Home A - Z
Page Index

An Otter Wiki

Blame

62c67d João Lopes 2025-03-13 12:01:58 1 
![](./fa4pc54n.png)
374ad8 João Lopes 2025-03-13 11:57:41 2
3
> Departamento de Engenharia Química
4
>
5
> Mestrado Integrado em Engenharia Química
6
>
7
> Integração e intensificação de processos
8
>
9
> Shell and Tubes Heat Exchangers
10
>
11
> **Docente** **Responsável:**
12
>
13
> Nuno Manuel Clemente de Oliveira
14
>
15
> **Integrantes** **do** **grupo:**
16
17
João Victor Vieira
18
19
> Matteo Gecchele
20
21
22
> **Introduction** **&** **Structure**
23
>
24
> The most common type of heat exchanger is the shell-and-tube, usually
25
> used in a lot of industrial applications. This type of heat exchanger
26
> has large number of tubes, sometimes several hundred, packed in a
27
> shell with their axes parallel to that of the shell. The heat transfer
28
> takes place between two fluid, one flowing inside the tubes and one
29
> flowing outside the tubes through the shell. Baffles are commonly
30
> placed in the shell to force the shell-side fluid to flow across the
31
> shell to enhance heat transfer, to maintain uniform spacing between
32
> the tubes and, also in order to maintain the turbulent flow inside the
33
> exchanger. The baffle spacing is usually not greater than a distance
34
> equal to the inside diameter or closer than a distance equal to
35
> one-fifth the inside diameter of the shell.
36
>
37
> Usually the shell-and-tube heat exchangers have large size and weight,
38
> and for this reason they are not using in automotive and aircraft
39
> applications. At both ends of the shell, the tubes open to some large
40
> flow areas, called headers, where the tube-side fluid accumulates
41
> before entering the tubes and after leaving them.
62c67d João Lopes 2025-03-13 12:01:58 42
>![](./xvze01o4.png)
5b9f6d João Lopes 2025-03-13 12:02:35 43
>
374ad8 João Lopes 2025-03-13 11:57:41 44
> Shell-and-tube heat exchangers are further classified according to the
45
> number of shell and tube passes involved. Heat exchangers in which all
46
> the tubes make one U-turn in the shell, for example, are called
47
> one-shell-pass and two-tube-passes heat exchangers. Likewise, a heat
48
> exchanger that involves two passes in the shell and four passes in the
49
> tubes is called a two-shell- passes and four-tube-passes heat
50
> exchanger.
51
62c67d João Lopes 2025-03-13 12:01:58 52
![](./tnci3l31.png)![](./cncqllod.png)
53
374ad8 João Lopes 2025-03-13 11:57:41 54
55
> **Operation** **principle**
56
>
57
> In order to calculate the temperature difference ∆𝑡 in a 1-2
58
> exchanger, it is necessary to make some assumptions:
59
>
60
> 1\. The shell fluid temperature is an average isothermal temperature
61
> at any cross section
62
>
63
> 2\. There is an equal amount of heating surface in each pass 3. The
64
> overall coefficient of heat transfer is constant
65
>
66
> 4\. The specific heat of each fluid is constant 5. The flowrate of
67
> each fluid is constant
68
>
69
> 6\. There are not phase change (evaporation or condensation) in a part
70
> of the exchanger
71
>
72
> 7\. Heat losses are negligible
73
>
74
> The overall heat balance where ∆𝑡 is the true difference of
75
> temperatures, is:
76
>
77
> 𝑄 = 𝑈𝐴∆𝑡 = 𝑊𝐶(𝑇 − 𝑇 ) = 𝑤𝑐(𝑡2 − 𝑡1) where U is the heat transfer
78
> coefficient and A is the surface of contact.
79
>
80
> Shell-and-tube heat exchangers are complicated devices and the
81
> simplified approaches should be used with care. In fact, it is assumed
82
> that the overall heat transfer coefficient U is constant throughout
83
> the heat exchanger and that the convection heat transfer coefficients
84
> can be predicted using the convection correlations. However, in some
85
> practical application, the predicted value of U can exceed 30 percent.
86
> Thus, it is natural to tend to overdesign the heat exchangers in order
87
> to avoid unpleasant surprises.
88
>
89
> Heat transfer enhancement in heat exchangers is usually accompanied by
90
> increased
91
>
92
> pressure drop, and this causes higher pumping power. Therefore, any
93
> gain from the enhancement in heat transfer should be balanced against
94
> the cost of the accompanying pressure drop. Also, some thought should
95
> be given to which fluid should pass through the tube side and which
96
> through the shell side. Usually, the more viscous fluid is more
97
> suitable for the shell side (larger passage area and lower pressure
98
> drop) and the fluid with the higher pressure for the tube side.
99
>
100
> Usually, it is convenient to relate the equivalent temperature
101
> difference to the log
102
>
103
> mean temperature difference relation for the counter-flow case as
104
>
105
> ∆ 𝑙𝑚 = 𝐹∆ 𝑙𝑚,𝐶𝐹
106
107
where *F* is the correction factor**,** which depends on the geometry of
108
the heat exchanger and the inlet and outlet temperatures of the hot and
109
cold fluid streams. The
110
111
> ∆𝑇𝑚,𝐶𝐹 is the log mean temperature difference for the case of a
112
> counter-flow heat exchanger with the same inlet and outlet
113
> temperatures.
114
>
115
> The correction factor *F* for a shell-and-tube heat exchanger is shown
116
> in the figures below versus two temperature ratios *P* and *R* defined
117
> as
118
>
119
> 𝑡2 − 𝑡1 𝑇 − 𝑡1
120
>
121
> 𝑇 − 𝑇 𝑡2 − 𝑡1
122
>
123
> where the subscripts 1 and 2 represent the inlet and outlet*,*
124
> respectively. Note that for
125
>
126
> a shell-and-tube heat exchanger, *T* and *t* represent the shell-side
127
> and tube-side temperatures, respectively.
128
62c67d João Lopes 2025-03-13 12:01:58 129
![](./gklfx0zi.png)
374ad8 João Lopes 2025-03-13 11:57:41 130
131
> **Factors** **that** **influence** **performances** *Fouling:*
132
>
133
> The performance of heat exchangers usually deteriorates with time as a
134
> result of accumulation of deposits on heat transfer surfaces. The
135
> layer of deposits represents additional resistance to heat transfer
136
> and this causes a decrease of the rate of heat transfer in a heat
137
> exchanger. The net effect of these accumulations on heat transfer is
138
> represented by a fouling factor, which is a measure of the thermal
139
> resistance introduced by fouling.
140
>
141
> For a shell-and-tube heat exchanger it possible to write the overall
142
> heat transfer relation as
143
>
144
> 𝑈𝐴𝑠 = 𝑈𝐴𝑖 = 𝑈0𝐴0 = 𝑅 = ℎ𝑖𝐴𝑖 + 𝐴𝑖𝑖 + ln⁡𝑈0𝐴0 𝑖) + 𝐴0 + ℎ0𝐴0
145
>
146
> where 𝐴𝑖 = 𝐷𝐿 and 𝐴0 = 𝐷0𝐿 L are the areas of inner and outer
147
> surfaces, and 𝑅,𝑖 and 𝑅,0 are the fouling factors at those surfaces.
148
>
149
> *Heat* *transfer* *rate:*
150
>
151
> The heat transfer rate is the most important parameter of a heat
152
> exchanger. A heat exchanger should be capable of transferring heat at
153
> the specified rate in order to achieve the desired temperature change
154
> of the fluid at the specified mass flow rate.
155
>
156
> *Size* *and* *Weight:*
157
>
158
> The heat exchanger is better if it is smaller and lighter, in
159
> particular, in the automotive and aerospace industries, where size and
160
> weight requirements are most stringent. For this reason,
161
> shell-and-tube heat exchangers cannot be used in this type of
162
> application. Also, a larger heat exchanger normally carries a higher
163
> price tag. The space available for the heat exchanger in some cases
164
> limits the length of the tubes that can be used.
165
>
166
> *Material:*
167
>
168
> The thermal and structural stress effects need not be considered at
169
> pressures below 15 *atm* or temperatures below 150*°C*. But these
170
> effects are major considerations above 70 *atm* or 550*°C* and
171
> seriously limit the acceptable materials of the heat exchanger.
172
>
173
> A temperature difference of 50*°C* or more between the tubes and the
174
> shell will probably pose differential thermal expansion problems and
175
> needs to be considered. In the case of corrosive fluids, we may have
176
> to select expensive corrosion-resistant materials such as stainless
177
> steel or even titanium.
178
>
179
> **Cost**
180
>
181
> The purchase cost of a shell and tube depends on the rear head type
182
> and on the heat transfer
183
>
184
> area (size factor). The relationship between the purchase cost and the
185
> size factor is
186
>
187
> represented in the graph below
188
62c67d João Lopes 2025-03-13 12:01:58 189
![](./s1x5d1ti.png)
374ad8 João Lopes 2025-03-13 11:57:41 190
191
> Both fluids are usually forced to flow by pumps or fans that consume
192
> electrical power. The annual cost of electricity associated with the
193
> operation of the pumps and fans can be determined from
194
>
195
> 𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔⁡𝐶𝑜𝑠𝑡 = 𝑃𝑢𝑚𝑝𝑖𝑛𝑔⁡𝑃𝑜𝑤𝑒𝑟⁡\[𝑘𝑊\] × 𝐻𝑜𝑢𝑟𝑠⁡𝑜𝑓⁡𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛⁡\[ℎ\] ×
196
> 𝑃𝑟𝑖𝑐𝑒𝑜𝑓⁡𝐸𝑙𝑒𝑐𝑟𝑖𝑐𝑖𝑡𝑦⁡\[\$ 𝑘𝑊ℎ\]
197
>
198
> where the pumping power is the total electrical power consumed by the
199
> motors of the pumps and fans.
200
>
201
> Minimizing the pressure drop and the mass flow rate of the fluids will
202
> minimize the operating cost of the heat exchanger, but it will
203
> maximize the size of the heat exchanger and thus the initial cost. As
204
> a rule of thumb, doubling the mass flow rate will reduce the initial
205
> cost by half but will increase the pumping power requirements by a
206
> factor of roughly eight. Typically, fluid velocities encountered in
207
> heat exchangers range between 0.7 and 7 m/s for liquids and between 3
208
> and 30 m/s for gases. Low velocities are helpful in avoiding erosion,
209
> tube vibrations, and noise as well as pressure drop.
210
>
211
> **Advantages**:
212
>
213
> *Size:*
214
>
215
> Shell-and-tube heat exchangers are capable of providing a larger
216
> surface area for heat transfer to take place while having a shorter
217
> length overall due to presence of multiple tubes.
218
>
219
> *Heat* *duty:*
220
>
221
> Shell-and-tube heat exchangers can handle higher temperatures and
222
> pressures and hence higher heat duty. This is because besides
223
> providing a higher overall heat transfer coefficient, additions can
224
> also be made to negate thermal expansion effects and the thickness can
225
> also be varied (more in the next point).
226
>
227
> *Versatility:*
228
>
229
> From the design point of view, shell-and-tube heat exchangers are the
230
> most versatile of all heat exchangers. Being tubular in shape, heads /
231
> closures of required shape and thickness can be used. The number of
232
> tubes and tube pitch can be selected according to operating
233
> conditions. Expansion bellows can be used to negate thermal expansion
234
> effects, baffles if different cuts and spacings can be used to
235
> influence the overall heat transfer coefficients and there's even
236
> something called a floating head which can be added to negate thermal
237
> expansion of the tubes. The number of passes on shell side and tube
238
> side can be altered as well.
239
>
240
> **Disadvantages**:
241
>
242
> *Size:*
243
>
244
> This can also be a disadvantage as at lower heat duty, there are more
245
> compact heat exchangers such as plate type exchanger. Also, the
246
> absence of hairpin bends causes shell-and-tube heat exchangers to take
247
> up more space than double pipe heat exchangers in some cases.
248
>
249
> *Maintenance:*
250
>
251
> Cleaning of tubes is difficult and fouling is always an issue when
252
> overall heat transfer coefficient is addressed. This requires periodic
253
> cleaning of the shell as well as the tubes. Cleaning tubes may be more
254
> difficult if the pitch is triangular.
255
>
256
> **Utilities**
257
>
258
> The selection of utilities to be used in the shell and tubes tube
259
> exchanger takes into
260
>
261
> account the type of industry in which it is being operated and the
262
> desired parameters, such as the required power, thermal stability and
263
> thermal capacity.
264
>
265
> *Cooling* *Water*:
266
>
267
> Cooling water is used to cool and/or condense currents. The cooling
268
> water circulates inside heat exchangers. About 80% of the temperature
269
> reduction is due to the evaporation of the cooling water and the
270
> transfer of heat to the surrounding air.
271
>
272
> *Steam:*
273
>
274
> Steam is the most common heat utility used in the chemical industry
275
> and can be used to power pumps, compressors and heat exchangers. Using
276
> steam allows a more efficient heat source since the heat of
277
> condensation of the steam is quite high, which translates into a high
278
> yield per utility mass, at a constant temperature. Another reason is
279
> that steam is non-flammable, non-toxic and inert to various process
280
> fluids (more safe than other utilities like oil).
281
>
282
> **Conclusion**
283
>
62c67d João Lopes 2025-03-13 12:01:58 284
> The simple design of a shell and tube heat exchanger makes it an ideal
374ad8 João Lopes 2025-03-13 11:57:41 285
> cooling solution for a wide variety of applications and as a
286
> consequence shell-and-tube heat exchangers are very popular and
287
> commonly found in industrial use.
288
>
289
> **References**
290
>
291
> \[1\] Notes on Transfer Phenomena II, Professor Maria Graça Carvalho,
292
> 2018/2019;
293
>
294
> \[2\] Warren D. Seider, University of Pennsylvania
295
>
296
> \[3\] Heat Transfer by Changel 2nd Edition
297
>
298
> \[4\] Heat Transfer by Holman 6th Edition