Comfort thermal

Humans seek and want thermal comfort, even at work in industrial settings. Clothing, activities, posture, location, and shelter are chosen, adjusted, altered, and sought consciously and unconsciously to reduce discomforts and enable us to focus more on the other tasks of life. Discomfort can contribute to mistakes, productivity decreases, and industrial accidents. Thermal discomfort results from the physiological strain of thermoregulation. The strain can be in the form of altered body temperatures, sweating and excessive skin moisture, muscle tension and stiffness, shivering, and loss of dexterity. A small 

thermal comfort is clearly desirable and important to the well-being and productivity, and thereby the financial health, of industry. An understanding of the principles of thermal comfort and discomfort can help guide a designer s efforts in creating and operating industrial environments that are both energy-efficient and thermally acceptable to the occupants. 

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. 

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Thermal asphalt Thermal blacks Thermal bonding Thermal chlorination Thermal coatings Thermal comfort Thermal conductivity... 

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. 

The guarded hot-plate method can be modified to perform dry and wet heat transfer testing (sweating skin model). Some plates contain simulated sweat glands and use a pumping mechanism to deUver water to the plate surface. Thermal comfort properties that can be deterrnined from this test are do, permeabihty index (/ ), and comfort limits. PermeabiUty index indicates moisture—heat permeabiUty through the fabric on a scale of 0 (completely impermeable) to 1 (completely permeable). This parameter indicates the effect of skin moisture on heat loss. Comfort limits are the predicted metaboHc activity levels that may be sustained while maintaining body thermal comfort in the test environment. 

HVAC the HVAC system is not able to control existing air contaminants and ensure thermal comfort (temperature and humidity conditions that are comfortable for most occupants). 

In contrast, variable air volume (VAV) systems condition supply air to a constant temperature and ensure thermal comfort by varying the airflow to occupied spaces. Most early VAV systems did not allow control of the outdoor air quantity, so that a decreasing amount of outdoor air was provided as the flow of supply air was reduced. More recent designs ensure a minimum supply of outdoor air with static... 

Humans and the other warm-blooded animals have developed thermoregulatory systems to carefully control body temperature to levels that enable them to function and survive effectively. In general, thermal comfort occurs when the physiological effort to control body temperature is minimized for the activity. Table. 5.1... 

In general, when a person is thermally comfortable, the person s thermal sensation for the whole body is at or near neutral as depicted in Fig. 5.7a. As we have seen, the thermal conditions necessary for comfort are affected by clothing insulation. Figure 5.7b shows the range of temperatures and humidities... 

Fanger, P. O. (1.972). Thermal Comfort, McGraw-Hill, New York. 

Cunningham, D, and Berglund, L. G. (1985). Skin wettedness under clothing and its relationship to thermal comfort in men and women. In CLIMA 2000 Indoor Climate, 4, VSS Kongres, Copenhagen, pp. 91-96. 

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. 

The numbers of dissatisfied persons in Table 6.3 are not additive. Some of the people experiencing general thermal comfort (PMV-PPD) may be the same as the people experiencing local thermal discomfort. In practice, a higher or lower number of dissatisfied persons may be found using subjective questionnaires in field investigations (ISO 10551). 

Li, Z. H. 1994. Fundamental studies on ventilation for improving thermal comfort and I.AQ. Ph.D. thesis. University of Illinois. 

The prevention of unnecessary heat loss or gain, causing poor thermal comfort due to large glazing areas and infiltration, should also he considered. 

Requirements for indoor environment quality must be discussed and decided before the air conditioning design is performed. Criteria for acceptable indoor air quality and thermal comfort must be set. 

In the current review, the term effectiveness of air distribution will be used to describe the ratio of the occupied zone area (where thermal comfort and contaminant concentration are within ranges required by standards and codes) to the total occupied zone area. This hygienic criterion allows one to judge how well the HVAC system fulfills its main task—creating thermal comfort conditions and controlling contaminants in the occupied zone. 

Air movement is ideal in the working zone, both for thermal comfort and pollution control. 

It is essential to ensure that the air distribution in a space provides satisfactory air movement for the occupants thermal comfort. 

A workbench makes use of a local air supply in conjunction with exhaust air to ensure good control of the contaminants generated on a bench process. The local exhaust removes the contaminants, while the local supply air protects the operator and/or the products against airborne contaminants. The local supply air improves the thermal environmental conditions by introducing cool dehumidified air in a hot environment. This ensures that the operator s thermal comfort is maintained in areas of high temperature, where full air conditioning of the entire workspace is nor economically feasible. 

Supply Air When designing workbenches, it is essential that the supply air face area be large enough to cover the contained area. Therefore it is important to have some indication of the operator s range of movements for all intended operations. Moreover, for efficient protection the supply airflow must be adequate to get a stable flow field that will not be affected by ambient disturbances. In industrial applications the suitable mean supply air velocities are typically between 0.2 and 0.45 m s h Low velocities should be used when the distance between the supply air unit and the operator is small or for cool supply air. High velocities are applicable at greater distances and in hot environments, with thermal comfort being considered. 

The flow field created within the protection zone depends mainly on the density difference between supply air and room air (Fig. 10.90). With vertical flow the supply air should be isothermal or cooler than ambient air. If it were warmer, the extension of the controlled flow would be reduced due to buoyancy effects, resulting in the supply air not reaching the operator s breathing zone. As the. supply air cannot be used for heating, the operator s thermal comfort should be maintained, preferably with radiant heaters in cold environments. If the supply air temperature is lower than the room air, the denser supply air accelerates down to the operator, and for continuity reasons the supply flow contracts. Excessive temperature differences result in a reduced controlled flow area with thermal discomfort, and should only be used in special cases. 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

Airflow in space, air Cjuality at all points in space, local age of ait, entraininent of jets, heating. and cooling by airstreams (heat transfer), buoyancy of tvarm and cold jets, thermal comfort at arbnra v points... 

If the flow is isothermal, there is no need to solve for the temperature equation (Eq. (11.6)). In this case the last term in Eq. (11.5) is also dropped. If, however, the thermal comfort is simulated, then the temperature equation must be solved. In ventilation the temperature variations are normally small, which means that it is sufficient to account for density variation only in the gravitation term (the last term in Eq. (11.5)). The gravitation term acts in the vertical direction, and in Eq. (11.5) it is assumed that the xj coordinate is directed vertically upward. denotes a reference temperature, which should be constant. It does not influence the predicted results, except that the pressure level is changed. It could, however, affect convergence rate (i.e., increase the number of required iterations required to reach a converged solution), and it should be chosen to a reasonable value, such as the inlet temperature. 

The probability density function of u is shown for four points in Fig. 11.16, two points in the wall jet and two points in the boundary layer close to the floor. For the points in the wall jet (Fig. 11.16<2) the probability (unction shows a preferred value of u showing that the flow has a well-defined mean velocity and that the velocity is fluctuating around this mean value. Close to the floor near the separation at x/H = I (Fig. 11.16f ) it is hard to find any preferred value of u, which shows that the flow is irregular and unstable with no well-defined mean velocity and large turbulent intensity. From Figs. 11.15 and 11.16 we can see that LES gives us information about the nature of the turbulent fluctuations that can be important for thermal comfort. This type of information is not available from traditional CFD using models. 

Thermal comfort assessments room air and operative temperatures (bur not draft risk evaluations)... 

Combined thermal and multizone airflow models are needed for problems such as thermal comfort analysis in naturally ventilated buildings, determination of heat-removal capacity by natural ventilation, design and evaluation of passive cooling by nighttime ventilation. This is outlined in more detail in Section 11.5. 

For integral building design and performance assessments (thermal comfort, indoor air quality, visual comfort), either integrated building simulation tools must be used or the thermal building simulation must be complemented by airflow and daylighting simulations. 

Radiation from internal heat sources is not directly considered in thermal comfort calculations. 

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