Thermodynamics

the environment is close to 100 percent. Prolonged sweating without anyfluid intake causes dehydration and reduced sweating, which may lead to arise in body temperature and a heat stroke.Another important factor that affects human comfort is heat transfer byradiation between the body and the surrounding surfaces such as walls andwindows. The sun’s rays travel through space by radiation. You warm up infront of a fire even if the air between you and the fire is quite cold. Likewise,in a warm room you feel chilly if the ceiling or the wall surfaces are at aconsiderably lower temperature. This is due to direct heat transfer betweenyour body and the surrounding surfaces by radiation. Radiant heaters arecommonly used for heating hard-to-heat places such as car repair shops.The comfort of the human body depends primarily on three factors: the(dry-bulb) temperature, relative humidity, and air motion (Fig. 14–19). Thetemperature of the environment is the single most important index of comfort.Most people feel comfortable when the environment temperature isbetween 22 and 27°C (72 and 80°F). The relative humidity also has a considerableeffect on comfort since it affects the amount of heat a body candissipate through evaporation. Relative humidity is a measure of air’s abilityto absorb more moisture. High relative humidity slows down heat rejectionby evaporation, and low relative humidity speeds it up. Most people prefer arelative humidity of 40 to 60 percent.Air motion also plays an important role in human comfort. It removes thewarm, moist air that builds up around the body and replaces it with freshair. Therefore, air motion improves heat rejection by both convection andevaporation. Air motion should be strong enough to remove heat and moisturefrom the vicinity of the body, but gentle enough to be unnoticed. Mostpeople feel comfortable at an airspeed of about 15 m/min. Very-high-speedair motion causes discomfort instead of comfort. For example, an environmentat 10°C (50°F) with 48 km/h winds feels as cold as an environment at7°C (20°F) with 3 km/h winds as a result of the body-chilling effect of theair motion (the wind-chill factor). Other factors that affect comfort are aircleanliness, odor, noise, and radiation effect.Chapter 14 | 72923°Cf = 50%Air motion15 m/minFIGURE 14–19A comfortable environment.© Reprinted with special permission of KingFeatures Syndicate.14–7 AIR-CONDITIONING PROCESSESMaintaining a living space or an industrial facility at the desired temperatureand humidity requires some processes called air-conditioning processes.These processes include simple heating (raising the temperature), simple cooling(lowering the temperature), humidifying (adding moisture), and dehumidifying(removing moisture). Sometimes two or more of these processes areneeded to bring the air to a desired temperature and humidity level.Various air-conditioning processes are illustrated on the psychrometricchart in Fig. 14–20. Notice that simple heating and cooling processes appearas horizontal lines on this chart since the moisture content of the air remainsconstant (v constant) during these processes. Air is commonly heatedand humidified in winter and cooled and dehumidified in summer. Noticehow these processes appear on the psychrometric chart.CoolingHumidifyingCooling anddehumidifyingDehumidifyingHeating andhumidifyingHeatingFIGURE 14–20Various air-conditioning processes.