## What is a shell and tube heat exchanger?

Shell-and-tube heat exchanger is also called tube-and-tube heat exchanger. A wall-to-wall heat exchanger with the wall of the tube bundle enclosed in the shell as the heat transfer surface. The heat exchanger is simple in structure and reliable in operation. It can be manufactured with various structural materials (mainly metal materials) and can be used under high temperature and pressure. It is the most widely used type at present.

Figure 1. Shell and tube exchanger.

## Structure and Type of Shell-and-Tube Heat Exchanger

The shell-and-tube heat exchanger is composed of shell, heat transfer tube bundle, tube plate, baffle and tube box. The shell is mostly cylindrical, with tube bundles inside, and both ends of the tube bundles are fixed on the tube plate. There are two kinds of heat and cold fluids for heat transfer, one is in-pipe flow, which is called tube-side fluid; the other is out-of-pipe flow, which is called shell-side fluid. In order to improve the heat transfer coefficient of the fluid outside the tube, several baffles are usually installed in the shell. The baffle can increase the velocity of fluid in the shell side, force the fluid to cross the tube bundle several times according to the prescribed distance, and enhance the turbulence degree of the fluid. The heat exchanger tubes can be arranged in an equilateral triangle or square on the tube sheet. The equilateral triangle arrangement is compact, the turbulence degree of the fluid outside the tube is high, and the heat transfer coefficient is large. The square arrangement is convenient for cleaning outside the tube and suitable for scaling-prone fluids.

Each time a fluid passes through a tube bundle is called a tube pass, and every time it passes through a shell is called a shell pass. The figure shows the simplest single-shell and single-tube heat exchanger, referred to as type 1-1 heat exchanger. In order to increase the fluid velocity in the pipe, separators can be installed in the boxes at both ends, and all the pipes can be divided into several groups. In this way, the fluid passes through only part of the tube at a time, and thus it goes back and forth many times in the tube bundle, which is called multi-pipe. Similarly, in order to increase the flow velocity outside the tube, the longitudinal baffle can also be installed in the shell, forcing the fluid to pass through the shell space many times, which is called multi-shell path. Multi-pipe and multi-shell can be used in conjunction.
Because the temperature of the fluid inside and outside the tube is different, the temperature of the shell and tube bundle of the heat exchanger is also different. If the two temperatures are very different, great thermal stresses will occur in the heat exchanger, which will cause the tube to bend, break, or pull off the tube sheet. Therefore, when the temperature difference between the tube bundle and the shell exceeds 50 C, appropriate compensation measures should be taken to eliminate or reduce the thermal stress. According to the compensation measures adopted, shell-and-tube heat exchangers can be divided into the following main types:

## Fixed tubesheet heat exchanger

Fixed tubesheet heat exchanger’s tube bundles at both ends of the tubesheet and shell are integrated, simple structure, but only suitable for heat transfer operation when the temperature difference between cold and hot fluids is not large and the shell side does not need mechanical cleaning. When the temperature difference is slightly larger and the pressure on the shell side is not too high, an elastic compensation ring can be installed on the shell to reduce the thermal stress.

The floating tube sheet at one end of the floating head heat exchanger tube bundle can float freely, completely eliminating the thermal stress, and the whole tube bundle can be withdrawn from the shell for easy mechanical cleaning and maintenance. Floating head heat exchanger is widely used, but its structure is complex and its cost is high.

## U-tube heat exchanger

Each heat exchanger tube is bent into a U-shaped shape, and both ends are fixed in the upper and lower parts of the same tubesheet. With the help of the partition in the tubesheet, it is divided into two chambers of inlet and outlet. The heat exchanger completely eliminates the thermal stress, and its structure is simpler than that of the floating head type, but the pipe side is not easy to clean.

## Packed funnel heat exchanger

The structural characteristics of packed box heat exchanger are that only one end of the tube sheet is fixed with the shell, and the other end is sealed by the packed box. The tube bundle can expand freely without producing temperature difference stress caused by temperature difference between shell wall and tube wall. The advantages of packed funnel heat exchanger are simpler structure, easier manufacture, less material consumption and lower cost than floating head heat exchanger. The tube bundle can be pulled out from the shell, and the tube and the tube can be cleaned and maintained conveniently. The disadvantage is that the pressure resistance of the packing box is not high, generally less than 4.0 MPa; shell-side media may leak through the packing box, which is not suitable for flammable, explosive, toxic and valuable media. The packed funnel heat exchanger is suitable for the occasion where the temperature difference between the tube and shell wall is large or the medium is easy to scale, and the pressure is not high and the cleaning needs are frequent.

## Kettle heat exchanger

The structure of kettle heat exchanger is characterized by setting appropriate evaporation space on the upper part of the shell, and also having the function of steam chamber. The bundles can be fixed tubesheet, floating head or U-shaped. The kettle heat exchanger is easy to clean and maintain. It can deal with unclean and scaly media, and can withstand high temperature and high pressure. It is suitable for liquid-vapor heat transfer and can be used as the simplest structure of waste heat boiler.

### Shell and Tube Exchanger: Geometric Terminology

The main components of a shell and tube exchanger are shown in Figure 2 a, b and c and described in Table 1.

Source: thermopedia.com

Figure 2. Type BEM, CFU and AES exchangers. © 1988 by Tubular Exchanger Manufacturers Association.

Table 1. Shell and tube geometric terminology

 1 Stationary (Front) Head—Channel 20 Slip-on Backing Flange 2 Stationary (Front) Head—Bonnet 21 Floating Tubesheet Skirt 3 Stationary (Front) Head Flange 22 Floating Tubesheet Skirt 4 Channel Cover 23 Packing Box Flange 5 Stationary Head Nozzle 24 Packing 6 Stationary Tubesheet 25 Packing Follower Ring 7 Tubes 26 Lantern Ring 8 Shell 27 Tie Rods and Spacers 9 Shell Cover 28 Transverse Baffles or Support Plates 10 Shell Flange—Stationary Head End 29 Impingement Baffle or Plate 11 Shell Flange—Rear Head End 30 Longitudinal Baffle 12 Shell Nozzle 31 Pass Partition 13 Shell Cover Flange 32 Vent Connection 14 Expansion Joint 33 Drain Connection 15 Floating Tubesheet 34 Instrument Connection 16 Floating Head Cover 35 Support Saddle 17 Floating Head Flange 36 Lifting Lug 18 Floating Head Backing Device 37 Support Bracket 19 Split Shear Ring

### Tema Designations

The popularity of shell and tube exchangers has resulted in a standard nomenclature being developed for their designation and use by the Tubular Exchanger Manufactures Association (TEMA). This nomenclature is defined in terms letters and diagrams. The first letter describes the front header type, the second letter the shell type and the third letter the rear header type. Figure 2 shows examples of a BEM, CFU, and AES exchangers while Figure 3 illustrates the full TEMA nomenclature.

Many combinations of front header, shell and rear header can be made. The most common combinations for an E-Type Shell are given in Table 2 but other combinations are also used.

Table 2. Shell and tube geometric terminology

 Fixed tubesheet exchangers U-tube exchangers Floating head exchangers AEL AEU AES AEM CEU BES AEN DEU BEL BEM BEN

Essentially there are three main combinations

• Fixed tubesheet exchangers

• U-tube exchangers

## Heat transfer mechanism of shell-and-tube heat exchangers

Generally speaking, shell and tube heat exchangers are easy to manufacture, low production cost, wide selection of materials, easy cleaning, strong adaptability, large processing capacity, reliable operation, and can adapt to high temperature and high pressure. Although it can not be compared with plate-fin heat exchangers in terms of compactness, light heat transfer and unit metal consumption, it still plays a leading role in the application of chemical industry, petroleum energy and other industries because of its advantages mentioned above.
Shell-and-tube heat exchangers connect tubes to tubesheets and then fix them with shells. Its types can be divided into fixed tubesheet type, kettle type floating head type, U type tubesheet type, sliding tubesheet type, packing function and casing type, etc. We briefly introduced them before. According to the type of medium, pressure, temperature, dirt and other conditions, the structural characteristics of the connection between tube sheet and shell, the shape and condition of heat transfer tube, the cost and the convenience of maintenance and inspection, the design and manufacture of various shell and tube heat exchangers are selected.

## Structure and Manufacturing Standard of Shell and Tube Heat Exchangers

Generally speaking, shell and tube heat exchangers are easy to manufacture, low production cost, wide selection of materials, easy cleaning, strong adaptability, large processing capacity, reliable operation, and can adapt to high temperature and high pressure. Although it can not be compared with plate-fin heat exchangers in terms of compactness, light heat transfer and unit metal consumption, it still plays a leading role in the application of chemical industry, petroleum energy and other industries because of its advantages mentioned above.
Shell-and-tube heat exchangers connect tubes to tubesheets and then fix them with shells. Its types can be divided into fixed tubesheet type, kettle type floating head type, U type tubesheet type, sliding tubesheet type, packing function and casing type, etc. We briefly introduced them before. According to the type of medium, pressure, temperature, dirt and other conditions, the structural characteristics of the connection between tube sheet and shell, the shape and condition of heat transfer tube, the cost and the convenience of maintenance and inspection, the design and manufacture of various shell and tube heat exchangers are selected.
Shell-and-tube heat exchanger: Interwall heat exchanger with the wall of the tube bundle enclosed in the shell as the heat transfer surface. This kind of heat exchanger is simple in structure and reliable in operation. It can be manufactured with various structural materials (mainly metal materials), and can be used under high temperature and pressure. It is the most widely used type at present. (Design and manufacture follow standards: foreign TEMA ASME domestic GB151, GB150)

Selection Principle of Heat Exchanger Head

• 1. Does the shell side need to be cleaned?
• 2. Does thermal expansion need to be considered?
• 3. Does the bundle need to be moved?

Front Header Types: A, B, C, D, N
Rear Head Types: L, M, N, P, S, T, W
The rear head is divided into fixed type, floating type and U type tube. Compared with fixed type, floating type has higher cost, larger shell diameter and lower heat transfer effect (due to the existence of leakage flow C). The advantage is that one end has a degree of freedom to deal with thermal expansion.

• Type A Head: It is suitable for dirty pipe flow and needs frequent cleaning.
• Type B Head: Single flange is the best economic type, because it is easy to purchase, it is the most commonly used head.
• C-type head: with tubesheet and removable cover, easy cleaning on the side of the tube, can handle high pressure and high-risk media (appropriate), suitable for heavy shell side tube bundles and shell side needs cleaning.
• D-type head: special high-pressure type, suitable for special high-pressure working conditions (pipe box welded on tube sheet)
• N-type head: with tubesheet and removable cap, tube bundle is not removable, this kind of head has the best economy, easy access to tubesheet, and can handle high-risk media on shell side.

Compared with B-type head, A-type head has one more flange, and its pressure resistance is not as good as B-type head. Its advantage is that it is not allowed to take off the head when the heat exchanger is repaired, which is more convenient than B-type head. The tube bundles in C-type and N-type heat exchangers are withdrawable, and the tube sheets and tube boxes in C-type heat exchangers are welded together.

• U-shaped: U-shaped tube bundle, movable, easy to clean the shell side; excellent thermal expansion treatment, economic (not blue); shortcomings are that the tube side can not be cleaned, difficult to replace the bundle, elbow easy to scour damage.
• P-type head and W-type head have been eliminated and are not in use.
• S-type head: Its size characteristic is that the rear head is bigger than the shell diameter. Its advantage is that it can solve two problems in the process of heat exchanger design. One is to eliminate the thermal stress of heat exchanger, the other is that the shell side of heat exchanger can be cleaned.
• T-type head and S-type head are similar, but the size of the later head and the diameter of the shell are the same, and the inner head and tube bundle can be pulled out directly. However, compared with S-type head, the stress of T-type head is not as good as S-type head. The only advantage is that core pulling is convenient, and T-type head is not generally used in engineering design.

Heat Exchanger Shell

• E-shell: It is a one-way shell, which is generally preferred in the design process. It is suitable for all situations. Single-phase heat transfer is better, but the disadvantage is that the pressure drop is larger.
• F-shell: It is suitable for the situation that the site is limited and double-shell side is needed. It is more suitable for single-phase heat transfer, pure counter-current heat transfer and large temperature difference of heat transfer. The disadvantage is that F-shell has split partition, where leakage occurs, and the pressure difference and temperature difference between the inlet and outlet of the shell side are the largest, temperature leakage will occur and split partition is easy to occur. Deformation. Therefore, the F-type shell is suitable for cases where the pressure difference and temperature difference are not large.
• G-shell: It belongs to parallel flow heat exchanger. The outlet temperature of hot fluid of this heat exchanger can be lower than that of cold fluid. It is suitable for horizontal thermosiphon reboiler and condenser which need to be strengthened by shell side.
• H-shell: Double parallel flow heat exchanger, mainly used for condensation and evaporation, and the shell does not use baffles. The advantage of G/H shell is that the difference of heat transfer temperature is larger than that of E shell.
• J-type shell: shunt shell, one is suitable for the case of large pressure drop in the gas phase of the shell, which can not be solved by vibration; the other is used for reboiler, which makes the heat transfer effect more stable compared with E-type; the third is used for partial condensation, whose shortcoming is that the heat transfer temperature difference is small and the heat transfer coefficient is not large.
• K-type shell: It is mainly used for heat medium of pipe engineering and evaporation of shell side. It is used under the condition of waste heat recovery.
• X-shell: The cold and hot fluids are cross-flow, and its advantage is that the pressure drop is very small. When other shell vibrates, and the vibration can not be eliminated by adjusting the parameters of heat exchanger, the shell can be used. The disadvantage is that the fluid distribution is not uniform, and X-shell is not often used.

## Baffle plate of heat exchanger

• Single arch baffle: The advantage is that the maximum cross flow can be achieved. The disadvantage is that the pressure drop is high, and the tube bundle of the window is prone to vibration. The design point is that the circular defect ratio of the baffle is between 17% and 35%, and the baffle spacing is between 0.2 and 1.0 times the shell diameter. This type of baffle is suitable for most occasions.
• NITW: The baffle window does not distribute pipes, the pipe supports perfectly, and does not cause vibration of the tube bundle. The disadvantage is the same shell diameter, fewer pipes, and the required shell diameter is large. Design essentials: 15% baffle circle defect rate. Suitable occasions are limited gas vibration and pressure drop.
• Double arch baffles: advantages are pressure reduction, better avoidance of vibration; disadvantages are large window flow area; design points: 5% – 30% of the circular defect rate, default overlap of two rows of tubes; suitable for occasions when vibration and pressure constraints heat exchangers (compared with single arch baffles).
• Helical baffles: divided into single helical baffles and double helical baffles, the advantages are good heat transfer, low pressure, uniform flow; the disadvantages are difficult to manufacture; the design point is the helical angle of 5-45 degrees, suitable occasions when pressure drop is limited, easy to scale occasions.
• Baffle rod: Advantage is excellent support, uniform flow, pressure reduction basically no vibration problem; Disadvantage is low heat transfer effect; Tube layout can only be 45 degrees and 90 degrees; Suitable for low pressure drop gas condensation and heat transfer.
• Nest type: excellent support, uniform flow, low pressure; shortcoming is that the heat transfer effect is not good, the design is basically not required.
• Egg frame type: good support, economical manufacture; defect is deformation under high temperature stress; design is basically not required.

## Factors to be considered in the design of shell-and-tube heat exchangers

There are many types of heat exchangers. For each specific heat transfer condition, the most suitable type of equipment can be obtained by optimizing the selection. If this type of equipment is used in other conditions, the effect of heat transfer may be greatly changed. Therefore, it is very important and complex to select the type of heat exchanger according to the specific working conditions. For the design of shell-and-tube heat exchangers, the following factors should be considered.
1. Selection of Flow Velocity
Flow rate is an important variable in heat exchanger design. Increasing flow rate increases heat transfer coefficient and pressure drop and power consumption. If pumping fluid is used, the pressure drop should be consumed on the heat exchanger rather than on the regulating valve as far as possible, so the heat transfer effect can be improved by increasing flow rate.
There are two advantages of using a higher flow rate: one is to increase the total heat transfer coefficient, thereby reducing the heat transfer area; the other is to reduce the possibility of fouling on the surface of the tube. But it also increases the consumption of resistance and power, so economic comparison is needed to determine the appropriate flow rate.
In addition, the structural requirements must also be taken into account in the selection of flow velocity. In order to avoid serious wear and tear of equipment, the calculated flow rate should not exceed the maximum allowable empirical flow rate.
2. Selection of Permissible Pressure Drop
Choosing a larger pressure drop can increase the flow rate, thereby enhancing the heat transfer effect and reducing the heat transfer area. But the larger pressure drop also increases the operating cost of the pump. Appropriate pressure drop should be calculated by adjusting the size of the equipment repeatedly, aiming at the annual total cost of the heat exchanger.
In most equipment, it may be found that the thermal resistance of one side is significantly higher than that of the other side, and the thermal resistance of this side becomes the control thermal resistance. When the thermal resistance of the shell side is the control side, the method of increasing the number of baffle plates or reducing the shell diameter can be used to increase the fluid flow velocity and reduce the heat transfer resistance on the shell side. However, the reduction of the baffle spacing is limited, generally not less than 1/5 or 50 mm of the shell diameter. When the thermal resistance of the pipe is the control side, the fluid flow rate is increased by increasing the maturity of the pipe.
When dealing with viscous materials, if the fluid is in laminar flow, the material will be removed from the shell. Because the fluid flow in the shell side can easily reach turbulent state, it can obtain higher heat transfer rate and improve the control of pressure drop.
3. Determination of Fluid in the Shell and Tube Side
According to the operating pressure and temperature of the fluid, the pressure drop that can be used, the structure and corrosion characteristics, as well as the selection of the required equipment and materials, it is considered which way the fluid is suitable to go. The following factors may be considered when choosing:
Fluids suitable for piping are water and steam or highly corrosive fluids; toxic fluids; fluids with easy structure; fluids operated at high temperature or high pressure, etc.
Fluids suitable for shelling include condensation of tower top distillates, condensation and reboiling of hydrocarbons, fluid controlled by pressure drop of pipe fittings, fluid with high viscosity, etc.
When the above situation is eliminated, the choice of which way the medium goes should focus on improving the heat transfer coefficient and making full use of the pressure drop. Because the flow of medium in the shell side is easy to reach turbulence (Re < 100), it is generally advantageous to move the fluid with high viscosity or low flow rate, that is, the fluid with low Reynolds number. On the contrary, if the fluid can reach turbulence in the pipeline, it is more reasonable to arrange the pipeline. Considering the pressure drop, it is generally reasonable to have a low Reynolds number.
4. Determination of Final Heat Transfer Temperature
The final heat transfer temperature is generally determined by the need of the process. When the final heat transfer temperature can be selected, its value has a great influence on whether the heat exchanger is economical and reasonable. When the outlet temperature of hot and cold fluids is equal, the heat utilization efficiency is the highest, but the effective heat transfer temperature difference is the smallest and the heat transfer area is the largest.
In addition, when determining the temperature of the logistics outlet, we do not want to cross the temperature, that is, the temperature of the hot fluid outlet is lower than that of the cold fluid outlet.
5. Selection of Equipment Structure
For certain technological conditions, the first step is to determine the form of equipment, such as fixed tubesheet or floating head.
In the process of heat exchanger design, the general objectives of heat transfer enhancement are as follows: to reduce the size of heat exchanger under given heat transfer rate; to improve the performance of existing heat exchangers; to reduce the temperature difference of flowing refrigerants; or to reduce the power of pumps.
Heat transfer process refers to the process of heat exchange between two fluids through the wall of hard equipment. According to the heat transfer mode of fluids, it can be basically divided into two types: phase-free and phase-change. The research of heat transfer enhancement technology without phase change process is generally based on the control of thermal resistance side, and the corresponding measures are taken: such as using expansion tube or outer surface of tube; using foreign matter in tube; changing the form of tube bundle support; adding low boiling point additives which are insoluble, etc. to enhance the heat transfer effect.

In the tube type, the threaded tube belongs to the type of expanding surface outside the tube. The threaded low fins are rolled on the outer wall of the ordinary heat exchanger tube to increase the heat transfer area outside. The surface area of the threaded tube is 1.6-2.7 times larger than that of the smooth tube. Compared with the smooth tube, when the outer flow velocity is the same, the heat transfer resistance of the shell side can be reduced by a corresponding multiple, while the pressure drop of the fluid in the tube will increase slightly as the diameter of the tube decreases. The threaded tube is more suitable for the case where the heat transfer coefficient of shell side is equal to 1/3-3/5 of that of tube side.

Performance Characteristics of Corrugated Tube Heat Exchanger

Corrugated tube and interpolated tube are the typical tubes to change the fluid flow state and enhance the heat transfer effect. Bellows are machined without cutting. The ribs are extruded in the tube, which changes the flow state of the stagnant layer on the inner wall of the tube, reduces the heat transfer resistance of the fluid and enhances the heat transfer effect.
Performance characteristics of baffle rod heat exchanger

Baffle rod heat exchanger, double bow plate heat exchanger, disc-ring heat exchanger and swirl heat exchanger all belong to the purpose of strengthening heat transfer by means of shell-side tube bundle support, drastically reducing resistance, increasing flow rate or changing flow mode. Baffle rod heat exchanger is fixed in four directions of each heat exchanger tube with baffle rod, which has good seismic performance.

Source: Network Arrangement – China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References:

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Summary
Article Name
What is a shell and tube heat exchanger?
Description
Shell-and-tube heat exchanger is also called tube-and-tube heat exchanger. A wall-to-wall heat exchanger with the wall of the tube bundle enclosed in the shell as the heat transfer surface. The heat exchanger is simple in structure and reliable in operation. It can be manufactured with various structural materials (mainly metal materials) and can be used under high temperature and pressure. It is the most widely used type at present.
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www.steeljrv.com
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