THE USE OF THE CORRECTED EFFECTIVE TEMPERATURE INDEX

Transcription

1 AN ASSIGNMENT ON THE USE OF THE CORRECTED EFFECTIVE TEMPERATURE INDEX BY OMOBUDE IGHODALO FELIX. ARC/05/5640. COURSE CODE: ARC COURSE TITLE: BUILDING CLIMATOLOGY. COURSE LECTURER: PROF. O. OGUNSOTE. 1 August, 2011.

2 TABLE OF CONTENTS Abstract. 1.0 Introduction. 2.0 Thermal Comfort. 3.0 Thermal Indices. 4.0 Effective Temperature index. 5.0 Corrected Effective Temperature index. 6.0 Use of the Corrected Effective Temperature index. 7.0 Conclusion. 8.0 References. 2

3 ABSTRACT Thermal comfort is an important concept in the design of buildings. With the knowledge of the basic concept of thermal comfort buildings can be designed in such a way that it will guarantee about 70% of comfort of its occupants. In attaining this fate certain indices have been developed by people over the decade to have a common template for rating the thermal comfort in a particular region. Some of this template include; Effective temperature(et), corrected effective temperature(cet), the Operative temperature(ot), equivalent temperature, Mahoney scale, Standard effective temperature(set), the Equivalent Warmth(EW), the Resultant Temperature (RT) just to measure a few. This paper centres on the Corrected Effective Temperature (derived from the Effective Temperature) and also elaborates on its uses. 3

4 1.0. INTRODUCTION Comfort is a subjective sensation. It is that state of mind that expresses satisfaction with the thermal environment. Alternatively, it is that state of mind that does not express dissatisfaction with the thermal environment. It is equivalent to conditions in which human beings can sleep soundly and work comfortably and when there is a minimum demand on the thermo-regulatory mechanisms of the body. Outside comfort exist discomfort, which is characterised by the degree and duration of thermal stress. In designing buildings and evaluating the thermal comfort conditions of a particular location certain parameters are used. These parameters are basically in the form of thermal indices which are derived from various climatic factors. Some of this index includes Effective temperature (ET), corrected effective temperature (CET), the Operative temperature (OT), equivalent temperature, Mahoney scale, Standard effective temperature (SET), the Equivalent Warmth (EW), the Resultant Temperature (RT). Their usefulness is very important in thermal comfort. The body attempts to maintain a stable internal temperature by balancing heat loss and heat gain (homeostasis). The body gains heat mainly through basal and muscular metabolism and additionally by convection, conduction and radiation. Heat is lost through work performance by conduction, radiation, convection, evaporation (sweat secretion) and skin diffusion, as well as by latent and dry respiration. 2.0 THERMAL COMFORT. Thermal comfort is defined as: that condition of mind which expresses satisfaction with the thermal environment. According to this definition comfort is a subjective sensation. Based on ASHRAE definition the zone of thermal comfort is the span of conditions where 80% of sedentary or slightly active persons find the environment thermally acceptable. In terms of climatic conditions the acceptable ambient temperature of comfort would be slightly higher in the summer than in the winter, being C and C, respectively. Fanger (1970) defined 3 parameters for a person to be in thermal comfort: a. the body is in heat balance; b. sweat rate is within comfort limits; c. mean skin temperature is within comfort limits. These conceptual requisites for determining thermal comfort can be expressed by measurable terms such as: body-core temperature within a very narrow range of C, a skin temperature of 30 C at the extremities and C at body stem and head, and the 4

5 body will be free of sweating. Any deviation from these assertions results in sensation of discomfort. Thermal comfort will be attained when the rate of heat dissipation from the body by means of radiation and convection (cardiovascular tone) will equal the rate of metabolic heat production and, consequently, heat storage. Factors Affecting Thermal Comfort. There are six major factors that determine comfort. They are ambient air temperature, humidity, radiation, air movement, intrinsic clothing and level of activity. Other factors that may have some effect on thermal comfort are age, sex, body shape, state of health, ethnic grouping, diet, sleep, colour of clothing, acclimatisation, availability of fresh air, transients, colour of a space enclosure and noise. An indication of the relative importance of these other factors is the fact that when all the six major factors are within an acceptable and optimal range, about 70% of the population will be comfortable THE THERMAL INDICES. Knowledge of the way different variables affect thermal comfort have been used to formulate thermal indices or thermal scales that indicate the effects of combining the different variables on comfort. Over thirty of these indices have been devised although their definitions and ranges of applicability differ widely. An ideal index should reasonably and accurately predict the consequences of any combination of the six major factors affecting comfort. It should be applicable both indoors and outdoors and it should be capable of indicating the degree of discomfort. One of the most popular indices is the Effective Temperature Index (ET) from which the Corrected Effective Temperature index (CET) is derived. Others include, the Operative temperature(ot), equivalent temperature, Mahoney scale, Standard effective temperature(set), the Equivalent Warmth(EW), the Resultant Temperature (RT) just to measure a few. 5

6 4.0. THE EFFECTIVE TEMPERATURE. The Effective Temperature (ET) is defined as the temperature of a still, saturated atmosphere which would, in the absence of radiation, produce the same effect as the atmosphere in question. It indicates the combined effects of relative humidity, air velocity, air temperature and clothing*. The major merit of the index is that it indicates the effects of most of the major factors on comfort. In addition, the nomogram is simple and easy to use. It however has some limitations. It does not indicate the effect of radiation or show the degree of discomfort directly. There are monograms for only two categories of clothing: normal indoor clothing and stripping to the waist. Analyses carried out by Glickman et al, Koch et al, Smith and Yaglou shows that the index overestimates the effects of humidity under cool and comfortable conditions. They also claim that it underestimates the effects of humidity at high temperatures and that it exaggerates the stress imposed by air velocity in hot environments. Givoni shows that the index made adequate allowance for air movement only below an ET value of 32 degrees Celsius. The index is valid for air temperatures between 0 and 45 degrees Celsius; wet bulb temperatures between 0 and 45 degrees Celsius and air velocity between 0.1 and 7 metres per second. The index requires that comfort limits should be established for the location, zone or region and degrees are assumed for the Tropics. 6

7 Figures 1: Nomogram for the Effective Temperature index and the Corrected Effective Temperature Index. 5.0 CORRECTED EFFECTIVE TEMPERATURE (CET). The Corrected Effective Temperature summates the separate environmental factors of air temperature, humidity and air velocity and also an allowance is made for radiant heat. It refers to standard conditions of still and saturated air and hence useful in the comparison of different thermal environments. It was derived by Vernon in

8 ASHVE (1932) published a nomogram representation of the ET index, which included air velocity effects and showed that over about 100 F (37.8 C) and 100% RH, air movement increases the thermal load (hence the reversal of the air velocity lines). Vernon (1932) included the effect of radiation by substituting globe temperature values for the dry bulb temperature scale, adopted also by Bedford (1940). This became known as the CET nomogram. As clothing has a large influence on radiation and wind effects, he produced two monograms: for people wearing 1 clo clothing (normal scale) and for people stripped to the waist (basic scale): The following expressions approximate the values at 0.1 m/s air speed: Normal: CET = (1.21 GT WBT) / [ (GT-WBT)] Basic: CET = (0.944 GT WBT) / [ (GT-WBT)] It was discovered that in hot environments the effect of humidity is underestimated and that the adverse effect of m/s air velocities at high temperatures is overestimated. Givoni (1963) however established that above 32 C air movements produced a greater heating effect than that suggested by the ET USE OF THE CORRECTED EFFECTIVE TEMPERATURE. In 1932 Vernon and Warner substituted the dry-bulb temperature with a black-globe temperature to allow radiation to be taken into account (the corrected effective temperature (CET)). Since then many modifications were made to this basic index. For the present discussion two indices, which are in daily use for many years are regarded. The wet-bulb globe temperature (WBGT) index: The wet-bulb globe temperature (WBGT) is by far the most widely used heat stress index throughout the world. It was developed in the US Navy as part of a study on heat related injuries during military training. The WBGT index, which emerged from the corrected effective temperature (CET) consists of weighting of dry-bulb temperature (Ta), Wet-bulb temperature (Tw) and black-globe temperature (Tg), in the following manner: WBGT=0.7Tw+0.1Ta+0.2Tg For indoor conditions the index was modified as follows: WBGT=0.7Tw+0.3Tg (For indoor purposes, when Tg Ta, t h e n WBGT=0.7Tw+0.3Ta) 8

9 The coefficients in this index have been determined empirically and the index has no physiological correlates; but, it was found that heat casualties and the time lost due to cessation of training in the heat were both reduced by using this index. This index is recommended by many international organizations for setting criteria for exposing workers to hot environment and was adopted as an ISO standard (ISO 7243). Corrected Effective Temperature can be used to calculate the Code of Measures for dealing with thermal stress of workers in outdoor worksites during the summer months. This is explained below: Code of Measures for dealing with thermal stress of workers in outdoor worksites during the summer months 1. General Measures The following general measures aim to reduce the harmful effects of thermal exposure of workers at outdoor worksites during the summer months: Issuing and using suitable head cover Issuing potable cool water (10-15 C) and in general issuing abundant supply of cool water to workers Configuring / selecting shady worksites or erecting suitable canopies for carrying out work, wherever this is possible. Designing the work schedule in such a way that labour intensive activities are conducted when temperatures are lower. 2. Adjustments To avoid exposure to solar radiation, adjustments must be made to include interruption or change in the time frame for carrying out work, when dry bulb temperatures combined with relative humidity are as shown in TABLE I below. 9

10 TABLE I (Conditions requiring adjustment of work schedule) Dry Bulb Temperature ( C) Relative Humidity Corrected Effective Temperature ( C) Explanatory notes (a) The Corrected Effective Temperature listed in the third column in Table I corresponds to the Corrected Effective Temperature calculated under conditions of negligible wind speed. (b) Using data provided by the Meteorological Service, it is concluded that the appearance, during the afternoon hours, of conditions of serious scorching heat and therefore conditions which necessitate the regulation of work conditions, can be forecasted if at 9.00 am conditions of average scorching heat prevail (Corrected Effective Temperature approx. 26 C). (c) Using the aforementioned information, it is possible to forecast early in the morning conditions where the upper limit values in Table I will be exceeded and hence inform employers and employees if measures need to be taken by means of an announcement. Confirmation of the forthcoming scorching heat can be carried out if, starting at 9.00 am, temperature and humidity conditions are as in Table II. 10

11 TABLE II (Scorching Heat forecasting conditions). Dry Bulb Temperature ( C) Relative Humidity Corrected Effective Temperature ( C) (d) The upper limit values of safe exposure (3rd column in Table I) are calculated according to the fact that workers bear light summer clothing. In cases where special clothing is required for carrying out specialised work, then the above upper limit values are not valid and become stricter. (e) The above upper limit values concern workers who are not included in high risk groups, i.e. those who do not belong to these categories: Patients suffering cardiological problems Patients suffering respiratory problems Those suffering from general ailments which negatively affect the workers psychosomatic health (diabetes, anaemia, arterial pressure disturbance, renal failure, psychological disorders) Pregnant and breast-feeding mothers. 11

12 7.0. CONCLUSION. In conclusion, the corrected effective temperature has been fully explained and the use has been discussed in this paper. It has been discovered that the use of the corrected effective index is important in achieving thermal comfort in the design of buildings. Its usefulness also encompasses the dealing with thermal stress of workers in outdoor worksites during the summer months. REFERENCE Andris Auliciems and Steven V. Szokolay. (2007. Thermal Comfort, Second revised edition, Oxford Brookes University press, UK. Ogunsote, O. O. and Prucnal-Ogunsote, B. (2002). Comfort Limits for the Effective Temperature Index in the Tropics: A Nigerian Case Study. Architectural Science Review, 45:2, , Sydney, Australia. Yoram EPSTEIN* and Daniel S. MORAN (2006). Thermal Comfort and the Heat Stress Indices, McGraw Hill press, London. comfort accessed on the 25 th day of August effective temperature accessed on the 25 th day of August Gagge AP, Nishi Y. (1976). Physical indices of the thermal environment. ASHRAE J 18,