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Design & Analysis of Shell and Tube Heat Exchanger

 

Heat exchanger designs are regularly incorporated in the flow sheets of process equipment in a wide range of industrial plants, refineries, and factories and allow the transfer or exchange of heat in industrial process equipment. What are Shell and Tube Heat Exchangers? Let’s take a look into the Design and Analysis of Shell and Heat exchangers.

Shell and tube heat exchangers are one of the most widely used heat exchangers in the processing industries (they account for 65% of the market, according to H. S. Lee’s book, Thermal Design, and are suitable for a wide range of operating temperatures and pressures. STHE’s (Shell and Tube heat exchangers) comprise a cylindrical shell (a pressure vessel) enclosed with a bundle of hollow tubes fitted inside.

These tubes are composed of thermally conductive elements, which enable the exchange of heat between the hot fluids flowing outside the tubes and the coolant flowing through the tubes. Baffles are implemented in the shell to direct the fluid flow and support the tubes. Support rods and spacers hold the baffle and tube assembly together.

Shell and tube heat exchangers are regarded as one of the most efficient kinds of heat exchangers and are widely used across industries like refining, power, chemical, marine, Oil & Gas, Food and many more.

Primary Components of Shell and Tube Heat Exchangers

1. Tubes

Shell and tube heat exchangers aren’t complete without tubes. Tubes help in providing a heat transfer surface between a fluid flowing inside the tube and a fluid flowing around the tube’s outside. Tubes are usually made of copper or steel alloys and can be seamless or welded. For specific applications, other nickel, titanium, or aluminum alloys can be needed. Extrusion produces seamless tubing; welding produces welded tubing by rolling a strip into a cylinder and welding the seam. Welded tubing is usually more economical. 

The most common tube diameters are 5/8”, 34”, and 1”. Smaller tubes may be used, but cleaning them mechanically is more difficult. Larger diameter tubes are often used to allow mechanical cleaning or to reduce pressure drop. The average tube wall thickness is 12 to 16 BWG (from 0.109 inches to 0.065 inches thick). Thinner-walled tubes (18 to 20 BWG) are used.

2. Tubesheets

The tubes are held in place by inserting them into holes in the tubesheets and then expanding them into grooves cut into the holes or welding them to the tubesheet where the tube extrudes from the surface. This prevents the mixing of the fluid on the shell side and the fluid on the side of the tube. The pipe sheet usually consists of a round single metal board that was properly drilled and ground to take the pipes, joints, spacer rods, and the circle of the bolt where the pipe has been attached to the shell.

The tube pitch is the distance between the center of the tube holes; the tube pitch is typically 1.25 times the outside diameter of the tubes. Other tube pitches are commonly used to control the velocity of the shell side fluid as it flows across the tube bundle and reduces shell side pressure drop. Triangular pitch is most commonly used due to the increased heat transfer and compactness it provides. The square pitch makes mechanical cleaning of the tubes’ exteriors easier.

Except for U-tube bundles, two tubesheets are required. A rolled joint is a tube-to-tube sheet joint formed by mechanical expansion of the tube against the tubesheet. This joint is most commonly made with roller expanders, hence the name “rolled joint.” Hydraulic processes are used less frequently to expand tubes to affect a mechanical bond. Tubes can also be welded to the tubesheets front or inboard face.

Strength welding denotes that the mechanical strength of the joint is primarily provided by the welding procedure, and the tubes are only lightly expanded against the tubesheet to eliminate the crevice that would otherwise exist. Seal welding denotes that the mechanical strength of the joint is primarily provided by tube expansion, with the tubes welded to the tubesheet for improved leak protection.

Increased reliability, lower maintenance costs, and fewer process leaks are commonly used to justify the cost of seal-welded joints. When using clad tubesheets, tubes with wall thicknesses less than 16 BWG (0.065 inches), and some metals that cannot be adequately expanded to achieve an acceptable mechanical bond, seal welded joints are required (titanium and Alloy 2205 for instance).

A double tubesheet can be used to prevent mixing between the two fluids. Because the space between the tubesheets is exposed to the atmosphere, any leakage of either fluid should be detected as soon as possible. Aside from mechanical requirements, the tubesheet must be corrosive resistant to both fluids in the heat exchanger and electrochemically compatible with the tube and all tube-side material.

3. Tube-Side Channel and Nozzles

The tube-side channel and nozzles simply control the flow of tube-side fluid into and out of the exchanger’s tubes. Because the tube side fluid is typically more corrosive, these channels and nozzles are frequently made of alloy materials (compatible with the tubes and tubesheet of course). Instead of solid alloy, they could be clad.

Channel covers are provided at the channel ends. They are round plates that bolt to the channel flanges and can be removed for tube inspection without causing any disruption to the piping on the tube side. Bonnets with flanged nozzles or threaded connections for the tube-side piping are frequently used instead of channels and channel covers in smaller heat exchangers.

4. Shell and Shell-Side Nozzles

The shell is simply a container for the fluid on the shell’s side, and the nozzles are the inlet and exit ports. The shell has a circular cross-section and is typically made by rolling a metal plate of appropriate dimensions into a cylinder and welding the longitudinal joint together.

Small diameter shells can be made by cutting the desired diameter pipe to the appropriate length. The roundness of the shell is critical in determining the maximum diameter of the baffles that can be inserted and, as a result, the effect of shell-to-baffle leakage.

An impingement plate is often set just below the inlet nozzle to divert the incoming fluid jet from impacting directly at high velocity on the top row of tubes. This type of impact can result in erosion, cavitation, and/or vibration. It may be necessary to omit some tubes from the full circle pattern in order to insert the impingement plate while still leaving enough flow area between the shell and plate for the flow to discharge without excessive pressure loss.

5. Pass Dividers

For an exchanger with two tube-side passes, a pass divider is required in one channel or bonnet, and they are required in both channels and bonnets for an exchanger with more than two passes. If the channels or bonnets are cast, the dividers are integrally cast and then faced to provide a smooth bearing surface on the gasket between the divider and the tubesheet. The dividers are welded in place if the channels are rolled from a plate or built up from a pipe.

6. Baffles

The baffle pitch is the distance between segmental baffles. The cross-flow velocity, and thus the rate of heat transfer and pressure drop, are determined by the baffle pitch and baffle cut. The spacing of the baffles should be chosen so that the free flow areas through the “window” (the area between the baffle edge and the shell) and across the tube bank are roughly equal.

When installing a heat exchanger horizontally, the orientation of the baffle cut is critical. The baffle cut should be horizontal when the shell side heat transfer is sensible heating or cooling with no phase change. This causes the fluid to move up and down, preventing stratification with warmer fluid at the top of the shell and cooler fluid at the bottom. The baffle cut for segmental baffles for shell side condensation is vertical to allow condensate to flow towards the outlet without the baffle causing the significant liquid holdup. Depending on the service, the baffle cut for shell side boiling can be vertical or horizontal.

The single segmental baffle configuration produces an unfavorable high shell side pressure drop for many high-velocity gas flows. The double segmental baffle is one method for retaining the structural advantages of the segmental baffle while reducing the pressure drop (and, regrettably, to some extent, the heat transfer coefficient).

7. Tie Rods

Tie rods and spacers are used for two reasons: to keep the baffle assembly together and to keep the baffle spacing consistent.

The tie rods are attached to the tubesheet at one end and the last baffle at the other. They are responsible for holding the baffle assembly together. To maintain the selected baffle pitch, spacers are placed over the tie rods between each baffle. The diameter of the shell, as well as the size of the tie rods and spacers, determine the minimum number of tie rods and spacers required.

Accurate, Complaint & Affordable Designs

Benefits of Shell and Tube Heat Exchangers

  • When compared to plate-type coolers, they are less expensive.
  • Can be used in frameworks with higher working temperatures and weights.
  • They can be designed and manufactured to withstand extremely high pressures.
  • They can withstand thermal shocks.
  • Pressure loss is minimal and can be kept to a minimum under the process’s objectives.
  • Tubular coolers in the refrigeration framework can go about as recipients also
  • The use of sacrificial anodes protects the entire cooling framework from erosion.
  • Because of the weight difference, tube coolers may be preferred for oil cooling.
  • Changes can be made to the pipe diameter, pipe number, pipe length, pipe pitch, and pipe arrangement. As a result, the designs of tube heat exchangers are quite adaptable.
  • They are simple to disassemble and reassemble for maintenance, repair, and cleaning. Because pressure testing is simple, tube leaks are easily found and stopped.

INDOVANCE Inc is a specialized CAD partner of AEC companies and provides a wide range of Heat Exchanger design and drafting services to Industrial, Commercial, Oil & Gas, Power, Marine, Cold Chain, and Process Industries. Our team is skilled, experienced, well-versed with industry standards, and aware of the industry’s needs and evolution. 

We have successfully delivered comprehensive 2D drawings and 3D models or illustrations for the manufacturing of different types of heat exchangers across various industries. We offer Calculation and structural analysis of heat exchangers, heat exchanger designing and detailing of different grades and capacities with detailed BOM list, 3D modeling of heat exchangers for digital illustration, comprehensive and accurate 2D heat exchanger drawings, CFD & FEA analysis.

For more queries regarding any of the above-mentioned topics, feel free to connect with us on our website www.indovance.com, or contact us at +1-919-238-4044.

 

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