THE EVALUATION ON INDOOR THERMAL COMFORT INDEX

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1 The 2005 World Sustainable Building Conference, THE EVALUATION ON INDOOR THERMAL COMFORT INDEX Xie Yingbai Ph.D Yu Zhun Yang Xianliang Institute of Energy and Power Engineering, North China Electric Power University, No. 204 Qingnian Road, Baoding , China, Keywords: thermal comfort, index, evaluation Summary Thermal comfort is that condition of mind which expresses satisfaction with the thermal environment. In the early twentieth century, people have entered into researching on comfortable sense, air-conditioning engineers and investigators of indoor air quality wish to forecast people s comfortable sense. These years, a large number of indexes of thermal comfort have been brought forward; different national officials and special organizations recommend different indexes. Therefore, it is very available for us to master these indexes and their phylogeny in whole. For China, because of the expansive border and unconstant climate, meanwhile, as different areas adopt different standards, especially in the areas which latitudes are dissimilar, thus it makes special sense to spread research on these aspects. With the improvement of people s standard of living, the request about indoor thermal comfort is becoming more and more important. The author introduces the origin and the characteristic of the pivotal indexes of thermal comfort as well as analysis, synthesize them. The main conclusion of this paper: in southward of China, we can adopt ET* to be the primary evaluating tool and index; In northward of China, we can adopt the comfort equation and PMV to confirm the comfortable conditions. At the same time of stating the above conclusion, the author also give the opinion on the developing aspect of thermal comfort and the job which should be taken. It can be used for reference by craft brothers. 1. Introduction Thermal comfort is an important index defined as the condition of mind which expresses satisfaction with thermal environment. In the early twentieth century, people have entered into researching on comfortable sense, air-conditioning engineers and investigators of indoor air quality wish to forecast people s comfortable sense quantificationally. These years, a large number of indexes of thermal comfort have been brought forward, different national officials and special organizations recommend different indexes. Therefore, it is very available for us to master these indexes and their phylogeny in whole. For China, because of the expansive border and unconstant climate, meanwhile, as different areas adopt different standards, especially in the areas which latitudes are dissimilar, thus the text makes a brief analyse on some main indexes, at the same time, the author gives his own opinion on it combining with the situation of China and existing relative researching results. 2. Definitions and characteristics of diversified indexes 2. 1 catarrh cooling ability The first index was put forward by Leonard Hill in 1914 and based on thermal loss. Katathermometer is composed of alcohol glass-stem thermometer which has a big columniform thermal package, its length is 40 millimeters and diameter is 20 millimeters. The pole of thermometer has two label strings: 35 centigrade and 38 centigrade. When the thermometer is used, its alcohol pole should be heated beyond 38 centigrade, then you must hang it in floating air and measure the time which alcohol pole spends during it descends from 38 centigrade to 35 centigrade. Thus environmental cooling ability can be calculated in term of the time and the revising coefficient of each thermometer. Most of experiments put up in 1930s adopted katathermometer, it synthesizes the influence of mean radiative temperature air temperature and air speed barring humidity eupatheostat and equivalent temperature

2 Dufton has developed a synthetical thermostat. It can keep room temperature while air temperature radiation and air speed are variable, this equipment is intituled eupatheostat. After this, Dufton defined equivalent temperature, it s the temperature of an symmetrical close object, in this object, the heat loss of a black cylinder which height is 550 millimeters and diameter is 190 millimeters equals to the heat loss of actual environment. The exterior temperature of cylinder is the precise function of the heat loss of cylinder, also, this temperature will be lower than 37.8 centigrade in any symmetrical interspace, and the difference is two third of the value that between 37.8 centigrade and the temperature of close interspace. Equivalent temperature doesn t give an analytic expression according to basic environmental variables, meanwhile, the eupatheostat is a big and very unwieldy equipment, and thus it was rarely used Effective temperature In early American air-conditioning projects, engineers wanted to know how humidity influenced thermal comfort and they needed the tried datum of it. The problems and some else impelled ASHRAE to build a laboratory. The lab worked from 1919 in Pittsburgh, ET was the first index researched and it came into being that time. It was defined as follow: an environmental index combines two or more parameters, such as air temperature, mean radiant temperature, humidity or air velocity, into a single variable, it equals to the temperature of resting saturated air which produces same feeling in numerical value. ET has been adopted by many official and professional groups, especially in criterions for thermal environment. ASHARE reshipped the index till 1967, but ET emphasizes the influence of humidity excessively in low temperature and contrarily in high temperature. These days, no main official organizations recommend to use ET, and ASHRAE commends to make use of ET* instead of ET. Still, it is the most remarkable index in early indexes for it was admitted popularly, at the same time, it has a number of experimental data. Foundation of ET is an excellent achievement and it was used by air-conditioning engineers as a normal index for fifty years. 2.4 New effective temperature (ET*) This index was brought forward by Gagge in 1971, what is called ET* is that the temperature of an environment at 50% rh that results in the same total heat loss as in the actual environment. It combines the effect of three factors (radiation, convection and evaporation) and is adopted popularly. The curve of equivalent effective temperature is as following figure: Relative humidity 100% ET=15 ET=20 ET=25 ET=30 Air temperature Fig.1 The curve of equivalent effective temperature 2. 5 Fanger s equation Comfortable condition P.O.Fanger has brought forward a synthetical comfort index, that is to say, the physical parameters which can ensure human comfortable state are relative to body but not environment. Above all, Fanger made three comfortable conditions, the first condition is that hominine body must be in thermal balance, thus the heat loss to environment will equal to the heat produced in hominine body, the second condition is that skin temperature should be corresponding to thermal comfort, the third is that hominine body should possess the

3 best sweat, sweat is the function of metabolism. Whereafter, Fanger summed up many literatures and got all the expressions for each variable in thermal balance equation. Thus we can get comfort equation by combining thermal balance equation with other two comfortable conditions. Any set of variables which can meet comfort equation must satisfy three comfortable conditions, so the equation is the necessary condition but not sufficient condition. We can assume such an instance than can meet comfort equation but unsatisfactory, just as a changeful environment. Comfort equation was proved successful, but the testee of most experiments for it were healthy American youngling or Europe undergraduates, so we can t assume that comfort equation is adaptive to other races Predicted Mean Vote(PMV) While a set of environmental variables meets the comfort equation, it will come to the best comfortable sense. If not, the environment won t be the best of all, but the equation hasn t told us that how the environment makes people feel uncomfortable. Fanger developed the comfort equation ulteriorly and expressed an index which can predict any fixed environmental variable by a formula, the index is called PMV, professor Fanger divided it into seven grades, the meaning of each grade is as following figure: Table 1 Fanger seven grade index PMV Predicted thermal sense heat warm slightly warm comfort slightly cool To extend the applied range of this idea, Fanger suggested that the thermal sense of a certain activity is the function of human heat load (heat load is the difference between bodily heat production and actual environmental heat loss) and put forward the equation of PMV through experiment. The equation for PMV hasn t been proved by experimental data of diversified clothes and activities, when people are sitting and wearing light clothes it can give a good result, but the data of high metabolism is unsatisfactory. 2.6 Standard effective temperature(set) Before SET is introduced, we must know the concept of skin wettedness (w). It is the ratio of actual skin evaporative loss and the largest value in same environment. The largest value means that skin is wet completely. The concept of w is very important to ET*, combining this index with the temperature and humidity of air, we can supply an index which is adaptive to people who are sitting and wearing normal clothes. Based on it, engineers extended the main content of ET* to be the same with various activities and thermal resistances, thus SET is brought forward. SET should include skin temperature and skin wettedness to ensure a person s thermal state. There are two steps to ensure the SET of a certain state, firstly, we must calculate a person s skin temperature and skin wettedness, it can be finished by actual measurement, secondly, we should calculate the normal environmental temperature of producing same skin temperature and skin wettedness, it can be finished by analysis to heat transfer. SET is a quite general index among this article, although at first it was used to predict the uncomfortable sense while people was sweating, it can apply to diversified clothes activities and environmental variables after being developed. But it must be calculated by computers for its complexity, so the use of it is limited greatly. 2.7 Subjective temperature Most early indexes were abandoned and replaced by more perfect indexes such as comfort equation and SET. However, new indexes have common shortcomings while they have merits, firstly, the complexity of formula brings on the use of computers, secondly, though they include all the variables, it makes the indexes less use in fact. Based on these reasons, engineers defined another new index, the use of this index makes the problem of stylist who want to design a comfortable environment as the centre. It needs two kinds of data: which temperature the occupants want to get and which combination of physical variables can produce this temperature. This index is subjective temperature. Its definition is as follow: a temperature of average enclosure space, its air temperature equals to average radiant temperature, its air speed is 0.1 meter per second and relative humidity is 50%, the environment will make the same warm sense as actual environment. The definition of subjective temperature lies on subjective warm sense quite a degree, the formula of environmental variables can be accounted at any time with existing data, so it s an experiential formula. cool cold

4 3. Evaluation for indexes of thermal comfort The article enumerates the main indexes of warm sense and thermal comfort. After concerned activities and thermal insulation are got, the formula for subjective temperature will make an estimate of comfortable temperature fleetly and truly. But it can t be used to predict uncomfortable sense and feeling, if necessary, we can use PMV. The most universal index is SET, it is used to get the influence of high temperature and humidity specially. Also, it can follow the variational course of body temperature and skin temperature, it will even give the thermal sense and uncomfortable sense which are the function of bodily state, however, the cost of it is complicated computer program. While temperature is too high to influence people s health, we must adopt a heat stress index which is designed for it specially. Chinese south area has a popular climatic feature that winter is cold while summer is hot and the whole year is damp, based on daily experience, the author think that the reason of frowziness in summer and cold in winter is dampness, so humidity is the main factor which should be controlled, especially the redundant vapor of indoor air (water in gaseity, which belongs to air pollution). To analyse the selected index, we adopt experimental data as following table, table 2 is data processing for backgrounds of testee: Ventilation type Sex Age Table 2 Analysis and statistic of background data Sample capacity 210 Air-conditioning 138(65.7%) Non- air-conditioning 72(34.3%) Male 110(52.4%) Female 100(47.6%) Average value Standard deviation 8.33 Max 67 Min 15 Time of inhabitation Average value 14.4 Thermal insulation Average value 0.43 Activity Average value 1.22 Frequency distribution of indoor physical condition with air-conditioning is as following figure (calculated by ET*): Frequency distribution(%) ET*(ºC) Fig.2 Frequency distribution of indoor physical condition with air-conditioning

5 We can get the frequency distribution of air-conditioning room from figure 2. According to ASHRAE , ET* for comfortable area is between 23 centigrade and 26 centigrade and humidity is less than 60%, thus the range of temperature and humidity calculated by it is quite adaptive, meanwhile, temperature and humidity are two most important factors to thermal comfort, based on it, ET* is recommended to be the main index. Chinese north area is rather dry relative to southward, especially in winter. These years, with the development of economy, air-condition and central heating are in common use in daily life, it makes rooms warmer and dryer, moreover, people are used to wearing thin clothes, accordingly thermal insulation is not too big. The following table is the experimental data and analysis for background of testee. Table 3 Background data in winter Sample capacity 74(male: 41,55%) Parameter Mean value Standard deviation Age 27 7 Time of inhabitation Relative humidity Air speed Thermal insulation Activity Table 4 Comfortable area in winter Predicting value Actual value Classify Result Fitting equation PMV= ET* Neutral temperature(et*) 21.8ºC 80% satisfying area 19.1ºC-24.5ºC 90% satisfying area 20.2ºC -23.4ºC Fitting equation ASH= ET* Neutral temperature(et*) 18.8ºC 80% satisfying area 16.5ºC-22.5ºC 90% satisfying area 17.5ºC-20.6ºC From the table we find that the predicting value is different from the actual value. It has two reasons, firstly, it s very difficult to confirm the value of PMV in real life, there are many factors influence thermal comfort, for example, clothes mood and emotion, and so on, each one has different assessment even if environment is familiar. Next, the error of fixing equation is rather big. Contraposing the first reason, we can use PPD to express the unsatisfactory percent, for the second reason, we can improve the comfortable equation or do a number of experiments, thus a more exact equation will be fixed. To sum up, comfortable equation and PMV are commended to ensure comfortable condition for northward. 4. Conclusion The border of our country is very expansive and the nation is numerous, thus the requirement to thermal comfort is different, the author think that each region should improve the index of thermal comfort combining with climatic feature racial characteristic habit bodily diathesis, etc, and get the index which adapts to itself. Reference D.A. McINTYRE. 1988, Indoor Climate. Applied Science Publishers Faye C. McQuiston, Jerald D. parker. 1981, Analysis and design for heating and ventilation and air-conditioning Yizai Xia, Rongyi Zhao, Yi Jiang. 1999, Research for residential thermal comfort in Beijing. HVAC, 29,

6 pp.1-5 Fang Lv.Thermal comfort and energy-saving. 2000, Tianjin: Tianjin University Xiaoping Yu, Xiangzhao, Fu. 2000, Quality control of indoor thermal environment in the area which summer is hot and winter is cold. Journal of Changdu Textile College,17, pp Satish kumar, Ardeshir Mahdevi. 2001, Intergrating thermal comfort field data analysis in a case-based building simulation environment. Building and environment, 36, pp