Shell and Tube Heat Exchanger Parts and Functions

The shell and tube heat exchanger parts include the shell, tubes, and tube sheets. They also include baffles, tie rods, sealing strip, pass partition plates, and spacers. The shell provides the outer casing, while the tubes house the fluid that needs to be cooled or heated. Tube sheets hold the tubes in place, and baffles direct the fluid flow to enhance heat transfer efficiency. These heat exchangers are valued for their robustness, versatility, and ability to handle high pressures and temperatures.

List of Shell and Tube Heat Exchanger Parts

1. Shell
2. Tube
3. Baffles
4. Tube Sheet
5. Shell Side Pass Partition Plate
6. Tube Side Pass Partition Plate
7. Sealing Strip
8. Tie Rods
9. Spacers

Shell

Shell is the outer casing of the shell and tube heat exchanger. It allows to pass shell side fluid.

Shell is the costliest part of the heat exchanger. Cost of shell and tube heat exchanger sensitively changes with change in the diameter of shell.

The TEMA (Tubular Exchanger Manufacturers Association) standard specifies shell sizes. They range from 6 in (152 mm) to 60 in (1520 mm). Standard pipes are available up to 24 in size (600 mm NB). If shell size is greater than 24 in, it is fabricated by rolling a plate.

Shell diameter depends on tube bundle diameter. For fixed tube sheet shell and tube heat exchanger, the gap between shell and tube bundle is minimal. It ranges from 10 to 20 mm. For pull through floating head heat exchanger, it is maximum, ranging from 90 to 100 mm.

These are typically made of materials like carbon steel, stainless steel, or other alloys. The choice depends on the operating conditions, including temperature, pressure, and the corrosive nature of the fluids.

Tube

The tubes in a shell-and-tube heat exchanger have a critical role. One of the two fluids involved in heat transfer flows through them. They are housed inside the shell. They facilitate heat exchange between the fluids on the tube side and the shell side.

Tube size range from 1/4 in (6.35 mm) to 2.5 in (63.5 mm) in shell and tube heat exchanger. Data for standard tubes are given in TEMA standard.

For the standard tubes, its size is equal to outer diameter of tube. Thickness of standard tubes are expressed in BWG (Birmingham Wire Gauge). Increase in the value of BWG means decrease in tube thickness.

For no phase change heat exchangers and condensers, 3/4 in (19.05 mm) OD tube is widely used in practice. While for reboiler 1 in (25.4 mm) OD tube size is common.

Tubes are available in standard lengths like 6 ft (1.83 m), 8 ft (2.44 m), 12 ft (3.66 m), 16 ft (4.88 m) and 6 m.

Typically made of metals with good thermal conductivity, like copper, stainless steel, or nickel alloys.

For corrosive environments, materials like titanium or non-metallic materials like Teflon are used.

Baffles

Baffles are structural components installed inside the shell of a shell-and-tube heat exchanger. They direct and control the flow of shell-side fluid. They enhance heat transfer by increasing turbulence. Baffles also give mechanical support to the tube bundle, preventing vibration and sagging.

Functions of Baffles

There are two functions of baffles:

1. Baffles are used in the shell to direct the fluid stream across the tubes. This increases the velocity of shell side flow. As a result, it improves the shell side heat transfer coefficient. In other words, baffles are used in shell to increase the turbulence in shell side fluid. This function is useful only if there is no phase change in shell side fluid.
2. Baffles indirectly support the tubes and thereby reduce the vibrations in tubes. If shell side liquid velocity is more than 3 m/s, conduct vibration analysis calculations. This will check whether baffle spacing is sufficient or not. If the velocity of gas or vapour is very high, perform a vibration analysis calculation. Also, consider this for baffle spacing higher than shell ID. This helps to check the baffle spacing. Vibration analysis calculations are given in TEMA standard.

Different types of baffles are used in shell and tube heat exchangers

1. Segmental baffle
2. Nest baffle
3. Segmental and Strip baffle
4. Disk and Doughnut baffle
5. Orifice baffle
6. Dam baffle

Know more: Different Types of Baffles in Heat Exchanger

Tube Sheet

Tubes and one end of tierods are attached to tube sheet (also called tube plate). Hence, entire load of tube bundle is transferred to one or two tube sheets.

In U tube shell and tube heat exchanger only one tube sheet is used. While in fixed tube sheet shell and tube heat exchanger, two tube sheets are used.

One surface of tube sheet is exposed to tube side fluid and other surface is exposed to shell side fluid. This point is very important in the choice of material for tube sheet and also in determining tube sheet thickness.

In majority cases tube to tube sheet joints are two types; (a) Expanded joint, and (b) Welded joint.

Tubesheet holes are drilled to accommodate expanded type joints. These tube holes are slightly greater in diameter than the tube OD (Outside Diameter). Two or more grooves are cut in the wall of each hole. The tube is placed inside the tube hole and a tube roller is inserted into the end of the tube. Roller is slightly tapered. On application of the roller, tube expands and tube material flows into grooves and forms an extremely tight seal.

Welding joint is used only for the cases where leakage of fluid can be disastrous

Shell Side Pass Partition Plate

A single pass shell is used in the most of the cases. Two-pass shell is rarely used. It is recommended where shell and tube temperature difference is unfavorable for the single shell side pass. For such cases, normally two or more
smaller size 1-1 heat exchangers, connected in series, are recommended.

Shell shell-side pass partition plate is not provided to improve the shell-side heat transfer coefficient. It is provided to avoid the unfavorable temperature difference. It also prevents the crossing of temperatures between hot fluid and cold fluid.

Tube Side Pass Partition Plate

Tube side passes are provided to decrease the tube side flow area and to increase tube side fluid velocity. This improves the tube side heat transfer coefficient at the expense of pressure drop.

This is true only if there is no phase change on tube side. Hence, more tube side passes are recommended. This is only advisable if there is no phase change in the tube side fluid.

For example, at the design stage, if the number of tube side passes is increased from one to two. Then the flow area becomes half for the given volumetric flow rate. As a result, the velocity becomes double.

Since, tube side heat transfer coefficient, (where u_t is tube side fluid velocity). On increasing number of tube side passes from 1 to 2, h_i nearly becomes 1.74 times.

But, , so the pressure drop×increases by 6.96 times.

Increase in means decrease in heat transfer area required and decrease in fixed cost.

Increase in means increase in power required for pumping the tube side fluid and increase in operating cost.

Hence, ideally optimum number of tube side passes must be decided.

Many a times, tube side velocity in a single pass is calculated very low (say 0.3 to 0.5 m/s). Under such circumstances, two passes or four passes will be beneficial.

It is recommended that u_t > 1 m/s. This is essential if tubeside fluid is cooling water; otherwise rate of fouling will be higher.

Tube side passes are very common and are advantageously used for improving tube side heat transfer coefficient.

Sealing Strip

It is a shell side part. Sealing strips are attached on the inside surface of shell throughout the length of shell.

Functions of Sealing Strips

There are two functions of sealing strips

1. Sealing strips reduce the amount of bypass stream of shell side fluid. This occurs through the clearance between the shell inside diameter and tube bundle diameter. As a result, they improve the shell side heat transfer coefficient. (This is valid only if there is no phase change of shell side fluid).
2. Sealing strips also make the removal of tube bundle from the shell easy. Hence, they are also known as sliding strips.

In Kern’s method, the entire shell side fluid flows across the tube bundle. It moves between the baffles.

Actually, shell side fluid is flowing in various ways. In all latest methods of process design, shell side fluid flow is divided in five streams. Among these streams, one of the streams is flowing through the clearance between shell ID and tube bundle.

This clearance is significant in case of pull through floating head heat exchanger (about 90 to 100 mm). In addition, this clearance provides low pressure drop path for shell side fluid. Hence, actual fraction of shell side fluid bypassed through this clearance is considerably higher.

Sealing strip partially blocks the gap between bundle and shell and thereby reduces the amount of this bypass stream considerably. In case of pull through floating head heat exchanger, the latest computer programs suggest using a sealing strip. Programs like HTRI also recommend it.

Also in such programs shell side heat transfer coefficient is a function of dimension and numbers of sealing strips provided.

Do not use Kern’s method to predict the heat transfer coefficient. It is also not suitable to predict the pressure drop for a pull through floating head heat exchanger.

Tie Rods

Baffles are supported by tie rods. Tie rods are made from solid metal bar.

Normally four or more tie rods are required to support the baffles. Diameter of tie rod is less than the diameter of tube.

Diameter and number of tie rods required for given shell diameter are specified by TEMA standard and IS:4503.

Spacers

Spacers are used to maintain the space between baffles. Spacers are the pieces of pipes. In most cases, they are pieces of extra available tubes.

Spacers are passed over the tie rods. Because of the spacers, baffles do not slide over tie rods under the force of fluid.

Hence, spacers fix the location of baffles and maintain the space between them. Length of spacer is equal to space between the baffles.

Conclusion

The shell and tube heat exchanger is versatile. It is widely used in the chemical process industries. This is due to its efficiency, durability, and ability to handle a wide range of operating conditions.

Understanding its parts provides valuable insight into its operation and maintenance. These parts include the shell, tubes, and baffles. They also consist of tube sheets, tube side and shell side pass partition plate. Additionally, there are tie rods and spacers. Each component plays a critical role in optimizing heat transfer and ensuring the reliability of the system.

Proper design, material selection, and maintenance of these parts can significantly enhance the exchanger’s performance and lifespan. Engineers must have detailed knowledge of its components. This knowledge is crucial to maximize efficiency and adapt the heat exchanger to specific industrial requirements.

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