Heat exchangers operations are guided by the fact that heat transfer is a product of temperature variation between cold process stream and hot process stream. A thin solid layer is used in separation of the two streams. …
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The wall has to be conducive to allow heat exchange and still be sufficiently strong to withstand fluid/gas pressures. In shell and tube heat exchangers, two closed process streams move across the unit; one move inside the tube and the other moves on the shell side. Convection and conduction allows heat to pass from hot stream to cold stream from the side of the tube side or from shell side.
As temperature variation between the process streams rise, heat exchange rate for every surface area unit also rises. Conversely, heat exchangers per surface are unit drops non-linearly as temperature difference between the two process streams drops. Increasing the effective surface area of the entire system helps in maintenance of the total transfer of heat between two streams although eventually the system reaches a point where extra surface area has no effect on extra heat transfer.
The other variable which affects heat exchange in shell and tube exchanger is each process stream’s velocity. This velocity directly contributes to a rise in convection cold process and hot process streams. Raising the velocity also raises heat exchange, more especially, in countercurrent design. Finally, velocity increments are limited by maximum permitted for a specific metallurgy constituting shell or tube. For carbon steel, for instance, velocity cannot exceed 6 ft. /sec. whilst for the case of stainless and high-alloy steel; rate is 12 ft. /sec. for liquids. The three conventional types of shell and tube heat exchangers are parallel, cross flow and countercurrent flow types. The names are derived from the process stream directions in relation to each other. In countercurrent heat exchanger type, average temperature variation between the process streams is optimized over the exchanger’s length, showing the highest heat transfer rate efficiency over a surface area unit. With respect to existing temperature variations observed during operation, parallel heat exchangers exhibit the lowest heat transfer rates, and then cross flow heat exchangers, and finally, countercurrent heat exchangers. Counterflow and parallel heat exchangers are illustrated below, Figure 1: Counterflow and parallel heat exchangers The design of shell and tube heat exchanger depends on flow pattern through the respective heat exchanger. It is however the most widely used heat exchanger in industries and can adopt counter-flow, parallel flow or cross-flow pattern. However, heat transfer area is a major factor in design calculation. Theoretically though, shell and tube heat exchanger flow patter is conventionally not specifically counter-flow, or parallel. Rather, it incorporates a mixture of counter-flow, parallel flow and cross-flow. Log mean temperature variation, used for design of shell and tube heat exchanger, works best for varied flow patterns occurring in this kind of heat exchanger. Shell and tube heat exchangers Shell and tube heat exchangers in their various construction modifications are probably the most widespread and commonly used basic heat exchanger configuration in the process industries. The reasons for this general acceptance are several. The shell and tube heat exchanger provides a comparatively large ratio of heat transfer area to volume and weight. It provides this surface in a form which is relatively easy to construction in a wide range of sizes and which is mechanically rugged enough to withstand normal shop fabrication stresses, shipping and field erection stresses, and normal operating conditions. There are many modifications of the basic configuration, which can be used to solve special problems. The shell and tube exchanger can be reasonably easily cleaned, and those components most subject to failure -
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(“Shell and tube heat exchanger design Coursework”, n.d.)
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(Shell and Tube Heat Exchanger Design Coursework)
“Shell and Tube Heat Exchanger Design Coursework”, n.d. https://studentshare.org/design-technology/1498811-shell-and-tube-heat-exchanger-design.
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