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Heat transfer - Wikipedia
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Basic Heat Transfer
Upcoming SlideShare. Like this presentation? Why not share! Embed Size px. Start on. Show related SlideShares at end. WordPress Shortcode. Full Name Comment goes here. Are you sure you want to Yes No. Aqel Albari. No Downloads. A spiral heat exchanger is constructed by rolling two relatively long metal strips around a mandrel to form two concentric spiral channels. The channels are alternately welded on opposite ends to form a hot and cold channel. Welding the channels eliminates the potential for any cross-contamination of fluids and is analogous to a welded tube to tube-sheet joint in a shell-and-tube heat exchanger.
On one side, the hot fluid enters the center nozzle of the hot cover and flows in a spiral outward to a nozzle on a peripheral header. The cold fluid simultaneously enters a peripheral header and flows countercurrently to the hot fluid to the center nozzle on the cold side cover.
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Removable cover heads with full-face gaskets are used to seal the open end of the channels and prevent bypassing of a respective fluid from the peripheral header to the center nozzles. The heads are easily removed to allow access to all heat transfer surfaces. The countercurrent monochannels give exceptionally high convective heat transfer coefficients due to the high turbulence and secondary flow effects eddy currents and vortices.
The monochannel also minimizes the potential for fouling to occur because any buildup in the channel results in an increase in local velocity at that point, an action that tends to flush the deposit away. When a spiral heat exchanger does require cleaning, all heat transfer surfaces are readily accessible by simple removing the heads.
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Spiral heat exchangers are particularly effective for handling sludges, liquids in suspension including slurries, and a wide range of viscous fluids. Their design and fabrication make them well suited for controlling viscosity, a vital parameter when abrasive or corrosive fluids must be handled. The spiral heat exchanger is also used as a condenser and evaporator.
The plate heat exchanger consists of a series of thin, corrugated alloy plates, which are gasketed and compressed together inside a carbon steel frame.
Once compressed, the plate pack forms an arrangement of parallel flow channels. The two fluids hot and cold flow countercurrent to each other in alternate channels.
Basics and Practice
Each plate is fitted with a gasket to direct the flow, seal the unit, and prevent fluid intermixing. Plate heat exchangers are frequently found in a wide range of heating and cooling applications in the chemical, petrochemical, petroleum, pulp and paper, and pharmaceutical industries, as well as in many wastewater treatment applications. The proper choice of gasket materials is important for the reliable operation of plate heat exchangers. For nearly 60 yr, these units have primarily used elastomer gaskets to seal the unit, direct the flow, and prevent fluid intermixing. Commonly used elastomers today are variations of three basic materials: nitrile, ethylene propylene diene terpolymer EPDM , and Viton.
Nitrile is the most common and is suitable for fluids such as water, oils, and foodstuffs. EPDM is used for fluids such as water, steam, dilute acids, amines, and strong alkalines. Viton is the most expensive material and is typically used for aggressive fluids such as concentrated acids and some petroleum oils.
The goal is to minimize the surface area requirement, and thus the cost, of a given heat exchanger. The heat transfer area is minimized by maximizing the U value and LMTD for a given heat transfer duty. Examining the various parts of this equation, the heat transfer requirement or duty is usually user defined. It is expressed as a desire to heat or cool a certain flow-rate of fluid by a given amount. Duty is calculated in this way:. The arithmetic average can be used, but does not account for the diminishing returns effect caused by close temperature approaches see illustration above.
In general, countercurrent flow gives the greatest LMTD values with cocurrent giving the smallest.
The overall heat transfer coefficient, or U value, is calculated as the sum of various resistances to heat transfer that might be encountered. Its basic form is:. The way to maximize the overall heat transfer coefficient, U, is by maximizing the individual heat transfer coefficients, h, and by minimizing the resistance due to fouling, Rf.
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The film coefficients are affected by the physical properties of the fluids viscosity, thermal conductivity, and specific heat , and the degree of turbulence of the fluid. Both plate and spiral heat exchangers enhance the effectiveness of heat transfer by inducing turbulence into the fluid see illustration above, right. The fouling resistance, Rf, is minimized by limiting the buildup of fouling on the heat transfer surface. This condition is primarily controlled by wall shear stress. Again, both plate and spiral heat exchangers have inherently higher wall shear stresses than conventional tubular equipment.