Thermal comfort zone

It should be noted that no Ihcnnal environment will please everyone. No matter what we do, some people will expres.s some discomfort. Tire thermal comfort zone is based on a 90 percent acceptance rate. Thai is, an environment is deemed comfortable if only 10 percent of the people are dissatisfied with it. Metabolism decreases somewhat with age, but it has no effect on the comfort zone. Research indicates that there is no appreciable difference between ihe environments preferred by old and young people. Experiments also show that uieu and women prefer almost the same envirorunent. The metabolism rate of women is somewhat lower, but lliis is compensated by their slightly lower skin lemperature and evaporative loss. Also, there is no significant varialion in the comfort zoue from one part of the world to another and from winter to summer. Therefore, the same thermal coinfort conditions can be used llirouglwut the world in any season. Also, people cannot acclimatize themselves to prefer different comfort conditions. 

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. 

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. 

Optimum comfort would be in the center of each zone. Moving away from the center, some people would be expected to have thermal sensations approaching - 0.5 and -i-0.5 at the cooler and warmer ET borders. The zones of Fig. 5.7b are for sedentary or slightly active ( M 1.2 met) people. If the activity level is higher than that, then the ET" line borders can be shifted about 1.4 K lower per met of increased activity. Similarly, if the clothing is different than the 0.9 and 0.5 do vales of Fig. 5.7a, the temperature boundaries can be decreased about 0.6 K for each 0.1 do increase in clothing insulation. Another, similar way to adjust the comfort zone for both different activity levels and do values is to shift the zone centered on the optimum temperature at... 

Conditions that are warmer than the applicable still-air comfort zone of Fig. 5.7b can often be made comfortable by increasing the air speed. If the conditions are 1 to 6 °C warmer than the still-air comfort zone of Fig. 5.7b, the necessary air speed v) to restore thermal balance and comfort can be estimated from Fig. 5.8, where Tis the temperature difference between the environment and the still-air comfort temperature. Though the increased air speed will bring the whole-body thermal sensation to the comfort level, air motions above 0.8 m/s or so may cause other kinds of discomfort frojn... 

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. 

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. 

Due to the methods and limitations outlined in Section 11.3..3, in thermal comfort analysis, draft risk evaluations cannot be performed using this type of room model. Analysis of air temperature stratification and thermal comfort for the occupant zone can be achieved only by using multi-air-node room models. 

For thermal comfort evaluations room air and/or operative temperatures in the zones... 

The thermal comfort was evaluated with hourly mean values of the air temperature in the occupied zone, plotted against the maximum I h mean outdoor temperature value of the day. Only the period from April 1 to October 30 and only working hours (7 a.m. to 6 p.m. are considered. 7 his evaluation method is based on the Swiss standard SIA V382/2. The minimum and maximum allowable comfort temperatures are adapted to the usual activity and clothing levels of the workers in the hall (see Figs. 11.55 and 11.56). 

In industrial ventilation the majority of air velocity measurements are related to different means of controlling indoor conditions, like prediction of thermal comfort contaminant dispersion analysis adjustment of supply airflow patterns, and testing of local exhausts, air curtains, and other devices. In all these applications the nature of the flow is highly turbulent and the velocity has a wide range, from O.l m in the occupied zone to 5-15 m s" in supply jets and up to 30-40 m s in air curtain devices. Furthermore, the flow velocity and direction as well as air temperature often have significant variations in time, which make measurement difficult. 

ASHRAE 55, Thermal Environmental Conditions for Human Occupancy. This standard covers several areas, including temperature, humidity, and air movement. Important aspects of the standard include the definition of acceptable thermal comfort. It provides information on environmental parameter considerations. The standard makes recommendations for summer and winter comfort zones for humidity and temperature. It also contains guidelines for conducting measurements. 

Yao, R., Li, B., Liu, J. (2009). A theoretical adaptive model of thermal comfort— Adaptive Predicted Mean Vote (aPMV). Building and Environment, 44(10), 2089-2096. doi 10.1016/j.buildenv.2009.02.014. Zingano, B. (2001). A discussion on thermal comfort with reference to bath water temperature to deduce a midpoint of the thermal comfort temperature zone. Renewable Energy, 23(1), 41 7. doi 10.1016/ 80960-1481(00)00101-4. 

Zone, comfort The area or volume ot a space that has its thermal and acoustic environment held at a set standard for the comfort of occupants. 

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