Thermal environment

A brief review of the figures of merit (1) for thermal imaging (4) and gas detection is given to show the various trades-off required to image the thermal environment and detect atmospheric contamination. 

Thermal comfort may be defined as "that condition of mind in which satisfaction is expressed with the thermal environment" (4). It is thus defined by a statistically vaUd sample of people under very specific and controlled conditions. No single environment is satisfactory for everybody, even if all wear identical clothing and perform the same activity. The comfort zone specified in ASHRAE Standard 55 (5) is based on 90% acceptance, or 10% dissatisfied. 

Recent experiments (4) have shown that there are no significant age or gender-related differences in thermal environment preference when all other factors such as weight of clothing and activity level are the same. Whereas people often accept thermal environments outside of their comfort range, there is no evidence that they adapt to these other conditions. Their environmental preference does not change. Similarly there is no evidence that there is any seasonal or circadian rhythm influence on a person s thermal preference. 

Eigure 3 shows the winter and summer comfort zones plotted on the coordinates of the ASHRAE psychrometric chart. These zones should provide acceptable conditions for room occupants wearing typical indoor clothing who are at or near sedentary activity. Eigure 3 appHes generally to altitudes from sea level to 2150 m and to the common case for indoor thermal environments where the temperature of the surfaces (/) approximately equals air temperature (/ and the air velocity is less than 0.25 m/s. A wide range of environmental appHcations is covered by ASHRAE Comfort Standard 55 (5). Offices, homes, schools, shops, theaters, and many other appHcations are covered by this specification. 

The cold-junction temperature can be fixed by immersing the cold junctions into some known thermal environment an ice bath or a properly maintained water triple-point cell. A temperature-controlled oven at a temperature above ambient may be used. 

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

Air conditioning systems control air quality and thermal environment for both human occupancy and processes. 

A commonly expressed definition is Thermal Comfort is that condition of mind that expresses satisfaction with the thermal environment. The definition implies that the judgment of comfort is a mental process that results from physical, physiological, and psychological factors and processes. Dissatisfac tion can lead to complaints and other undesirable side effects. 

Both primary factors and lesser secondary factors affect our sense of satisfac tion with the thermal environment. The primaiy factors have significant reproducible effects and directly affect heat transfer and the occupant s thermal state, Secondar factors that may affect one s sense of satisfaction with a space are conditions such as color and ambiance, local climate, age, physical fitness, sound, food, and illness. These secondary factors have smaller to negligible effects on one s thermal state and will not be discussed here, but such information is available. ... 

TABLE 5.1 Thermal Environment and Physiological Responses of Thermoregu lation... 

Berglund, L. G. and Cain, W. S. (1989). Perceived air quality and the thermal environment. In The Human Equation Health and Comfort. Proceedings of ASHRAE/SOEH Conference lAQ 89. ASHRAE, Atlanta, pp. 93-99. 

The main purpose for the heating and air conditioning of work spaces is to provide an environment that is acceptable and does not impair the health and performance of the occupants. During production processes and in the external environment it may be necessary to work in unacceptable conditions for a limited time period. However, it must be ensured that these conditions do not impair the health of the employees. Light, noise, air quality, and the thermal environment are all factors that influence the acceptability of conditions for and performance of the occupants. This section will only deal with the thermal environment. Several standards dealing with methods for the evaluation of the thermal environment have been published by international standard organizations such as ISO and CEN. 

The basic philosophy has been to standardize evaluation methods, with recommended limit values for the different parameters or indices listed in informative annexes. These or other values may then be adapted in national rules for the thermal environment. 

Aside from the general thermal state of the body, a person may find the thermal environment unacceptable or intolerable if local influences on the body from asymmetric radiation, air velocities, vertical air temperature differences, or contact with hot or cold surfaces (floors, machinery, tools, etc.) are experienced. 

TABLE 6.2 Developments of International Standards for the Ergonomics of the Thermal Environment... 

ISO EN 1EI99 General presentation of the set of standards in terms of principles and application Ergonomics of the thermal environment Principles and application of intcnianonri standards... 

ISO EN 7730 Ciomfort evaluation in moderate environ-nienrs Mode.tate thermal environments Deterrnina tion of the PMV and PPD index and specification of the conditioirs for thermal comfoi r... 

IS( ) EN 7726 Requirements for measuring instruments Thermal environments instruments fov measuring physical quancirie.s... 

ISO DIS 12894 Selection of an appropriate system of medical supervision for different types of thermal exposure EIrgonomics of the thermal environment Medical supervision of individuals exposed to hot or cold environment ... 

ISO ms 14415 People with special requirements Ergonomics of the thermal environment The application of internanonal standards for people with special requirements... 

The main standards for comfortable thermal environment are ISO EN 7730 and ASHRAE 55-92. The research that forms the basis for these two standards is mainly performed under environmental conditions similar to those for commercial and residential buildings, with activity levels of 1 to 2 met, normal indoor clothing (0.5 to 1.0 do), and a limited range of environmental parameters. 

ISO EN 7730 standardizes the PMV-PPD index as the method for evaluation of moderate thermal environments. To quantify the degree of comfort, the PMV (predicted mean vote) index gives a value on a 7-point thermal sensation scale -t-3 hot, +2 warm, +1 slightly warm, 0 neutral, -I slightly cool, -2 cool, -3 cold. An equation in the standard calculates the PMV index based on the six factors (clothing, activity, air and mean radiant temperatures, air speed, and humidity). 

The PMV index can be used to check whether a given thermal environment complies with specified comfort criteria and to establish requirements for different levels of acceptability. By setting PMV = 0, an equation is established that predicts combinations of activity, clothing, and environmental parameters that will provide a thermally neutral sensation. Figure 6.1 shows the optimal operative temperature as a function of activity and clothing for different levels of acceptability. 

Target Values for Acceptable Thermal Environments for Comfort... 

Thermal comfort is defined as the condition of mind that expresses satisfaction with the thermal environment. Dissatisfaction may be caused by thermal discomfort of the body as a whole as expressed with the PMV and PPD indices, or it may be caused by unwanted cooling (or heating) of a particular part of the body. Due to individual differences, it is impossible to specify a thermal environment that will satisfy everybody. There will always be a percentage of dissatisfied occupants, but it is possible to specify an environment predicted to be acceptable by a certain percentage of the occupants. 

Due to local or national priorities, technical developments, and climatic regions, in some cases a higher thermal quality (fewer dissatisfied) or a lower quality (more dissatisfied) may be sufficient. In both cases the PMV and PPD indices, the model of draft, and the relation between local thermal discomfort parameters and the expected percentage of dissatisfied people may be used to determine different ranges of parameters for the evaluation and design of the thermal environment. 

The three categories in Table 6.3 apply to spaces where persons are exposed to the same thermal environment. It is advantageous if some kind of individual control over the thermal environment can be established for each person in a space. Individual control of the local air temperature, mean radiant temperature, or air velocity may contribute to reducing the rather large differences between individual requirements and therefore provide fewer dissatisfied. 

FIGURE 6.6 Acceptable mean air velocity as a function of local air temperature and turbulence intensity for the three categories of thermal environment. 

TABLE 6.5 Range of Floor Temperature for Three Categories of Thermal Environment ... 

A computer program is provided for ease of calculation and efficient use of the standard. This rational method of assessing hot environments allows identification of the relative importance of different components of the thermal environment, and hence can be used in environmental design. The WBGT index is an empirical index, and it cannot be used to analyze the influence of the individual parameters. The required sweat rate (SW. ) has this capability, but lack of data may make it difficult to estimate the benefits of protective clothing. 

The application of the above standards requires measurement or estimation of a number of parameters. The supporting and complementary standards described below provide information that is required for the application of standards for assessing thermal environments. They can also be used independently in ergonomics and other investigations. 

All assessments of thermal environments require an estimate of the metabolic heat production of the occupants. ISO EN 8996 presents three types of methods. The first is by use of tables, where estimates are provided based on a description of the activity. These range from a general description (light. 

ISO EN 9886 presents the principles, methods, and interpretation of measurements of relevant human physiological responses to hot, moderate, and cold environments. The standard can be used independently or to complement other standards. Four physiological measures are considered body core temperature, skin temperature, heart rate, and body mass loss. Comments are also provided on the technical requirements, relevance, convenience, annoyance to the subject, and cost of each of the physiological measurements. The use of ISO 9886 is mainly for extreme cases, where individuals are exposed to severe environments, or in laboratory investigations into the influence of the thermal environment on humans. 

Subject scales are useful in the measurement of subjective responses of persons exposed to thermal environments. They are particularly useful in moderate environments and can be used independently or to complement the use of the objective methods (e.g., thermal indices) that were described previously. ISO EN 10551 presents the principles and methodology behind the construction and use of subjective scales and provides examples of scales that can be used to assess thermal environments. 

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