Heating , cooling, lighting - Lechner ch 4
adaptive comfort occurs in three ways: behavioral, physiological and psychological.
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for thermal comfort, the body must eliminate waste heat by means of conduction, convection, radiation, and evaporation.
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ADIABATIC - an important and common phenomenon since this is what happens in evaporative cooling, in which the evaporation of water converts sensible heat to latent heat and the total -heat content remains the same. thus, although the air becomes cooler, it also becomes more humid. it is equally true that whtn water vapor condenses into water, the temperature rises, since latent heat is being converted to an equal amount of sensible heat.
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DPT is also an indication of how much moisture is in the air at any temperture. the higher the DPT, the more moisture. Thus, the DPT can be used to describe the actual amount of moisture in the air.
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Heating and humidifying an air sample increases both its sensible and latent heat. the total heat gain can be read directly from the enthalpy scale.
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MRT - when MRT differs greatly from the air temperature, its effect must be considered. even though the air temperature is a comfortable 75°F, you actually might feel too warm. this is because the sun's rays raised the MRT to a level too high from comfort. as soon as the sun sets, however, you will probally feel cold even though the air temperature in the room is still 75°F. this time the cold window glass lowered the MRT too far, and you experience a net radiant loss.
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a certain set of temperatures and coincident humidities is called the comfort zone on the psychometric chart.
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air movement - affects the heat-loss rate by both convection and evaporation. consequently, air velocity has a very pronounced effect on heat loss. in the summer, it is a great asset and in the winter a liability. the comfortable range is from about 20 to 60 feet/ min (FPM). about 60 to about 200 fmp, air motion is noticeable but acceptable depending on the activity being performed. above 200 fpm, the air motion can be slightly unpleasant and disruptive. a draft is an undersirabel local cooling of the human body by air movement, and it is a serous thermal comfort problem. air motion is also required to prevent excessive stratification, which tends to make heads warmer and feet colder - exactly the opposite of what is comfortable.
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air temperature - the air temperature will determine the rate at which heat is lost to the air, mostly by convection. above 98.6°F (37°C), the heat flow reverses and the body will gain heat from the air. the comfort range for most people (80%) extends from 68°F in winter to 78°F in summer. the range is this large mostly because warmer clothing is worn in the winter.
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although air movement from a breeze is usually desirable in the summer, it is not in very hot and dry climates. if the air is above 98.6°F, it will heat the skin by convection while it cools by evaporation. the higher the temperature, the less the total cooling effect.
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among the benefits of utilizing adaptive comfort are the reduction of energy consumption and greater comfort for occupants. bldg use less energy because the temperature difference between indoors and outdoors is less both summer and winter. occupants are more comfortable because they have greater control over their thermal environment to meet their individual needs, since there are always some people who want it hotter or cooler.
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behavioral adaptations include such behavioral adaptions include strategies as opening and closing windows and adjusting blinds for more or less sunshading. it also includes wearing appropriate clothing.
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certain combinations of air temperature, RH, air motion, and MRT will result in what most people consider thermal comfort. when combinations of air temperature and RH that are comfortable are plotted on psychrometric chart, they define an area known as the comfort zone.
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certain combinations of these four factors result in what is called thermal comfort, which can be represented by the comfort zone on charts such as the psychrometric chart
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comfort zone should be the goal of the thermal design of a building because it defines those conditions that 80% of people in our society find comfortable.
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dew point temperature (DPT) - at this point the air is fully saturated (100% RH) and cannot hold any more moisture. any cooling beyond this pint results in condensation where some of the water comes out of solution in the air as whenb dew is formed. this phenomenon is also seen in rain, snow, fog, hoafrost and the "sweating" of a cold glass of water.
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four factors of the environment together determine how easily the body can eject the waste heat. a. air temperature: 68° to 78°F (20° to 25C°) b. relative humidity: 20 to 80% in the winter and 20 to 60% in the summer. c. air velocity: 20 to 60 fmp d. MRT (near air temperature)
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if the air is humidified to 80% RH, the moisture content will increase and the temperature will decrease. since the loss of sensible heat equals the gain in latent heat, the total-heat content is the same for point G as it was for point A.Note that htere is no change on the total-heat scale. A change in the air that does not result in a change of total-heat content is called an ADIABATIC change.
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in cold climats, windchill factors are often given on weather reports because they better describe the severity of the cold tthan temperature alone. the windchill factor is equal to the still-air temperature that would have the same cooling effect on a human being as does the combined effect of the actualo temperature and wind speed
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in general, the goal is to maintain the MRT close to the ambient air temperature. In a well-insulated and shaded building, the MRT is usually close to that of the indoor air temperature. since the MRT is not always close to the air temperature and yet has a large impact on comfort, the concept of Operative temperature was developed. the operative temperature is a combination of the air temperature and the MRT.
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interior wall colors and furnishings could be chosen to promote comfort during the dominant season. for example, warm colors (red, orange, and yellow) would be used in cold climates and cool colors (blue and green) in hot climates.
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movement to the right increases not only temperature but also the sensible-heat content of an air sample. this is also not a suprise because temperature is an indicator of sensible-heat content.
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moving up is wetter and moving down is dryer or the air is dehumidified when the specific humidity (humidity ratio) is reduced (actual amount of moisture removed from each pound of dry air)
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moving vertically up on the chart indicates that moisture is being added to that air sample, while a downward motion on the chart represents water vapor removal (dehumidification). Movement to the right indicate that the air sample is beaing heated and movement to the left indictes cooling of the air. thus, if a sample of air at 80°F and 40% RH (point A) is cooled to 60°F, the point representing the air sample will move horizontalloy to the left on the psychromentric chart to point B. its RH, however, has increased to about 78%even though there was no change in the moisture content of the air (no vertical movement on the chart). the RH increased because cool air can hold less moisture than warm air, and the existing moisture level is now a larger percentage of waht air can hold at that cooler temperature.
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on the other hand, if the air t point A is heated to 100°F, then its relative humidity will be about 22%. the RH changed because warm air can hold more moisture than cool air, and the existing moisture level is now a smaller percentage of what the air can hold at the higher temperature.
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physiological adaptation (acclimatization) includes the body's strategy to pump more or less blood to the skin. more blood, for example, will increase the skin's temperature and thereby increase heat loss. another adaptation strategy is to regulate the amount of evaporation from the skin, which at high rates is called sweating. Psychological adaptation is shifting expectations. people's satisfaction with the thermal environment is heavily influenced by what they expect at a certain location and time. for example, people expect higher indoor temperatures in the summer than in the winter. as a result, what people consider to be comfortable indoor temperatures is related to outdoor temperatures, where the indoor comfort zone rises along with the outdoor temperature.
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pscychrometric chart can also be used to describe the sensible-, latent-, and total-heat content of an air sample. the total-heat or ENTHALPY (sensible plus latent heat) scale is a standard part of the psychrometric chart..
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psychrometric chart - a useful and convenient way to understand some of the interrelationships of the thermal conditions of the environment.
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recent research on adaptive comfort has shown that the standard comfort zone should be modified for naturally ventilated buildings where the occupants have some control over their immediate environment.
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relative humidity - evaporation of skin moisture is largely a function of air humidity. dry air can readily absorb the moisture from the skin, and the resulting rapid evaporation will effectively cool the body. on the other hand, when the RH reaches 100 %, the air is holding all the water vapor it can and cooling by evaporation stops. for comfort, the RH should be above 20% all year, below 60% in the summer. and below 80% in the winter. high Humidity not only reduces the evaporative cooling rate but also encourages the formation of skin moisture (sweat), which the body senses as uncom fortable. furthermore, mildew growth is frequently a serious problem when the humidity is high.
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shifting of the comfort zone due to increase in the MRT. Cooler air temperatures are required to compensate for the increased heating from radiation. Likewise, a low MRT would have to be offset by an increase in the air temperature. For example, a room with a large expense of glass must be kept warmer in the winter and cooler in the summer than a room with a more modest window area.
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since the psychrometric chart relates only temperature and humidity, the other two factors (air motion and MRT) are held fixed. the MRT is assumed to be near the air temperature, and the air motion is assumed to be modest.
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the building bio climatic chart shows which architectural design strategies are appropriate for different climates, as determined by temperatures and their coincident humidities.
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the diagram has two boundaries that are absolute limits. the bottom edge describes air that is completely dry (0 percent RH), and the upper curvedboundary describes air that is completely saturated with water vapor (100% RH). the upper boundary is curved because as air gets warmer, it can hold more water vapor. even if we know how much water vapor is already in the air. we cannot predict how much more it can hold unless we also know the temperature of the air. the RH is affected by changes in either the amount of moisture or the temperature of the air.
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the four environmental conditions that allow the heat to be lost: 1. Air temperature (°F) (C°) 2. relative humidity 3. air movement (feet/ min) (m/s) 4. mean radiant temperature (MRT) all of these conditions affect the body simultaneously
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the horizontal axis describes the temperature of the air, the vertical axis describes the actual amount of water vapor in the air called humidity ratio or specific hunmidity, and the curved lines describe the ralative humidity (HR)
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the large window area creates a high MRT in the summer and a low MRT in the winter. for every degree increase or decrease in MRT, the air temperature must be adjusted a degree in the opposite direction. window shading and better insulated windows can have tremendous efffects on the MRT.
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the psychrometric chart also presents information on the heat content of a sample of air. heat is gained by either an increase in temperature (sensible heat) or an increase in moisture (latent heat) or both.
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the psychrometric chart describes the combined effect of temperature and its coincident humidity.
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the shift of the comfort zone is due to increased air velocity. the cooling effect of the air motion is offset by an increase in the air temperture. we usually make use of this relationship in the reverse situation. when the air temperature is too high for comfort, we often use air motion (open a window or turn on a fan) to raise the comfort zone so that it includes the higher air temperature. for every increase of 15 fpm of air speed, the comfort zone will shift 1 degree warmer (to the right).
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there is also a shift of the comfort zone due to physical activity. cooler temperatures are required to help the body dissipate the increased production of heat. Gymnasiums, should always be kept significantly cooler than classrooms. Thus the comfort zone shifts down to the left when physical activity is increased.
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thermal comfort - occurs when body temperatures ar held within narrow ranges, skin moisture is low, and the body's effort at temperature regulation is minimizsed
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to maintain vital thermal equilibrium, our bodies mujst lose heat at the same rate at which the metabolic rate produces it. this heat production is partly a function of outside temperature but mostly a function of activity.
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upward movement on the chart increases not only the moisture content but also the latent-heat content. this is not a supprise if you remember that water vapor is a form of latent heat.
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web-bulb temperature (wbt) - another way ot describe the amount of moisture in an air sample. WBT is determined by slinging two thermometers side by side through the air. one thermometer has its bulb covered with a wet sock. if this sling psychrometer is slung around in dry air , the temperature of the wet-bulb thermometer will drop significantly below the temperature of the dry-bulb thermometer because of the olarge evaporation of water. simillary if the air is humid, the wet-bulb temperature will drop only a little. and of course, if the air is at 100% RH, no veaporation will take place, and the wet-bulb and dry-bulb temperatures will be the same.
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when one or more of the four factors of the environment is somewhat outside its comfort range, the remaining factors can be adjusted up or down to compensate, thereby restoring thermal comfort
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the amount of waste heat produced is mostly a function of the physical activity being performed.
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