Thermal effect of temperature gradient in a field environment chamber served by displacement ventilation system in the tropics

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1 Building and Environment ] (]]]]) ]]] ]]] Thermal effect of temperature gradient in a field environment chamber served by displacement ventilation system in the tropics W.J. Yu a,,k.w.d. Cheong a,k.w. Tham a,s.c. Sekhar a,r. Kosonen b a Department of Building, National University of Singapore, Singapore b Halton Oy, Haltonintie 1-3, Kausala 47400, Finland Received 3 August 2005; received in revised form 25 August 2005; accepted 7 September 2005 Abstract The effect of vertical air temperature gradient on overall and local thermal comfort at different overall thermal sensations and room air temperatures (at 0.6 m height) was investigated in a room served by displacement ventilation system. Sixty tropically acclimatized subjects performed sedentary office work for a period of 3 h during each session of the experiment. Nominal vertical air temperature gradients between 0.1 and 1.1 m heights were 1,3 and 5 K/m while nominal room air temperatures at 0.6 m height were 20,23 and 26 1C. Air velocity in the space near the subjects was kept at below 0.2 m/s. Relative humidity at 0.6 m height was maintained at 50%. It was found that temperature gradient had different influences on thermal comfort at different overall thermal sensations. At overall thermal sensation close to neutral,only when room air temperature was substantially low,such as 20 1C,percentage dissatisfied of overall body increased with the increase of temperature gradient. At overall cold and slightly warm sensations,percentage dissatisfied of overall body was non-significantly affected by temperature gradient. Overall thermal sensation had significant impact on overall thermal comfort. Local thermal comfort of body segment was affected by both overall and local thermal sensations. r 2005 Elsevier Ltd. All rights reserved. Keywords: Temperature gradient; Air temperature; Overall thermal sensation; Displacement ventilation; Tropics 1. Introduction Numerous researches have demonstrated displacement ventilation (DV) system provides better indoor air quality and lower energy consumption than mixing ventilation (MV) system [1 3]. DV system has become increasingly popular over recent years as a greener alternative to other common forms of air conditioning system [4]. However, cold discomfort on the foot,ankle and leg due to draft and vertical air temperature difference is often reported with DV system [5,6]. One of the reasons is due to the vertical air temperature gradient in the space. To avoid the local discomfort,iso 7730 [7] recommends the air temperature difference between 0.1 and 1.1 m levels above the floor be less than 3 K,whereas ASHRAE [8] recommends that temperature difference between 0.1 and 1.7 m levels be less Corresponding author. Tel.: ; fax: address: g @nus.edu.sg (W.J. Yu). than 3 K. This restriction on temperature gradient beyond 3 K/m in current standards limits the heat removal capability of DV system. For typical office accommodation,the application of DV is limited to a room heat gain no greater than 25 W/m 2 [9]. Limited researches on the effect of vertical air temperature gradient have been reported in publication. McNair and Fishman [10] conducted an experiment in the Europe to study the subjective effect of vertical air temperature gradients by exposing 48 sedentary subjects to four gradients of 0,1.3,2.7 and 4 K between head (1.1 m) and ankles (0.1 m) for 1 h. It was found that differences between thermal sensation of the head and feet were slight and hence non-significant for all practical purposes. The 1-h exposure may not be sufficient for subjects to reach steady state. Eriksson [11] investigated the thermal effect of vertical air temperature in tractor cabins. Fifteen sedentary subjects wearing heavy outdoor clothing ( clo) were involved in this study. It was found that positive /$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi: /j.buildenv

2 2 ARTICLE IN PRESS W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] differences,i.e.,higher air temperature at head level than at ankle level would cause discomfort if they exceeded 4 6 K. Negative differences,i.e.,highest air temperature at ankle level,would cause discomfort if they exceeded 6 8 K. However,these experiments were not conducted in a building. In addition,the clothing worn by subjects was 1.5 clo.,which is higher insulation than in an office environment. The influence of radiation is not reported as well. So,the results would not be applicable to building environment. The imposition of a 3 K/m limit on the vertical air temperature gradient arose from research work conducted by Olesen et al. [12]. In the study,16 subjects (8 females and 8 males) were individually exposed to 4 different vertical air temperatures (0.4,2.5,5.0 and 7.5 K) between head and ankle level with highest temperature at head level. The subjects were sedentary and wore light standard clothing (0.6 clo.). Subjective judgments of the thermal sensation,local discomfort,and skin temperature were recorded. A curve was established showing the percentage of people feeling discomfort due to a vertical temperature difference between head and ankle levels. However,this study was not based on DV system. Furthermore,some of the thermal discomfort expressed in the experiment had been caused by a difference in radiant temperature rather than a difference in air temperature. If the radiant temperature of the upper and lower half of the room had been the same,the subjects would probably have tolerated a higher vertical air temperature difference. Another systematic experiment regarding the thermal effect of vertical air temperature gradient was conducted by Wyon and Sandberg [13]. The experiments were carried out in a chilled/displacement ventilation environment. It was concluded that thermal gradients due to displacement ventilation up to at least 4 K/m were likely to be acceptable if air quality is satisfactory and individual control of whole-body heat loss is provided by some means for sensitive individuals. The discomfort due to dry eyes increased significantly at a thermal gradient above 2 K/m. However,these experiments were conducted in the temperate climate. Relative humidity (RH) was in the range of 20 30% during the experiment. Literature review shows all studies on thermal effect of temperature gradient were conducted in the temperate climate. However,tropical climate is very different from the temperate climate and tropically acclimatized occupants may have different sensation as compared to occupants in the temperate climate. In addition,almost all the above-mentioned researches were not conducted in a space served by DV system. Different mechanism of airconditioning system may lead to different thermal effect. Therefore,objective of this study is to investigate the thermal influence of temperature gradient on overall and local thermal comfort of tropically acclimatized occupants at different room air temperatures (at 0.6 m height) and overall thermal sensations. 2. Methodology 2.1. Experimental facilities This study was conducted in a Field Environment Chamber (FEC) at the National University of Singapore between July and September Fig. 1 shows the chamber,11.12 m (L) 7.53 m (W) 2.60 m (H),which simulates a typical office environment. The east-facing wall comprises of large glass panels,which are insulated with aluminium foil externally and furnished with blinds internally to reduce heat conduction and radiation. Airconditioning and mechanical ventilation (ACMV) system of the chamber is capable of switching between DV and MV modes. Sixteen identical workstations were arranged equally into two rows and positioned at the centre of the chamber. Each workstation had one computer and one chair with backrest Experimental design The experiment was divided into two stages. In Stage 1, 30 subjects were allowed to adjust their clothing to achieve thermal comfort during the first two hours. Jackets were available for subjects to adjust their clothing. In Stage 2, another 30 subjects were exposed to cold sensation at room temperatures of 20 1C and slightly warm sensation at room temperature of 26 1C. Subjects in this group were not Fig. 1. Layout of FEC.

3 W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] 3 allowed to adjust their clothing during the experiment. For both stages,test conditions were blind to all subjects. Skirts and trousers were compulsory for females and males, respectively and all subjects were required to wear openedtoe shoes with socks. Subjects were performing sedentary office work over a period of 3 h for each session. Air velocity in the space near the subjects was kept at below 0.2 m/s. RH at 0.6 m height and outdoor air provision were maintained at 50% and 10 l/s/p,respectively throughout the experiment. The independent variables were temperature gradients between 0.1 and 1.1 m heights and room temperatures at 0.6 m height Objective measurements Room air temperature was measured using type T thermocouple wire with accuracy of C at 0.1,0.6, 0.8,1.1,1.7 and 2.5 m heights. RH was measured using portable sensor with accuracy of 75% at the same heights. Air velocity was measured at 0.1,0.6,0.8 and 1.1 m heights near the subjects using omni-directional hot-wire type of anemometer probes with accuracy of 0.01 m/s. Global temperature was measured at 0.6 m height in the middle of the chamber using black globe thermometer. All measurements were taken continuously throughout each experiment Subjective assessment The ASHRAE scale,( 3) cold,( 2) cool,( 1) slightly cool,(0) neutral,(+1) slightly warm,(+2) warm and (+3) hot,was used for the assessment of subjects Overall thermal sensation (OTS) and local thermal sensation (LTS) of 14 specific body parts (head,chest,back,stomach,right and left arms,hands,thighs,calves and feet). Meanwhile, Overall thermal comfort (OTC) and local thermal comfort (LTC) of body segments was assessed using the Bedford s scale,( 3) much too cold,( 2) too cold,( 1) comfortable cool,(0) neither warm nor cool,(+1) slightly warm,(+2) too hot and (+3) much too hot. Subjects were requested to complete one set of questionnaire at every 30-min interval Subjects A total of 60 subjects,30 for Stage 1 and 30 for Stage 2, were selected for the experiments. Subjects who are acclimatized to the tropical climate were selected for this study. Table 1 shows the anthropometric data of subjects for this study. 3. Results and discussion 3.1. Experimental conditions Table 2 shows the nominal and actual experimental conditions of this study. For nominal room air temperatures (T r ) of 20,23 and 26 1C at 0.6 m height,respective actual values were in the ranges of , and C. For nominal temperature gradients (Dt) of 1, 3 and 5 K/m between 0.1 and 1.1 m heights,respective actual values were in the ranges of 1 1.4,3 3.3 and K/m. For nominal RH of 50% at 0.6 m height, actual value was in the range of %. The results showed that actual experimental conditions were very close to nominal conditions. It appears that the experimental conditions were well monitored during the period of experiment. Air velocity (V) at 0.1 m height near the subjects was less than 0.2 m/s. For Cases 1 9 in Stage 1, subjects were allowed to adjust their clothing to achieve thermal neutral sensation. For Cases in Stage 2,the subjects were not allowed to adjust their clothing Profiles of room air temperature and velocity in the space Typical profiles of spatial temperature and velocity for Cases 1,2 and 3 are shown in Fig. 2 and Fig. 3, respectively. Results show that temperature gradients were steeper below 1.1 m height than beyond 1.1 m height. Air velocity at 0.1 m height was the highest and reduced with the increase in height. At 0.1 m height,the velocity was around 0.1 m/s OTS, OTC and clothing value Table 3 shows average values of OTS,OTC and clothing value for all the cases. For Cases 1 9 of Stage 1 in which overall thermal neutrality was expected,subjects were allowed to adjust their clothing. Cases 1 9 have average votes of OTS were within the range of 0.6 to 0.10 which is close to thermal neutrality. Average votes of OTC were within the range of 0.6 to For Cases of Stage Table 1 Anthropometric data of subjects Stage 1 2 Gender Females Males Total Females Males Total Numbers Age (years) Height (cm) Weight (kg)

4 4 ARTICLE IN PRESS W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] Table 2 Experimental conditions Stage Case Nominal value Actual value T r (1C) (0.6 m height) Dt (K/m) RH (%) T r (1C) (0.6 m height) Dt (K/m) RH (%) V (m/s) (0.1 m height) Mean S.D. Mean S. D. Mean S. D. Mean S. D Height (m) Height (m) Case 1 Case 2 Case Temperature ( C) Fig. 2. Temperature profiles for Cases 1 3. Case 1 Case 2 Case Velocity (m/s) Fig. 3. Air velocity profiles for Cases ,the subjects were not allowed to adjust their clothing. Overall cold sensation was expected for Cases with clothing values between 0.73 and Average votes of OTS were around 2 corresponding to cold sensation. Average votes of OTC were within the range of 2.1 to Overall hot sensation was expected for Cases with clothing values in the range of Average votes of OTS were approximately around 1.1 corresponding to slightly warm sensation. Average votes of OTC were within the range of Values of OTS were non-significantly different for temperature gradients of 1,3 and 5 K/m at room air temperature of 20,23 and 26 1C (p40.05) except for 1 pair of test conditions under overall cold thermal sensation. Values of OTS for Cases 10 and 12 were significantly different (p-value ¼ 0.01). For Cases 10,11 and 12 with respective temperature gradient of 1,3 and 5 K/m at overall cold thermal sensation,values of OTS increased with the increase of temperature gradient. Similarly,values of OTC were non-significantly different for temperature gradients of 1,3 and 5 K/m at room air temperature of 20,23 and 26 1C (p40.05) except for 1 pair of test conditions under overall cold sensation. Values of OTC for Cases 10 and 12 were significantly different (p-value ¼ 0.019). For Cases at overall cold sensation,values of OTC increased with the increase of temperature gradients Percentage dissatisfied of overall body Fig. 4 shows the percentage dissatisfied of overall body (PDO). A summary of w 2 test is shown in Table 4. At overall thermal sensation close to neutral with room temperatures 23 and 26 1C,values of PDO were 0% for temperature gradients of 1,3 5 1C. At overall thermal sensation close to neutral with room temperature 20 1C, values of PDO were 0,0 10% for temperature gradients of 1,3 and 5 1C,respectively (p ¼ 0.045). Results showed that at OTS close to neutral,only when room air temperature was substantially low,such as 20 1C,values of PDO increased with the increase of temperature gradient. This finding is similar with that of Olesen et al. [12] in which percentage of people feeling discomfort increased with the increase of vertical temperature difference between head

5 W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] 5 Table 3 Values of overall thermal sensation,overall thermal comfort and cloth value Stage 1 2 Cases OTS S.D OTC S.D Cloth value (clo.) S.D Percentage Dissatisfied (%) cold-20 slightly warm-26 neutral-20 neutral-23 neutral-26 cold-20 slightly warm neutral-20 neutral-23 0 neutral Gradient (K/m) Fig. 4. Percentage dissatisfied of overall body. Note: cold-20 means overall cold thermal sensation room air temperature of 20 1C. and ankle levels at room air temperature around 24 1C. Room air temperature had significant impact on PDO (p ¼ 0.048) at overall thermal sensation close to neutral. At overall cold sensation,values of PDO were much higher than at OTS close to neutral and slightly warm. Values of PDO decreased with the increase of temperature gradient with non-significant difference (p ¼ 0.530). At overall slightly warm sensation,values of PDO were nonsignificantly different for temperature gradients of 1,3 and 5 K/m (p ¼ 0.237). Values of PDO for all the cases at different overall thermal sensations were significantly different (po0.001). The findings demonstrated that overall thermal sensation had significantly impact on PDO. At overall thermal sensation close to neutral,values of PDO decreased with the increase of room air temperature. The impact of temperature gradient on OTC was different at different overall thermal sensations and air temperatures. At OTS close to neutral,only when room air temperature was substantially low,such as 20 1C,values of PDO increased with the increase of temperature gradient. At overall cold and slightly warm sensations,values of PDO were non-significantly affected by temperature gradient Local percentage dissatisfied (LPD) of upper body segments Values of LPD of upper body segments are shown in Fig. 5. Summary of w 2 test is shown in Table 5. For OTS close to neutral,values of LPD of upper body segments except the hand were quite low in the range of 0 3.3%, which were non-significantly different at different gradients of 1,3 and 5 K/m. Values of LPD at the hand were in the ranges of 20 30%,6.7 10% and 0 3.3% at room temperature of 20,23 and 26 1C,respectively. The results showed that LPD at the hand increased with the decrease of room air temperature (po0.001). Values of LPD at upper body segments with overall cold sensation for Cases were much higher than at Cases 1 9 with OTS close to neutral. There were non-significant differences between LPD of body segments at different temperature gradients. The highest values of LPD were in the range of % at the hand. For slightly warm sensation,the highest values of LPD were at the back in the range of % whereas the lowest values of LPD were at the hand. Values of LPD at the back (p ¼ 0.044),chest (p ¼ 0.009),stomach (p ¼ 0.009) and arm (p ¼ 0.008) were significantly different for gradients of 1,3 and 5 K/m. Overall thermal sensations had significantly impact on values of LPD of body segments (po0.001) as shown in Table 5. The results show that values of LPD of upper body segments were nonsignificantly affected by temperature gradient at close to neutral and cold overall thermal sensations. Values of LPD at the back,chest,stomach and arm were significantly different for gradients of 1,3 and 5 K/m at overall slightly warm thermal sensation. At OTS close to neutral,lpd at the hand was significantly affected by room air temperature. This could be due to the hand was exposed to the surrounding air directly during the experiment. Values of LPD of upper body segments were significantly affected by OTS Local percentage dissatisfied of lower body parts Fig. 6 shows values of LPD of lower body segments. Summary of w 2 test is shown in Table 6. For Cases 1 9 at

6 6 ARTICLE IN PRESS W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] Table 4 Summary of w 2 test for percentage dissatisfied of overall body OTS Gradient Temp. OTS Close to neutral Cold Slightly warm Close to neutral Cold to slightly warm Tr( C) χ p <0.001 Fig. 5. Local percentage dissatisfied of upper body segments. Table 5 w 2 test for local percentage dissatisfied of upper body segments Body segment OTS Gradient Temp. OTS Close to neutral Cold Slightly warm Close to neutral Cold to slightly warm Tr( C) Head χ p <0.001 Back χ p <0.001 Chest χ p <0.001 Stomach χ p <0.001 Arm χ p <0.001 Hand χ p <0.001 <0.001 OTS close to neutral with room temperatures of 20,23 and 26 1C,values of LPD at the thigh,calf and foot were non-significantly different at gradients of 1,3 and 5 K/m. Conversely,LPD at the thigh (p ¼ 0.002),calf (po0.001) and foot (po0.001) increased with decrease of room air temperature even though the overall thermal sensations were close to neutral. For Cases with overall cold sensation,values of LPD were much

7 W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] 7 Table 6 w 2 test for local percentage dissatisfied of lower body parts Fig. 6. Local percentage dissatisfied of lower body segments. Body segment OTS Gradient Temp. OTS Close to neutral Cold Slightly warm Close to neutral Cold to slightly warm Tr( C) Thigh χ p <0.001 Calf χ p <0.001 <0.001 Foot χ p <0.001 <0.001 higher than for Cases 1 9 with OTS close to neutral and Cases with overall slightly warm sensation. Values of LPD at the foot were the highest while values of LPD at the thigh were the lowest. Results showed that at overall cold sensation with room air temperature of 20 1C, thermal gradient had non-significant impact on LPD of body segments as shown in Table 6. In addition,more than half of subjects felt uncomfortable at calf or foot. For Cases at overall slightly warm sensation with room temperature of 26 1C,LPD at the thigh (p ¼ 0.024) and calf (p ¼ 0.045) were significantly different for gradients of 1,3 and 5 K/m. Overall thermal sensation had significant impact on values of LPD of lower body segments (po0.001). The results indicate that values of LPD of lower body segments were non-significantly affected by temperature gradient at close to neutral and cold overall thermal sensations. Values of LPD at the thigh and calf were significantly different for gradients of 1,3 and 5 K/m at overall slightly warm thermal sensation. At OTS close to neutral,values of LPD of the thigh,calf and foot were significantly affected by room air temperature. Similar to LPD of upper body segments, LPD of lower body segments were significantly affected by OTS. Percentage Dissatisfied (%) cold-20 slightly warm-26 neutral-20 neutral-23 neutral Percentage dissatisfied (PD) of any segment cold-20 neutral-20 neutral slightly warm-26 0 neutral Gradient (K/m) Fig. 7. Percentage dissatisfied of any segment. Note: cold-20 means overall cold thermal sensation room air temperature of 20 1C. Values of PD of any segment in all the cases are shown in Fig. 7. PD of any segment in this paper is defined as percentage of subjects feeling uncomfortable at any segment. Table 7 summarizes the w 2 test for all the cases.

8 8 ARTICLE IN PRESS W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] Table 7 Summary of w 2 test for percentage dissatisfied of any segments OTS Gradient Temp. OTS Close to neutral Cold Slightly warm Close to neutral Cold to slightly warm Tr ( C) χ p <0.001 <0.001 Table 8 Correlation of percentage dissatisfied with thermal sensation Body segment Regression R 2 Any segments PD ¼ OTS OTS Overall body OPD ¼ OTS OTS Head LPD ¼ LTS LTS Back LPD ¼ LTS LTS Chest LPD ¼ LTS LTS Stomach LPD ¼ LTS LTS Arm LPD ¼ LTS LTS Hand LPD ¼ LTS LTS Thigh LPD ¼ LTS LTS Calf LPD ¼ LTS LTS Foot LPD ¼ LTS LTS For Cases at overall cold sensation with room air temperature of 20 1C,values of PD were higher than at OTS close to neutral and overall slightly warm sensation. Values of PD were non-significantly affected by temperature gradients as shown in Table 7. Conversely,values of the PD were significantly different at different room air temperatures (po0.001) and overall thermal sensations (po0.001). Drawing a comparison among values of PD of any segment,overall body,upper and lower body segments,it was found that values of PD of any segment were the highest. It is worth noting that at OTS close to neutral at room air temperature of 20 1C,values of PDO were in the range of 0 10% whereas values of LPD of the hand and foot were in the range of 20 30% and %. The results showed that even though the percentage of subjects feeling overall thermal discomfort was low,actually much higher percentage of subjects felt local discomfort. Therefore,it is much more difficult to provide local thermal comfort for all body segments,especially in thermally nonuniform environment. This finding is in accordance with that of Fanger et al. [14] in which it is possible to be in thermal comfort at a global level but still feel some local discomfort Correlation of percentage dissatisfied with thermal sensation Table 8 shows the correlation of PD of overall body and any segment with OTS and correlation of LPD of body segments with LTS. The results showed that PDO had strong correlation with OTS (R 2 ¼ ). LPD of body segment had strong correlations with LTS of respective body segment. The regression equations of LPD were similar with the equation of PDO. The results demonstrated that LTC was affected by LTS in the similar way that OTC was affected by OTS. 4. Conclusion Thermal influence of temperature gradient at different room air temperatures (at 0.6 m height) on overall and local thermal comfort of tropically acclimatized occupants has been extensively investigated in the Field Environment Chamber at cold,close to neutral and slightly warm sensations. The results show that the impact of temperature gradient on overall thermal comfort (OTC) was different at different overall thermal sensations (OTS) and air temperatures. OTS had significant impact on percentage dissatisfied of overall (PDO). At overall cold and slightly warm sensations,values of PDO were non-significantly affected by temperature gradient. At OTS close to neutral, values of PDO decreased with the increase of room air temperature. In the previous studies on the effect of temperature gradient conducted in temperate climate [12,13],the impact of room air temperature on the effect is not mentioned. The results of this study show that at OTS close to neutral,only when room air temperature was substantially low,such as 20 1C,values of PDO body increased with the increase of temperature gradient. These indicate that even though at thermal neutral state,at different room air temperature,temperature gradient had different impact on OTC. The results show that values of local percentage dissatisfied (LPD) of upper and lower body segments were non-significantly affected by temperature gradient at close to neutral and cold overall thermal sensations. Values of LPD of the upper body segments of the back,chest, stomach and arm and lower body segments of thigh and calf were significantly different for gradients of 1,3 and 5 K/m at overall slightly warm thermal sensation. For OTS close to neutral,values of LPD at the hand,thigh,calf and foot were affected by room air temperature. Similar with those of PDO,values of LPD of upper and lower body segments were significantly affected by OTS. Values of percentage dissatisfied (PD) of any segment were nonsignificantly affected by temperature gradients. Conversely,

9 W.J. Yu et al. / Building and Environment ] (]]]]) ]]] ]]] 9 values of the PD were significantly different at different room air temperatures and overall thermal sensations. The results show that even though the percentage of subjects feeling overall thermal discomfort was low,actually much higher percentage of subjects felt local discomfort. The results demonstrated that local thermal comfort was affected by both overall and local thermal sensations. The findings suggest that during the design stage of DV system,room air temperature,temperature gradient and occupants thermal sensations need to be taken into account holistically. Acknowledgements This research is funded by the National University of Singapore,Building and Construction Authority (Singapore) and the Oy Halton Group Ltd (Finland). References [1] Skistad H,Mundt E,Nielsen PV,Hagstro m K,Railio J. Displacement ventilation in non-industrial premises. Guidebook No 1: REHVA; [2] Xu M,Yamanaka T,Kotani H. Vertical profiles of temperature and contaminant concentration in rooms ventilated by displacement with heat loss through room envelopes. Indoor Air 2001;11(2): [3] Yuan X,Chen Q,Glicksman LR. Performance evaluation and design guidelines for displacement ventilation. ASHRAE Transactions 1999; 105(1): [4] Riffat SB,Zhao X,Doherty PS. Review of research into and application of chilled ceilings and displacement ventilation systems in Europe. International Journal of Energy Research 2004;l28: [5] Melikov A,Nielsen J. Local thermal discomfort due to draft and vertical temperature difference in rooms with displacement ventilation. ASHRAE Transactions 1989;95(2): [6] Pitchurov G,Naidenov K,Melikov A,Langkilde G. Field survey of occupants thermal comfort in rooms with displacement ventilation. In: Proceedings of Roomvent 2002,Copenhagen,2002. p [7] ISO Standard Moderate thermal environments determination of PMV and PPD indices. Geneva: International Organization for Standardization; [8] ASHRAE Standard 55. Thermal environmental conditions for human occupancy. Atlanta,GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers; [9] Sandberg M,Blomqvist C. Displacement ventilation systems in office rooms. ASHRAE Transaction 1989;95(2): [10] McNair HP,Fishman DS. A further study of the subjective effects of vertical air temperature gradients. British Gas Corporation,Report No. WH/T/R & D/74/2,London,1974. [11] Eriksson HA. Heating and ventilating of tractor cabs. In: Presented at the 1975 winter meeting of American Society of Agriculture Engineers. Chicago: American Society of Agriculture Engineers; [12] Olesen BW,Scholer M,Fanger PO. Discomfort caused by vertical air temperature differences. In: Fanger PO,Valbjorn O,editors. Indoor climate. Copenhagen: Danish Building Research Institute; p [13] Wyon DP,Sandberg M. Discomfort due to vertical thermal gradients. In: Proceedings of the indoor air 96,vol. 6 (1). Japan, Nagoya p [14] Fanger PO. Local discomfort to the human body caused by nonuniform thermal environments. The Annals of Occupational Hygiene 1977;20(3):