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1、Man and His Environment Body heat losses. The human body, like any other heat engine , consumes fuel(food) to generate heat. The heat leaves the body in the form of radiation and convection from the skin ,evaporation of moisture from pores in the skin ,and in the breath. Radiation and convection occ
2、ur only if the air temperature or the surroundings are below blood heat. If approaching or above this condition, the sweat glands expand and heat is rejected solely by evaporation. High relative humidity coupled with high temperature renders life increasingly difficult.At low temperatures, the blood
3、 vessels near the surface of the skin close up, leaving the epidermis dry and acting as an insulator. Shivering is a provision of nature to cause muscular activity and hence increase blood circulation.Human comfort . Four factors affect comfort: air temperature, radiation, relative humidity, and air
4、 movement. Most thermometers measure air temperature but are not influenced by infrared (long-wave) radiation, because their glass envelopes are opaque to such radiation. The human body, however , may receive heat from a radiant source ; if so, air temperature must be reduced if body heat release is
5、 to remain normal. Conversely , if the body is losing heat by radiation to its surroundings , such as to the cold walls of a room , the air temperature must be raised for an equal degree of comfort .The study of radiant effects is complex . The mean radiant temperature in an enclosed space is the me
6、an of all the surfaces bounding the space in proportion to their areas and temperature , the latter being calculable from the known conductivity of the materials and an assumed temperature difference between inside and outside . This calculation gives only an approximation ; such factors as the effe
7、ct of a large area of window , which may be cold in winter and hot in summer, produce variations in different parts of a room . Moreover , the body itself is sizeable, and its response varies according to differences of clothing. The matter is further complicated by the introduction of heated surfac
8、es to warm the space and by the entrance of heat through windows via sunshine.A close approximation to mean radiant temperature may be obtained by the use of globe thermometer (Fif.1), consisting of an ordinary thermometer in a blackened , hollow metal sphere about six inches (15 centimeters) in dia
9、meter. If suspended in an enclosure in which all surfaces are at the same temperature as the air , the globe thermometer reads the same as an ordinary thermometer , regardless of air movement; but ,if the surface and air temperatures differ ,the globe becomes warmer or cooler in proportion to the po
10、sitive or negative radiation falling on it. Given the globe temperature and air temperature , mean radiant temperature may be calculated.Research in great Britain on factory workers doing light sedentary work established that globe thermometer temperatures of 62-68F ( 17-20C) and air temperatures of
11、 60-68F (16-20C) were regarded as comfortable . Persons seated at rest required temperatures a few degrees higher, as did those persons who were used to warmer climates.The above criteria take no account of humidity. In colder climates, such as in northern Europe ,variations in relative humidity wit
12、hin the range 40-70 percent have little effect on comfort. In warmer climates ,in which sweating plays an important part in heat release ,humidity must be taken into account .The effective temperature scale was developed in the United States in a long series of subjective tests in carefully conditio
13、ned rooms ,from which lines of equal comfort or discomfort-effective temperature lines-were derived. Limits within which individuals feel comfortable may also be charted ,establishing what is known as the “comfort zone”, which differs for summer and winter.The effective temperature scale takes no ac
14、count of radiation. This can be included simply by using a globe thermometer instead of an ordinary thermometer. The scale so derived is the corrected effective temperature scale, which is no longer widely used. Another scale, resultant temperature, takes account of air temperature ,radiation, and a
15、ir speed. It is measured by a small blackened globe and is more sensitive to air temperature and movement and less to radiation than the globe thermometer. The wet resultant temperature takes account of humidity by using a wet-bulb thermometer a useful device for determining relative humidity. It co
16、nsists of a thermometer with the bulb wrapped in cloth kept moistened by a small reservoir of water. Evaporation causes cooling, the amount of cooling depending on the relative humidity.Another scale is known as environmental temperature; it is weighted in that it is equivalent to two-thirds mean radiant temperature and one-third air temperature. It takes no account of air movement or humidity. Its application is in the study of the thermal properties of buildings.Oth
