HEAT TRANSFER - salemmbrothers6传热salemmbrothers.ppt
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1、6. HEAT TRANSFER,Heat Exchange,There are two methods of heat exchange: Indirect transfer The transfer of heat through a tube wall is called indirect transfer. Examples heat exchangers, furnaces, boilers Direct transfer Another method by which heat exchange can be accomplished is without the benefit
2、of transfer surface. The type of heating/cooling is called direct transfer and brought about by intermingling the hot and cold fluids. Examples fractionators, cooling towers, strippers,Heat Exchange,Flow of heat: Steady state Rate of heat transfer remain constant and is unaffected by time Unsteady s
3、tate Rate of heat transfer at any point varies with time Examples batch process, cooling and heating of materials, certain types of regeneration, curing or activation process.,Heat Transfer,Methods of Heat Transmission Convection transports heat by carrying hot portions of a fluid from one place to
4、another Example home water heater Q = U * A * (t2 t1),Convection,All system of heating and ventilating depend upon what are called convection currents, which, in turn, depend upon the expansion of liquids and gases. Any gas or liquid expands when heated, volume increases, density decreases. In a con
5、vection current, the lighter fluid is pushed upward by the heavier surrounding fluid. Since the rising part of a convection current is warmer than the returning part there is a transfer of heat from the heat source to the cooler parts of the fluid at the top.,Heat Transfer,Methods of Heat Transmissi
6、on Conduction the passage of heat from one part of a substance to another part of the same substance or from one substance to another in physical contact with it, without the movement of particles of either substance. Example metal spoon in hot coffee Q = K/L * A * (t2-t1),Conduction,Heat that passe
7、s from one part of a substance to another part of the same substance or from one substance to another in physical contact with it, without the movement of particles of either substance, is said to flow by conduction. Day to day example of heat transfer by conduction - Metal handle of a sauce pan bec
8、omes extremely hot even though only the bottom of the pan is exposed to a flame - We often found the handle of spoon in hot coffee or soup too hot to be lifted from the cup barehanded. These experiences are example of heat being transmitted by conduction.,Conduction,There are some substances, such a
9、s wood, wool, cork which are poor conductor of heat. They are called heat insulators All metals such as silver, copper, brass, iron are good conductor as compared to non-metals. Metals vary in their respective abilities to conduct heat. This ability or inability is referred to as conductivity. This
10、physical property to conduct heat is called coefficient of thermal conductivity of the materials.,Heat Transfer,Method of Heat Transmission Radiation the transfer of heat through space by heat waves that travel in straight lines. Example heat from an electrical light bulb Q = o eA * (T4r1 T4 r2),Rad
11、iation,If an iron ball is heated and hung up in the room the heat can be felt when the hand is held under the ball. this cannot be due to convection because hot air currents rise It cannot be due to conduction because gases are poor conductors A lighted electric bulb feels hot if the hand is held ne
12、ar it, but when the light is turned off, the sensation stops very quickly - the glass in the bulb is a poor conductor and there is very little air in the bulb - Therefore, the sensation, of heat cannot be due neither to convection or conduction - if a book or screen is placed between heat source and
13、 the hand the sensation immediately ceases. These effects are caused by the travel of heat waves in straight lines through the atmosphere similar to light. Radiation is the transfer of heat through space by heat waves that travel in straight lines.,Factors Affecting Heat Transfer,Factors that effect
14、 the transfer of heat energy from one substance to another 1. Temperature Transfer of heat is proportional to its driving force temperature difference. Heat is always transmitted from a warmer body to a colder one The greater the difference in temperature of the 2 bodies, the greater the amount of h
15、eat exchanged 2. Thermal Resistance Transfer of heat is opposed by a factor called thermal resistance Opposition is caused by the material of the container (heat exchanger) stagnant films, scale or dirt on the tube wall.,Thermal Resistance,Stagnant Film In indirect heat transfer, flow of heat is ini
16、tially being accompished by convection. However, as the tube wall is approached the effect of convection is steadily reduced until heat is transmitted through the stagnant fluid film by conduction almost exclusively. Since liquids or gases are relatively poor conductor, the flow of heat is impeded.
17、Heat transfer by conduction is inversely proportional to the thickness through which it flows. Decreasing the stagnant film thickness is the only means by which greater heat flow can be effected. This can be done by increasing the speed or velocity at which the fluid is passing over the tube wall.,T
18、hermal Resistance,2. Scale After passing through the stagnant fluid film, heat encounters its second resistance scale. Here mechanism is heat flow by conduction. Since scale is relatively porous it is a good insulator, not a conductor. There are no means of reducing the scale thickness on the run. S
19、teps must be taken to reduce the tendency of scale formation and periodically clean the tube surface.,Thermal Resistance,3. Tube wall Thermal resistance offered by the tube wall is very often very negligible compared with resistance offered by the stagnant films and scales Heat flow through the tube
20、 wall is dependent upon conduction, this means heat transfer is improved by reduced thickness of impeding material. It is for this reason that thin-walled tubes are used in heat exchangers. Selection of better conductor for tube material can help reduce this resistance.,Heat Transfer Coefficient,In
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