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<img src="./fa4pc54n.png"
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> Departamento de Engenharia Química
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>
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> Mestrado Integrado em Engenharia Química
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>
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> Integração e intensificação de processos
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>
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> Shell and Tubes Heat Exchangers
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>
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> **Docente** **Responsável:**
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>
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> Nuno Manuel Clemente de Oliveira
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>
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> **Integrantes** **do** **grupo:**
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João Victor Vieira
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> Matteo Gecchele
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> **Introduction** **&** **Structure**
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>
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> The most common type of heat exchanger is the shell-and-tube, usually
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> used in a lot of industrial applications. This type of heat exchanger
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> has large number of tubes, sometimes several hundred, packed in a
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> shell with their axes parallel to that of the shell. The heat transfer
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> takes place between two fluid, one flowing inside the tubes and one
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> flowing outside the tubes through the shell. Baffles are commonly
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> placed in the shell to force the shell-side fluid to flow across the
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> shell to enhance heat transfer, to maintain uniform spacing between
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> the tubes and, also in order to maintain the turbulent flow inside the
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> exchanger. The baffle spacing is usually not greater than a distance
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> equal to the inside diameter or closer than a distance equal to
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> one-fifth the inside diameter of the shell.
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>
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> Usually the shell-and-tube heat exchangers have large size and weight,
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> and for this reason they are not using in automotive and aircraft
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> applications. At both ends of the shell, the tubes open to some large
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> flow areas, called headers, where the tube-side fluid accumulates
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> before entering the tubes and after leaving them.
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>
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> Shell-and-tube heat exchangers are further classified according to the
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> number of shell and tube passes involved. Heat exchangers in which all
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> the tubes make one U-turn in the shell, for example, are called
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> one-shell-pass and two-tube-passes heat exchangers. Likewise, a heat
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> exchanger that involves two passes in the shell and four passes in the
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> tubes is called a two-shell- passes and four-tube-passes heat
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> exchanger.
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<img src="./tnci3l31.png"
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> **Operation** **principle**
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>
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> In order to calculate the temperature difference ∆𝑡 in a 1-2
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> exchanger, it is necessary to make some assumptions:
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>
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> 1\. The shell fluid temperature is an average isothermal temperature
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> at any cross section
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>
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> 2\. There is an equal amount of heating surface in each pass 3. The
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> overall coefficient of heat transfer is constant
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>
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> 4\. The specific heat of each fluid is constant 5. The flowrate of
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> each fluid is constant
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>
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> 6\. There are not phase change (evaporation or condensation) in a part
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> of the exchanger
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>
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> 7\. Heat losses are negligible
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>
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> The overall heat balance where ∆𝑡 is the true difference of
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> temperatures, is:
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>
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> 𝑄 = 𝑈𝐴∆𝑡 = 𝑊𝐶(𝑇 − 𝑇 ) = 𝑤𝑐(𝑡2 − 𝑡1) where U is the heat transfer
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> coefficient and A is the surface of contact.
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>
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> Shell-and-tube heat exchangers are complicated devices and the
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> simplified approaches should be used with care. In fact, it is assumed
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> that the overall heat transfer coefficient U is constant throughout
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> the heat exchanger and that the convection heat transfer coefficients
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> can be predicted using the convection correlations. However, in some
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> practical application, the predicted value of U can exceed 30 percent.
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> Thus, it is natural to tend to overdesign the heat exchangers in order
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> to avoid unpleasant surprises.
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>
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> Heat transfer enhancement in heat exchangers is usually accompanied by
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> increased
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>
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> pressure drop, and this causes higher pumping power. Therefore, any
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> gain from the enhancement in heat transfer should be balanced against
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> the cost of the accompanying pressure drop. Also, some thought should
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> be given to which fluid should pass through the tube side and which
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> through the shell side. Usually, the more viscous fluid is more
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> suitable for the shell side (larger passage area and lower pressure
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> drop) and the fluid with the higher pressure for the tube side.
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>
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> Usually, it is convenient to relate the equivalent temperature
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> difference to the log
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>
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> mean temperature difference relation for the counter-flow case as
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>
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> ∆ 𝑙𝑚 = 𝐹∆ 𝑙𝑚,𝐶𝐹
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where *F* is the correction factor**,** which depends on the geometry of
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the heat exchanger and the inlet and outlet temperatures of the hot and
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cold fluid streams. The
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> ∆𝑇𝑚,𝐶𝐹 is the log mean temperature difference for the case of a
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> counter-flow heat exchanger with the same inlet and outlet
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> temperatures.
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>
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> The correction factor *F* for a shell-and-tube heat exchanger is shown
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> in the figures below versus two temperature ratios *P* and *R* defined
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> as
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>
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> 𝑡2 − 𝑡1 𝑇 − 𝑡1
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>
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> 𝑇 − 𝑇 𝑡2 − 𝑡1
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>
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> where the subscripts 1 and 2 represent the inlet and outlet*,*
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> respectively. Note that for
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>
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> a shell-and-tube heat exchanger, *T* and *t* represent the shell-side
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> and tube-side temperatures, respectively.
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<img src="./gklfx0zi.png"
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> **Factors** **that** **influence** **performances** *Fouling:*
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>
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> The performance of heat exchangers usually deteriorates with time as a
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> result of accumulation of deposits on heat transfer surfaces. The
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> layer of deposits represents additional resistance to heat transfer
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> and this causes a decrease of the rate of heat transfer in a heat
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> exchanger. The net effect of these accumulations on heat transfer is
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> represented by a fouling factor, which is a measure of the thermal
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> resistance introduced by fouling.
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>
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> For a shell-and-tube heat exchanger it possible to write the overall
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> heat transfer relation as
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>
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> 𝑈𝐴𝑠 = 𝑈𝐴𝑖 = 𝑈0𝐴0 = 𝑅 = ℎ𝑖𝐴𝑖 + 𝐴𝑖𝑖 + ln⁡𝑈0𝐴0 𝑖) + 𝐴0 + ℎ0𝐴0
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>
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> where 𝐴𝑖 = 𝐷𝐿 and 𝐴0 = 𝐷0𝐿 L are the areas of inner and outer
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> surfaces, and 𝑅,𝑖 and 𝑅,0 are the fouling factors at those surfaces.
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>
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> *Heat* *transfer* *rate:*
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> The heat transfer rate is the most important parameter of a heat
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> exchanger. A heat exchanger should be capable of transferring heat at
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> the specified rate in order to achieve the desired temperature change
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> of the fluid at the specified mass flow rate.
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> *Size* *and* *Weight:*
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> The heat exchanger is better if it is smaller and lighter, in
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> particular, in the automotive and aerospace industries, where size and
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> weight requirements are most stringent. For this reason,
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> shell-and-tube heat exchangers cannot be used in this type of
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> application. Also, a larger heat exchanger normally carries a higher
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> price tag. The space available for the heat exchanger in some cases
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> limits the length of the tubes that can be used.
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>
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> *Material:*
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>
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> The thermal and structural stress effects need not be considered at
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> pressures below 15 *atm* or temperatures below 150*°C*. But these
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> effects are major considerations above 70 *atm* or 550*°C* and
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> seriously limit the acceptable materials of the heat exchanger.
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>
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> A temperature difference of 50*°C* or more between the tubes and the
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> shell will probably pose differential thermal expansion problems and
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> needs to be considered. In the case of corrosive fluids, we may have
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> to select expensive corrosion-resistant materials such as stainless
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> steel or even titanium.
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> **Cost**
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> The purchase cost of a shell and tube depends on the rear head type
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> and on the heat transfer
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> area (size factor). The relationship between the purchase cost and the
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> size factor is
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>
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> represented in the graph below
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<img src="./s1x5d1ti.png"
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> Both fluids are usually forced to flow by pumps or fans that consume
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> electrical power. The annual cost of electricity associated with the
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> operation of the pumps and fans can be determined from
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>
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> 𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔⁡𝐶𝑜𝑠𝑡 = 𝑃𝑢𝑚𝑝𝑖𝑛𝑔⁡𝑃𝑜𝑤𝑒𝑟⁡\[𝑘𝑊\] × 𝐻𝑜𝑢𝑟𝑠⁡𝑜𝑓⁡𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛⁡\[ℎ\] ×
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> 𝑃𝑟𝑖𝑐𝑒𝑜𝑓⁡𝐸𝑙𝑒𝑐𝑟𝑖𝑐𝑖𝑡𝑦⁡\[\$ 𝑘𝑊ℎ\]
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>
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> where the pumping power is the total electrical power consumed by the
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> motors of the pumps and fans.
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> Minimizing the pressure drop and the mass flow rate of the fluids will
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> minimize the operating cost of the heat exchanger, but it will
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> maximize the size of the heat exchanger and thus the initial cost. As
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> a rule of thumb, doubling the mass flow rate will reduce the initial
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> cost by half but will increase the pumping power requirements by a
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> factor of roughly eight. Typically, fluid velocities encountered in
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> heat exchangers range between 0.7 and 7 m/s for liquids and between 3
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> and 30 m/s for gases. Low velocities are helpful in avoiding erosion,
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> tube vibrations, and noise as well as pressure drop.
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>
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> **Advantages**:
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>
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> *Size:*
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> Shell-and-tube heat exchangers are capable of providing a larger
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> surface area for heat transfer to take place while having a shorter
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> length overall due to presence of multiple tubes.
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>
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> *Heat* *duty:*
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>
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> Shell-and-tube heat exchangers can handle higher temperatures and
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> pressures and hence higher heat duty. This is because besides
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> providing a higher overall heat transfer coefficient, additions can
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> also be made to negate thermal expansion effects and the thickness can
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> also be varied (more in the next point).
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>
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> *Versatility:*
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>
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> From the design point of view, shell-and-tube heat exchangers are the
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> most versatile of all heat exchangers. Being tubular in shape, heads /
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> closures of required shape and thickness can be used. The number of
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> tubes and tube pitch can be selected according to operating
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> conditions. Expansion bellows can be used to negate thermal expansion
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> effects, baffles if different cuts and spacings can be used to
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> influence the overall heat transfer coefficients and there's even
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> something called a floating head which can be added to negate thermal
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> expansion of the tubes. The number of passes on shell side and tube
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> side can be altered as well.
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>
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> **Disadvantages**:
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>
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> *Size:*
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>
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> This can also be a disadvantage as at lower heat duty, there are more
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> compact heat exchangers such as plate type exchanger. Also, the
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> absence of hairpin bends causes shell-and-tube heat exchangers to take
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> up more space than double pipe heat exchangers in some cases.
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>
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> *Maintenance:*
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> Cleaning of tubes is difficult and fouling is always an issue when
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> overall heat transfer coefficient is addressed. This requires periodic
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> cleaning of the shell as well as the tubes. Cleaning tubes may be more
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> difficult if the pitch is triangular.
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>
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> **Utilities**
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> The selection of utilities to be used in the shell and tubes tube
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> exchanger takes into
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> account the type of industry in which it is being operated and the
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> desired parameters, such as the required power, thermal stability and
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> thermal capacity.
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> *Cooling* *Water*:
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>
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> Cooling water is used to cool and/or condense currents. The cooling
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> water circulates inside heat exchangers. About 80% of the temperature
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> reduction is due to the evaporation of the cooling water and the
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> transfer of heat to the surrounding air.
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>
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> *Steam:*
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> Steam is the most common heat utility used in the chemical industry
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> and can be used to power pumps, compressors and heat exchangers. Using
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> steam allows a more efficient heat source since the heat of
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> condensation of the steam is quite high, which translates into a high
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> yield per utility mass, at a constant temperature. Another reason is
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> that steam is non-flammable, non-toxic and inert to various process
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> fluids (more safe than other utilities like oil).
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>
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> **Conclusion**
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> The simple design of a shell and tube heat exchanger makes it an ideal
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> cooling solution for a wide variety of applications and as a
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> consequence shell-and-tube heat exchangers are very popular and
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> commonly found in industrial use.
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>
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> **References**
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>
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> \[1\] Notes on Transfer Phenomena II, Professor Maria Graça Carvalho,
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> 2018/2019;
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> \[2\] Warren D. Seider, University of Pennsylvania
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>
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> \[3\] Heat Transfer by Changel 2nd Edition
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> \[4\] Heat Transfer by Holman 6th Edition