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Human Comfort and Health Requirements 13357_Bradshaw_3p_c01.f.qxd 3/15/06 10:19 AM Page 3 Chapter 1 Human Comfort and Health Requirements COMFORT CONDITIONS THERMAL COMFORT STANDARDS HUMAN PHYSIOLOGY THERMAL INDICES HEAT VS. TEMPERATURE THE COMFORT CHART BODY TEMPERATURE CONTROL ASHRAE’S THERMAL COMFORT STANDARD DESIGN CONSIDERATIONS HEAT BALANCE INDIVIDUAL VARIABILITY EVAPORATION RADIATION TEMPERATURE AND HUMIDITY EXTREMES CONVECTION-CONDUCTION HEAT STRESS COMBINED EFFECTS RESPONSE TO EXTREME COLD METABOLISM AIR QUALITY AND QUANTITY CLOTHING AIR CONTAMINANTS ENVIRONMENTAL FACTORS ODORS DRY-BULB TEMPERATURE VENTILATION HUMIDITY MEAN RADIANT TEMPERATURECOPYRIGHTED MATERIAL AIR MOVEMENT 3 13357_Bradshaw_3p_c01.f.qxd 3/15/06 10:19 AM Page 4 4 THERMAL CONTROL CONCEPTS Thermal and atmospheric conditions in an enclosed space need for large windows, which provided light and ventilation are usually controlled in order to ensure (1) the health and in older commercial and institutional buildings. Although comfort of the occupants or (2) the proper functioning of windows are still important for aesthetics, daylighting, and sensitive electronic equipment, such as computers, or certain natural ventilation, windowless interior spaces may now be manufacturing processes that have a limited range of tem- used to a much greater extent. Air conditioning allows for perature and humidity tolerance. The former is referred to as more compact designs with lower ceilings, fewer windows, comfort conditioning, and the latter is called process air less exterior wall areas, and less land space for a given en- conditioning. The conditions required for optimum opera- closed area. Conditioned air, which is cleaner and humidity- tion of machinery may or may not coincide with those con- controlled, contributes to reduced maintenance of the space. ducive to human comfort. As a testament to the importance placed on air condition- Process air conditioning requirements are highly specific ing, over one-third of the entire U.S. population presently to the equipment or operation involved. Specifications are spends a substantial amount of time in air-conditioned envi- generally available from the producer or manufacturer, and ronments. And all of this represents growth since the com- the ASHRAEa Handbook of Applications provides a descrip- mercialization of refrigeration cooling in the early 1950s. tion of acceptable conditions for a number of generic indus- On the other hand, this improvement in comfort has come trial processes. about at the expense of greater equipment installation, main- Once the necessary conditions for process or machinery tenance, and energy costs. A substantial portion of the operation are established, attention must be paid to provid- energy consumed in buildings is related to the maintenance ing acceptable comfort, or at least relief from discomfort or of comfortable environmental conditions. In fact, approxi- physiological stress, for any people also occupying the mately 20 percent of the total U.S. energy consumption is space. directed toward this task. Although human beings can be considered very versatile But this doesn’t have to continue to be the case. With an “machines” having the capacity to adapt to wide variations understanding of the factors that determine comfort in rela- in their working environment while continuing to function, tion to climate conditions, designers may select design their productivity does vary according to the conditions in strategies that provide human comfort more economically. their immediate environment. Benefits associated with Thus, prior to investigating the energy-consuming mechani- improvements in thermal environment and lighting quality cal systems in buildings, we will begin by discussing the include: concepts of human comfort. • Increased attentiveness and fewer errors • Increased productivity and improved quality of prod- Comfort Conditions ucts and services Besides being aesthetically pleasing, the human environ- • Lower rates of absenteeism and employee turnover ment must provide light, air, and thermal comfort. In addi- • Fewer accidents tion, proper acoustics and hygiene are important. Air • Reduced health hazards such as respiratory illnesses requirements and thermal comfort are covered in this chap- Indeed, in many cases, air conditioning costs can be justi- ter, while illumination and acoustical considerations will be fied on the basis of increased profits. The widespread avail- presented in later chapters. ability of air conditioning has also enabled many U.S. Comfort is best defined as the absence of discomfort. companies to expand into the Sun Belt, which was previ- People feel uncomfortable when they are too hot or too cold, ously impractical. or when the air is odorous and stale. Positive comfort condi- Air conditioning and electric lights have eliminated the tions are those that do not distract by causing unpleasant sen- sations of temperature, drafts, humidity, or other aspects of a The American Society of Heating, Refrigerating, and Air-Conditioning the environment. Ideally, in a properly conditioned space, Engineers, Inc. (ASHRAE), a professional organization of engineers, con- people should not be aware of equipment noise, heat, or air ducts basic research of importance to the progress of the industry. Its hand- motion. books are recognized references and data from them are widely used in this The feeling of comfort—or, more accurately, discom- text. A major function of ASHRAE is to promulgate national voluntary con- fort—is based on a network of sense organs: the eyes, ears, sensus standards relating to the work of its members. Based on authoritative research and assembled with great care, ASHRAE standards are accepted nose, tactile sensors, heat sensors, and brain. Thermal com- throughout the industry and usually serve as the basis for state and local fort is that state of mind that is satisfied with the thermal building codes. environment; it is thus the condition of minimal stimulation 13357_Bradshaw_3p_c01.f.qxd 3/15/06 10:19 AM Page 5 HUMAN COMFORT AND HEALTH REQUIREMENTS 5 of the skin’s heat sensors and of the heat-sensing portion of Heat the brain. By definition, heat is a form of energy that flows from a The environmental conditions conducive to thermal point at one temperature to another point at a lower temper- comfort are not absolute, but rather vary with the individ- ature. There are two forms of heat of concern in planning for ual’s metabolism, the nature of the activity engaged in, and comfort: (1) sensible heat and (2) latent heat. The first is the the body’s ability to adjust to a wider or narrower range of one we usually have in mind when we speak of heat. ambients. For comfort and efficiency, the human body requires a Sensible Heat. Sensible heat is an expression of the fairly narrow range of environmental conditions compared degree of molecular excitation of a given mass. Such excita- with the full scope of those found in nature. The factors that tion can be caused by a variety of sources, such as exposure affect humans pleasantly or adversely include: to radiation, friction between two objects, chemical reaction, 1. Temperature of the surrounding air or contact with a hotter object. 2. Radiant temperatures of the surrounding surfaces When the temperature of a substance changes, it is the 3. Humidity of the air heat content of the object that is changing. Every material 4. Air motion has a property called its specific heat, which identifies how 5. Odors much its temperature changes due to a given input of sensi- 6. Dust ble heat. 7. Aesthetics The three means of transferring sensible heat are radia- 8. Acoustics tion, convection, and conduction. All bodies emit thermal 9. Lighting radiation. The net exchange of radiant heat between two bodies is a function of the difference in temperature Of these, the first four relate to thermal interactions between the two bodies. When radiation encounters a mass, between people and their immediate environment. In order to one of three things happens: (1) the radiation continues its illustrate how thermal interactions affect human comfort, the journey unaffected (in which case it is said to be transmit- explanation below describes the body temperature control ted), (2) it is deflected from its course (in which case it is mechanisms and how environmental conditions affect them. said to be reflected), or (3) its journey comes to an end (and it is said to be absorbed). Usually, the response of radiation to a material is some combination of transmission, reflec- tion, and absorption. The radiation characteristics of a mate- HUMAN PHYSIOLOGY rial are determined by its temperature, emissivity (emitting characteristics), absorptivity, reflectivity, and transmissivity. Heat vs. Temperature Conduction is the process whereby molecular excitation spreads through a substance or from one substance to The sense of touch tells whether objects are hot or cold, but another by direct contact. Convection occurs in fluids and is it can be misleading in telling just how hot or cold they are. the process of carrying heat stored in a particle of the fluid to The sense of touch is influenced more by the rapidity with another location where the heat can conduct away. The heat which objects conduct heat to or from the body than by the transfer mechanisms of radiation, conduction, and convec- actual temperature of the objects. Thus, steel feels colder tion are elaborated on in Chapter 2. than wood at the same temperature because heat is con- ducted away from the fingers more quickly by steel than by Latent Heat. Heat that changes the state of matter from wood. solid to liquid or liquid to gas is called latent heat. The latent As another example, consider the act of removing a pan heat of fusion is that which is needed to melt a solid object of biscuits from an oven. Our early childhood training would into a liquid. A property of the material, it is expressed per tell us to avoid touching the hot pan, but at the same time, we unit mass (per pound or per kilogram). The latent heat of would have no trouble picking up the biscuits themselves.
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