The Thermal Environment

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. 

Existing methods for evaluation of the general thermal state of the body, both in comfort and under heat or cold stress, are based on an analysis of the heat balance for the human body  

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 

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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. 

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. ... 

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. 

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... 

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. 

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. 

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. 

Body thermal sensation The response of the body to changes in the thermal environment, relating to moisture, air movement, or temperature. 

The design and development of kinematic mounts is a rich and complex field, but we only introduce the subject and its basic ideas here. When dealing with optics, stress-free mounts are often essential in order to avoid distorting the optic. Possible disturbances that must be considered include changes in gravity vector and changes in the thermal environment. 

Mench et al. developed a technique to embed microthermocouples in a multilayered membrane of an operating PEM fuel cell so that the membrane temperature can be measured in situ. These microthermocouples can be embedded inside two thin layers of the membrane without causing delamination or leakage. An array of up to 10 thermocouples can be instrumented into a single membrane for temperature distribution measurements. Figure 32 shows the deviation of the membrane temperature in an operating fuel cell from its open-circuit state as a function of the current density. This new data in conjunction with a parallel modeling effort of Ju et al. helped to probe the thermal environment of PEM fuel cells. 

DSC has the advantage of being fast (results typically obtained within 1 hr) and suitable for any consistency of fat. However, perhaps because of the difficulty in accurately weighing the small (5 to 15 mg) samples required, precision and reproducibility tend to be poor. DSC instruments can be programmed to accurately reproduce the thermal environment the fat will... 

Meniscus-Defined Crystal Growth Systems. In most conventional meniscus-defined growth systems, a seed crystal is dipped into a pool of melt, and the thermal environment is varied so that a crystal grows from the seed as it is pulled slowly out of the pool. Two examples of meniscus-defined growth are shown in Figure 1. The Czochralski (CZ) method (Figure lb) and the closely related liquid-encapsulated Czochralski (LEC) method are batchwise processes in which the crystal is pulled from a crucible with... 

The same phenomenology must be important locally on Earth, too, where thick evaporite deposits of hydrated salts and local thick beds of methane clathrate in permafrost or seafloor sediments should influence the thermal environment of the crust. The predicted control on the crust s thermal state by hydrate deposits should have consequences for the localization of hydrothermal springs around and within evaporite basins, hydrothermal metamorphism... 

It is known that the DLC heat management problems depend on the power solicitation and on the thermal environment of the device [72], Therefore, an optimum heat loss can be achieved by maximizing the surface-to-volume ratio. This condition is in opposition with the needs of high energy... 

The term Tml — (Em -+ A , — )) + zT, ] in the denominator of Eq. (6.2) allows us to tune cw, so that only a select few of the thermally populated levels Eh Jh Mt) are excited. That is, making this denominator small (i.e. achieving this resonance condition) establishes coherence, despite the thermal environment. 

Environmental Conditions. The last area of discussion concerns those studies that emphasize environmental factors indoors and their interrelationship with clothing. Fanger s multivariate equation for predicting thermal comfort indoors, which he defines as thermal neutrality, is based on statistical analysis of 1,300 Danish and American subjects and consists of six parameters metabolic activity of occupants, clothing insulative value (clo), air temperature, mean radiant temperature, relative humidity, and air velocity ( 8, TjO An instrument based m these parameters and the statistical analysis is available (Figure 2) a reading for the parameters is integrated and the percent of occupants satisfied with the thermal environment is displayed. 

To allow for the superposition of effects of seasonally changing temperatures with depth on daily temperatures, we can incorporate both daily and annual additive terms in an equation like Equation 7.28. Each term contains its appropriate d and p, with rsurf then coming from the annual case. In any case, soil properties markedly affect the thermal environment of roots, which can represent about half of a plant s biomass, as well as the temperatures in animal burrows and even in certain wine cellars. 

The temperature in deep space is close to absolute zero, which presents thermal challenges for the astronauts who do space walks. Propose a design for the clothing of the astronauts that will be most suitable for the thermal environment in space. Defend the selections in your design. 

Harmonic oscillators are often used as approximate models for realistic systems. A common application is their use as convenient models for the thermal environments of systems of interest (see Section 6.5). Such models are mathematically simple, yet able to account for the important physical attributes of a thennal bath temperature, coupling distribution over the bath normal modes, and characteristic timescales. Their prominence in such applications is one reason why we study them in such detail in this chapter. 

In fact what is needed for Eq. (6.17) to be a meaningful transition rate is that thermal relaxation (caused by interaction with the thermal environment) in the manifold of initial states is fast relative to r. See Section 12.4 for fiirther discussion. 

In many applications we use models that are more explicit about the nature of the initial and final states involved in this transition. A common model (see Chapter 12) is a two-level system that interacts with its thermal environment. The lineshape of interest then corresponds to the photon-induced transition from state 1 to state 2, dressed by states of the thermal environment. The initial and final states are now z) = 11, a and f = 2, a where a and a are states of the bath. Equation (6.21) can then be rewritten as ... 

The spectral density (see also Sections (7-5.2) and (8-2.5)) plays a prominent role in models of thermal relaxation that use harmonic oscillators description of the thermal environment and where the system-bath coupling is taken linear in the bath coordinates and/or momenta. We will see (an explicit example is given in Section 8.2.5) that /(co) characterizes the dynamics of the thermal environment as seen by the relaxing system, and consequently determines the relaxation behavior of the system itself. Two simple models for this function are often used ... 

The function (Z) describes the effects of random collisions between our subsystem (henceforth referred to as system ), that may sometimes be a single particle or a single degree of freedom, and the molecules of the thermal environment ( bath ). This force is obviously a stochastic process, and a full stochastic description of our system is obtained once we define its statistical nature. 

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