A review of ventilation and air-conditioning technologies for energy-efficient healthy buildings in the tropics.
Clear associations are being established between ventilation rates, indoor air quality (IAQ), and the productivity of the workforce in various types of buildings, most significant of which is the commercial and office building sector. While source control is commonly advocated as the fundamental approach to eliminating or containing the contaminant levels inside the building, a more practical and often necessary approach is likely to be exposure control. Thus, ventilation plays an important role in providing a good quality built environment. However, climatic conditions pose considerable challenges in the design of air-conditioning systems, and, in a hot and humid climate, this can translate to significant energy penalty. Two considerations are highlighted in order to achieve "good" IAQ and energy efficiency--the enhanced dehumidifying performance of cooling coils and the effectiveness of air distribution strategies. This paper reviews some of the current and future technologies for air conditioning and air distribution that can collectively contribute to the design of energy-efficient healthy buildings. The air-conditioning technologies reviewed include outside air pretreatment systems, single-coil twin-fan (SCTF) systems employing a compartmented cooling coil, desiccant dehumidification systems, and heat pipes. The air distribution systems reviewed include an SCTF system with independent "ventilation" and "thermal cooling" on zone-based demand, a personalized ventilation system coupled with secondary ambient air distribution system, a displacement ventilation system, an underfloor air distribution system, and a dedicated outdoor air system coupled with radiant chilled ceiling. Applications of some of these technologies, such as the SCTF air-conditioning and air distribution systems and personalized ventilation systems, in Singapore are presented.
A recent study in the United States involving available indoor environmental data indicated that improving building environments may result in significant health benefits for more than 15 million of the 89 million US indoor workers, with estimated economic benefits of $5 to $75 billion annually (Mendell et al. 2002). It is important to address the issues related to indoor air quality (IAQ) from a holistic viewpoint that commences at the conceptual design stage of a building. The concept of "source control" stems from the basic philosophy of eliminating the source of contamination, if possible, or alternatively mitigating the source of contamination. This may involve several strategies, ranging from the selection of non-emitting or low-emitting building materials to localized extraction of the contaminant at the source before it is airborne. While source control can be seen as the fundamental approach to minimize the buildup of indoor contaminants, "exposure control" through proper ventilation provision and strategies is inevitable. There are several contaminants that are best mitigated through dilution, examples of which are carbon dioxide (C[O.sub.2]) and bacterial contaminants. The design and operation of appropriate ventilation systems is, therefore, crucial to the successful management of IAQ in any building, but in particular, tropical buildings where increased ventilation requirements are usually associated with an energy penalty. In this regard, two fundamental considerations are stressed as being of paramount importance in the delivery of good IAQ:
* The air-conditioning system, especially the cooling and dehumidifying coil
* The air distribution system
This paper will review the following technologies that are envisaged to lead to the design of energy-efficient healthy buildings:
* Air-conditioning system
* Outside air pretreatment systems
* Single-coil twin-fan (SCTF) system employing a compartmented cooling coil
* Desiccant dehumidification systems
* Heat pipes
* Air distribution system
* SCTF system with independent "ventilation" and "thermal cooling" on zone-based demand
* Personalized ventilation system coupled with secondary ambient air distribution system
* Displacement ventilation system
* Underfloor air distribution system
* Dedicated outdoor air system (DOAS) coupled with radiant chilled ceiling
It is pertinent to note that research in most of the above areas is in its infancy, and very limited practical applications exist for a holistic comparison and evaluation. In view of this limitation, specific cost/energy benefit analysis of the various systems discussed is beyond the scope of this paper, although information, where available, has been included to give a general idea about both the quantitative performance metrics as well as the practical viability of such systems.
OUTSIDE AIR TREATMENT SYSTEM
Figure 1 illustrates an outside air treatment system based on the high driving potential concept (Luxton and Marshallsay 1998). The hot, humid air is first conditioned to the required off-coil conditions in a separate air-handling unit employing a dedicated cooling and dehumidifying coil, following which the chilled water is sequentially fed to the secondary cooling coil in the other floor-based air-handling units that essentially handle the sensible cooling loads of the recirculated air. By treating the hot, humid outdoor air in this manner with the greatest enthalpy potential possible allows the off-coil dewpoint temperature to be substantially reduced, thus enabling significantly larger dehumidification to be achieved. Incorporating a heat recovery feature such as an air-to-air heat exchanger or a wrap-around coil not only gives a very low off-coil dew-point temperature but also provides the possibility of precooling the incoming outdoor air, thus resulting in an overall energy-saving potential.
[FIGURE 1 OMITTED]
SINGLE-COIL TWIN-FAN (SCTF) SYSTEM
The SCTF concept involves two variable-air-volume (VAV) systems employing one compartmented cooling and dehumidifying coil. A prototype unit was developed and installed to serve conditioned air to two rooms of an IAQ chamber in the Department of Building at the National University of Singapore (Sekhar et al. 2004). A diagram of the SCTF air-conditioning and air distribution system is shown in Figure 2. The fresh air (F/A) is conditioned in the "fresh air" compartment of the air-handling unit (AHU) and distributed to the various VAV boxes that form part of the air distribution network. Each of these F/A VAV boxes is controlled by its own localized C[O.sub.2] sensor, which will ensure an adequate ventilation (F/A) provision at all times. As the main purpose of the F/A VAV box is to ensure adequate fresh air quantity based on occupant density, it helps in achieving energy conservation in the event of reduced occupant loads.
The return air from the various zones of the same distribution network is conditioned in the "recirculated air" (R/A) compartment of the same AHU and distributed to a separate set of the various VAV boxes. Each of these R/A VAV boxes is controlled by its own localized zone thermostat, which addresses diversity in cooling loads and consequently helps in achieving significant energy savings at part-load operating conditions resulting from non-occupancy-related factors. Based on our findings with the prototype unit, energy savings up to 12% in conjunction with significantly improved IAQ have been observed in comparison with conventional air-conditioning systems (Sekhar et al. 2004). The conditioned fresh air and the conditioned return air travel in parallel ducts and do not mix until just before the supply air diffusers in the mixing chamber of the modified VAV box.
[FIGURE 2 OMITTED]
The compartmented coil is aimed at achieving the required psychrometric performance of the two separate airstreams throughout the operating range based on the concept of two VAV systems but one cooling coil. The significant advantage over conventional means of achieving the requisite cooling and dehumidifying performance with two separate coils is the relative ease of operation of the compartmented coil, which is based on a single feed of the chilled water.
Following a successful series of empirical measurements in late 2002 with a small prototype system built for two small rooms with a total floor area of about 50 [m.sup.2] (Sekhar et al. 2004), a field trial of the SCTF system has recently begun (Sekhar et al. 2005a). This involves the design and installation of the SCTF system for a 2,500 [m.sup.2] floor area in a newly constructed office building located in the western part of Singapore. One half of the third floor of this building, approximately 2,500 [m.sup.2], is used as the test-bed for the new air-conditioning and air distribution system. The office layout, shown in Figure 3, comprises a combination of individual office rooms, low-level office cubicles, conference room, meeting rooms, and open-plan spaces. The return air is ducted from the various zones to the AHU room. The basis of design of the SCTF air distribution system is as follows:
* All the temperature thermostat and C[O.sub.2] sensors are duct-mounted near the return air grille of the concerned occupied zones.
* Every individual office room has its own temperature thermostat and C[O.sub.2] sensor.
* All meeting rooms and conference rooms have their own temperature thermostat and C[O.sup.2] sensor.
* The open-plan areas are divided into clusters of imaginary thermal zones as shown in Figure 3. For each open-plan zone, a set of temperature thermostat and C[O.sub.2] sensor is to be provided that will control the requirements of R/A and F/A, respectively.
IAQ measurements of the SCTF AHU in the above test-bed have just commenced and preliminary findings are tabulated in Table 1. These were measurements conducted in five indoor locations, as indicated in Figure 3, and an outdoor ambient air location. Four measurements were conducted at half-hour intervals in each of the sampling locations between 11.00 a.m. and 12.30 p.m. on May 24, 2006. It is seen from Table 1 that both thermal comfort parameters and key indoor pollutant concentration levels are maintained within acceptable limits, established by ASHRAE and local standards/guidelines (ASHRAE 2004b; ENV 1996). More detailed IAQ measurements on a continuous monitoring mode and energy measurements are currently in progress and will be reported in the future.
[FIGURE 3 OMITTED]
DESICCANT DEHUMIDIFICATION SYSTEM
A comprehensive discussion of various types of chemical dehumidification systems is available in Mazzei et al. (2005). Figure 4 shows a simple diagram and the principle of operation of a typical rotary solid desiccant dehumidifier (Harriman and Judge 2002). The desiccant can be balls or beads of granular material packed into a bed, or it can be finely divided and an integral component of a structured medium, which may look like corrugated cardboard rolled over a drum. Desiccant material itself can be silica gel or dry lithium chloride mixed with zeolites. As the range of dehumidification applications is large, considerable flexibility in the selection of desiccants is necessary to minimize installation and operating costs. Among several variables that affect desiccant dehumidifier performance, the most important are
* inlet air temperature,
* moisture content, and
* velocity at face of the desiccant bed (to maximize residence time).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Figure 5 shows the psychrometrics of a liquid desiccant dehumidification system. The process air (outdoor air) is desiccated from A to B, following which post cooling is adopted to yield the appropriate comfort temperature. The desiccant must be periodically regenerated with the continuous introduction of hot humid air.
A rooftop-type application of a conventional system integrated with a desiccant wheel hybrid system has been reported for a supermarket of about 3700 [m.sup.2] area in Rome (Mazzei et al. 2004). In this study, a comparison of the savings was attempted in terms of the operating costs, and a simple payback of two to three years has been estimated with respect to the additional cost of the hybrid system. It was also found in this project that the ambient relative humidity was maintained at lower levels. A reduction of approximately 30% supply airflow rate has also been reported in the same project that consequently reduced system and operating costs as well as space needed by ducts.
Another application of the above system has been reported for a theater of 1200 [m.sup.2] area in Rome. Operating cost savings on the order of 23% and 38% were observed (Bellia et al. 2000). The reduction of electric power demand was in the range of 44%-50%. It was also found that the ambient relative humidity was better controlled with hybrid systems in comparison to conventional systems alone. The percentage of hours where ambient relative humidity exceeded 60% was around 20% for conventional systems and less than 0.6% for the desiccant wheel systems.
Heat pipes are passive devices that recover energy from cold air with the aim of precooling the incoming outdoor air that is required for ventilation. Conventional heat pipes with sintered metal powder wick inside, saturated with liquid, are commonly used as heat transfer devices (Vasiliev 2005). A schematic of the heat pipe is shown in Figure 6. A key feature of the heat pipe is to transfer a large amount of energy over its length with a small temperature drop through liquid evaporation in the evaporator section of the heat pipe (heat source), vapor condensation in the condenser (heat sink), and liquid movement in the opposite direction inside a wick by capillary force. They resemble finned-tube heat exchangers except that the evaporator section and the condenser section are physically decoupled and connected by means of a set of pipes that ensures the closed-loop cycle for the refrigerant. Typically, heat pipes are seen as sensible heat recovery devices; however, they can also be used for latent cooing applications. In a hot and humid climate, heat pipes can enhance dehumidification potential by providing precooling of the hot, humid outdoor air, thus enabling the off-coil conditions of the cooling coil to be at a much lower dew-point temperature than would otherwise be possible; this allows for increased condensation on the coil surfaces. The inherent recovery of the "cool energy" from this cold air to precool the incoming hot and humid outdoor air provides an attractive overall energy-efficient option.
[FIGURE 6 OMITTED]
Recent research and development of heat pipes (conventional heat pipes, heat pipe panels, loop heat pipes, vapor-dynamic thermosyphons, micro/miniature heat pipes, sorption heat pipes, etc.) have been driven by industry-specific application needs (Vasiliev 2005). Heat pipes have been sparingly studied for the control of relative humidity in air-conditioning systems in lieu of conventional reheating systems. Wu et al. (1997) concluded that a thermosyphon heat pipe heat exchanger (HPHE), with no wicks, acting in association with the cooling coil can be used in conjunction with an air-conditioning system to derive an overall cooling capability enhancement of 20% to 32.7% under the test conditions. However, they reported that an HPHE increases pressure losses (increased fan power), manufacturing cost, and overall size of the air-conditioning unit.
With a view to improve IAQ, Martinez et al. (2003) conducted a study of a mixed energy recovery system, heat pipes, and indirect evaporative equipment for air conditioning. A latent heat storage unit incorporating heat pipes embedded in phase-change material (PCM) has been described and tested for a novel application in low-energy cooling of buildings (Turnpenny et al. 2000, 2001). It should be noted that the limited HP studies in the literature pertain to conventional cooling coils.
Studies have been reported where reasonable energy efficiency has been achieved with the use of heat pipes (Martinez et al. 2003). Integrating the heat pipes into conventional cooling coil design also makes it possible to reduce the size of the cooling coil.
A recent study in the tropics demonstrated that for all cases examined, the overall SHR of the HVAC system was reduced from the maximum of 0.688 to the minimum of 0.188 by the heat pipe heat exchanger (HPHE) as inlet dry-bulb temperature to the HPHE evaporator increased, thereby implying an increasing potential to remove moisture (Yau 2006).
A research project is currently being developed that is aimed at integrating heat pipes with the compartmented coil of the SCTF system, as shown in Figure 7. The development of the heat pipe integrated compartmented coil (HPICC) is envisaged to result in an overall enhanced performance of the SCTF system from energy, dehumidification, and IAQ perspectives. Due to the increased efficiency of dehumidification potential of the compartmented coil, it is postulated that the SCTF system would become even more attractive and cost-effective.
PERSONALIZED VENTILATION (PV) SYSTEM
Before the development of personalized ventilation (PV) systems, task/ambient conditioning (TAC) systems were explored to provide occupants with control of a local supply of air for adjusting their individual thermal environment. Early research using TAC systems has shown that it is possible to improve ventilation at the breathing zone (relative to ventilation with perfect mixing) by employing desk-mounted (Faulkner et al. 1993) or floor-mounted (Fisk et al. 1991; Faulkner et al. 1995) air supply outlets. However, these studies reported a small degree of enhancement (20%-40%) and were also observed mainly during operating conditions that are likely to be uncomfortable owing to high air velocities. Subsequent research has shown not only improved ventilation efficiency with TAC systems but also increased thermal comfort and occupant satisfaction (Bauman et al. 1998). A detailed comparison of two desk-mounted TAC systems in conjunction with a conventional ceiling-supply and return HVAC system in terms of air change effectiveness (ACE) and pollutant removal efficiency (PCE) is presented in Faulkner et al. (1999).
The concept of PV is at the cutting edge of technological developments in the area of air conditioning and is fundamentally based on improving ventilation to every individual in the built environment (Kaczmarczyk et al. 2002, 2004; Melikov et al. 2002). PV leads to improved IAQ by its inherent method of supplying conditioned air to the occupants and is essentially aimed at improving the health, comfort, and productivity of the workforce. The PV concept has tremendous potential in enhancing the acceptability of ventilation, IAQ, and thermal comfort in air-conditioned buildings by supplying clean fresh air directly to the occupant breathing zone without mixing with recirculated air, which is usually contaminated with indoor pollutants. The inability of conventional air-conditioning systems to do so often leads to occupant dissatisfaction.
[FIGURE 7 OMITTED]
The PV concept is also envisaged to lead to flexibility in the form of individual control of the microenvironment at the occupant level as well as significant energy-saving potential through reduction of absolute fresh air quantities in the light of an efficient and effective air distribution system. Figure 8 illustrates an integrated PV system in conjunction with either a separate ceiling supply or underfloor air distribution system. It is seen from Figure 8 that the primary PV system provides outdoor air alone while the secondary system supplies conditioned recirculated air to handle the sensible cooling loads.
Preliminary findings from a pilot study conducted at the National University of Singapore suggest that the use of a PV system in conjunction with a secondary air-conditioning system significantly enhances thermal comfort and IAQ acceptability as well as the perception of freshness in the air (Sekhar et al. 2005b). Figure 9 shows the PV experimental setup in the IAQ chamber, and Figure 10 indicates that the ventilation effectiveness of the PV system is significantly higher than that of a conventional mixing ventilation system. Based on the responses of thermal comfort and IAQ acceptability, it is evident that the subjects do find the PV system to be significantly better than the total mixing system. The pilot study has also indicated that the PV system has a potential to save energy in tropical designs.
[FIGURE 8 OMITTED]
DISPLACEMENT VENTILATION SYSTEM
Displacement ventilation systems have been widely used in Europe and North America, where considerable research has been undertaken to examine the potential benefits in terms of improved thermal comfort, IAQ, and energy efficiency. A study by Xing et al. (2001) showed good agreement between experimental data and predictions, which also showed improved air quality in the breathing zone due to the ability of the human body to draw uncontaminated fresh air from the lower zone in the case of displacement ventilation. The same study found that perceived air quality in the breathing zone (represented by the mean age of air) for a seated mannequin in a room ventilated using displacement systems was between 35% and 50% better than the average air quality in the occupied zone. It was also observed that the local air exchange index was the highest close to the DV unit, and the index was greater at the breathing zone than at most other points in the room. Another study in Sweden concluded that temperature gradients and convection flows are of great importance in sizing a displacement ventilation system (Mundt 1995). In comparison with conventional mixing ventilation, displacement ventilation has been found to provide better thermal comfort and IAQ, achieve considerably higher ventilation efficiency, and is more energy efficient (Zhang et al. 2005; Zhao et al. 2004).
[FIGURE 9 OMITTED]
A study was recently conducted in Singapore in which tropical subjects were surveyed with respect to their thermal sensations under different room conditions in either displacement ventilation or mixing ventilation (Yu et al. 2003). Objective measurements, such as room air temperature, air velocity, and relative humidity, were measured at different heights in the chamber. It is observed that tropical subjects can accept a relatively high RH level and low supply air temperature with a displacement ventilation system. When room temperature is maintained at about 23[degrees]C, supply air temperature for displacement ventilation can be as low as 16.2[degrees]C without sacrificing the thermal sensations for the tropical subjects. Tropical subjects are observed to be thermally comfortable with an air temperature of 23.2[degrees]C and RH of 77%. The acceptance of higher than expected RH level could be attributed to the acclimatization of tropical subjects. Hence, the range of acceptable RH levels may be wider than for subjects in a temperate climate, although this needs further substantiation. This study found that the room neutral temperatures for DV and MV systems are 23.8[degrees]C and 23[degrees]C, respectively. DV systems can, thus, produce cooler sensation than TV systems, and preliminary findings suggest the possibility of energy-saving potential for DV systems in the tropics (Yu et al. 2003).
The study also revealed that average votes of dry air sensation for most of the cases were on the dry side, implying that an RH of 50% at 0.6 m height was quite low for tropically acclimatized subjects (Cheong et al. 2006). The results also showed that thermal gradient had insignificant impact on perceived air quality (PAQ), sick building syndrome (SBS) symptoms, and their corresponding percentage dissatisfied. This implies that a temperature gradient of 5 K/m should not cause detrimental effects for tropically acclimatized occupants in terms of PAQ and SBS, although it is beyond the restriction of 3 K/m, as specified in ASHRAE Standard 55 (ASHRAE 2004b). Dry air sensation, irritations, and freshness decreased with increase of room air temperature, which could be attributed to the fact that average values of absolute humidity and enthalpy at the breathing level increased with an increase in room air temperature.
[FIGURE 10 OMITTED]
It should be noted that the improved efficiency of the DV system would depend, to a large extent, on relatively tall occupied enclosures.
UNDERFLOOR AIR-CONDITIONING (UFAC) SYSTEM
A UFAC system is comparable to a DV system in that both systems occasionally supply cold air from floor-mounted diffusers. While DV systems typically supply air at low velocities with the aim of minimizing mixing and maximizing displacement and stratification, UFAC systems can reach higher velocities that are aimed to achieve mixing in the occupied zone and stratification in the upper zone. The findings from a tropical study conducted in the late 1990s show a reasonable level of IAQ acceptability associated with a UFAC system (Sekhar and Ching 2002). The study also revealed the issue of localized thermal discomfort, especially cold feet, near the UFAC air supply outlets. Another recent study also observed undesirably high air velocities and high draft rates within a small region near the supply outlet and concluded that a clearance zone may be necessary as a design consideration for locating the outlets relative to occupants to avoid undesirable draft discomfort to occupants (Chao and Wan 2004).
In designing a UFAC system, one needs to consider the velocity of supply air as it exits from UFAC outlets. Close proximity of a sedentary occupant in an office environment to a UFAC outlet may pose problems of cold thermal discomfort. One of the strategies to overcome such concerns is to limit the exit velocity to no more than 0.1 m/s and then determine the number of UFAC outlets in a given space for a known supply air volume.
DEDICATED OUTDOOR AIR SYSTEM (DOAS)
A dedicated outdoor air system (DOAS) is a 100% outdoor air constant-volume system designed to deliver the volumetric flow rate of ventilation air to each conditioned space. It is used to place the required and conditioned ventilation air directly into the space without first mixing it with stale building air as is the current practice, thus always meeting the requirements of ASHRAE Standard 62.1 (ASHRAE 2004a). The integration of DOASs with parallel terminal systems has been hailed as a significant paradigm shift and the general layout of a DOAS, consisting of a preheat coil, an enthalpy wheel, a deep cooling coil, a sensible heat exchanger, and the prime movers, has been discussed by Mumma and his research team at Pennsylvania State University (Mumma and Shank 2001; Mumma 2001).
This paper has reviewed some of the current and emerging technologies that are aimed to provide good IAQ as well as achieve energy efficiency. The technologies that are presented have a bias toward hot and humid climates, where the dehumidification challenge is most critical. The notion of a decoupled ventilation, or independent ventilation, system is beginning to be seen as a paradigm shift in our quest to improve IAQ and thereby improve the comfort, health, and productivity of workers in nonindustrial indoor environments.
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Lew Harriman, Director of Research and Consulting, Mason-Grant Consulting, Portsmouth, NH: Please comment on your observations and opinions regarding the thermal acceptability, for tropically acclimatized occupants of a higher dry-bulb temperature combined with a lower dew point, as a means of reducing energy consumption and as a means of improving thermal comfort in hot/humid climates.
S.C. Sekhar: Thank you for your comment. In most tropical designs, the indoor space conditions are often found to be maintained at fairly low temperatures, say around 22[degrees]C, sometimes even lower. The corresponding RH level could be 60%-65%. In my opinion, this results from the need to overcool to extract the moisture from the air, and active reheating would, in most cases, not be allowed for comfort air-conditioning applications. It is again my opinion that a warmer air temperature, say 24[degrees]C-25[degrees]C (accompanied by a lower dew-point temperature) may find equal or better acceptance among tropically acclimatized occupants. There has been limited research to test this hypothesis in field conditions--simply because there are not very many tropical buildings designed and operated as such. Under such conditions, increased air movement may also be a technique for improving thermal comfort. In a study involving personalized ventilation (conditioned outdoor air only) together with a secondary air distribution system (recirculated air) of the mixing ventilation type, it was found that tropically acclimatized occupants do prefer an elevated air velocity without complaining of draft (Sekhar et al. 2005). (1)
The warmer air temperature and lower dew-point temperature can be made possible with energy-efficient technologies, such as desiccant dehumidification and heat pipes. It is also to be noted that an indoor design condition of 24[degrees]C or 25[degrees]C will result in lower cooling load compared to designing for 22[degrees]C.
(1). Sekhar, S.C., N. Gong, K.W. Tham, K.W. Cheong, A.K. Melikov, D.P. Wyon, and P.O. Fanger. 2005. Findings of personalised ventilation studies in a hot and humid climate. HVAC & R Research 11(4):603-20.
S.C. Sekhar, PhD
S. C. Sekhar is an associate professor in the Department of Building, National University of Singapore.
Table 1. Preliminary Measurements of IAQ During Field Trial IAQ Parameters Location 1 Location 2 Location 3 Space temperature, 22.4-22.8 21.6-22.4 22.7-23.1 [degrees]C Space humidity, %RH 55.5-57.2 55.9-57.5 55.7-57.2 Carbon dioxide, ppm 767-820 768-825 720-800 Carbon monoxide, ppm < 1 < 1 < 1 Total volatile organic 0.343-0.377 0.350-0.375 0.340-0.352 compounds, ppm Ambient IAQ Parameters Location 4 Location 5 (Outdoor Air) Space temperature, 22.2-22.9 21.1-21.7 34 [degrees]C Space humidity, %RH 54.6-55.6 54.7-56.2 77.2 Carbon dioxide, ppm 750-815 720-770 400 Carbon monoxide, ppm < 1 < 1 1 Total volatile organic 0.355-0.369 0.330-0.348 0.160 compounds, ppm
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|Date:||Jan 1, 2007|
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