Shell and Tube Heat Exchanger

Shell and tube heat exchangers are widely used in various industries to transfer heat between two fluid streams. Their robust design and versatility make them suitable for a range of applications, from power generation to chemical processing. In this blog, we will delve into the intricacies of shell and tube heat exchanger design, focusing on key considerations such as heat transfer mechanisms, design parameters, material selection, and Recent technological advancements in shell and tube heat exchangers . By understanding the fundamental principles and design factors involved, engineers can optimize the performance, efficiency, and reliability of shell and tube heat exchangers.

fig.1 shell and tube heat exchanger schematic diagram

The design of a shell and tube heat exchanger involves determining the appropriate dimensions of the exchanger to ensure that it can transfer the required amount of heat between the two fluids. The following are some of the basic formulas used in the design of shell and tube heat exchangers:

Heat transfer rate formula:

Q = U × A × ΔTlm

Where,

                Q = Heat transfer rate

                U = Overall heat transfer coefficient

                A = Heat transfer area

                ΔTlm = Logarithmic mean temperature difference

Logarithmic mean temperature difference formula:

ΔTlm = (ΔT1 - ΔT2) / ln(ΔT1 / ΔT2)

Where,

              ΔT1 = Temperature difference between the hot fluid inlet and outlet

              ΔT2 = Temperature difference between the cold fluid inlet and outlet

Reynolds number formula:

Re = ρ × V × D / μ

Where,

              Re = Reynolds number

              ρ = Density of the fluid

              V = Velocity of the fluid

             D = Diameter of the tube

             μ = Viscosity of the fluid

Pressure drop formula:

ΔP = f × L × (V^2 / (2 × g × D))

Where,

          ΔP = Pressure drop

          f = Friction factor

          L = Length of the tube

         V = Velocity of the fluid

         g = Acceleration due to gravity

         D = Diameter of the tube

Heat transfer coefficient formula:

1 / U = (1 / h_i) + (Δx / k) + (1 / h_o)

Where,

          U = Overall heat transfer coefficient

          h_i = Heat transfer coefficient on the hot fluid side

          h_o = Heat transfer coefficient on the cold fluid side

         Δx = Thickness of the tube wall

         k = Thermal conductivity of the tube wall material

These formulas can be used to calculate the design parameters of shell and tube heat exchangers such as the heat transfer rate, overall heat transfer coefficient, and pressure drop, among others.

Design of heat exchanger

Software modelling : A shell and tube heat exchanger was modelled in CAD software, the corresponding geometric specifications used is as follows.

1. Baffle arrangement and final assembly with shell.

Fig. 1

       2. Tube bundle and channel cover/tube-side nozzles.

Fig. 2

CFD analysis :

1. Geometry

Fluid model is created using CAD software and created under Fluent in Ansys Workbench and is as shown in Figure 3.

Fig. 3

2. Meshing :

Tetrahedral and hexahedral mixed cells with triangle and rectangle shapes were generated in a large mesh size (see Fig. 4). This was produced using an automatic method that the ANSYS meshing client offers. To obtain precise results, proximity is chosen for the sizing preference and a coarse mesh is constructed. To acquire accurate findings, 300 iterations were chosen under hybrid initialization to get the heat exchanger outlet. The energy below 107 for continuity and the X, Y, Z, and K velocities should all be below 104 in order to produce convergent solutions. To achieve convergence, 600 iterations were performed.

Fig. 4

Solution setup :

The solution is built up in a cell zone with a fluid type change to the water option. Ansys Fluent software was used to run the simulation. The necessary materials are chosen from the fluent material database, and the calculation phase has been adjusted to account for viscous realisable and standard walls when calculating energy (heat transfer). Every tube inlet and shell inlet have their default temperatures and boundary conditions configured for velocity inlets.

Streamline plot :

The necessary streamlines are plotted in CFD-Post, and results are produced. It is shown how a temperature contour streamline moves along shell tube heat exchangers.

In Results and discussion we get Geometrical specifications ,Effect of materials utilized on heat exchanger, effect of baffle spacing etc.

Checklist for shell and tube heat exchanger maintenance

• Depressurize the apparatus

• removing the headers and scraping the tube sheets

• free any obstructions in the tubing (this may also entail the use of acids, high pressure water, drill rods, scrapers, calcite cleaners or scale remover) 

• Look around for leaks.

• replace the gaskets 

• Check the accuracy of the pressure gauge and thermometer.

• If there are strainers, check and clean them.

• Examine the working safety equipment.

• When necessary, clean the outside surfaces.

• Check the sump tank's functionality.

• Adjust all mechanical connections' torques to the specified levels.

Tube material selection

The tube material must have strong thermal conductivity to be able to transport heat effectively. There is a temperature difference across the breadth of the tubes because heat is transported from a hot to a cold side through the tubes. To reduce deterioration such as corrosion, the tube material should also be compatible with both the shell-and-tube side fluids for extended periods of time under the operating circumstances (temperatures, pressures, pH, etc.). 

                The careful selection of robust, thermally conductive, corrosion-resistant, high-quality tube material is required to meet all of these characteristics. Typical metals include titanium, Inconel, nickel, Hastelloy, stainless steel, carbon steel, non-ferrous copper alloy, and aluminum. Due to their strong resilience to extreme temperatures, fluoropolymers like perfluoroalkoxy alkane (PFA) and fluorinated ethylene propylene (FEP) are also utilized to make the tube material. Inadequate tube material selection could lead to a leak between the shell and tube sides, which would cause fluid cross-contamination and possibly pressure loss.

Although shell and tube exchangers are typically built of metal, other materials, including glass, graphite, and plastic, may be utilised for specialised applications (such as those requiring strong acids or pharmaceuticals).

Advantages

• less expensive than plate heat exchangers.

• relatively easy to maintain and a simple design.

• superior than plate heat exchangers in that it can withstand higher pressures and temperatures.

• Compared to a plate heat exchanger, the pressure drop (delta P/P) is smaller.

•  Leaking tubes are simple to locate and isolate.

• To lessen the possibility of the fluid from the shell side seeping into the tube side fluid, tubes might be "double walled." (or vice versa).

• Sacrifice anode installation is simple.

• not as susceptible to fouling as plate heat exchangers.

Disadvantages

• a less effective heat exchanger than a plate. extra room is needed in order to open and remove tubes.

• A plate heat exchanger's capacity can be increased, but cooling capacity cannot.

Application and utilization

A shell-and-tube heat exchanger is the best cooling option for a wide range of applications due to its straightforward design.

• The cooling of hydraulic oil and fluid in engines,
• transmissions, and
• hydraulic power packs

is one of the most widespread uses. They can also be used to charge air or cool or heat

other media, including swimming pool water, with the appropriate choice of materials. Shell-and-tube technology has significant benefits over plates.

One of the major benefits of employing a shell-and-tube heat exchanger is that they are frequently simple to maintain, especially with types that come with a floating tube bundle.(where the tube plates are not welded to the outer shell).The housing's cylinder shape makes it incredibly pressure-resistant and enables applications for all pressure ranges.

fig.2 alfa laval shell and tube heat exchanger

Recent technological advancements in shell and tube heat exchangers include the following:

1. Complex geometries for heat transfer surfaces can be made using additive manufacturing (3D printing) processes, which improves heat transfer performance and makes better use of available space.

2. Heat transfer surfaces are being developed with greater thermal conductivity and increased resistance to fouling and corrosion using cutting-edge materials like graphene and carbon nanotubes. 3. The design of shell and tube heat exchangers is being improved using computational fluid dynamics (CFD) simulations, which enable more accurate fluid dynamics and heat transfer performance prediction.   

4. Shell and tube heat exchangers are integrating intelligent sensors and control systems, enabling real-time monitoring and adjusting of working parameters to optimise heat transfer efficiency and lower energy usage. 5. To fulfil the growing demand for more effective and space-saving heat transfer equipment in diverse industries, such as data centers, power plants, and chemical processing facilities, compact shell and tube heat exchangers with reduced footprints are being developed.

In conclusion, the design of a shell and tube heat exchanger requires consideration of various factors such as the properties of the fluids being exchanged, the required heat transfer rate, and the available space and materials. The basic formulas used in the design include the heat transfer rate formula, logarithmic mean temperature difference formula, Reynolds number formula, pressure drop formula, and heat transfer coefficient formula. These formulas help to determine the appropriate dimensions of the heat exchanger, including the heat transfer area, tube diameter, and tube length, to ensure efficient heat transfer and minimal pressure drop. A well-designed shell and tube heat exchanger can provide efficient and cost-effective heat transfer in a variety of industrial applications.

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