外文翻译--热带地区地板空调系统的室内空气品质和热舒适性的调查 英文版

编号： 毕业设计 (论文 )外文翻译 （原文） 院 （系）： 职业技术学院 专 业： 机电一体化工程 学生姓名： * 学 号： *自考准考证号 * 指导教师单位： 职业技术学院 姓 名： * 职 称： * 2012 年 10 月 8 日 1 Indoor air quality and thermal comfort studies of an Under-Floor air-conditioning system in the tropics Abstract This paper reports thermal comfort and indoor air quality (IAQ) studies of an under-floor air-conditioning (UFAC) system in hot and humid climate. Thermal comfort parameters were measured at predetermined grid points within an imaginary plane to predict the airflow pattern of the supply air jet as well as to determine the occurrence of thermalstratification in the office space. Fangers Thermal Comfort Analysis and Applications in Environmental Engineering, McGraw-Hill, New York, 1970 thermal comfort index was also computed to detect the occupants thermal sensation. Besides, the concentration levels of dust and carbon dioxide were recorded with the intention to examine the quality of the indoor air. Statistical methods were applied to derive the relationship between air velocity and the other parameters as mentioned earlier. The main findings from the study revealed reasonable level of acceptability of IAQ associated with the UFAC system. However, occupants are likely to experience localised thermal discomfort near the supply diffusers due to the existence of large temperature gradients. In addition, a stagnant zone is discovered at sedentary level, which is caused by the parabolic airflow nature of the primary air jet. Keywords: under-floor air-conditioning systems； thermal comfort； Indoor Air Quality； Singapore； office, field trips 2 1 Introduction This concept has been used for many years the concept of cold air from the raised floor computer room. However, these areas rarely occupier and no real attempt to apply for the same air standard office environment comfortable movement. For example, in the early 1960s, the Federal Republic of Germany, (UFAC) under-floor air-conditioning system is the first to introduce high heat production room, such as the electric light bulb factory, the drying device and data processing centers in the textile industry . This is a purchase and then, in the mid-1970s, began to be used, this system general offices mainly in European countries. Open office concept innovation is a change of office layout constraints, traditional ceiling-based air-conditioning system is adopted. The layout of the diffuser, in this case, usually before the establishment of the position of the workstation and the thermal load has been determined. In addition, the partition does not consider the location of air diffusion under surface-enhanced Raman spectroscopy, tend to restrict the flow of air space, causing discomfort to the crew 1. Based floor air distribution system, residents can enjoy a great deal of flexibility to add, delete, or relocation of supply channels to meet their individual preferences Allocation, as well as any office layout rearrangement. In addition, some systems will automatically thermostat spread on the ground, so that the occupants are required to adjust the temperature. This not only provides personal control, but also caused occupant comfort 2,3. Then, with the advent of electronic or automated office to improve the installation and service of floor, lay the cable under, to adapt and to conceal. Since the cavity between the concrete slab and flooring create a raised floor tiles, floor invalid and can also be used as a function of the supply air chamber. Therefore, the conditioned air through the floor of the upper chamber from the supply of office space grille. The depth ceiling ventilation system so that you can reduce or eliminate the pipeline system and terminals are no longer there. Therefore, the new building can benefit from a significant height reduction or additional storeys, which in turn led to the huge savings cost 4. However, the people to move towards IT-based working environment, the use of computer data storage and other work-related functions is essential. Together with other electronic equipment, they are likely to heat load of office space a major contributor. As the temperature of the air is supplied from the floor outlet is usually 3 lower than the indoor air, the high-velocity air, i.e. from the following computer equipment in a cooling faster rate than cold air from a conventional ceiling-based systems. This effectively eliminates the heat engine, otherwise, will be set up, along with the time. However, a potential drawback, in this case, is likely to draft by the households and the feeling, machine condensate 5. Plenary Session of the concept of supply of cold air from under the floor is relatively new in Singapore and small studies have carried out this study the applicability of the system in the office buildings of the city. Use the upper limit of the local system based on this system have been explored in limited circumstances, common complaints, such as cold, still very common paper is designed to explore the performance of the system related to indoor air quality (IAQ) and thermal comfort of office space in the case of the thermal comfort Next, it is limited in its scope: 1, the room space temperature measured empirical data collected, the speed and relative humidity； 2, calculate the predicted mean vote (PMV) and predicted percentage dissatisfied with (PPD) based on measured values； 3, the correlation analysis and the simple linear regression between the airflow rate and other variables, namely: 3.1, relative humidity； 3.2, temperature； 3.3, PMV and 3.4, PPD The purpose here is to carry out a comprehensive audit of the indoor air quality on indoor air quality, 6, the air quality level meters, will provide an in-depth understanding of the performance of the floor analysis based system choice. The results will also be subsequently used to support this hypothesis the airflow pattern in the modeling. Therefore, the study of indoor air quality include: 1, concentration data collected particles and carbon dioxide； 2, correlation analysis and simple linear regression of variables, that: 2.1, particle counting, and the air flow rate and 2.2, carbon dioxide and an air flow rate. 4 2 UFAC system-a review An UFAC system is comparable to a displacement ventilation (DV) system in that both system sometimes supply cold air from diffusers that are mounted on the floor.However,for the latter system,supply air can also be supplied through diffusers that are mounted in walls near the floor where the return air is subsequently exhausted through the ceiling grilles,as shown in Fig. 1. While DV system generally supply lower velocity air with the goal of minimising mixing and maximising displacement and stratification, UFAC systems purposely supply higher velocity air with the goal of promoting mixing in the occupied zone and allowing stratification in the upper zone.The higher velocities in the UFAC system provide air movement for occupant cooling to offset higher ambient temperatures and the system also offers an opportunity for user control. Nevertheless,both systems aim to create to supply air conditions in the occupied zone,which is defined as a zone between planes 75 and 1800 mm above the floor and more than 600 mm from the walls 7. 2.1. Thermal stratification The aim of air diffusion in air-conditioning system is to create the proper combination of temperature,humidity and air motion in the occupied zone of the conditioned space. Nevertheless,a floor-based air-conditioning system is often found to be associated with large thermal gradients (thermal stratification) between the feet and the head as well sa local thermal discomfort due to draft 8.In the case of dissatisfaction as a function of the vertical air temperature difference between head and ankles, it has been observed that the percentage dissatisfied varies exponentially, with 2% dissatisfied at 2 C and rises to as high as 60% at 8 C 9. 5 Temperature gradient refers to the difference between temperatures at any two points. ASHRAE Standard 55- 1992 10 recommends that the maximum temperature gradient should not exceed 3 C. Therefore, an ideal con- dition would be uniform room temperature from the floor to about 1800 mm above the floor 11. However, a gradient of 2 C should be acceptable to about 85% of the occupants to remain thermally comfortable. A field study involving the installation of desktop task/ ambient air-conditioning system at 42 selected locations within three San Francisco office buildings revealed higher occupant satisfaction levels in terms of thermal, acoustic and air quality 12. Hanzawa and Nagasawa subjected 26 people in a test room that utilised the UFAC system in which the return grilles were mounted in the ceiling 13. These people were then surveyed on the sensation of draft and air movement. The air velocity around them was also measured. It was found that the average room-air temperature differences between 0.1 and 1.1 m above the floor in the occupied zone were around 1 C. In addition, it is found that the system could provide little draft risk in the occupied zone of the airconditioned space. Besides, satisfactory thermal environmental condition was also observed in an intelligent building in Tokyo 14. It was found from the field measurements that the UFAC system has no serious problem with extreme vertical temperature distribution and draft, although some occupants (usually women) complained of cold feet. None- theless, findings do not seem to be consistent with Melikov and Nielsen 8. Akimoto et al. 15 conducted a comprehensive study on such system in an experimental chamber located in a controlled environment chamber with the return air leaving through the ceiling grilles 15. Temperature distribution tests were carried out to investigate the effects of lighting, occupants, electrical equipment, walking and change of air volume for thermal stratification. In addition, qualitative and quantitative studies for the system were performed to investigate the airflow characteristics and ventilation efficiencies in comparison to a conventional ceiling-based system. In terms of thermal stratification, the observation was made that the temperature profiles of such system could be divided into two zones, namely, the steep temperature gradient part that was measured from the floor to 1-1.5 m high, and the gentle part that was above this zone to the ceiling. From the study, however, it was seen that in one of the experimental conditions that simulate a modern office, the steep temperature difference limit of 3 C. As a result, sedentary occupants would be expected to feel cold discomfort at the feet and warm discomfort at the head level. 2.2. Thermal comfort indices A practical approach to assessing thermal environments for the comfort of the occupants is provided by Fanger 16. A thermal sensation index was developed to predict the mean thermal sensation vote on a 7-point standard scale for a large group of people. The index depends on the four thermal environmental variables (air temperature, relative humidity, mean radiant temperature (MRT) and 6 relative air velocity), the activity level and the clo-value of the clothing worn by the occupants. Meanwhile, it seems more meaningful to state what percent of persons can be expected to be decidedly dissatisfied than just giving the predicted mean vote as an expression for the thermal environment. Therefore, the PPD curve can also be used as a basis for an evaluation of thermal environments. Some of the researchers have conducted investigations on the effect of such system on the thermal sensation of the occupants. For instance, Yokoyama and Inoue evaluated the performance between the UFAC system and the ceilingsupply air-conditioning system 1. The evaluation was carried out by measuring the room environment in a test chamber and by surveying sensation responses from ques- tionnaire in the operating offices. PMV values were calculated and the results revealed that the thermal sensation consensus was neutral； comfort consensus was slightly comfortable； and general satisfaction was also slightly satisfactory for the UFAC system. Besides this, another study by Hanzawa was also carried out on the floor-based system with a controlling system based on PMV that has been installed in an office building in Tokyo 17. The mean vote values were derived from the field measurements and it was found that the ambient condition was almost sufficiently comfortable. On the other hand, the result of the actual mean vote from the questionnaire survey on the occupants did not seem to be satisfactory as compared to the former. 2.3. Indoor air quality Air quality in offices has been a major concern in the past two decades,particularly due to the increasing number of reports on the sick building syndrome. Although several research efforts have been carried out, most of the investigations on the IAQ are performed on the conventional ceilingsupply air-conditioning systems. In the case of UFAC systems, limited information is available in the literature 12. Although air quality is not directly considered a thermal comfort parameter, it is closely related. For example, high relative humidity in the office space not only brings about occupants discomfort but also promotes the growth of moulds and fungi. On the contrary, humidity level less than 30% can lead to increased airborne dust levels as well as static electricity, which in turn may lead to comfort and IAQ problems. Under the ENV guidelines 18, it is recommended that the relative humidity level be less than 70% although ASHRAE Standard 62-89 7 specifies levels between 30 and 60% to be acceptable. Carbon dioxide is commonly used as an indicator of reduced air quality and ventilation efficiency. In the occupied rooms, CO2 will increase if the air supply rate per person is inadequate. It is therefore an effective surrogate for the adequacy of fresh air being brought into a building. Studies 14,15,19 have shown that the UFAC system, when compared to the conventional system, provides for better ventilation efficiency. This is mainly due to the fact that the partially enclosed cubicles in the open-plan office do not restrict the 7 distribution of the supply air. An interesting finding by Gabrielsson and Wiljanen 20 shows that the contaminant removal ventilation effectiveness is strongly influenced by the way of supplying air from the floor level and exhausting alternatively from floor or ceiling level. Result shows that a low velocity floor supply with floor return has a ventilation effectiveness of 0.33 whereas the one with a ceiling return measures at 1.07. From the study, it can be observed that the UFAC system that takes after a DV nature is able to remove contaminants more effectively. There is a concern for the suspended particulates that follow the supply air stream from the floor. The size range of concern when human health effects and IAQ are considered is from 0.1 to 10 mm in aerodynamic diameter. Small particles that reach the thoracic or lower regions of the respiratory tracts are responsible for most of the adverse health effects. Local IAQ guidelines 18 recommend the limit of 150 mg/m 3 for suspended particulate matter. Particulate matter is easily collected within the raised floor if regular maintenance is not carried out. As a result, these particles may be blown up to the occupied zone together with the cold air. In addition, occupant movement and air circulation above the floor may capture the particulate and hold it in suspension, thus giving rise to poor IAQ. Stymne et al. 21 and Loudermilk 22 suggested that the occupants draw uncontaminated air from the lower zone and, therefore, experience better air quality at the breathing level. Similar findings were reported by Matsunawa et al. 14 and he rationalised that the removal of dust is promoted by piston-like airflow pattern. Besides this, Akimoto et al. 15 also found from the experiment that, the mass concentration of dust of the under-floor system was slightly less than that of the conventional system, with the former measuring 0.045 mg/m and the latter 0.07 mg/m 3. However, Shute 23 observed that the concentration of the suspended particulate is the same for the two different systems. Advantages of the floor-based system enumerated by most of the proponents include flexibility, cost-saving in the long run, individual control over the temperature, better air quality and thermal comfort in the workplace. On the contrary, some have raised doubts on the effectiveness of the system. For instance, an under-floor system with the return grille at the ceiling may cause local thermal discomfort and large temperature gradient whereas the one with return air exhausting through the floor plenum may end up losing the displacement flow characteristic 24,25. On the other hand, with both supply and return on the floor, it is possible to encounter certain amount of short-circuiting which may hence reduce the effectiveness of the ventilation system 26. As most of the research works are based on UFAC systems that resemble displacement flow characteristic, field studies that are carried out on systems that have both supply and return on the floor are rare. Indeed, such a system will be examined in this paper. 8 3 UFAC system description UFAC systems come in different designs although the location of the return air grille is limited to be only either on the floor or the ceiling plenum. Nevertheless, the test system here is designed with both the supply and return on the raised floor. It is now intended to provide a general description of the system and an understanding of the way the air is distributed in the occupied zone. The UFAC system directly makes use of the floor void as a plenum for the distribution of air. Each floor is fitted with raised access flooring and steel encapsulated floor panels. Beneath this, movable airtight baffles delineate the supply and return air channels. A conditioning unit, which is usually located in the office, conditions the outside air that is brought into the building. The air is then distributed under the floor to the fan terminal units (FTUs), which ventilate individual work areas with the conditioned air. Subsequently, secondary room air is returned through the return air grilles that are mounted on the floor, which flows back to the conditioning unit. Besides this, the building is generally divided into several zones per floor and these zones are defined by the means of fire barriers placed in the floor void. Each conditioning unit is a vertically arranged air handling unit containing a filter, copper pipe/aluminium fin cooling coils, direct drive fans and comprehensive controls system. The unit is made with self-supporting steel framework clad with acoustically lined steel panels. In addition, it is finished in standard powder coat finish and stands on a purpose made base frame. The FTU draws air from the plenum and introduces it into the space above in accordance with the requirements of their own on-board controls system. In addition, a multi-functional control permits personalised temperature and airflow control. The FTU measures 600 mm in length and 300 mm in breadth and is recessed into the floor to offer greater space gain. It can replace an access floor tile anywhere in the room and can be used as part of the walking surface. Each unit also contains twin scroll fans driven from a single motor, an open/shut control damper, an optional electric heating coil, and a self contained on-board control system. The units require no ductwork connections and are easily relocated in a matter of minutes. Return grilles constructed in the same material as those used for the FTU are 600 mm long and 150 mm wide. They are positioned in the floor over the return plenum to permit the return of secondary room air to the conditioning unit for re-conditioning. They comprise of solid parallel aerofoil blades supported with transverse bars and coated in neutral silver anodised finish as standard. The grilles have the same height adjustment feature as the FTU grilles for easy alignment with the final floor covering and may be supplied with a collecting tray, opposed blade control dampers and acoustic attenuator as additional options. 9 4 Description of test site The building in which the experiment is carried out is a double-storey 6-year-old single unit factory with a gross floor area of approximately 5000 . However, the area that utilises the UFAC system is only about 300 and this is found in the corporate office, shown in Fig. 2. The total number of staff working in the office does not exceed 10 persons. In other words, every occupant occupies an approximate area of 30 . The UFAC system is not equipped with a proper outside air provision and is dependent on infiltration during opening and closing of the doors. For the remaining of the building,i.e.the marketing department and reception area,conventional ceiling-mounted air-conditioning system is used. As for the production floor, it is not air-conditioned. Instead, natural ventilation is provided. 10 5 Research methodology A methodology involving the measurement of thermal comfort parameters, chemical and particulate contamination is adopted in this research study. The office space that utilises the UFAC system is divided into three zones namely Zones A, B and C with each zone served by a different . conditioning unit, as shown in Fig. 3(a)-(c). As it is not within the scope to study the airflow pattern, air quality and hermal stratification at every location where the floor grille exists, two sampling locations were selected from Zones A and B, respectively. Only one sampling location was taken from Zone C, as this is the only location that is free from obstruction by the furniture. Each sampling location comprises a set of floor diffuser and a return grille in which the supply air is likely to flow to. Apart from this, it is seen from Fig. 3(b) that the real-time monitoring of carbon dioxide over a continuous period of 48 h is also set up at 11 point X in Zone B at a height of 1.2 m above the floor. In the experiment that was carried out to determine the various thermal comforts and IAQ parameters of the UFAC system, an imaginary plane, consisting of horizontal and vertical gridlines, was set up in the sampling location. Within the plane, readings were taken at every grid point as show in Fig.4. 5.1. Measurement procedures For the measurement of temperature, air velocity and relative humidity, the intervals along the horizontal, as well as vertical gridlines, were set at 0.5 m with the highest point at 2.5 m above the raised floor. Readings were thus recorded at the various grid points using the appropriate equipment accordingly. As for the measurement of dust concentration, the horizontal distance between the supply and return grilles was divided into the intervals of approximately 1.2 m at all the sampling locations except Locations 2 and 4, in which the intervals were set at 1.5 m. In addition, at each interval, the vertical gridline was divided into intervals of 0.8 m, with the highest point at 2.4 m above the raised floor. Measurements of the total suspended particles and PM10 were, therefore, carried out at all the intersection points of the gridlines. A total of 210 observations each were recorded for temperature, relative humidity and air velocity in the office space for the five sampling locations. As for carbon dioxide and dust measurements, 32 and 64 readings were noted, respectively. Each measurement for carbon dioxide and particulate matter was measured for a time period of 15 min. The following sections describe the various types of equipment that are needed for the field measurement. The research strategy and experimental design will also be illustrated together with the procedures for data analysis. 5.2. Analytical techniques Readings for all the parameters are tabulated according to the location as well as depicted in the form of contours. The results are subsequently used for analyses and comparison against guidelines and standards. 12 It will be inadequate to assess the level of thermal comfort in the office space if it is only based on the physical quantities obtained from objective measurement. To enhance the preliminary findings, simulation can also be carried out to detect how the occupants feel in the indoor environment. One method is to adopt the thermal index developed by Fanger 16. This involves the computation of PMV, which then leads to the determination of values for PPD. In this research, the values of PMV are computed for all the grid points in the five sampling locations. In order to find out the PMV for a particular grid point, readings collected on temperature, relative humidity and air velocity at the similar point are employed in the comfort equation 16. MRT is assumed to be equal to the dry bulb temperature. Metabolic rate is assumed to be 50 kcal/h and this indicates an occupant who is quietly carrying out some clerical work in a sedentary position. The clothing value in this case is taken as 0.5 clo, which is quite typical of office-goers in a hot and humid climate. Statistical tools are employed to explore whether the values of the primary air velocity, measured at the grid points, are correlated to the relative humidity and temperature in the room space at those corresponding points. This also includes PMV and PPD. Furthermore, it is speculated that the velocity of the primary air jet may also have certain impact on the levels of carbon dioxide and dust particles that are lying in its air path. Correlation analyses for the five locations are carried out for the following pairs of variables: temperature and relative humidity； temperature and air velocity； relative humidity and air velocity； PMV and air velocity； PPD and air velocity； particle count (TSP/PM10) and air velocity and carbon dioxide and air velocity. As some of the grid points of dust and carbon dioxide measurements do not coincide with the grid points of the velocity readings, mathematical interpolations were carried out for consistency. This enabled the readings for velocity and dust particles, as well as carbon dioxide, to be subjected to correlation study on the same basis. 5.3. Instrumentation 5.3.1. Chemical measurement An indoor environment monitor is used for the measurement of CO 2. The standard unit comes with sensors for monitoring three basic comfort and ventilation parameters namely carbon dioxide, temperature and relative humidity. However, in this experiment, the purpose of using such a monitor is to measure the concentration level of carbon dioxide in the various sampling locations. The instrument, employing a non-dispersive infrared detector, measures the gas within the range of 0-5000 ppm and has a resolution of 1 ppm. 5.3.2. Particulate measurement 13 The concentration of the suspended particulates in this experiment is measured using a dust monitor, which has been designed to provide continuous gravimetric measurement of dust as well as virtually any other aerosol and/or particulate-as these materials exist suspended in the ambi- ent air. In addition, it provides real-time continuous documentation of the size (or aerosol diameter) distributions of these ambient particulates. This environmental laser aerosol spectrometer is capable of measuring particles with diameter between 0.1 and 15 mm, with an accuracy of 5%. In general, the instrument is suitable for monitoring dust and/or aerosols all the way from the ultra-low concentration levels of a clean room, to the relatively higher levels commonly encountered in the average workplace. 5.3.3. Thermal comfort measurement The three thermal comfort parameters, namely temperature, relative humidity and air velocity, which are essential for tracing the airflow pattern of the supply air from the UFAC system and also to determine any thermal stratifica- tion in the office space, are measured with the help of two hand-held instruments. The MRT is assumed to be equal to the dry bulb temperature in this study. The hot wire anem- ometer is used specifically for the measurement of the velocity of the supply air from the diffusers as it measures airflow velocity ranging from 0 to 20.0 m/s. A relative humidity and temperature meter is used to measure these parameters inside the office space. The measuring range for humidity in this case is 0-100% RH, with an accuracy of 2%. As for temperature, the range is between 20 and 60 C with an accuracy of 0.1 C. 14 6 Results and discussion 6.1. Empirical results 6.1.1. Temperature The maximum, minimum and the mean temperatures for the five sampling locations are shown in Table 1 and a sample of the temperature results is presented in Fig. 5. It is seen that majority of the values fall within the acceptable temperature range of 20-27 C as stated in ASHRAE Standard 55-1992 10. Singapore Standard Code of Practice 13 27 requires the comfort temperature to be within 23-25 C while ENV Guidelines 18 recommends temperature range between 22.5 and 25.5 C for acceptable IAQ. Based on the local standards, the average room temperatures for all the locations are satisfactory except temperature readings that are near or at the supply diffusers. Those readings are much lower than the rest of the recorded values and similar outcomes are observed in the rest of the locations. Consequently, this could lead to potential cold feet problem and localised discomfort. The vertical temperature distribution at every vertical axis in the sampling locations is examined so as to determine the extent of the thermal layer, which is a feature of the UFAC system 14. It is found from the experiment that large temperature gradient usually occurs at the supply diffuser while temperature differentials at other vertical axes along the plane appears to be less than 1 8C on 15 the average. Temperature difference between the grid points at supply diffuser, and 2.5 m above the floor for Locations 1, 2 and 5 is observed to be 2.3, 4.7 and 2 C, respectively. Locations 3 and 4 are both found to have a temperature difference of 3.5 C. ISO Standard 7730 28 recommends that the vertical air temperature difference should be less than 3 8C between the 0.1 and 1.1 m levels, and ASHRAE Standard 55 10 specifies less than 3 8C between the 0.1 and 1.7 m levels. It is seen that the values of vertical temperature difference for all the locations, at the levels set out by the respective standards mentioned above, satisfy the requirement of the thermal comfort. It is, however, to be noted that the limits recommended in the standards are based on the conventional complete mixing system. 6.1.2. Humidity A sample of the relative humidity levels recorded is presented in Fig. 6. The mean relative humidity readings for all the five locations, as indicated in Table 2, are observed to fall below the limit of 70% specified by ENV Guidelines, as well as between the range of 55 and 65% stated in Singapore Standard CP 13 18,27. It is, however, found that most of the grid points in close proximity to the supply grilles deviate from the comfort requirement. Furthermore, it is seen that all the grid points at the diffusers turn out to have the highest humidity values as compared to the rest. 6.1.3. Air velocity Draft is defined as an unwanted local cooling of the human body caused by air movement. Strong draft was felt when an occupant stood directly above the supply diffuser during the experiment. A sample of the air velocity plot is 16 presented in Fig. 7. It is observed that the air velocity at a point directly on top of the supply diffuser at Location 1 was 2.9 m/s； Locations 2 and 3, 2.0 m/s； Location 4, 5.1 m/s； and Location 5, 2.1 m/s. In addition, high air movement was also observed at grid points that are near the supply. It can be seen that these velocity readings highly exceed the recommended limit for the average air movement of 0.25 m/s 10,27,18. It can be deduced from the results that, occupants who are either standing or sitting within a radius of approximately 0.5 m away from the air supply could actually sense the strong air movement. It is known that the risk of draft increases with increasing mean air velocity and decreasing air temperature. As low temperature and high velocity readings were recorded near the region where the supply diffuser was located, the risk of draft and hence occupants complaints are found inevitable, unless the diffuser is shut. Apart from this, the airflow pattern is studied with the aim to find out whether the airflow creates any uncomfortable region in the office space. Result shows that the distribution of the air velocity over the vertical plane follows a parabolic path from the supply to the return grille. During the experiment, primary air jet was observed to be thrown vertically upwards, which then hit and travelled along the ceiling, and subsequently began to drop after a distance of about 1.5 m away from the supply grille. Stagnant zo ne, where there is no air movement, was also discovered at a distance of 1-2 m away from the supply grille, and this is usually up to a height of 1.5 m above the raised floor, as shown in Fig. 8. The implication of such airflow pattern is that, occupants who are sitting inthe middle of the supply and return grilles, with breathing level at 1.2 m above the floor, are less likely to receive fresh air from the supply. 17 6.1.4. Thermal sensation The measured values of temperature, humidity and air velocity are then used in the computation of PMV and PPDvalues. The expected thermal comfort of the occupants can be represented as a plot of PPD values at the various grid points and a sample plot is presented in Fig. 9. The PMV values indicate that 72% of the calculated mean votes in Location 1 falls within the range of neutral and slightly cool while almost every mean vote along the vertical axis above the supply diffuser is termed cold. Besides this, no other neutral zone is found within the vertical plane. Eighty-two percent of the predicted mean votes in Location 2 is observed to be neutral with the other 12% being slightly cool； only the grid point that is directly on the supply grille is voted as cold. As for Location 3, it is seen that 86% of the mean votes are classified as neutral and 6% as slightly cool. In this case, two grid points directly above the diffuser are termed cold. For both, Locations 4 and 5, two-third of the grid points along the vertical axes at the supply grilles is found to be cold. In spite of this, 62% of the mean votes from the former are observed to be neutral whereas 21% are categorised as slightly cool. On the other hand, Location 5 has 19% of the mean votes falling under the range of neutral and 61% under slightly cool. Generally, the thermal sensation of the occupants is noted to be within the range of neutral to cold. No warm discomfort is experienced. It could be predicted that occu- pants are likely to feel cold when they are standing on top of or near the diffusers due to strong air movement and low temperature. However, as they move across the vertical. plane and away from the supply air, they would tend to feel thermally more comfortable and less dissatisfied. According to the ISO comfort standard 28, PPD should be lower than 10% for thermal comfort. However, it is observed that, the average PPD in Location 1 is 52.4%； Location 2, 11.64%； Location 3, 16.3%； Location 4, 23.6%； and Location 5, 44.4%. 6.1.5. Particulate matter A sample plot of total suspended particulate (TSP) and Fig. 8. Airflow pattern in office space. PM10 is shown in Fig. 10. The ranges of TSP and PM10 measurements 18 in a vertical grid at the other four locations are given as follows: Location1 : TSP 3 32mg/m ； PM10 2 15mg/m Location3 : TSP 1 18mg/m ； PM10 1 8mg/m Location4 : TSP 1 60mg/m ； PM10 1 21mg/m Location5 : TSP 2 37mg/m ； PM10 2 15mg/m It is seen from the above measurements that the amount of particulate matter at all the five sampling locations satisfies the standard of 150 mg/m provided by the local ENV Guidelines. Nevertheless, a phenomenon is observed from the distribution of dust particles within the vertical plane. In places where there is no human traffic, for instance Locations 1 and 3, it is found that, the amount of particle counts decreases when the readings are taken at increasing room height. Conversely, for the rest of the locations where human traffic is present, the particle counts actually increases.The explanation for the cause of such phenomenon may be attributed to the occupant movement and air circulation above the floor, which capture the particulate matter and hold it in suspension 23. 6.1.6. Carbon dioxide Although not entirely definitive, carbon dioxide is widely used as an indicator of IAQ and a limit of 1000 ppm is recommended to satisfy the comfort (odour) criteria. The carbon dioxide levels at all the grid points satisfy the recommended thresholds and a sample plot of carbon dioxide is presented in Fig. 11. 19 The ranges of carbon dioxide measurements in a vertical grid at the other four locations are given as follows: Location 1 : 842 950 ppm Location 3 : 758 790 ppm Location 3 : 758 790 ppm Location 5 : 772 848 ppm 6.2. Statistical analysis The empirical measurements obtained at the various locations are statistically analysed in which bivariate corre- lation studies are attempted to validate the expected relation- ships between the various comfort and air quality parameters for an UFAC system. This includes an examination of the strength of association, regression equations and R 2 values between the variables. 20 6.2.1. Temperature, relative humidity and velocity The values of the correlation coefficient of room space temperature, relative humidity and air velocity for the five sampling locations are presented in Table 3. The degree of correlation between temperature and relative humidity for all the locations, except Location 1, is observed to be high. It is seen that, an increase in the air temperature leads to a drop in the relative humidity in the room space and vice versa. Similarly, for temperature and air velocity, a strong negative correlation is found for all the five locations. This implies that, as the velocity of the supply air detected at the grid points becomes stronger, the corresponding temperature decreases. Furthermore, it can be inferred that, at places where air movement is present, lower temperature readings are usually being recorded. On the other hand, it is observed that there exists a direct relationship between relative humidity and velocity. Coefficients for all the five locations, except Location 1, are relatively high with an average value of 0.7. A linear regression shows that the relationship between velocity and temperature is negative. It can be predicted that when there is no air movement, the temperature in the office space is about 24.2 8C. In addition, every 0.1 m/s increase in velocity of supply air will lead to a drop in temperature of about 0.2 8C. The significance of the relationship is also supported by a moderately high correlation coefficient of -0.66. In the case of velocity and relative humidity, the linear regression depicts a positive gradient, which implies that an increase in velocity by 0.1 m/s will give rise to an increase in humidity reading by approximately 0.4%. The correlationcoefficient 21 is found to be 0.61, which suggests that the relationship is significant. However, the value of R 2 is found to be rather low, indicating that the regression line did not fit the data very well. 6.2.2. Velocity and carbon dioxide Table 4 summarises the correlation coefficients for the respective variables at the five sampling locations.Based on the results shown, it seems that there is an inconsistency among the correlation coefficients between the variables carbon dioxide and air velocity. One possible explanation is that, the concentrations of carbon dioxide at the selected locations within the vertical plane are not affected by the airflow pattern. However, it may also be due to the variation of occupants in the various locations. No significant relationship is observed between velocity and carbon dioxide. 6.2.3. Velocity and particulate matter It is seen from Table 4 that there is an inverse relationship between the air velocity and particulate matter. However, the opposite happens in Location 1. Even though similar trend between the two variables has occurred in four out of the five sampling locations, the strength of correlation among those four varies. As a result, it is difficult to predict whether the supply airflow pattern affects the distribution of dust particles in an UFAC system. It is again seen that no significant relationship is observed between velocity and particulate matter. 6.2.4. Velocity and PMV/PPD It is seen that a negative correlation exists between PMV and velocity. Four out of five sampling locations show that the correlation coefficients are ranged between -0.8 and -1.0. It is also evident that the relationship between the two variables is very strong, which is attributed to the fact that the PMV equation is a function of the air velocity. As a result, the stronger the velocity of the cold air, the larger the number of people that will feel uncomfortably cold and hence dissatisfied； and this positive relationship is found to match the correlation study between velocity and PPD as shown in Table 4. It is seen that the relationship between the velocity and PPD is positive. The correlation coefficient is also found to be 0.73. This explains that, the percentage of people feeling dissatisfied increases every time by approximately 3.7% whenever the supply air velocity increases by 0.1 m/s. 22 7 Conclusion A study is carried out on the UFAC system with an aim of examining its impact on the indoor office environment in relation to IAQ and thermal comfort. Experiments were conducted in an office to measure the temperature, relative humidity, air velocity, carbon dioxide and dust particulate at the selected grid points within an imaginary plane for a total of five sampling locations. Based on the empirical data, the PMV and PPD are computed. Besides this, simple linear regression and correlation analyses were also attempted to derive the relationship between air velocity and the rest of the variables as mentioned in the earlier paragraphs. It is found that strong correlation exists between air velocity and the other variables such as temperature, relative humidity, PMVand PPD. In addition, temperature and relative humidity are also highly associated. However, dust concentration and air velocity are only moderately correlated. Nevertheless, it seems that there is a phenomenal behaviour of distribution of particle counts. Higher concentration of dust is found when measurements are taken at higher room space in locations that are prone to human traffic whereas the opposite occurs in unoccupied rooms. For the former, the phenomenon may be ascribed to the agitation of dust caused by occupants movement in conjunction with the strong draft from the floor outlet. Apart from this, the correlation results for the five locations between carbon dioxide and air velocity are found to be inconsistent and thus cannot be concluded that they are interdependent. In other words, air velocity cannot be used to predict the behaviour of carbon dioxide. In relation to occupants comfort, thermal stratification study shows that no large thermal gradient exists in the room space except at the supply diffuser. The results of PMV and PPD also reveal that cold discomfort is always felt at grid points that are in close proximity to the supply. Therefore, unless occupants are seated away from the supply, localised thermal discomfort is unavoidable. It is also observed from the study that the airflow pattern follows a parabolic path. However, the disadvantage of such airflow pattern is the presence of stagnant zone beneath the parabolic path, which usually exists in the region where occupants are seated. In conclusion, the IAQ associated with the UFAC system is found to be within reasonable limits as recommended by the local guidelines as well as the international standards. How