6. HEAT TRANSFER Heat Exchange There are two methods of heat exchange: 1. Indirect transfer • The transfer of heat through a tube wall is called indirect transfer. • Examples – heat exchangers, furnaces, boilers 2. Direct transfer • Another method by which heat exchange can be accomplished is without the benefit 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: 1. Steady state • Rate of heat transfer remain constant and is unaffected by time 2. Unsteady state • 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 1. Convection – transports heat by carrying hot portions of a fluid from one place to 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 convection 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 Transmission 2. 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 passes 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 becomes 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 as 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 physical property to conduct heat is called coefficient of thermal conductivity of the materials. Heat Transfer Method of Heat Transmission 3. 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) Radiation 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 near 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 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 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 heat 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 1. • • • Stagnant Film In indirect heat transfer, flow of heat is initially 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. 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. Thermal 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. Steps 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 wall is dependent upon conduction, this means heat transfer is improved by reduced thickness of impeding material. It is for this reason that thinwalled tubes are used in heat exchangers. • Selection of better conductor for tube material can help reduce this resistance. Heat Transfer Coefficient • In the study of heat exchange and the design of heat transfer equipment there must be a means of evaluating the performance of a given exchanger. • In heat transfer work, this evaluating factor is called the overall heat transfer coefficient • It indicates the amount of heat an exchanger can interchange during a given period of time for a given amount of surface and with a given temperature difference between the hot and cold fluids. • In engineering practice this value is referred us “U” • Unit of measure is BTU/Hr/Ft2/oF. Overall Heat transfer Coefficient Clean coefficient U = 90 U (100 % efficiency) U = 63 after 3 months ops 70 % efficiency Months in operations Fouling Factor • After heat transfer equipment has been in service for some time, dirt or scale may form on the heat transfer surfaces, causing additional resistance to the flow of heat • To compensate for this, a resistance called dirt, scale or fouling factor is included in determining an overall coefficient of heat transfer Heat Transfer Equipment The 3 general categories of heat transfer equipment are: 1. Exchangers – transfer heat from a hot process stream to a cold process stream. 2. Cooler – transfer heat from a hot medium to cooling wate 3. Condenser – condense vapor Heat Transfer Equipment Process use 1. Exchangers – conserve heat that would otherwise be wasted i.e. taking heat from one liquid that needs cooling and giving it to another which needs heating 2. Coolers – to cool materials leaving a unit to a temperature which is safe for storage or loading. 3. Condensers – remove heat from vapors, condense these vapor and heat the cooling water/ Heat Transfer Equipment As a background in the use of heat transfer equipment, an understanding of the various types of equipment on the market is helpful. 1. Double pipe heat exchanger In this type, one fluid flows in the small, inner pipe and the other fluid in the space between the small, inner pipe and the larger, outer pipe. Finned tubes are often used to improve the heat transfer efficiency of a unit 2. Air cooled Heat Exchanger This is an air cooled heat exchanger in which the hot fluid is cooled by air blown past the tubes by the fan below Heat Transfer Equipment 3. Shell and Tube heat Exchanger The majority of heat transfer equipment is the shell-and-tube type of heat exchanger. A shell and tube exchanger consists of a number of parallel tubes enclosed in a cylindrical shell. One fluid flows inside the tubes and is called the tube side fluid The other fluid flows outside the tubes and is called the shell side fluid. All shell and tube heat transfer equipment is composed of the same basic parts, but some of these parts are arranged in such a way as to produce the desired results. Shell and Tube Exchange Shell and Tube Exchanger Parts Identification 2, 2 heat exchangers Reboiler Baffles Baffles are used to increase the rate of heat transfer by increasing the velocity of the shell side fluid. • Two (2) general types of baffles 1. Transverse 2. Longitudinal • In order to be effective, baffles must be installed so that there is a minimum of by-passing around baffles. This is done by reducing to a minimum the clearance between the baffles and the shell side fluid. Baffle Arrangements in Exchangers Cooler Cause of Fouling • Fouling of a heat exchanger may result from scale, or particles lodging in the exchanger. • Water residue may foul the tubes of coolers and condensers. • Usually, oil side fouling material cannot be readily removed short of mechanical means. When this is necessary, the tube bundle must be removed. However, water residue can be removed without pulling the tube bundle from the shell. Water residue may be dirt, mud, silt, and/or scale. Cause of Fouling IF THE FOULING MATERIAL IS DIRT: • The use of backwash connections can substantially improve operating efficiency • By reversing the direction of flow momentarily, the force of the water will knock the adhering trash from the tube ends and remove some mud and silt from the tube walls. • Such an arrangement makes it possible to regain a degree of design performance without a shutdown Causes of Fouling Water scale formation can be removed without dismantling the exchanger by utilizing chemical cleaning. Water residue is removed mechanically and is accomplished by water jets or by drilling through which water passes as a scavenging agent. Mechanical cleaning of oil side scale can be done in a number of ways. Drilling, steaming, and sandblasting are used to clean the tube side. The shell side scales are usually hand sawed and washed with high pressure water. It is also possible to chemically remove oil scales. Tips to Reduce Maintenance OPERATION TECHNICIANS ARE IN A POSITION TO MAKE A SUBSTANTIAL CONTRIBUTION TO REDUCE MAINTENANCE AND IMPROVED OPERATING EFFICIENCY. By RECOGNIZING PROPER OPERATING PROCEDURES AND REALIZING THE EFFECT OF INCORRECT PROCEDURES ON EQUIPMENT LIFE, HE/SHE CAN PREVENT MATERIAL FAILURE AND EQUIPMENT INOPERABILITY Some example of recommended procedures by which we can help reduce maintenance and extend the life and efficiency of exchangers: • Maintain proper cooling water outlet temperature (not in excess of 125oF) Tips to Reduce Maintenance Failure to observe this recommendation may result in: 1. excessive scaling of tube wall. 2. accelerated corrosion of exchanger parts. 3. mechanical failure of exchanger in tube rolls. • Avoid introducing a high temperature stream into an exchanger before circulation of the cooling medium has been established. This is to prevent undue stress on the metal. • Maintain adequate flow rates through exchangers to wash fouling media from the bundle and maintain heating rates within the limits of thermal design to prevent overheating. • Operate within mechanical design limits to prevent overstress of metals. Tips to Reduce Maintenance • Utilize steam traps on steam heaters to maintain maximum steam pressure within exchanger. By passing traps results in excessive steam consumption and reduced overall transfer coefficient. • Guard against the use of superheated steam in exchangers designed for saturated steam heat. Superheated steam in such tube bundles reduces the overall transfer rate 10 times and may results in over heated equipment.