All About Shell and Tube Heat exchanger

 Shell and tube heat Exchanger-


A mechanical Static equipment or device having two different compartments – shell compartment and tube compartment, where two different fluids/ media /substance, with different temperature, flow without mixing with each other to pass a heat from one to another by convection & conduction  

Here's how it typically works:

Shell Side /compartment- One fluid flows through the shell, surrounding a bundle of tubes. This fluid gains or loses heat as it passes over the outer surface of the tubes. The heat transfer happens through convection between the fluid in the shell and the outer surface of the tubes.

Tube Side/compartment- The other fluid flows through the tubes. As it passes through the tubes, it gains or loses heat through convection with the inner surface of the tubes. Heat is transferred from one fluid to the other through the tube walls by conduction.

This setup maximizes the surface area available for heat transfer between the two fluids, allowing efficient exchange of thermal energy.




Coming back to shell and tube heat exchanger, a shell is a cylindrical-shaped body, typically a large pipe with a diameter of 4 inches or more. It serves as the outer casing and acts as a pressure vessel.




and Tubes are small pipes, plain tubes, or finned tubes, bundled and placed inside a large cylindrical shell, compactly and symmetrically spaced together.




 This is a widely used in big size industries such as Oil & Gas, Refineries, chemical plants due to its efficiency and suitability for high and wide pressure and temperature ranges.

Distinction by Phases :- Single Phase shell and tube heat exchanger ,Two Phase Shell and tube heat exchanger

The heat exchanger, where two fluids undergo phase changes from liquid to gas or vapor form, and from vapor or gas form, condense back into liquid, is known as a two-phase shell and tube heat exchanger. Examples- Condensers, Evaporators steam Boilers

In instances where there is no phase change, and liquids remain in liquid form while gases remain in gas form on both sides of the shell and tube heat exchanger, this type of heat exchanger is referred to as a single-phase or one-phase shell and tube heat exchanger. Examples – oil coolers, air intercoolers aftercoolers.

 

 

 

Geometry & Design :-

The shell and tube heat exchangers are designed and categorized in three types:-

1)      U Tube Heat Exchanger ( Have only one head / bonnet & U tubes bundle, both removable )

Heat exchanger with one stationary tube sheet attached to the shell and channel. The heat exchanger contains a bundle of Utubes attached to the tube sheet





 

 

 

2)      Fixed Tube Sheet Heat Exchanger (with removable front & rear heads (Both) )

Heat exchanger with two stationary tube sheets, each attached to the shell and channel. The heat exchanger contains a bundle of straight tubes connecting both tube- sheets 



 

3)      Floating Head Heat Exchanger with floating head (with removable  front & rear heads + floating head & tube bundle too)

Heat exchanger with one stationary tube sheet attached to the shell and channel, and one floating tube sheet that can move axially. The heat exchanger contains a bundle of straight tubes connecting both tube sheets

 


Images source: -  ASME BPVS SEC VIII-1

 

The terminology of shell and tube heat exchanger & components as given in ASME BPVC sec Viii Div1 is as below:-

 

1)      Channel cover ( bolted flat cover)                9) Baffles or support plates

2)      Channel                                                          10) Floating head backing device

3)      Channel flange                                               11) Floating tube sheet

4)      Pass partition                                                 12) Floating head

5)      Stationery tube sheet                                    13) Floating head flange

6)      Shell flange                                                     14) Shell cover

7)      Tubes                                                               15) Expansion joint

8)      Shell                                                                 16) Distribution or vapor belt

                                                       

 

 

"Designations for Shell and Tube Exchangers

Three classes of Mechanical Standards, R, C, and B, reflecting acceptable designs for various service

applications, are presented. The user should refer to the definition of each class and choose the one that best fits the specific need.

TEMA CLASS “R” “C” “B” Type Shell and Tube Heat exchangers

 

Definition of Class “R” Heat exchanger:-

The TEMA mechanical standard for class “R” heat exchanger specify Design & Fabrication of unfired shell and tube heat exchangers for the generally severe requirements of petroleum and process application.

Definition of Class “C” Heat exchanger:-

The TEMA mechanical standard for class “C” heat exchanger specify Design & Fabrication of unfired shell and tube heat exchangers for the generally moderate requirements of commercial and general process application.

Definition of Class “B” Heat exchanger:-

The TEMA mechanical standard for class “B” heat exchanger specify Design & Fabrication of unfired shell and tube heat exchangers for chemical process service

The widespread adoption of shell and tube exchangers has led to the establishment of a standardized naming system developed by the Tubular Exchanger Manufacturers Association (TEMA). This system utilizes letters and diagrams to describe various aspects of the exchangers. The initial letter denotes the type of front header, the second letter indicates the shell type, and the third letter specifies the rear header type. For instance, examples like BEM, AEP, AES, and CFU exchangers are illustrated in below shown Figure 1, Figure 2, Figure 3, and figure 4 while Figure A provides a comprehensive overview of the complete TEMA nomenclature."

 

 

In TEMA standard:- based on front end stationery head , type of shells and rear end head types classification is as show in below table .

Front end stationery head types are A,B,C,N and D 

Shell types are E,F,G,H,J,K and X

Rear end head types are L,M,N,P,S,T,U and W

 

Bellow given Figure A provides a comprehensive overview of the complete TEMA nomenclature."

 Figure A




Image Source- TEMA 10th Edition

 

 Components or parts name of heat exchanger :-

1)      Stationery Head Channel                                                               21.Floating head cover External

2)      Stationery Head Bonnet                                                                 22. Floating Tube sheet skirt

3)      Stationery Head Flange Channel or Bonnet                                23. Packing box

4)      Channel Cover                                                                                 24. Packing

5)      Stationery Head Nozzle                                                                 25. Packing Gland

6)      Stationery Tube Sheet                                                                    26. Lantern ring.

7)      Tubes                                                                                                27. Tierods and spacers

8)      Shell                                                                                                  28. Transverse baffle or support plate

9)      Shell Cover                                                                                      29. Impingement plate.

10)   Shell flange Stationery Head End                                                30. Longitudinal baffle

11)   Shell flange rear head end                                                            31. Pass partition

12)   Shell Nozzle                                                                                    32. Vent Connection

13)   Shell Cover flange                                                                          33. Drain connection

14)   Expansion Joint                                                                              34. Instrument connection

15)   Floating Tube sheet                                                                        35. Support saddle

16)   Floating head cover                                                                        36. Lifting lug

17)   Floating head cover flange                                                            37. Support Bracket

18)   Floating head backing device                                                       38. Weir

19)   Split shear ring                                                                                39. Liquid Level connection.

20)   Slip on backing Flange                                                                   40. Floating head support

 

BEM – Bonnet(integral cover) (B type)+Shell E Type+ Fixed tube sheet like “b” stationery head(M type)

Figure 1


Image Source- TEMA 10th Edition


AEP- Channel and removable cover (A type)+ Shell E Type + Outside packed floating head (P type)

Figure 2


Image Source- TEMA 10th Edition

AES- Channel and removable cover (A type)+ Shell E Type+ Floating head with backing device (S type)

Figure 3


Image Source- TEMA 10th Edition

CFU- Channel integral with tube sheet and removable cover (C type)+ shell F type + U tube bundle

Figure 4


Image Source- TEMA 10th Edition

 

Fixed Tube sheet Heat Exchanger :-

 

In this particular heat exchanger design, the shell and both side tube sheets are welded together, resulting in the lowest manufacturing cost. This structural design contributes to its widespread popularity and extensive usage due to its cost-effectiveness. Cleaning the interior of the tubes involves removing both end covers or headers/dish ends, facilitating mechanical cleaning using lengthy brushes and pressurized water. However, cleaning the exterior of the tubes is limited to chemical methods. To accommodate varying temperature differentials, expansion joints or bellows are commonly integrated into the design to withstand thermal expansion effectively.

As illustrated in Figure A above, the rear end head types for the Fixed Tube Sheet type heat exchanger are labeled as L, M, and N.

 U Tube Heat Exchangers:-

 This particular exchanger features a single front head bolted to a tube bundle containing U tubes, enclosed within a K-type shell that is sealed at the rear with a dish end or welded pipe cap. The tube bundle can be easily removed from the shell, enabling cleaning of both the inner and outer surfaces of the tubes. While mechanical cleaning of the tubes internally is limited, pressurized water and chemical cleaning can effectively remove contaminants from both sides. The U-type configuration of the tube bundle allows ample space for thermal expansion, eliminating concerns regarding expansion in this design. This has more cost than fixed tube sheet heat exchanger.

The U-tube shell and tube heat exchanger employs 180-degree U-bend tubes in its manufacturing construction.

 Floating Head Heat Exchanger:-

 The Floating Head Heat Exchanger is characterized by both tube sheets, front and rear, not being welded to the shell. They possess varying outside diameters, allowing for easy disassembly from the front end for cleaning purposes. The tube sheets form a bundle with the tubes, while the shell features a shell body flange welded to it, which is then bolted to the front tube sheet, front bonnet, and rear bonnet. The front bonnet, front tube sheet, and shell flanges share the same outside diameter for bolted assembly. The floating head is attached to the rear tube sheet using a floating head backing device with a smaller outside diameter than the front side. The rear tube sheet has a smaller diameter than the inside diameter of the shell to facilitate removal. This design offers the most convenience for cleaning both the interior and exterior of the tubes as well as the interior of the shell. Additionally, the floating head allows for thermal expansion management. While this type encompasses the advantages of the previous two types, it incurs higher manufacturing costs due to its complexity.

As depicted in Figure A, the rear end header types for the Floating Head type heat exchanger are denoted by the letters P, S, T, and W.

 Both the Fixed Tube Sheet heat exchanger and the Floating Head heat exchanger utilize straight tubes in their construction.

 

 Let us study in detail front end stationery head types shown in figure A :-

Type A:- Front Header Type A features a header cover bolted directly onto it, allowing for easy removal of the cover for inspection, maintenance, and tube cleaning purposes. This design offers the advantage of not requiring the removal of associated piping for tube cleaning. However, it requires the use of two gaskets—one for the tube sheet and another for the cover—as well as double the set of fasteners, making it a more costly design option.

Type B:- Front Header Type B is characterized by its requirement of only one gasket joint and one set of fasteners, enabling it to withstand higher pressures. However, a disadvantage is that it necessitates the removal of associated piping for inspection, maintenance, and tube cleaning. Nonetheless, this design option is the least costly among alternatives.

C type front header – it has channel integral with tube sheet and removable cover along with removable tube bundle with two seal joints , 2  gaskets , 2 set of fasteners . header cover bolted directly onto it, allowing for easy removal of the cover for inspection, maintenance, and tube cleaning purposes. This design offers the advantage of not requiring the removal of associated piping for tube cleaning

N type front header features a channel integrated with the tube sheet and shell, along with a removable cover requiring only one gasket and one set of fasteners. The header cover is bolted directly onto it, enabling easy removal for inspection, maintenance, and tube cleaning. This design offers the advantage of not necessitating the removal of associated piping for tube cleaning.

D-Type front header, designed for very high-pressure applications, presents challenges in maintenance and is the most expensive option available. Despite its high cost, this design offers the advantage of not needing associated piping removal for tube cleaning. However, due to the integral nature of the tube bundle, maintenance and tube cleaning are intricate processes

 

 Shell Types :- 

Shell Type E: This design features a simple shell with one inlet at the top left side and one outlet at the bottom right side, providing a single pass for the shell-side fluid. It is widely utilized across various applications.

 Shell Type F: Incorporating a horizontal longitudinal baffle, this shell design divides the shell flow into two passes, with one inlet at the top left side and one outlet at the bottom left side, resulting in a double pass for the shell-side fluid.

 Shell Type G: Utilizing a horizontal longitudinal baffle positioned at the center, this shell design splits the shell flow on both sides, featuring one inlet at the middle of the top and one outlet at the middle of the bottom side, thus creating a split pass for the shell-side fluid.

 Shell Type H: With two horizontal longitudinal baffles in the center, this shell design divides the shell flow into two split passes on both sides. It has two inlets at the top and two outlets at the bottom side, offering a double split pass for the shell-side fluid.

 Shell Type J: This shell design includes one inlet at the middle of the top and two outlets at the bottom side, resulting in a divided pass for the shell-side fluid.

 Shell Type K: Featuring one inlet at the middle of the top and two outlets at the bottom side, this shell design is in a kettle shape with a baffle.

 Shell Type X: With one inlet at the middle of the top and one outlet at the middle of the bottom side, this shell design provides a cross pass for the shell-side fluid.

 

Rear End Head Types:

 

L Type: This type features a header cover bolted directly onto it, allowing for easy removal of the cover for inspection, maintenance, and tube cleaning. While it offers the advantage of not requiring the removal of associated piping for tube cleaning, it entails the use of two gaskets—one for the tube sheet and another for the cover—as well as double the set of fasteners, making it a more costly design option. It resembles a fixed tube sheet like A, forming a stationary head.

 M Type: Characterized by the requirement of only one gasket joint and one set of fasteners, this type can withstand higher pressures. However, it necessitates the removal of associated piping for inspection, maintenance, and tube cleaning. Nonetheless, this design option is the least costly among alternatives. It resembles a fixed tube sheet like B, forming a stationary head.

 N Type: This type features a channel integrated with the tube sheet and shell, along with a removable cover requiring only one gasket and one set of fasteners. The header cover is bolted directly onto it, enabling easy removal for inspection, maintenance, and tube cleaning. It offers the advantage of not necessitating the removal of associated piping for tube cleaning. It resembles a fixed tube sheet like N, forming a stationary head.

 P Type: In this type, the outside packed floating head allows for the removal of the bundle, with easy access to the header cover for inspection, maintenance, and tube cleaning. However, it has limitations for thermal expansion.

 S Type: This type features a floating head with a backing device, making it costly and challenging for dismantling but has no restrictions for thermal expansion. Due to removable bundles, more shell clearance is required.

 T Type: This type features a pull-through floating head, resulting in more shell clearance. It is less costly compared to other floating heads and has no restrictions on thermal expansion.

 U Type: With a U-tube bundle design, this type offers the lowest cost among removable bundle types. It allows for any thermal expansion and enables easy cleaning of tubes from inside and outside. However, it has limitations on counter flow even with F type shell and uneven tube passes.

 W Type: Featuring an externally sealed floating tube that allows for thermal expansion with no restriction, this type is less costly than other floating heads. It is easy to maintain, clean, and repair as the tube bundle is removable.

 

Now we can further design each based on shell and tube side passes as –

The configuration of shell and tube heat exchangers can vary in terms of passes, denoted as 1-1, 1-2, 1-4, and so forth, representing one, two, four, six, or eight passes, respectively. The first digit indicates the number of shells, while the second digit indicates the number of passes.

The number of passes signifies how many times the fluid circulates within the shell. For instance, in a single-pass heat exchanger, the fluid traverses the shell only once. With each increase in the number of passes, there's a corresponding increase in the heat transfer coefficient.

 1 shell side pass and 1 tube side pass (1-1)  -  Both the following figures shows this 1-1 type




2 shell side pass and 2 tube side pass – (2-2) .  


 

 

1 shell side pass and 2 tube side pass   - ( 1-2)


In addition to above three we can even go for more than three passes for tube side as four pass, five pass, six pass, seven pass, eight pass upto 12 passes. More passes are very common in chillers of  HVAC & R as a evaporators /coolers. We can have these with alternate pass partition placed in both side of head covers as shown in below diagram

    


 


Flow types are an integral part of categorizing shell and tube heat exchangers based on their characteristics, facilitating comprehension of their function and operation. One such characteristic is their flow type.

 

Shell and tube heat exchangers are categorized into three flow types: parallel, counter, and cross. These flow types are utilized in various combinations due to their distinct designs, operational requirements, and applications.

Parallel flow occurs when both the shell and tube sides enter the heat exchanger from the same end and flow directly towards the opposite end. In this configuration, the temperature change for each fluid is uniform, increasing or decreasing by equal amounts.


                                          

Counter flow, on the other hand, involves fluids flowing in opposite directions, entering the heat exchanger at opposite ends, and discharging at opposite ends. It is widely regarded as the most popular and efficient type of heat exchanger due to its superior performance.

 


Cross flow in a shell and tube heat exchanger entails fluids flowing perpendicular to each other at a 90-degree angle. One of the fluids undergoes a change in state, similar to a steam system condenser where cooling water absorbs steam, before being absorbed by the fluid that remains in its liquid state.

 

 

 

 

 

 

Now we can further design each based on shell and tube side circuits as – Typically for evaporators & condensers used in refrigeration & air-conditioning chillers .

1 shell side circuit and 2 tube side circuit . this is normally in evaporators  used in refrigeration & air-conditioning chillers .







1 shell side circuit and 3 tube side circuit  this is normally in evaporators  used in refrigeration & air-conditioning chillers .

 







 

2 shell side circuit and 1 tube side circuit  this is normally in condensers  used in refrigeration & air-conditioning chillers .

 










The TEMA Mechanical Standards are applicable to shell and tube heat exchangers that fall below certain criteria:

Inside diameters must not exceed 100 inches (2540 mm).

The product of the nominal diameter (in inches or millimeters) and the design pressure (in pounds per square inch or kilopascals) must not exceed 100,000 (17.5 x 10^5).

The design pressure should not exceed 3,000 psi (20684 kPa).

These criteria are set to ensure that the maximum shell wall thickness remains approximately 3 inches (76 mm), and the maximum stud diameter is around 4 inches (102 mm).

TUBE Pattern




                               Triangular            Rotated Triangular         Square           Rotated Square

Please take note in above figure: The flow arrows are oriented perpendicular to the edge of the baffle cut.

Triangular Pattern :- When mechanical cleaning of the shell side is required, triangular or rotated triangular patterns should be avoided.

Square Pattern :- In removable bundle units, if mechanical cleaning of the tubes is requested by the purchaser, the tube lanes should form a continuous pattern.

Tube Pitch for TEMA mechanical standard for class “R” - Tube spacing must adhere to a minimum center-to-center distance of 1.25 times the outside diameter of the tube. In instances where mechanical cleaning of the tubes is requested by the purchaser, minimum cleaning lanes of 1/4" (6.4 mm) should be included.

Tube Pitch for TEMA mechanical standard for class “C”- Tubes should be positioned with a minimum center-to-center distance of 1.25 times the outside diameter of the tube. However, in cases where the tube diameters are 5/8" (15.9 mm) or smaller and tube-to-tube sheet joints are solely expanded, the minimum center-to-center distance can be reduced to 1.20 times the outside diameter.

Tube Pitch for TEMA mechanical standard for class “B”- Tubes must maintain a minimum center-to-center distance of 1.25 times the outside diameter of the tube. If the purchaser specifies mechanical cleaning of the tubes and the nominal shell diameter is 12 inches (305 mm) or less, minimum cleaning lanes of 3/16" (4.8 mm) should be included. For shell diameters exceeding 12 inches (305 mm), minimum cleaning lanes of 1/4" (6.4 mm) are required.

 

Tube Length :- Commonly used tube lengths for both straight and U-tube heat exchangers include: 96 inches (2438 mm), 120 inches (3048 mm), 144 inches (3658 mm), 192 inches (4877 mm), and 240 inches (6096 mm).

Bare Tube Sizes :- Birmingham Wire Gage (BWG) standard




Tube wall thickness must be designated as either minimum or average.

For integrally finned tubes:



The nominal fin diameter must not surpass the outside diameter of the unfinned section.

Tubes must be specified with both the thickness under the fin and at the plain end.



Baffles & Suport Plates :-

In a shell and tube heat exchanger, baffles are used to direct the flow of fluid within the shell side of the exchanger. As the shell fluid circulates around the tubes, directed by baffles, the motion of the shell fluid—whether it's lateral or vertical—and the number of times it traverses over the tubes, are regulated by segmental baffles. These baffles play a vital role in optimizing heat transfer efficiency. They are typically perforated plates or rods placed perpendicular to the tube bundle. Baffles serve several purposes:

Enhanced Heat Transfer: By causing the fluid to flow around the tubes in a specific pattern, baffles promote turbulence, which increases heat transfer efficiency.

Prevention of Fluid Channeling: Baffles help prevent fluid from taking a direct path through the exchanger, ensuring that it is evenly distributed across the tube bundle. This helps in maximizing heat transfer rates and avoiding hot spots or cold spots.

Structural Support: Baffles also provide structural support to the tube bundle, helping to maintain its alignment and integrity, especially in high-pressure applications.

Tube Alignment: Support plates ensure proper alignment of the tubes within the tube sheet, preventing them from sagging or vibrating excessively, which could lead to mechanical failure or reduced heat transfer efficiency.

Overall, baffles and support plates play crucial roles in optimizing the performance and structural integrity of shell and tube heat exchangers, ensuring efficient heat transfer while maintaining mechanical stability.

Baffle Cuts for segmental baffle :-

             Horizontal                                             Vertical                                              Rotated




Baffle Cuts for Multi -Segmental baffle :-

                                                                                Double Segmental




Triple segmental








 

The segmental or multi-segmental type of baffle or tube support plate is standard, although other types of baffles are allowed. The baffle cut is defined as the opening height of the segment expressed as a percentage of the shell inside diameter or as a percentage of the total net free area inside the shell (which is the shell cross-sectional area minus the total tube area).

 

For multi-segmental baffles, the number of tube rows that overlap should be adjusted to ensure approximately equal net free area flow through each baffle. Baffles should be cut near the centerline of a row of tubes, a pass lane, a tube lane, or outside the tube pattern. Moreover, baffles should have a satisfactory machined finish on the outside diameter.

Typically, baffle cuts can be vertical, horizontal, or rotated, as illustrated in the figure above.

When determining the maximum and minimum spacing for baffles/support and unsupported tube lengths, it's essential to engage a designer proficient in the thermal/hydraulic design of shell and tube heat exchangers. Various factors such as operating conditions, heat load, prevention of flow-induced vibration, available tube length, and nozzle locations must be considered when establishing the spacing of baffle and support plates.

 

For minimum spacing, segmental baffles typically should not be positioned closer than 1/5 of the shell inside diameter or 2 inches (51 mm), whichever is greater. However, specific design requirements might necessitate a closer spacing.

Baffles normally shall be spaced uniformly, spanning the effective tube length. When this

is not possible, the baffles nearest the ends of the shell, and/or tube sheets, shall be located

as close as practical to the shell nozzles. The remaining baffles normally shall be spaced

uniformly.







Tie Rod & Spacers :-

To ensure the secure positioning of all transverse baffles and tube support plates, tie rods and spacers, or alternative methods with equivalent effectiveness, must be implemented to tie the baffle system together and securely in position.

It is recommended to use a specific number and diameter of tie rods based on the size of the heat exchangers. Alternative combinations of tie rod count and diameter with equivalent metal area are acceptable. However, a minimum of four tie rods and a diameter no smaller than 3/8" (9.5 mm) must be utilized. Additionally, each baffle segment requires at least three points of support. 

The suggested number of tie rods based on the size of the shell diameter  is as follows:For shell sizes up to 14 inches: 4 tie rods

For shell sizes from 16 inches to 33 inches: 6 tie rods

For shell sizes from 34 inches to 48 inches: 8 tie rods

For shell sizes from 49 inches to 60 inches: 10 tie rodsFor shell sizes from 61 inches to 100 inches: 12 tie rods        TIE RODS     




             SPACER TUBES    



           
BAFFLE ASSLY



Impingement protection is crucial for shell side inlet nozzles to safeguard the tube bundle against impinging fluids.

An Impingement insert means needed to prevent or minimize erosion of tube bundle components at the entrance and exit areas.

In this section,  v is defined as the linear velocity of the fluid in feet per second (meters per second)

and p is its density in pounds per cubic foot (kilograms per cubic meter)

However, if the product of  ρv2 in the inlet nozzle falls below the specified limits, impingement protection may not be necessary:

1500 (2232) for non-abrasive, single-phase fluids (liquids, gases, or vapors).

500 (744) for all other liquids, including those at their boiling points.

For all other gases, vapors (including steam), and liquid-vapor mixtures, impingement protection is required.

A well-designed diffuser can effectively reduce line velocities at the shell entrance. Distributor belt type diffusers may be utilized for this purpose.



Expansion Joint :-

An expansion joint in a shell and tube heat exchanger serves a crucial function in accommodating thermal expansion and contraction of the tubes. As the heat exchanger operates, the metal tubes are subjected to changes in temperature, causing them to expand and contract. Without an expansion joint, these thermal movements could lead to stress build-up, deformation, or even damage to the tubes and other components of the heat exchanger.

Expansion joints are typically installed at specific locations within the tube bundle assembly where thermal movement is expected to occur normally in shell. These joints consist of flexible elements, such as bellows or corrugated sections, which can flex and elongate in response to the changes in tube length due to temperature variations. By absorbing the thermal expansion and contraction of the tubes, the expansion joint helps to reduce mechanical stresses on the heat exchanger components and ensures the integrity of the system over its operational lifespan.




In summary, expansion joints are essential components in shell and tube heat exchangers, providing flexibility to accommodate thermal expansion and contraction of the tubes, thereby preventing damage and maintaining the efficiency and reliability of the heat exchanger.


The shell of the exchanger needs safeguarding from over-pressure through the use of pressure relief valves (PRV). The timing of these protective devices' activation has been identified as crucial for the effective protection of the exchanger from high pressures than designed. These safety mechanisms are installed directly onto the exchanger's shell and release pressure into a relief system.





Fluid Allocation :-

Fluid distribution:

Place the high-pressure stream within the tubes. Opt for high-pressure and high-temperature fluids on the tube side for optimal cost design.

Position corrosive, hazardous and toxic fluids , freezing fluids within the tubes.

Allocate the stream with the highest fouling tendency to the tube side.

Direct more viscous fluids to the shell side.

Assign streams with lower flow rates to the shell side. Allocate fluids requiring low pressure drop during operation to the shell side due to its larger flow area and lower resistance.

Place fluids undergoing phase change or transitioning from gas / vapor/liquid  on the shell side.

Route streams with lower heat transfer coefficients to the shell sideand high heat transfer coefficient in tube side.

Place toxic fluids within the tubes.

 


Thermal Design:-

The thermal design process for a heat exchanger involves several key steps:

Definition of Operating Conditions: Determine the design temperature, pressure, and maximum allowable pressure drop for both the product and service fluids.

Fluid Properties Analysis: Analyze the physical properties of the fluids involved, including density, specific heat, thermal conductivity, and viscosity.

Customer Input: Typically, the customer specifies the product's flow rate and desired entry and exit temperatures. Two out of three parameters (flow rate, entry temperature, exit temperature) are provided, and the third is calculated accordingly.

Geometry Definition: Define the heat exchanger's geometry, including the shell diameter, tube bundle specifications (number of inner tubes, inner tube diameter, wall thickness, and length), and dimensions of shell and tube side fluid connections. Material selection is also determined at this stage.

Thermal Calculation: Perform thermal calculations to determine the heat transfer coefficients for both the shell and tube sides. These coefficients are influenced by fluid properties and flow velocity. Using specific formulas tailored to the heat exchanger type, calculate the overall heat transfer coefficient. This value is used to calculate the total heat transfer area required for the application using the formula:   Area = Duty / (K × LMTD), where Duty is the total heat transferred, K is the overall heat transfer coefficient, and LMTD is the log mean temperature difference.

Pressure Drop Calculation: Calculate the pressure drop for both the shell and tube side fluids, considering factors such as Reynolds number, flow type (turbulent or laminar), and roughness values of the shell and inner tubes.

Interpretation of Thermal Calculation: Compare the calculated heat transfer area with the predefined area. If the calculated area exceeds the predefined area, consider redesigning the heat exchanger geometry by adjusting parameters such as length or adding inner tubes. Similarly, if the calculated pressure drop exceeds the maximum defined, redesign the geometry to reduce pressure drop.

Iterative Process: Repeat above steps as necessary until a satisfactory design with suitable geometry and within design limits is obtained.

This process ensures that the heat exchanger is designed to efficiently transfer heat while meeting the specified operating conditions and constraints.

Mechanical Design:-

 

the mechanical design calculations ensure its structural integrity and compliance with design pressure and conditions. These calculations typically include:

Shell Wall Thickness Calculation: Determine the required thickness of the shell to withstand the design pressure and maintain structural integrity.

Nozzle Wall Thickness Calculation: Calculate the thickness of the nozzle walls to ensure they can withstand the pressure and loads applied at the connections.

Inner Tube Wall Thickness Calculation: Determine the thickness of the inner tube walls to withstand the pressure and thermal stresses.

Expansion Joint Dimensions Calculation: Calculate the dimensions of the expansion joints to accommodate the differential expansion between the shell and tube sides due to temperature differences.

Tube Sheet Thickness Calculation: Determine the thickness of the tube sheet to support the tube bundle and withstand pressure and thermal stresses.

The mechanical design calculations may reveal parameters that do not align with the initial geometric design. In such cases, a new proposal for the geometry may be necessary, requiring repetition of steps four to seven.

Once all dimensions are finalized, manufacturing drawings are prepared which include detailed specifications of various components such as the shell, tubes, expansion joints, and connections, ensuring accurate fabrication of the heat exchanger.



A Brief about Material of construction ( MOC)

Shell and Tube Sheets:- shell material should also be able to withstand the operating conditions, including temperature, pressure, and potential corrosive environments. Common materials for the shell include carbon steel, stainless steel, and various alloys.

End Covers or Heads:-The end covers or heads seal the ends of the shell and are typically removable for maintenance and inspection. These components must be made from materials that provide a tight seal and can withstand the mechanical stresses and operating conditions. Materials commonly used for end covers include carbon steel, stainless steel, and alloy steels.

Tubes:- The selection of tube material is crucial for effective heat transfer. Good thermal conductivity is essential for efficient heat transfer, as heat moves from a hotter side to a colder side through the tubes, creating a temperature gradient along their length. Moreover, the tube material must be able to withstand thermal expansion differences at varying temperatures, which can induce thermal stresses during operation, alongside potential stress from fluid pressures. Furthermore, the chosen material should be compatible with both the fluids on the shell and tube sides over prolonged periods under operating conditions (including temperatures, pressures, and pH levels) to prevent deterioration such as corrosion. Therefore, it's imperative to carefully choose robust, thermally conductive, corrosion-resistant materials of high quality, typically metals like aluminium, copper alloys, stainless steel, carbon steel, non-ferrous copper alloys, Inconel, nickel, Hastelloy, and titanium. Additionally, fluoropolymers and Fluorinated ethylene propylene are utilized for tubing material due to their exceptional resistance to extreme temperatures.

Here are the uses and industries associated with each of the listed shell and tube heat exchanger applications:

 

  • 1)      Condenser - Uses: Condenses vapor into liquid; Industry: HVAC, refrigeration, power generation.
  • 2)      Evaporator - Uses: Evaporates liquid into vapor; Industry: Refrigeration, food processing, chemical processing.
  • 3)      Oil Cooler - Uses: Cools oil or other fluids; Industry: Automotive, industrial machinery, hydraulic systems.
  • 4)      Intercooler - Uses: Cools compressed air between turbocharger stages; Industry: Automotive, marine, aerospace.
  • 5)      Aftercooler - Uses: Cools compressed air after compression; Industry: Air compression systems, power generation, refrigeration.
  • 6)      Refrigerant Cooler - Uses: Cools refrigerant fluids; Industry: HVAC, refrigeration, chemical processing.
  • 7)      Steam Generator - Uses: Produces steam from water; Industry: Power generation, industrial processes, steam systems.
  • 8)      Boiler Feedwater Heater - Uses: Heats feedwater before entering a boiler; Industry: Power generation, boiler systems.
  • 9)      Gas Cooler - Uses: Cools gases in various industrial processes; Industry: Petrochemical, chemical processing, power generation.
  • 10)   Brine Cooler - Uses: Cools brine solutions in refrigeration systems; Industry: Refrigeration, cold storage facilities, food processing.
  • 11)   Process Heater - Uses: Heats gases or liquids in industrial processes; Industry: Chemical processing, petroleum refining, food processing.
  • 12)   Air Preheater - Uses: Heats combustion air in power plants and industrial boilers; Industry: Power generation, boiler systems, industrial processes.
  • 13)   Cooling Tower Water Heater - Uses: Heats water in cooling towers; Industry: HVAC, power generation, industrial cooling systems.
  • 14)   Heat Recovery Steam Generator (HRSG) - Uses: Recovers heat from exhaust gases to generate steam; Industry: Power generation, combined cycle power plants.
  • 15)   Hydraulic Oil Cooler - Uses: Cools hydraulic fluid in hydraulic systems; Industry: Automotive, aerospace, industrial machinery.
  • 16)   Lubricating Oil Cooler - Uses: Cools engine lubricants; Industry: Automotive, industrial machinery, marine.
  • 17)   Chemical Reactor Cooler - Uses: Controls temperatures in chemical reactions; Industry: Chemical processing, pharmaceuticals, petrochemicals.
  • 18)   Waste Heat Recovery Unit (WHRU) - Uses: Recovers waste heat from industrial processes; Industry: Energy production, chemical processing, manufacturing.
  • 19)   Marine Heat Exchanger - Uses: Cools marine engine coolant; Industry: Marine, shipping, offshore drilling.
  • 20)   Ethylene Glycol Heater/Cooler - Uses: Heats or cools ethylene glycol in HVAC systems; Industry: HVAC, refrigeration, chemical processing.
  • 21)   Heat Recovery Exchanger - Uses: Recovers waste heat for reuse; Industry: Industrial processes, power generation, chemical processing.
  • 22)   Feedwater Preheater - Uses: Preheats boiler feedwater; Industry: Power generation, boiler systems, industrial processes.
  • 23)   Ammonia Cooler - Uses: Cools ammonia gas or liquid; Industry: Refrigeration, chemical processing, food processing.
  • 24)   Chlorine Cooler - Uses: Cools chlorine gas; Industry: Chemical processing, water treatment, disinfection.
  • 25)   Glycol Reboiler - Uses: Re-boil glycol used in natural gas dehydration; Industry: Oil and gas, chemical processing, petrochemicals.
  • 26)   Heater Treater - Uses: Heats and separates oil-water emulsions; Industry: Oil production, petroleum refining, chemical processing.
  • 27)   Thermal Oil Heater - Uses: Heats thermal oil for industrial heating systems; Industry: Chemical processing, asphalt plants, food processing.
  • 28)   Deaerator - Uses: Removes dissolved gases from boiler feedwater; Industry: Power generation, boiler systems, chemical processing.
  • 29)   Feedwater Heater - Uses: Heats feedwater before entering a boiler; Industry: Power generation, boiler systems, industrial processes.
  • 30)   Gas-to-Gas Heat Exchanger - Uses: Transfers heat between two gas streams; Industry: Chemical processing, power generation, gas purification.
  • These descriptions provide insight into the specific uses and industries where shell and tube heat exchangers are commonly employed.



Standards & Regulations :-

The codes applicable for shell and tube heat exchanger design & manufacturing.


1) ASME 
American Society of Mechanical Engineers, Boiler and Pressure Vessel Code (BPVC) Sec VIII Div 1 Part UHX

2)TEMA is the Tubular Exchanger Manufacturers Association- most widely accepted and used.

3)API 660 API standards relate to the manufacturing of equipment and parts for the oil and gas industry

4) IS 4503 : 1967 indian standard

5) Canadian Registration Number. CRN - required for any boiler, pressure vessel will be in operation in Canada

5) PED Pressure Equipment Directive (PED) is mandatory in the European Union when manufacturing pressure equipment, or selling it into the EU

6) NORSK - Standards are by the Norwegian petroleum industry essential for manufacturing heat exchangers to sell in Norway.

These regulations and standards play a crucial role in ensuring the safety, reliability, and compliance of shell and tube heat exchangers across various industries and regions.

 

 

References:-

 

              Standards of the Tubular Exchanger Manufacturers Association (TEMA), 10th edition, 2019

              ASME Boiler and Pressure Vessel Code, (BPVC) Section VIII, Division 1, Part UHX



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