Application of displacement ventilation system to buildings in the tropics

... simplifies the computation of the supply air flow rate that is needed in the design of the displacement ventilation system for buildings with ceiling heights of about 2.6m in the tropics vii List of. .. 2.1 Introduction Since displacement ventilation was first applied to the welding industry in 1978, it has been gaining popularity in Scandinavia as a means of ventilation to provide good indoor... influence on the quality of the inhaled air Conversely, the rising stream had a negative influence on the quality of the inhaled air when the contaminants were generated in the lower part of the room

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z Ms Leow Hwee Ching, Ms Lai Yinghui, Mr Lee Chong Leong and Ms Ng

Yi Yu for their assistance in the data process and analysis

z All the lecturers who have shared their invaluable knowledge and experience

in the Building Science program

Finally, thanks to my friends whom I have been working with throughout the period of study in the National University of Singapore

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TABLE OF CONTENTS

SUMMARY vi

List of Tables viii

List of Figures ix

CHAPTER 1 INTRODUCTION 1

1.1 Background 1

1.2 Objectives and scope of research 3

1.3 Outline of dissertation 4

CHAPTER 2 LITERATURE REVIEW 5

2.1 Introduction 5

2.2 Gradients in occupied space 6

2.2.1 Temperature gradient 6

2.2.2 Concentration gradient 7

2.2.3 Humidity gradient 9

2.3 Thermal comfort 10

2.3.1 Standards on thermal comfort 11

2.3.3.1 Indices (ISO 7730, 1994) 11

2.3.3.1.1 Predicted mean vote (PMV) 11

2.3.3.1.2 Predicted percentage of dissatisfied (PPD) 11

2.3.3.1.3 Draft rating (DR) 12

2.3.3.2 Criteria in various standards 12

2.3.2 Findings of previous research 13

2.3.2.1 Thermal comfort studies on displacement ventilation system 13

2.3.2.2 Tropical area thermal comfort studies 14

2.4 Indoor air quality 16

2.4.1 Concentration distribution 16

2.4.2 Age of air 17

2.4.3 Ventilation effectiveness 18

2.5 Energy 19

2.6 Conclusions and hypotheses 21

2.6.1 Conclusions 21

2.6.2 Hypotheses 22

CHAPTER 3 PRELIMINARY STUDY 23

3.1 Methodology 23

3.1.1 Research design 23

3.1.1.1 Group 1 24

3.1.1.2 Group 2 25

3.1.1.3 Group 3 26

3.1.2 Methods of data collection 27

3.1.2.1 Subjective assessment 27

3.1.2.1.1 Subjects 27

3.1.2.1.2 Subjective assessment protocol 28

3.1.2.1.3 Questionnaire 29

3.1.2.2 Objective measurement protocol 29

3.1.2.2.1 Objective parameters 29

3.1.2.2.2 Instrumentation 30

3.1.2.2.2.1 Thermal chamber 30

3.1.2.2.2.2 Instruments 32

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3.1.2.2.3 Measuring locations 34

3.1.2.2.4 Measurement procedure 35

3.1.3 Data collection and processing 36

3.1.3.1 Objective data 36

3.1.3.2 Subjective data 37

3.1.4 Method of data analysis 37

3.2 Results and discussion 39

3.2.1 Gradients 39

3.2.1.1 Temperature gradient 39

3.2.1.1.1 General description 39

3.2.1.1.2 Effect of different supply air temperature and flow rate with the same room air temperature (Group 1) 40

3.2.1.1.3 Effect of different supply air temperature and flow rate with different room air temperature (Group 2) 41

3.2.1.2 Humidity gradient 42

3.2.1.2.1 Effect of different supply air temperature and flow rate with the same room air temperature (Group 1) 42

3.2.1.2.2 Effect of different supply air temperature and flow rate with different room air temperature (Group 2) 43

3.2.1.3 Concentration gradient of carbon dioxide 44

3.2.1.3.1 Effect of flow rate with the same room air temperature (Group 1) 44

3.2.1.3.2 Effect of flow rate with different room air temperature (Group 2) 45

3.2.2 Thermal comfort 45

3.2.2.1 Effect of different supply air temperature & relative humidity on thermal comfort (Group 1) 45

3.2.2.1.1 Overall Actual Mean Vote (AMV) and comfort acceptability 45

3.2.2.1.2 Effect of different supply air temperature on thermal comfort 47

3.2.2.1.3 Effect of distance from supply unit on thermal comfort

48

3.2.2.1.4 Effect of vertical temperature difference on local thermal sensation 48

3.2.2.2 Effect of different room air temperature on thermal comfort (Group 2) 49

3.2.2.2.1 Overall AMV and comfort acceptability 49

3.2.2.2.2 Effect of vertical temperature gradient on local thermal comfort 51

51

3.2.2.3 Comparison of DV with MV (Group 3) 52

3.2.2.3.1 AMV and comfort acceptability 52

3.2.2.3.2 Neutral temperature 54

3.2.2.3.3 Local thermal comfort 55

3.2.2.4 Application of ISO 7730 55

CHAPTER 4 CONFIRMATION STUDY 57

4.1 Methodology 57

4.1.1 Research design 57

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4.1.1.1 Group 1 57

4.1.1.2 Group 2 58

4.1.1.3 Group 3 59

4.1.2 Methods of data collection 59

4.1.2.1 Subjective assessment 59

4.1.2.1.1 Subjects 59

4.1.2.1.2 Subjective assessment protocol 60

4.1.2.1.3 Questionnaire 60

4.1.2.2 Objective measurement protocol 60

4.1.2.2.1 Objective parameters 60

4.1.2.2.2 Instrumentation 61

4.1.2.2.2.1 Thermal chamber 61

4.1.2.2.2.2 Instruments 62

4.1.2.2.3 Measuring locations 62

4.1.3 Data collection and analysis 64

4.1.3.1 Objective data 64

4.1.3.2 Subjective data 64

4.1.4 Method of data analysis 64

4.2 Results and discussion 65

4.2.1 Gradients 65

4.2.1.1 Temperature gradient 65

4.2.1.1.1 Effect of different supply air temperature and flow rate with the same room air temperature and humidity (Group 1) 65

4.2.1.1.2 Comparison between DV and MV (Group 3) 66

4.2.1.2 Humidity gradient 67

4.2.1.2.1 Effect of different supply humidity ratio with the same room air temperature (Group 2) 67

4.2.1.3 Concentration gradient 70

4.2.1.3.1 Effect of different supply air temperature and flow rate with the same room air temperature and humidity (Group 1) 70

4.2.2 Thermal comfort 72

4.2.2.1 Effect of different supply air temperature & flow rate on thermal comfort (Group 1) 72

4.2.2.1.3 Draft 75

77

4.2.2.2 Effect of different humidity levels on thermal comfort (Group 2) 78

4.2.2.3 Comparison of DV with MV (Group 3) 80

4.2.2.3.1 Overall AMV and comfort acceptability 80

4.2.2.3.3 Draft risk 82

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4.2.3 Energy 85

4.2.3.1 Comparison between DV and MV 85

4.2.3.1.1 Experiment conditions, system and conditioning process 85

4.2.3.1.2 Energy consumption analysis 86

4.2.3.2 Effect of different supply air temperature on energy consumption 88

4.2.3.2.1 Experimental conditions, system and conditioning process 88

4.2.3.2.2 Energy consumption analysis 88

4.2.4 Preliminary design guide 89

4.2.4.1 Model 89

4.2.4.2 Application of the model 91

4.2.4.3 Design procedure: 92

CHAPTER 5 CONCLUSIONS 93

5.1 Review and achievement of research objective 93

5.2 Recommendation 98

BIBLIOGRAPHY 101

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SUMMARY

Conventional mixing ventilation (MV) system is used widely in the hot and humid country like Singapore The nature of this system is to create a good mixture of the room and supply air in order to provide a uniform thermal and air quality environment Hence every occupant in the same space will be exposed to similar level of pollutants even though one is far away from the source of pollutant However, if occupants are exposed to harmful and/or excessively high concentration pollutants, they may fall ill and this will lead to a decrease in productivity

Displacement ventilation (DV) system can resolve this problem in a more efficient manner However, research on displacement ventilation has been mainly conducted in the Scandinavian countries To date there is limited research done in the tropics Furthermore, as the climate and building loads in this area are different from those in the Scandinavian countries, the results of such research may not be applicable

energy-in Senergy-ingapore It is therefore of great importance to conduct research to assess its viability in the Tropics The results and findings would be valuable to local practitioners when adopting the DV system in Singapore The results and findings would be valuable to researchers who are interested to carry out this area of study in the tropics

It is found that temperature gradient, humidity gradient and CO2 concentration gradient exist in all experiments of this study The profile of the gradient depends on the supply air flow rate and/or outdoor air flow rate Results show that supply air temperature, room air temperature and relative humidity have significant influence on subjects’ thermal sensation, but not on subjects’ acceptability ratings

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Generally subjects have cooler thermal sensation and lower acceptability with DV system than with MV system Occupants in an environment served by DV system have

1 ºC higher neutral temperature as compared to the MV system With proper design,

DV system may have lower draft risk as compared to the MV system

CO2 concentration gradient can be found in space served by DV system The concentration of CO2 at the height of 1.1m and below is always lower than in a space served by the MV system, under the same condition Hence, the ventilation efficiency

is always higher for DV system than for MV system

When conditions for both systems are the same, the cooling capacity for DV system is 5% lower than that for MV system In addition, with the presence of temperature and concentration gradients, the energy consumption of the DV system could be further reduced

A model, which computes the supply-return air temperature difference is developed based on the experimental results This model simplifies the computation of the supply air flow rate that is needed in the design of the displacement ventilation system for buildings with ceiling heights of about 2.6m in the tropics

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List of Tables

Table 2.1 The criteria stated in different standards 12

Table 3.1 Test conditions for Group 1 25

Table 3.4 Instrumentations 32

Table 3.5 Average clo value, overall AMV and average comfort acceptability 46

(Group 1 DV) 46

Table 3.6 Mean thermal sensation and comfort acceptability at the workstation closest to the supply unit (Group 1 DV) 47

Table 3.7 Mean thermal sensation and comfort acceptability between workstations nearest and furthest from the supply unit (Group 1 DV) 48

Table 3.8 Average clo value, overall AMV and average comfort acceptability (Group 2 DV) 50

Table 3.9 Mean thermal sensation and comfort acceptability between workstations nearest and furthest from the supply unit (Group 2 DV) 51

Table 3.10 Average clo value and AMV (Group 3) 53

Table 4.4 Average clo value, overall AMV and average comfort acceptability (Group 1) 73

Table 4.5 Mean thermal sensation and comfort acceptability between workstations closer and further from the supply unit (Group 1 DV) 77

Table 4.8 Experiment conditions for Group 3 85

Table 4.9 Experiment conditions 88

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List of Figures

Figure 2.1 Sketch of displacement ventilation 6

Figure 2.2 Temperature gradients in a thermal chamber with different cooling loads 7

Figure 2.3 The CO2 concentration gradients at various locations in a room 8

Figure 2.4 Relative humidity gradient and moisture ratio gradient with displacement ventilation 9

Figure 3.1 Total approach employed in this research study 24

Figure 3.2 Continuous scale used in the questionnaire 29

Figure 3.3 Chamber layout 31

Figure 3.4 Floor-standing, low velocity, semi-circular supply unit 31

Figure 3.5 Return grille 32

Figure 3.6 Supply diffuser 32

Figure 3.7 Portable RH sensor 33

Figure 3.8 Dew-point hygrometer 33

Figure 3.9 Hood 33

Figure 3.10 Photoacoustic spectrometer multi-gas analyser 33

Figure 3.12 Plan view of measuring points 35

Figure 3.13 Typical temperature gradient (1) 40

Figure 3.14 Typical temperature gradient (2) 40

Figure 3.15 Temperature gradients (Group 1) 41

Figure 3.17 Humidity gradients (Group 1) 42

Figure 3.19 Carbon dioxide gradients (Group 1) 44

Figure 3.20 Carbon dioxide gradients (Group 2) 45

Figure 3.21 Body parts’ thermal sensation (Group 1 DV) 49

Figure 3.23 Thermal comfort unacceptability (Group 3) 54

Figure 3.24 Neutral temperatures for DV and MV system (Group 3) 54

Figure 3.25 Body parts’ thermal sensation for DV and MV cases (Group 3) 55

Figure 4.1 Chamber layout 61

Figure 4.2 Plan view of measuring points 63

Figure 4.4 Temperature profiles (Group 3) 67

Figure 4.7 CO2 gradients (Group 1) 70

Figure 4.8 CO2 gradients (Group 3) 71

Figure 4.10 Draft at different heights (Group 1) 76

Figure 4.12 Body Parts’ Thermal Sensation (Group 3) 81

Figure 4.13 Draft at different heights (Group 3) 83

Figure 4.14 System for both ventilation modes 85

Figure 4.15 Conditioning process on psychrometric chart 87

Figure 4.16 Relationship between measured temperature (symbols) and predicted temperature (lines) 91

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There is another type of system, based on displacement ventilation (DV) strategy, to provide better air quality in a more energy-efficient way This system was first applied

to the welding industry in the Scandinavian countries in 1978 Since then it has been increasingly used Recently, this system has become popular not only in industrial facilities, but also in offices and other commercial spaces In 1989 in the Nordic countries, it was estimated that displacement ventilation accounted for a 50% market

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C A T R 1 N R D C I N

share in industrial application and 25% in office application This system can provide better air quality as it supplies cool air with low velocity near the floor level and exhausts at the ceiling level The air is then transported within the room by the rising convection flows from the heat sources, e.g human, PCs etc, which take the warm contaminated air from the lower parts of the room to the upper parts In this way, the person who has not created any pollutants will not be bothered by the pollutants created by others

In comparison with the conventional mixing ventilation, displacement ventilation can achieve considerably higher ventilation efficiency and is more energy-efficient However, research on displacement ventilation has been mainly conducted in the Scandinavian countries There is limited research done in the tropics As the ethnic groups and building loads in the tropics are not the same as those in Scandinavian countries, the results of such research may not be directly applicable

Moreover, in the tropics, due to the all-year-round hot and humid climate, there is high recirculation of air for most of the Air conditioning Mechanical Ventilation (ACMV) systems in order to save energy The ratio of the return air to the total supply air could range between 70% and 90%, depending on the types of building DV system has to comply with the energy-saving rule, i.e 70% to 90% of the exhaust air needs to be recirculated, if it is to be used in Singapore However, 100% outside air can be used in the Scandinavian countries for DV system due to their climatic condition Hence, the results of the research conducted in Scandinavian countries may not be applicable in Singapore

There is a growing trend of new buildings exploring the possibility of adopting new air-conditioning technology in Singapore The acceptance of new system can be

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difficult in the construction industry since the suitability of such systems is not extensively investigated in the tropics This can be detrimental during the operation of building if it cannot perform or under-perform Therefore, there is a need to assess the feasibility and viability of displacement ventilation system in the tropics It is therefore

of great importance to conduct research to assess its applicability in the Tropics

The results and findings of this study will help local practitioners to create a comfortable environment with suitable room air temperature, supply air temperature, relative humidity and ∆T (temperature difference between head and feet level) for the occupants The findings of this study will serve as preliminary directions for future research on DV system in the tropics This will in turn help local practitioners and ACMV system operators in creating and/or maintaining a comfortable indoor environment economically

1.2 Objectives and scope of research

The main objectives of the research study are:

a To investigate the stratification effect of the wall supply displacement ventilation system;

b To investigate the thermal comfort and energy performances for the wall supply displacement ventilation system;

c To compare performance of the wall supply displacement ventilation system against the conventional ceiling supply mixing ventilation system based on thermal comfort, indoor air quality and energy; and

d To develop a preliminary design guide that can be used for offices in the tropics

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1.3 Outline of dissertation

This chapter states the background, the objectives and scope of work of this study It is intended to point out that, though indoor air quality issues have a large scope of interest, this study would only focus on the basic aspects of indoor air quality, i.e concentration stratification and ventilation efficiency

Chapter two introduces the fundamentals of displacement ventilation This is substantiated by past research findings It is arranged in the following order: temperature, pollutant’s concentration and humidity gradients; thermal comfort fundamentals and research findings; and indoor air quality basics and research findings

Chapter three presents the methodology and results of the preliminary study The methodology includes research design, detailed experimental conditions description, methods of data collection and methods of data analysis The results are presented in two categories, namely gradients and thermal comfort

Chapter four presents the methodology and results of the second study with a larger sample size This study is to confirm the results and findings of the preliminary study Therefore, the basic methodology adopted is the same as the preliminary study

Chapter five summaries the concluding remarks following the data analysis and discussion A list of recommendations is ascertained from this study

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A typical displacement ventilation system provides cool air at a temperature several degrees below room air temperature and at a very low velocity of less than 0.5 m/s through large-area supply devices near the floor level and extracts air at the ceiling level The supply air spreads over the floor and then rises as it comes into contact with heat sources, e.g persons, computers, in the occupied space The rising air above the heat source is called a plume Plumes carry heat and contaminants and entrain the ambient air to the upper part of the room space Thus the airflow rate of plumes increases with height The flow rate in the convection flow equals the supply air flow rate at a certain height above the heat source In order to feed the convection flow above that height, the air in the upper part of the room is naturally recirculated In this way the air will be stratified with a lower zone of fresh cool air and an upper zone of mixed and contaminated warm air A schematic flow pattern is shown in Figure 2.1

(Yuan et al, 1998)

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C A T R 2 I E A U E R V E

This chapter will present the characteristics and mechanism of displacement ventilation in detail, and some findings of previous research studies They are categorized into: 1) gradients; 2) thermal comfort; 3) indoor air quality; and 4) energy

Figure 2.1 Sketch of displacement ventilation

(Source: Yuan et al, 1998)

2.2 Gradients in occupied space

2.2.1 Temperature gradient

As mentioned earlier, the principle of displacement ventilation is a supply of cool air at low velocity near the floor level and an exhaust at the ceiling level The air is transported within the room by the rising convection flow from the heat sources, which take the heated air from the lower parts of the room into the upper parts At a certain height, the air stratifies, thus forming two parts: one with warm and less dense air in the upper space and the other with cool and denser air in the lower space

Figure 2.2 shows an example of the vertical temperature profile in a thermal chamber

with different cooling loads (Xu et al, 2001) It is observed that the temperature profile

could be separated into two regions: (1) steep temperature gradient (floor level to 1.0

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-C A T R 2 I E A U E R V E

1.2m high), and (2) gentle temperature gradient (1.0-1.2m high to ceiling level) when the indoor heat load exists

These findings were consistent with those of the other researchers too For example,

Yuan et al (1999) conducted both measurements and computational fluid dynamics

(CFD) modelling and found that the temperature gradient at the lower part would be larger than at the upper part when most of the heat sources were in the lower part of

the room Murakami et al (1998) analyzed both flow and temperature fields around a

modelled standing human body using CFD program and found that the gradient became very steep between the feet and waist level

Figure 2.2 Temperature gradients in a thermal chamber with different cooling loads

(Source: Xu et al, 2001)

2.2.2 Concentration gradient

When the contaminant source is combined with the heat source (this is the usual case, for example, human being generates not only heat, but CO2 and bioeffluent), the plume will carry the contaminants over the heat source to the upper zone of the room The result is that the air in the upper zone will be polluted while the air in the lower zone is

as clean as the supply air It is necessary to note that the important characteristic of displacement ventilation system is the temperature stratification It suppresses the

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vertical mixing of air and therefore, vertical concentration can be maintained It should also be noticed that, normally, the concentration in the upper part of the room is larger than the average concentration, i.e it is larger than the concentration that occurs in a well mixed room

Figure 2.3 The CO2 concentration gradients at various locations in a room (a) concentration gradients (upper); (b) plan view of measuring locations (lower)

(Source: Xu et al, 2001)

An example of the concentration gradient is shown in Figure 2.3 (Xu et al, 2001)

where the non-dimensional concentration C* is plotted against the room height The C* is defined as C* = (Cp-Cs)/(Ce-Cs) where Cp is the CO2 concentration at point p

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exhaust air It shows that the steep concentration gradient occurs between 0.6~1.2m and below 0.6m, the concentration is almost the same and much lower than that at the upper zone (above 1.2m)

The results of Xu’s work were consistent with other studies, such as Yuan et al (1999a) and Murakami et al (1998) where CFD modelling was used, and Yuan et al (1999c)

where measurements and CFD modelling were both employed In all these studies, it was found that the CO2 concentration in the lower zone was lower than that in the upper zone

2.2.3 Humidity gradient

Plumes not only carry heat and contaminants, they also carry moisture It is a common perception that relative humidity is constant throughout the whole space with conventional mixing ventilation Applying this assumption to a space with displacement ventilation where air stratifies, having higher temperature and contaminant concentration in the upper part of the room, one may conclude that humidity also stratifies

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Figure 2.4 shows measurement data from field studies conducted in 1988 in a

food-processing facility located in Finland (Kosonen et al, 2001) The measurement data is

presented for full load and half load It was observed that both humidity ratio gradient and relative humidity gradient existed

2.3 Thermal comfort

Thermal comfort has been defined as "the condition of mind that expresses satisfaction with the thermal environment" (ISO 7730, 1994) The reference to "mind" emphasized that comfort is a psychological phenomenon It is therefore often "measured" using subjective methods—survey into man’s thermal sensation votes Man’s thermal sensation is mainly related to the thermal balance of their body as a whole This balance is influenced by his physical activity and clothing, as well as the environmental parameters: air temperature, mean radiant temperature, air velocity and air humidity Moreover, man’s thermal sensation can also be influenced by factors such as age, sex, body build, etc (Fanger, 1970) Over dozens of years, research on man’s thermal comfort has been carried out throughout the world, but mainly in mixing ventilation system Recently, studies in displacement ventilation system have been gaining popularity and large amounts of beneficial results have thus been obtained This section presents the criteria stipulated in thermal comfort standards Some findings from past research studies will also be presented in the latter portion of this chapter

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2.3.1 Standards on thermal comfort

2.3.3.1 Indices (ISO 7730, 1994)

2.3.3.1.1 Predicted mean vote (PMV)

The PMV is an index that predicts the mean value of the votes of a large group of persons on the following 7-point thermal sensation scale:

2.3.3.1.2 Predicted percentage of dissatisfied (PPD)

The PMV index predicts the mean value of the thermal votes of a large group of people exposed to the same environment However individual votes are scattered around this mean value and it is useful to predict the number of people likely to feel uncomfortably warm or cool

The PPD index establishes a quantitative prediction of the number of thermally dissatisfied people The PPD predicts the percentage of a large group of people likely

to feel too warm or cool, i.e voting hot (+3), warm (+2), cool (-2), or cold (-3), on the 7-point thermal sensation scale

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The PPD-index predicts the number of thermally dissatisfied person among a large group of people The rest of the group will feel thermally neutral, slightly warm, or slightly cool

2.3.3.1.3 Draft rating (DR)

Draft is an unwanted local cooling of the body caused by air movement The draft rating may be expressed as the percentage of people predicted to be bothered by draft The model of draft applies to people at light activity (mainly sedentary activity), with a thermal sensation for the whole body close to neutral The draft rating is also called the percentage of dissatisfied due to draft, or PD

2.3.3.2 Criteria in various standards

Table 2.1 The criteria stated in different standards

(Singapore)

5%

For comparison, Table 2.1 shows the criteria stated in different standards and

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compared to ISO 7730 in terms of Dissatisfied due to vertical temperature difference

and radiant temperature asymmetry, Dissatisfied due to floor temperature, operative

temperature (To), relative humidity (RH) and temperature difference between floor

level and breathing level (△T) It is seen that the ASHRAE Standard 55-1992 and ISO

7730 are much more detailed than the local standard (CP13, 1999)

2.3.2 Findings of previous research

2.3.2.1 Thermal comfort studies on displacement ventilation system

Melikov and Nielsen (1989) evaluated the thermal comfort conditions in 18 spaces

ventilated by the displacement ventilation principle developed in Scandinavia It was found that PD>15% was identified for 33% of the measured locations within the occupied zone and △t1.1-0.1>3 ºC for 40% of the locations For 18% of the measured locations within the occupied zone, PD>15% and △t1.1-0.1>3 ºC was registered However, the risk of discomfort due to draft and vertical temperature difference was low in some of the investigated rooms Hence, they concluded that when displacement ventilation system is well designed, it is feasible to create good thermal comfort in rooms

Yuan et al (1999a) evaluated the performance of traditional displacement ventilation

systems for small offices, large offices with partitions, classrooms, and industrial workshops under U.S thermal and flow boundary conditions using CFD program It was found that generally, the air velocity was less than 0.2m/s, the temperature difference between the head and foot level of a sedentary occupant was less than 2 ºC, and draft rating (PD), predicted percentage of dissatisfied (PPD) were less than 15% in the occupied zone, if the design used the guidelines shown in their paper The PD and

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‘neutral’ and a range 20.9°C < T < 28.0°C based on 80% acceptability criterion was proposed

Akimoto et al (1999) evaluated the performance of a floor-supply displacement

air-conditioning system in comparison to a DV system with a sidewall-mounted supply unit and a ceiling-based distribution system Thermal stratification was observed, as there was a greater vertical air temperature difference in both of the displacement system than in the ceiling-based system A large vertical temperature difference that may cause local thermal discomfort was observed in several cases for both of the displacement systems It was observed that the measured skin temperatures of the thermal manikin with both of the displacement systems were slightly lower than those

of the ceiling-based system However, this is not considered too low to cause local thermal discomfort

2.3.2.2 Tropical area thermal comfort studies

De Dear et al (1991) performed thermal comfort field experiments in Singapore

Results of the air-conditioned sample indicated that office buildings were overcooled, and one-third of their occupants experienced cool thermal comfort sensation The

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observation in these air-conditioned buildings was broadly consistent with the ISO, ASHRAE and local standards PMV model’s predicted neutralities were all slightly warmer than the empirically observed neutralities by approximately 1K

Busch (1992) conducted a field study in Thailand to explore whether there was justification for adopting a comfort standard that differs from those developed for office workers accustomed to temperate climates The neutral temperature was found

to be 24.7°C EHT for air-conditioned buildings The author determined the temperature limits of comfort zone for air-conditioned buildings – lower limit of the comfort zone was about 22°C and the upper limit reached about 28°C These limits were broader than that stipulated by the standards

Tan (1995) carried out field and chamber study to determine whether Singaporeans’ perception of thermal comfort differ from existing literature The neutral temperature was found to be 24.7°C which is slightly lower than 25.6°C that was found by Fanger The author also derived PMV-PPD characterization of Singaporeans and it was found

to be similar to Fanger’s PMV-PPD curve

Cheong et al (2003) performed a thermal comfort study of an air-conditioned lecture

theatre in Singapore using CFD, objective and subjective measurements It was found that thermal conditions were within limits of thermal comfort standards but the subjective responses were slightly biased towards the ‘cold’ section of the 7-point thermal sensation scale and the occupants were slightly uncomfortable at a 23°C environment The calculated PMV and PPD were close to the subjective result

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2.4 Indoor air quality

The concern about energy efficiency has increased since the 1970’s oil crisis This growing concern has led to many changes in the way buildings are constructed and operated The most conspicuous ones are the reduction in ventilation rate and increase

in air-tightness of buildings In the age of rapid technology development, more and more synthetic building materials are used to create a comfortable indoor environment These materials emit pollutants, such as formaldehyde, volatile organic compounds (VOCs) etc The reduction of the ventilation rate, coupled with these pollutants, can accumulate gradually and finally reach a level at which they can have adverse effects

on the occupants’ health

2.4.1 Concentration distribution

Concentration gradient does exist with displacement ventilation system as described in

Section 2.2.2 Concentration gradient In the studies by Yuan et al (1999a), where CFD

modelling was used, and Yuan et al (1999c), where measurements and CFD modelling

were both employed, it was found that the CO2 concentration in the lower zone was less than that in the upper zone It was found that as the convective flow around a human body brings the air at a lower zone to the breathing zone, the occupant actually breathes air with lower concentration of contaminant than those at the nose level in the

middle of the room

In Murakami’s study (1998) where CFD program was used, three cases of concentration distribution prediction were carried out at different locations of contaminant generation It was found that the rising stream around the body surface was not broken by the surrounding airflow The air quality at the breathing zone

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depended on the location of the contaminant generation When contaminants were generated in the upper part of the room above the breathing height, and the air in the lower part of the room was relatively clean, the rising stream of air had a positive influence on the quality of the inhaled air Conversely, the rising stream had a negative influence on the quality of the inhaled air when the contaminants were generated in the lower part of the room below the breathing height, and the air in the lower part of the room was relatively dirty

2.4.2 Age of air

The age of air is defined as the time for all air molecules to travel from the air supply device to a point in the space It can be derived from the measured transient history of the tracer gas concentration

Several field measurements with displacement ventilation system (Yuan et al, 1999; Murakami et al, 1998; Xing et al, 2001; Awbi, 1998; Seppanen et al, 1989) have been

performed using the age of air concept It was found that the mean age of air in the lower part of the room was much younger than in the upper part of the room Furthermore, Awbi (1998) reported from the measurement data that the age of air at the breathing zone is about 40% lower than the mean value of the occupied zone with displacement ventilation

Similar result was also found by Xing et al (2001) in their studies in which three types

of supply units were used in a series of tests: flat-faced wall unit, semi-circular wall unit, and floor swirl unit in a displacement ventilated environmental chamber It was found that the local mean age of air at the breathing zone of a seated mannequin was 35% and 50% lower than that in the occupied zone While for the standing mannequin

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s e

C C

ε

While in Yuan’s study (1999a) ventilation effectiveness was defined as:

s e

s h

C C

−

=

In the above formulas, C is the contaminant concentration, subscripts e, s, p and h refer

to point in exhaust, point in supply, point in a room and point at the head level of a sedentary person respectively

Xing et al (2001) carried out measurements with the presence of a heated mannequin

and other heat sources and found that the ventilation effectiveness at the breathing zone for both the seated and standing mannequins were greater than for a point at the same height in the chamber for the tests with all DV units, because the mannequin entrained fresh air from the fresh air layer on the floor into the breathing zone

Yuan et al (1999) studied 56 cases using CFD simulation program and found that the

ventilation effectiveness of these cases varied between 1.2 and 2 Since the ventilation

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T T

−

=

ε , T is temperature, (°C), the subscripts

i and o refer to inlet and outlet respectively and (¯) represents the mean value for the occupied zone) with displacement ventilation was almost twice the value with mixing ventilation

2.5 Energy

Annual energy consumption over a life-cycle is an important criterion for the evaluation of a ventilation system Almost all the energy analyses in the literature were done by numerical simulation because it is too expensive and time consuming to conduct hour-by-hour measurements for a building based on a yearly basis

Seppanen et al (1989) evaluated the energy performance of displacement and mixing

ventilation systems in a high-rise office building in the United States The study analyzed the north, south, and core zones of the buildings in four representative U.S climates and found that the energy consumed by displacement ventilation systems with heat recovery and variable-air-volume (VAV) flow control were similar to the energy consumption of conventional air distribution systems operated with recirculation

Chen et al (1990) used the cooling load program ACCURACY and energy analysis

program ENERK to calculate the space load and the annual energy consumption of a room based on the weather data of the Dutch short reference year It was found that in

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the variable-air-volume system with the same lowest supply air temperature (16 ºC), the DV system would save 26% energy on the chiller and the ventilator and 3% on the boiler, compared with the MV system The cost of annual energy consumption for the displacement ventilation system was 16% smaller than that for the well-mixed system However, if the lowest supply air temperature for the DV system remained at 16 ºC and that for the MV system was controlled at 12.5 ºC, the cost of annual energy consumption was nearly the same

Hensen et al (1995) carried out simulations using a computer model of a typical office

module located in The Netherlands (temperature sea climate) and found that applying displacement ventilation in a case of relative low casual gains (30 W/m2) resulted in energy savings of up to 14% for cooling during the summer months During the rest of the year hardly any saving was to be expected The overall annual energy consumption for cooling could be up to 10% lower At causal gains above about 35 W/m2 the energy consumption for cooling would be considerably higher than in the case of mixing system only

Hu et al (1999) used a detailed computer simulation method to study the energy

consumption of displacement and mixing ventilation systems for an individual office, a classroom, and a workshop for five U.S climatic regions The study showed that when free cooling was used for both ventilation systems in the shoulder seasons, when the supply air temperature for displacement ventilation system was 20 ºC while that for the mixing ventilation system was 12.8 ºC, the displacement ventilation system might use more fan energy and less chiller and boiler energy than the mixing ventilation system The total energy used was slightly less with displacement ventilation

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Livchak (2001) used displacement ventilation system selection software to compare the energy consumption for a computer room It was found that if 100% outside air was used during the free cooling season, displacement ventilation system with the supply air temperature of 18 ºC allowed a reduction of the chiller cooling capacity by 1.4 kW and its annual energy consumption by 33%, as compared to mixing ventilation system

2.6 Conclusions and hypotheses

2.6.1 Conclusions

Through the literature review, the following conclusions were drawn:

i Displacement ventilation can create a thermally comfortable environment that has low air velocity, a small temperature difference between the head and foot level, and a low percentage of dissatisfied people if the system is well designed;

ii Displacement ventilation can provide better indoor air quality with lower pollutant concentration and higher ventilation effectiveness in the occupied zone, if the system is well designed However, the improvement of IAQ could remain small at large recirculation ratios

iii Displacement ventilation can use less energy while providing thermally comfortable environment and better indoor air quality, if the system is well designed and controlled

iv Though there are large numbers of research studies done on displacement ventilation, research in the tropics is rather limited It is therefore of great importance to conduct research to assess its viability in the tropics

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2.6.2 Hypotheses

The following hypotheses can be drawn after the literature review

i Displacement ventilation system is applicable to the tropics, although there are geographical, climatic, and racial differences

ii Displacement ventilation can create a thermally comfortable environment and provide better indoor air quality as compared to mixing ventilation in the tropics iii Displacement ventilation can use less energy while providing a thermally comfortable and better indoor air quality environment in the tropical conditions

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The approach of the research design is shown in Figure 3.1 In the data collection stage, objective data such as temperature and velocity were measured using instruments in the experiments, and subjective data were collected by means of questionnaire survey

In data analysis stage, data from the objective measurements would be analyzed using normal methods such as normalization and tabulation Data from survey would be analyzed by statistical tools such as T test and ANOVA

To investigate the thermal and indoor air quality performances of displacement ventilation system, a total of 8 displacement ventilation cases and 4 mixing ventilation cases were formulated These cases are assigned to different groups depending on the supply air temperature, room air temperature and relative humidity

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This group of cases was formulated based on the following hypotheses:

i Subjects would have different thermal sensation when they are exposed to different room humidity levels;

ii Subjects would have different thermal sensation when they are exposed to different supply air temperatures; and

Research Hypothesis

Research Design

Data Collection

z Objective measurement

z Dry bulb Temperature

z Relative humidity / Dew point temperature

z Velocity

z Flow rate

z CO2 concentration

z Subjective assessment z Subjects survey

Data Analysis z Normal

Analysis

z Statistical Analysis Conclusions

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C A T R 3 R L M N R T D

iii Subjects would have different thermal sensation when they are seated near or far away from the supply unit

A total of 12 subjects were subjected to the condition in each case In each experiment,

3 subjects were seated in workstations (WS 1-3 in Figure 3.3) with each one having a computer to allow them to do their own work The duration of each experiment was 2 hours

Table 3.1 Test conditions for Group 1

In this group, both the supply air temperature and room air temperature were allowed

to vary while the room relative humidity at 1.3m height remained unchanged The ratio

of outdoor air flow rate to the total supply air flow rate was also kept constant at about 36% Table 3.2 shows the various conditions for Group 2

This group of cases was formulated based on the following hypotheses:

i Subjects would have different thermal sensation when they are exposed to different room air temperatures; and

ii Subjects would have different thermal sensation when they are seated near or far away from the supply unit

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6 subjects were involved with each workstation having a computer to allow them to do their own work The duration of each experiment was 2 hours

This group of cases was formulated based on the following hypothesis:

Subjects would have different thermal sensation with DV and MV systems even though the room air temperature and relative humidity are similar

6 subjects were involved with each workstation having a computer Cases in Group 3 with initial “D” refer to displacement cases while initial “M” denotes mixing cases The duration of each experiment was 2 hours

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Case Ts (°C) Tr (°C) RH (%) D1 16.8 22.2 65.0

A M1 15.7 22.0 65.9

D2 18.5 23.3 67.8

B M2 17.8 22.9 70.4

D3 18.7 24.2 65.5

C M3 17.3 24.0 66.5

D4 20.4 26.2 64.7

D M4 17.9 26.1 64.7

3.1.2 Methods of data collection

3.1.2.1 Subjective assessment

3.1.2.1.1 Subjects

Twelve (five male and seven female) college-age students participated in the experiments as subjects The subjects were recruited based on the following criteria: exposed to local tropical climate for more than 6 months, familiarity with a PC, impartiality to the chamber in which the study was carried out, and absence of chronic diseases, asthma, allergy and hey-fever etc The statistical summary of these subjects is shown in Appendix A1 All subjects were volunteers who were paid for taking part in these experiments Subjects were instructed to eat normally before arrival at the thermal chamber No intakes of alcohol or drugs were allowed 24 hours prior to each experiment During the experiments, subjects were asked to be dressed in typical office

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to avoid biased results They were exposed to the test conditions in groups of three or six as stated in Section 3.1.1

3.1.2.1.2 Subjective assessment protocol

Each experiment proceeded as follows:

i Subjects arrived at the chamber 15 min prior to the commencement of the experiment They were seated in the control room and briefed about the procedure During this period, as they acclimatized to the environment, they started to answer Section 1 of the questionnaire which inquired about their personal particulars, thermal sensation for the whole body and thermal comfort acceptability

ii After the acclimatization period, these subjects entered the chamber and started to answer Section 2 of the questionnaire on their thermal sensation for the whole body and thermal comfort acceptability

iii For every ten minutes thereafter, subjects would complete a questionnaire on their thermal sensation for different parts of the body, thermal comfort acceptability and air movement detection and acceptability

iv At the end of 60 minutes, subjects would complete the last questionnaire inquiring about their thermal sensation for different parts of the body, thermal comfort acceptability, air movement detection and acceptability, and the type of clothing the subjects were wearing

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3.1.2.1.3 Questionnaire

A set of the questionnaire is given in Appendix B1

A simple Yes/No category scale was used to ascertain the air movement sensation If the subjects indicated “Yes”, they would go on to the next question and mark on the related body part(s) where they felt the air movement

Divided continuous scale was used to determine the acceptability for thermal comfort and air movement This scale is divided into two parts with “Just Acceptable” and

“Just Unacceptable” in the middle A sample of the scale is shown in Figure 3.2 This

is to allow the subjects to make a definite choice and grade the degree of acceptability

Figure 3.2 Continuous scale used in the questionnaire

(a) Divided continuous scale (upper); (b) Undivided continuous scale (lower)

3.1.2.2 Objective measurement protocol

Objective measurement started 5~15 minutes prior to the experiments and ended 5~15 minutes after the experiment During each experiment the following parameters were

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measured:

i Temperature: supply air temperature (Ts), return air temperature (Te), room air temperature (Tr), wall surface temperature (Tw), ceiling surface temperature (Tc), floor surface temperature (Tf);

ii Room air relative humidity and dew point temperature;

iii Air velocity at the area near the supply unit;

iv Supply and return air flow rates; and

v Concentration of CO2, TVOC and formaldehyde

3.1.2.2.2 Instrumentation

3.1.2.2.2.1 Thermal chamber

The experiments were carried out in the thermal chamber, 6.6m (L) x 3.7m (W) x 2.6m (H), at the School of Design and Environment, National University of Singapore The Air-Conditioning and Mechanical Ventilation (ACMV) system is capable of controlling the air temperature and airflow rates by adjusting the off coil temperature and fan speed using the computer controller to achieve the required room conditions It can be operated in either mixing (MV) or displacement (DV) modes

The room is illuminated by 6 sets of twin double-battens fluorescent lights The power consumption of each fluorescent tube is 36W There are six workstations inside the chamber with two large fixed glass windows (W×H=1.47×1.17 m) on one side of the wall to simulate a typical office environment Each workstation consists of a table, a chair, and a personal computer (PC) Beside each workstation there is a partition panel, measuring 1.5m (Height), 1.0m (Wide) and 5.5cm (Thick) Figure 3.3 shows the layout

of the chamber

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DV unit

MV diffuser T1 T2

E2

Figure 3.3 Chamber layout

Note: WS denotes workstation number and E denotes extract grille number

In the DV mode, the air is supplied from a floor-standing, low velocity, semi-circular

unit at one end of the chamber and extracted from two ceiling grilles, E1 and E2 In the

MV mode, the air is supplied from two square ceiling diffusers, T1 and T2, and extracted from two ceiling grilles, E2 and E3

The floor-standing, low velocity, semi-circular supply unit, return grille and square ceiling diffuser are shown in Figures 3.4~3.6, respectively

Figure 3.4 Floor-standing, low velocity, semi-circular supply unit

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