Thermodynamics

728 | ThermodynamicsFIGURE 14–17We cannot change the weather, but wecan change the climate in a confinedspace by air-conditioning.© Vol. 77/PhotoDisc37°CWasteheat23°CFIGURE 14–18A body feels comfortable when it canfreely dissipate its waste heat, and nomore.It did not take long for people to realize that they could not change theweather in an area. All they can do is change it in a confined space such as ahouse or a workplace (Fig. 14–17). In the past, this was partially accomplishedby fire and simple indoor heating systems. Today, modern air-conditioningsystems can heat, cool, humidify, dehumidify, clean, and even deodorize theair–in other words, condition the air to peoples’ desires. Air-conditioning systemsare designed to satisfy the needs of the human body; therefore, it isessential that we understand the thermodynamic aspects of the body.The human body can be viewed as a heat engine whose energy input isfood. As with any other heat engine, the human body generates waste heatthat must be rejected to the environment if the body is to continue operating.The rate of heat generation depends on the level of the activity. For anaverage adult male, it is about 87 W when sleeping, 115 W when resting ordoing office work, 230 W when bowling, and 440 W when doing heavyphysical work. The corresponding numbers for an adult female are about15 percent less. (This difference is due to the body size, not the bodytemperature. The deep-body temperature of a healthy person is maintainedconstant at about 37°C.) A body will feel comfortable in environments inwhich it can dissipate this waste heat comfortably (Fig. 14–18).Heat transfer is proportional to the temperature difference. Therefore incold environments, a body loses more heat than it normally generates,which results in a feeling of discomfort. The body tries to minimize theenergy deficit by cutting down the blood circulation near the skin (causing apale look). This lowers the skin temperature, which is about 34°C for anaverage person, and thus the heat transfer rate. A low skin temperaturecauses discomfort. The hands, for example, feel painfully cold when theskin temperature reaches 10°C (50°F). We can also reduce the heat lossfrom the body either by putting barriers (additional clothes, blankets, etc.)in the path of heat or by increasing the rate of heat generation within thebody by exercising. For example, the comfort level of a resting persondressed in warm winter clothing in a room at 10°C (50°F) is roughly equalto the comfort level of an identical person doing moderate work in a roomat about 23°C (10°F). Or we can just cuddle up and put our handsbetween our legs to reduce the surface area through which heat flows.In hot environments, we have the opposite problem—we do not seem tobe dissipating enough heat from our bodies, and we feel as if we are goingto burst. We dress lightly to make it easier for heat to get away from ourbodies, and we reduce the level of activity to minimize the rate of wasteheat generation in the body. We also turn on the fan to continuously replacethe warmer air layer that forms around our bodies as a result of body heatby the cooler air in other parts of the room. When doing light work or walkingslowly, about half of the rejected body heat is dissipated through perspirationas latent heat while the other half is dissipated through convection andradiation as sensible heat. When resting or doing office work, most of theheat (about 70 percent) is dissipated in the form of sensible heat whereaswhen doing heavy physical work, most of the heat (about 60 percent) is dissipatedin the form of latent heat. The body helps out by perspiring or sweatingmore. As this sweat evaporates, it absorbs latent heat from the body andcools it. Perspiration is not much help, however, if the relative humidity of