upward flow above the computer, the lamps, and the manikins and a downward flow between the plumes from the textile terminal.
The location of the textile terminal directly above the two workplaces creates a large mixture in the two opposing plumes, as indicated in Figure 5a. The layout of the air distribution system with the textile terminal located close to a side wall (Figure 5b) generates a room air movement with displacement flow in large areas. The return openings are located at floor level in the layout with the textile terminal in the center plane and located close to the ceiling in the layout, with the textile terminal close to the side wall to support the displacement effect in the latter situation. The computational fluid dynamics (CFD) predictions in the figures are made in a three-dimensional rectangular grid with 295,630 cells. The predictions are also made with 57,960, 102,480, and 181,350 cells. Variables at monitoring points are in practice grid-independent for grids with more than 150,000 cells. A hybrid scheme with a k-e turbulence model is solved by the numerical method.
[FIGURE 5 OMITTED]
The CFD simulations in Figure 5 are confirmed by smoke experiments. Smoke is supplied to the supply flow, and mixing in the plumes is observed when the textile terminal is located in the center plane. On the other hand, smoke experiments with the textile terminal close to the side wall show large areas with displacement flow.
VENTILATION EFFECTIVENESS AND EXPOSURE
The ability of the air distribution system to remove airborne pollutants is expressed by the air quality index of the occupied zone:
[[epsilon].sub.oc] = [C.sub.R]/[C.sub.oc] (1)
where [C.sub.R] and [C.sub.oc] are the steady-state concentrations in the return opening and in the occupied zone, respectively (Mundt et al. 2004).
The level of protection of the target manikin is expressed by the personal exposure index:
[[epsilon].sub.exp] = [C.sub.R]/[C.sub.exp] (2)
where [C.sub.exp] is the mean concentration in the inhalation of the target manikin (Brohus and Nielsen 1996; Mundt et al. 2004). The personal exposure index is also called the air quality index in the inhaled air.
Figure 6 shows that the air quality index for the occupied zone is close to 0.8 when the center plane layout of the textile terminal is used. The index has a decreasing value for an increasing air change rate. A level of 0.8 is typical of a flow with some mixing effect.
[FIGURE 6 OMITTED]
The personal exposure index for the manikin in position B is about 1.0, which is also the value that can be obtained with mixing ventilation. The source manikin is located in position A.
A number of experiments with the source manikin located close to the table with the target manikin (position B) show also results close to 1.0 indicating a large mixing effect.
The air quality index of the occupied zone is improved when the textile terminal is located close to the side wall. An increase from 0.8 to 1.4 indicates displacement flow in the room, which also means that the pollutants are removed from the source in an efficient way. This is also reflected by the increased level of the personal exposure index, which is raised from 1.0 to 1.23. This experiment is only made for an air change rate of 5.5 [h.sup.[-1]].
Experiments with cross-infection between two persons in beds have been carried out in the same room with the same two layouts of the air distribution system (see Figures 3a and 3b). Qian et al. (2005) have shown that similar results are obtained, although the details around the manikins and the position of the manikins are different. The air distribution system with the terminal in the center plane generates a personal exposure index for the target manikin around 1.0. When the terminal is located close to the side wall, the personal exposure index is 0.94 when both manikins are lying face to face and 2.19 when the manikins are facing up. The air change rate is in both cases is equal to 4 [h.sup.[-1]].
Additional experiments with the hospital ward with the ceiling-mounted textile terminal located close to the side wall have been made by Buus et al. (2006). If it is assumed that the flow in the room is a fully developed turbulent flow, it is possible to express the personal exposure index as a single-value function of the Archimedes number Ar without considering the Reynolds numbers involved.
Ar = [[beta]gH[DELTA][T.sub.0]/[u.sub.0.sup.2] (3)
where [beta],g, and H are thermal expansion coefficient, gravity, and room height, respectively. difference between return and supply, and uo is the inlet velocity, defined as the flow rate divided by supply area of the textile diffuser. The room height H is selected as a characteristic length because the room dimensions are important in connection with the change in the flow pattern that develops at different Archimedes numbers.
Figure 7 shows the results for both manikins lying face to face. The layout of the room used by Buus et al. (2006) is slightly different from the layout used by Qian et al. (2005), and an extra standing manikin located under the terminal (health-care worker) is also applied by Buus et al. (2006).
[FIGURE 7 OMITTED]
The figure indicates a minimum value for the personal exposure index eexp for Archimedes numbers between 500 and 1000. A low Archimedes number, corresponding to a high flow rate, gives a high eexp, and a high Archimedes number, corresponding to low flow rate and large thermal forces, also gives a high eexp. In practice, it is necessary to use a high flow rate to obtain a low level of the concentration in the room, and this situation corresponds to the low Archimedes number in the graph. Table 1 shows the air change rate n and the temperature difference [DELTA][T.sub.o] connected to the Archimedes numbers given in Figure 7.
Table 1. Archimedes Number, Air Change Rate, and Corresponding Temperature Difference Ar 71 98 217 404 675 1024 2428 n, [h.sup.[-1]] 10.0 9.0 7.5 6.0 5.0 4.0 3.0 [DELTA][T.sub.0], K 1.4 1.6 2.5 3.0 3.5 3.4 4.5
Figure 6 shows that the personal exposure index in the office situation is equal to 1.22 for an air change rate of 5.5 [h.sup.[-1]] when the air terminal is located close to the side wall. This flow rate and load correspond to an Archimedes number of 120. This result is also in good agreement with the other measurements in Figure 7. The position of the manikin is different, but the exhalation is vertical in the office situation, as it is in the hospital ward situation in Figure 7 where the manikins are lying face to face.
Personalized ventilation has been tested in the office layout (see Figure 1). Personalized ventilation is able to protect the target manikin at the table (position B) from cross-infection from the source manikin. Figure 8 shows the personal exposure index for the target manikin with two different locations of the source manikin, namely, at the table (position A) and standing close to the target manikin (position B). The air distribution system in both cases is the design with the textile terminal located close to the side wall (see Figure 1b). This system gives the highest air quality index, as shown in Figure 6. All the experiments are made with an air exchange rate of 5.5 [h.sup.[-1]].
Figure 8 shows that the personalized ventilation improves the exposure index when the flow rate to the movable panel is increased from 5 to 8 L/s (10.60 to 16.95 [ft.sup.3]/min). A flow rate of 6 L/s (12.71 [ft.sup.3]/min) gives a velocity of 0.2 m/s in the constant velocity core of the jet from the movable panel. This velocity is sufficient to penetrate the boundary layer flow around the manikin and, therefore, improve the exposure index. A velocity of 0.2 m/s should not give rise to increased discomfort, as discussed by Yang et al. (2003).
Figure 8 also shows that the personal exposure index is especially large in situations where the source manikin is standing close to the target manikin. The reason for this is the fact that the air quality index for the occupied zone is strongly improved in this case because the exhalation from the source manikin is located above the target manikin and because it is close to the location of the return openings.
[FIGURE 8 OMITTED]
A ceiling-mounted textile terminal creates a fully mixed flow in large areas of a room. However, positioning of the textile terminal in areas without heat sources creates a displacement flow, which may improve the air quality index and, thus, the personal exposure index in some optimized situations.
Personalized ventilation improves the air quality as well as the protection of the person from cross-infection.
A high personal exposure index is obtained for an airflow rate of 6-8 L/s from the personalized ventilation device. The level of the personal exposure index is dependent on the air quality index of the room.
Bjoern, E., and P.V. Nielsen. 2002. Dispersal of exhaled air and personal exposure in displacement ventilated rooms. Indoor Air 12(3):147-64.
Bolashikov, Z., L. Nikolaev, A. Melikov, J. Kaczmarczyk, and P.O. Fanger. 2003. New air terminal devices with high efficiency for personalized ventilation application. Proceedings of Healthy Buildings 2003, Singapore 2:850-55. National University of Singapore, Department of Building.
Brohus, H., and P.V. Nielsen. 1996. Personal exposure in displacement ventilated rooms. Indoor Air: International Journal of Indoor Air Quality and Climate 6(3):157-67.
Buus, M., F.V. Winther, and P.V. Nielsen. 2006. Private communication. Aalborg University.
Jensen, R.L., D.N. Pedersen, P.V. Nielsen, and C. Topp. 2001. Personal exposure between people in a mixing ventilated room. Proceedings of the 4th International Conference on Indoor Air Quality, Ventilation & Energy Conservation I:33-40. Hunan University.
Kaczmarczyk, J, Q. Zeng, A. Melikov, and P.O. Fanger. 2002. Individual control and people's preferences in experiments with personalized ventilation system. Proceedings of Roomvent 2002, September, Copenhagen, Denmark, pp. 57-60.
Li, Y., X. Huang. I.T.S. Yu, T.W. Wong, and H. Qian. 2004a. Role of air distribution in SARS transmission during the largest nosocomial outbreak in Hong Kong. Accepted for publication in Indoor Air.
Li, Y., I.T.S. Yu, P. Xu, J.H.W. Lee, T.W. Wong, P.P. Ooi, and A. Sleigh. 2004b. Predicting super spreading events during the 2003 SARS epidemics in Hong Kong and Singapore. American Journal of Epidemiology 160:719-28.
Melikov, A.K., R. Cermak, and M. Mayer. 2002. Personalized ventilation: Evaluation of different air terminal devices. Energy and Buildings 34:829-36.
Melikov, A.K. 2004. Personalized ventilation. Indoor Air 2004 14(suppl. 7):157-67. Blackwell Munksgaard.
Mundt, E, H.M. Mathisen, P.V. Nielsen, and A. Moser. 2004. Ventilation effectiveness. REHVA Guidebook No. 2.
Nielsen, P.V., C. Topp, M. Soennichsen, and H. Andersen. 2005. Air distribution in rooms generated by a textile terminal--Comparison with mixing ventilation and displacement ventilation. ASHRAE Transactions 111(1):733-39.
Qian, H., Y. Li, P.V. Nielsen, C.E. Hyldgaard, T.W. Wong, and A.T.Y. Chwang. 2006. Dispersion of exhaled droplet nuclei in a two-bed hospital ward with three different ventilation systems. Indoor Air 16:111-28.
Qian, H., P.V. Nielsen, Y. Li, and C.E. Hyldgaard. 2005. Dispersion of exhalation pollutants in a two-bed hospital ward with downward ventilation system. Indoor Air, The 10th International Conference on Indoor Air Quality and Climate, Beijing, China.
Skistad, H, E. Mundt, P.V. Nielsen, K. Hagstrom, and J. Railio. 2002. Displacement ventilation in non-industrial premises. REHVA Guidebook No 1.
Yang, J, J. Kaczmarczyk, A. Melikov, and P.O. Fanger. 2003. The impact of a personalized ventilation system on indoor air quality at different levels of room air temperature. Proceedings of the 7th International Conference on Healthy Buildings 2003, Singapore, pp. 345-50.
Peter V. Nielsen is a professor and Carl Erik Hyldgaard is an associate professor in the Department of Civil Engineering, Aalborg University, Aalborg, Denmark. Arsen Melikov is an associate professor at the International Centre for Indoor Environment and Energy, Technical University of Denmark, Lyngby, Denmark. Heine Andersen is a development engineer with K.F. Ventilation ApS, [ANGSTROM]benraa, Denmark. Mads Soennichsen is a development engineer with KE Fibertec AS, Vejen, Denmark . ASHRAE's prior written permission.
Peter V.Nielsen, PhD
Carl Erik Hyldgaard
Fellow ASHRAE Fellow ASHRAE
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|Author:||Nielsen, Peter V.; Hyldgaard, Carl Erik; Melikov, Arsen; Andersen, Heine; Soennichsen, Mads|
|Publication:||HVAC & R Research|
|Date:||Jul 1, 2007|
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