hhhhr.pdf

hhhhr.pdf

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  1 Port Said University Facultyof Engineering Heat Exchanger Made by/ Abdelmgeed Mahmoud Abdelmgeed ID/ 20182014 Department/ Natural Gas Subject/ Heat Transfer Prof. Dr./ Taha Ibrahiem Farrag
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  2 Abstract The aim ofthis report is to define heat exchanger, how it works, the types of heat exchanger, the use of each type, advantages, and disadvantages of it, and what is the most common heat exchanger used, then which factors affect in the selection of the heat exchanger, finally the applications of heat exchangers. Table of contents 1. Introduction ……………………………………………………………… (3) 2. Overall …………………………………………………………………… (4) 3. Fouling factor ……………………………………………………………. (5) 4. Types of heat exchanger …………………………………………………. (6) 4.1 Double Pipe Heat Exchanger……………………………………………. (6) 4.2 Shell & Tube Heat Exchanger ……………………………………...…… (7) 4.3 Plate Heat Exchanger …………………………………………………… (9) 4.4 Condensers, Evaporators, and Boilers…………………………………. (11) 5. Selection of heat exchangers …………………………………………... (11) 6. Application of heat exchangers ………………………………………… (12) 7. References ……………………………………………………………… (13)
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  3 1. Introduction A heatexchanger is a device that is used for transfer of thermal energy (enthalpy) between two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid, at differing temperatures and in thermal contact, usually without external heat and work interactions. The fluids may be single compounds or mixtures. Typical applications involve heating or cooling of a fluid stream of concern, evaporation, or condensation of a single or multicomponent fluid stream, and heat recovery or heat rejection from a system. In other applications, the objective may be to sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control process fluid. In some heat exchangers, the fluids exchanging heat are in direct contact. In other heat exchangers, heat transfer between fluids takes place through a separating wall or into and out of a wall in a transient manner. In most heat exchangers, the fluids are separated by a heat transfer surface, and ideally, they do not mix. Such exchangers are referred to as the direct transfer type, or simply recuperators. In contrast, exchangers in which there is an intermittent heat exchange between the hot and cold fluids via thermal energy storage and rejection through the exchanger surface or matrix--are referred to as the indirect transfer type or storage type, or simply regenerators. Such exchangers usually have leakage and fluid carryover from one stream to the other. A heat exchanger consists of heat exchanging elements such as a core or a matrix containing the heat transfer surface, and fluid distribution elements such as headers, manifolds, tanks, inlet and outlet nozzles or pipes, or seals. Usually there are no moving parts in a heat exchanger; however, there are exceptions such as a rotary regenerator (in which the matrix is mechanically driven to rotate at some design speed), a scraped surface heat exchanger, agitated vessels, and stirred tank reactors. Heat exchangers have three (3) primary flow configurations: Parallel flow – the two fluids enter at the same end of the heat exchanger and flow in the same direction, parallel to one another. In this design, the temperature differences are large at the inlet, but the fluid temperatures will approach a similar value at the outlets. Counter flow – the two fluids enter at opposite ends of the heat exchanger and flow counter to one another. In this design, the temperature differences are less but more constant over the length of the exchanger. It is possible that the fluid being heated may leave the exchanger at a higher temperature than the exit temperature of the heating fluid. This is the most efficient design because of higher temperature differential over length of the exchanger.
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  4 Cross flow –the two fluids flow perpendicular to one another. Figure (1) 2. Overall A heat exchanger typically involves two flowing fluids separated by a solid wall. Heat is first transferred from the hot fluid to the wall by convection, through the wall by conduction, and from the wall to the cold fluid again by convection. Any radiation effects are usually included in the convection heat transfer coefficients. Figure (2)
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  5 where TA andTB are the fluid temperatures on each side of the wall. The overall heat-transfer coefficient U is defined by the relation q=U*A*T overall Heat transfer flow rate (q) Watt Overall heat transfer (U) Watt/m2 . K Area (A) m2 Temperature difference (T) K 3. FOULING FACTORS After a period of operation, the heat-transfer surfaces for a heat exchanger may become coated with various deposits present in the flow systems, or the surfaces may become corroded because of the interaction between the fluids and the material used for construction of the heat exchanger. In either event, this coating represents an additional resistance to the heat flow, and thus results in decreased performance. The overall effect is usually represented by a fouling factor, or fouling resistance, Rf, which must be included along with the other thermal resistances making up the overall heat-transfer coefficient. Fouling factors must be obtained experimentally by determining the values of U for both clean and dirty conditions in the heat exchanger. The fouling factor is thus defined as. Rf = 𝟏 𝑼𝒅𝒊𝒓𝒕𝒚 - 𝟏 𝑼𝒄𝒍𝒆𝒂𝒏
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  6 4. Types ofheat exchanger 1. Double Pipe Heat Exchanger 2. Shell & Tube Heat Exchanger 3. Plate Heat Exchanger 4. Condensers, Evaporators, and Boilers 4.1 Double Pipe Heat Exchanger A form of shell and tube heat exchanger, double pipe heat exchangers employ the simplest heat exchanger design and configuration which consists of two or more concentric, cylindrical pipes or tubes (one larger tube and one or more smaller tubes). As per the design of the shell and tube heat exchanger, one fluid flows through the smaller tube(s), and the other fluid flows around the smaller tube(s) within the larger tube. The design requirements of double pipe heat exchangers include characteristics from the recuperative and indirect contact types mentioned previously as the fluids remain separated and flow through their own channels throughout the heat transfer process. However, there is some flexibility in the design of double pipe heat exchangers, as they can be designed with concurrent or countercurrent flow arrangements and to be used modularly in series, parallel, or series-parallel configurations within a system. Figure (3)
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  7 Advantages • Simplest type •Can be easily assembled • Relative low-cost cost/ft2 • Suited to high pressure applications. Disadvantages • Leakages are very common • Require a lot of time in dismantling and clean • Small surface area of heat transfer • Space requirements are large 4.2 Shell and Tube heat exchanger Shell and Tube Heat Exchangers are the most used in the chemical processing industries today. It possesses large surface areas for heat transfer and can operate at high temperatures and pressures. Figure (4)
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  8 Two fluids ofdifferent starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the tube side. To transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this way, waste heat can be use. This is an efficient way to conserve energy. Baffles: used to increase the shell fluid turbulence and hence its heat transfer coefficient. Tubes: we use fluids with, high pressure, more fouling and cooling water or heated steam. Shells: we use fluids with more viscous and low Reynold number. Figure (5) Triangle pitch it’s the default and the most common Advantages • More compact for the same shell • Less flow area of shell, increase heat transfer coefficient Disadvantages • Difficult to clean • High shell side pressure drop
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  9 Square pitch Advantages • Pressuredrop is small • Outer surface tube clean is accessible • Used for high viscous or fouling shell side fluids Disadvantages • Less number of tubes so less heat transfer area Advantages and disadvantages of shell and tube heat exchanger Advantages • simple structure • ideal for heat transfer from steam to water Disadvantages • large design 4.3 Plate Heat Exchangers Also referred to as plate type heat exchangers, plate heat exchangers are constructed of several thin, corrugated plates bundled together. Each pair of plates creates a channel through which one fluid can flow, and the pairs are stacked and attached—via bolting, brazing, or welding—such that a second passage is created between pairs through which the other fluid can flow. The standard plate design is also available with some variations, such as in plate fin or pillow plate heat exchangers. Plate fin exchangers employ fins or spacers between plates and allow for multiple flow configurations and more than two fluid
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  10 streams to passthrough the device. Pillow plate exchangers apply pressure to the plates to increase the heat transfer efficiency across the surface of the plate. Some of the other types available include plate and frame, plate and shell, and spiral plate heat exchangers. • use even with minimal temperature differences • transfer between liquids and gases, between two liquids or between two gases • with and without phase change Advantages • large exchange area due to embossing of the plate surface • compact design, low filling volume • good convective heat transfer due to turbulent flow Disadvantages • high pressure loss • maintenance intensive Figure (6)
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  11 4.4 Condensers, Evaporators,and Boilers Boilers, condensers, and evaporators are heat exchangers which employ a two- phase heat transfer mechanism. As mentioned previously, in two-phase heat exchangers one or more fluids undergo a phase change during the heat transfer process, either changing from a liquid to a gas or a gas to a liquid. Condensers are heat exchanging devices that take heated gas or vapor and cool it to the point of condensation, changing the gas or vapor into a liquid. On the other hand, in evaporators and boilers, the heat transfer process changes the fluids from liquid form to gas or vapor form. 5. Selection of heat exchanger Heat exchangers are complicated devices, and the results obtained with the simplified approaches presented above should be used with care. For example, we assumed that the overall heat transfer coefficient U is constant throughout the heat exchanger and that the convection heat transfer coefficients can be predicted using the convection correlations. However, it should be kept in mind that the uncertainty in the predicted value of U can even exceed 30 percent. Thus, it is natural to tend to overdesign the heat exchangers in order to avoid unpleasant surprises. Heat transfer enhancement in heat exchangers is usually accompanied by increased pressure drop, and thus higher pumping power. Therefore, any gain from the enhancement in heat transfer should be weighed against the cost of the accompanying pressure drop. Also, some thought should be given to which fluid should pass through the tube side and which through the shell side. Usually, the more viscous fluid is more suitable for the shell side (larger passage area and thus lower pressure drop) and the fluid with the higher pressure for the tube side. Engineers in industry often find themselves in a position to select heat exchangers to accomplish certain heat transfer tasks. Usually, the goal is to heat or cool a certain fluid at a known mass flow rate and temperature to a desired temperature. Thus, the rate of heat transfer in the prospective heat exchanger is
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  12 Q max =m Cp (T in – T out) Maximum heat transfer flow rate (Q max) Watt Mass flow rate (m) Kg/m3 Specific heat at constant pressure (Cp) kJ/kg · K Temperature difference (T in-Tout) K which gives the heat transfer requirement of the heat exchanger before having any idea about the heat exchanger itself. An engineer going through catalogs of heat exchanger manufacturers will be overwhelmed by the type and number of readily available off-the-shelf heat exchangers. The proper selection depends on several factors. 1. Heat Transfer Rate 2. Cost 3. Size and Weight 4. Type of Fluid 5. Material 6. Other Considerations: There are other considerations in the selection of heat exchangers that may or may not be important, depending on the application. For example, being leak-tight is an important consideration when toxic or expensive fluids are involved. Ease of servicing, low maintenance cost, and safety and reliability are some other important considerations in the selection process. Quietness is one of the primary considerations in the selection of liquid-to-air heat exchangers used in heating and air-conditioning applications.
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  13 6. Application onheat exchanger Type of Heat Exchanger Common Industries and Applications Shell and Tube • Oil refining • Preheating • Oil cooling • Steam generation • Boiler blowdown heat recovery • Vapor recovery systems • Industrial paint systems Double Pipe • Industrial cooling processes • Small heat transfer area requirements Plate • Cryogenic • Food processing • Chemical processing • Furnaces • Closed loop to open loop water cooling Condensers • Distillation and refinement processes • Power plants • Refrigeration • HVAC • Chemical processing Evaporators/Boilers • Distillation and refinement processes • Steam trains • Refrigeration • HVAC 7. References https://ifsolutions.com/types-of-heat-exchangers-in-oil-gas-applications-how-heat-exchangers-work/ https://www.thomasnet.com/articles/process-equipment/understanding-heat-exchangers/ https://www.youtube.com/watch?v=2DN5cc444zY&list=PLsGufry5cP1EQDms3tpso_vWrbIOG1I3k Heat and Mass Transfer (Fundamentals and Applications), By: Yunus A. Engel Fundamentals of Heat and mass Transfer, By: Frank P. Incropera
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