Shell & Tube – What Is Piping

What is a Heat Exchanger? A Heat exchanger is a device to transfer heat from one fluid (Liquid/Gas) to another. There are various types of heat exchangers used in process piping. Shell and tube heat exchanger is the most widely used heat exchanger and is among the most effective means of heat exchange. Shell and tube heat exchanger is a device where two working fluids exchange heat by thermal contact using tubes housed within a cylindrical shell. The fluid temperature inside the shell and tube are different and this temperature difference is the driving force for temperature exchange. Used for wide temperature and pressure ranges, Shell and tube heat exchangers are compact in design, easy to construct and maintain and provide excellent heat exchange. As the name specified, it consists of a shell and a number of tubes. Shell is the housing of the exchanger and tubes are mounted inside the cylindrical shell. Working Principle of Shell and Tube Heat Exchanger The working of a shell and tube heat exchanger is fairly simple. One fluid flows inside the tubes and the other through the shell. While flowing they exchange heat which means the cold fluid gains the heat from the hot fluid. So one cold fluid enters the shell (or tube side or channel side) inlet nozzle and comes out of the outlet nozzle as hot fluid. Obviously, the other fluid will become cold in the outlet than in the inlet. The heat transfer in a shell and tube heat exchanger is determined by the exposed surface area that is decided by the number of thermally conductive metal tubes. The fluid flow inside the shell and tube heat exchanger can be parallel flow or crossflow. Fig 1 shows the typical working principle of a shell and tube heat exchanger. Fig. 1: Working principle of a Typical Shell and Tube Heat Exchanger The above figure shows both the inlet and outlet nozzle in the front header of the channel side. That means this exchanger consists of an even number of tube passes. However, there can be an odd number of tube passes. In that situation, the channel side outlet nozzle will be on the read header. Increasing the number of tube passes increases the heat transfer coefficient. To increase the fluid turbulence in the tube and shell side flow, turbulator and baffles are installed inside tubes and shells respectively. This increase the heat transfer between the fluids. Basic Components of Shell and Tube Heat Exchanger Typically a Shell and Tube Heat Exchanger consists of two-compartment / sections; one is shell side and the other is channel/tube side The shell side section consists of the following components: Shell, Cover, Body Flange, Nozzles, and Saddle support. Channel / Tube side section consists of the following components: Channel, Cover, Body Flange, Nozzles, Tube Sheet, and Tubes (Tube Bundle) Fig. 2: Components of a Shell and Tube Heat Exchanger The heat exchanger is supported by saddles in the shell part. Tube Bundle of Shell & Tube Heat Exchanger Tube Bundle (Fig. 3) consists of the following components Tube sheet Tubes Baffles Tie rods and Spacers Sliding strips Tube bundles are removed during maintenance. Standard practice is to flow the corrosive fluid inside the tubes so that if corroded they can be easily replaced or repaired. Fig. 3 below shows a typical tube bundle. Fig. 3: Typical tube bundle of a shell and tube heat exchanger Tube Pattern inside Shell & Tube Heat exchanger Normally tubes inside the exchanger are 0.5" to 2" in size and arranged in a triangular or square patterns as shown in Fig. 4 Fig. 4: Typical tube patterns Tube Pitch The tube shall be placed with a minimum center-to-center distance of 1.25 times the tube outside diameter of the tube. When mechanical cleaning of the tube is specified then a minimum cleaning lane of 6.4 mm shall be provided. Baffles Baffles are installed in the shell of the shell and tube heat exchanger to create more turbulence and increase the flow time so that better heat exchange is possible. Baffles support the tubes so that damage and vibration of tubes are minimized. Types of Shell and Tube Heat Exchangers TEMA shell and tube heat exchanger types based on application As per TEMA (Tubular Exchanger Manufacturers Association) Shell and Tube Heat Exchanger can be classified as Class R Exchangers - Refinery and Petrochemical Application Class C Exchangers - General Process Application Class B Exchangers- Chemical Process Application TEMA Shell and Tube Heat Exchanger Applicable Criteria Inside diameter less than 2540 mm (100 inches) Product of nominal diameter (mm) and design pressure (kPa) of 17.5 x 106 Design pressure of 3000 psig (20684 KPa) The reason behind such limitation is to keep the maximum shell wall thickness below 3 in. (76 mm), and the maximum stud diameter to below 4 in. (102 mm). Shell and Tube heat exchanger types based on construction Depending on various construction and configuration parameters following types of shell and tube heat exchangers are widely used in industries. Fixed Tube Sheet Heat Exchanger The tube sheet is fixed in the shell by welding and hence the term fixed tube sheet exchanger applies. This simple and economical construction allows the cleaning of the tube bores by mechanical or chemical means. An expansion bellow is installed in the shell when there is a large temperature difference between the shell and tube materials. Refer to Fig. 5 for an example of the fixed tube heat exchanger. Fig. 5: Example of a typical fixed tube heat exchanger Floating Head Heat Exchanger In floating head construction, the rear header can float or move as it is not welded to the shell. The tube bundle can easily be removed while maintenance. Fig. 6 shows an example of a floating head heat exchanger. Fig. 6:Example of a typical Floating Head Removable bundle heat exchanger Stationary Tube sheet with removable tube bundle Fig. 7 shows an example of a stationary tube sheet with a removable tube bundle. Fig. 7: Example of a typical stationary tube sheet type heat exchanger U-tube Heat exchanger U-tube exchangers are a type of shell and tube heat exchanger whose tube bundle is made of continuous tubes bent into a “U” shape. The bend side is free-floating and this helps in thermal expansion without requiring expansion joints. However, such bends are difficult to clean. Fig. 8: Typical representation of U-tube Heat exchanger Based on the number of times the tube-side/shell-side flows pass through the exchanger, the shell and tube heat exchanger is categorized as: Single-Pass exchangers and Multi-Pass exchangers The full TEMA classification of shell and tube heat exchanger types is provided in Fig. 9 below: Fig. 9: TEMA Shell and Tube Heat Exchanger types Depending on the application of shell and tube heat exchangers, they are known as various types as listed below: Reboilers Evaporators Condensers Aftercoolers Preheaters, etc. Stack Types of Configuration In this, two or three heat exchangers are placed one above the other. This is termed as 1 shell in parallel and 2 or 3 Shells in series. Refer to Fig. 10. Fig. 10: Figure showing stack arrangement of heat exchangers Codes and Standards for Shell and Tube Heat Exchangers The following codes and standards govern the design of shell and tube heat exchangers. API 660 / ISO 16812 ( Shell and Tube heat exchangers for general refinery service ) ASME SECT.VIII Div.1 (UHX) or Div.2, PD 5500, EN 13445, AD 2000 Merkblatt. TEMA -Tubular Exchanger Manufacturers Association Shell DEP 31.22.20.31 and DEP 31.21.01.30 Design of Shell and Tube Heat exchangers Process Design of Shell and tube Heat exchanger The design of the Shell and tube heat exchanger is a trial-and-error iterative process. In recent times, thermal design is carried out by the process team using engineering software. However, the logic behind the calculations should be clearly understood. The shell and tube heat exchanger design calculations are based on the initial selection of a preliminary exchanger configuration and certain initial decisions like the front and rear header type, shell type, the sides the fluids are allocated, baffle type and baffle pitch tube diameter, length, and tube layout shell diameter, and number of tube passes Further steps for the shell and tube heat exchanger design consist of Calculation of shell side flow distribution and heat transfer coefficient Estimation of tube side heat transfer coefficient and pressure drop Determination of wall resistance and overall heat transfer coefficient Calculation of Mean temperature difference (log mean temperature difference) from the inlet and outlet temperatures of the two fluids. Estimation of the required heat transfer area Comparison of the calculated area with the assumed geometry Comparison of shell and tube-side pressure drop with allowable pressure drop If the pressure drop is within the allowable pressure drop design is acceptable. Otherwise, adjust the assumed geometry and repeat the above steps. Once the requirements are met, a process datasheet is developed indicating all process design parameters of shell and tube heat exchanger design. General design Considerations for shell and tube heat exchanger Fluid Allocation: Shell side vs Tube Side The following table (Table 1) provides general guidelines for shell and tube side fluid allocation in a shell and tube heat exchanger: Fluid ParametersFluid Allocation-Shell SideFluid Allocation-Tube SideHigh-Pressure Fluid Stream XCorrosive Fluid XHigh-fouling fluid stream XMore Viscous fluidX Lower Flow Rate FluidX Fluid with a low heat transfer coefficientX Toxic Fluid XTable 1: Shell and Tube-side Fluid Allocation Fluid Velocity inside Shell and Tube High fluid velocities increase heat transfer coefficients and reduce fouling but cause erosion and increase pressure drop. So velocity selected should be just enough to prevent the settling of suspended solids. Typical fluid velocities considered for the design of shell and tube heat exchangers are given in the following table (Table 2): Fluid TypesFluid Velocity-Shell SideFluid Velocity-Tube SideLiquid0.3 to 1 m/s1 to 2 m/sGas /Vapor (Vacuum Pressure)50 to 70 m/s50 to 70 m/sGas /Vapor (Atmospheric Pressure)10 to 30 m/s10 to 30 m/sGas /Vapor (High Pressure)5 to 10 m/s5 to 10 m/sTable 2: Typical fluid Velocities in Shell and Tube Heat Exchanger Design Pressure Drop Consideration Typical pressure drop values considered for shell and tube heat exchanger design are: For Liquids with Viscosity<1 mN-s/m2, ΔP=35 kPa For liquids with Viscosity=1 to 10 mN- s/m2, ΔP= 50-70 kPa Liquids without phase change= 70 kPa Condensing streams= 14 kPa For Vapor and gas services: High vacuum Pressure: 0.4-0.8 kPa Medium vacuum Pressure: 0.1 x absolute pressure Pressure 1 to 2 bar: 0.5 x system gauge pressure Pressure above 10 bar: 0.1 x system gauge pressure Vapors without phase change= 14 kPa Boiling Streams = 7 kPa Software used for Thermal Design The most popular software used for the thermal design of shell and tube heat exchanger are listed below HTRI – Heat Transfer Research Institute HTFS – Heat Transfer Research and fluid flow service Mechanical design of Shell and Tube Heat exchanger Mechanical design of shell and tube heat exchangers consists of calculation of shell thickness, flange thickness, etc. Various codes like ASME Sec VIII, PD 5500, TEMA, etc provide guidelines for mechanical design. The following design guidelines can be followed: Minimum Shell Thickness (Fig. 11) as per TEMA for Class - R Fig. 11: Minimum Shell thickness of shell and tube heat exchanger Baffle clearance, Baffle spacing, and thickness as per TEMA table RCB -4.3 Tie rod size and nos. as per TEMA table R- 4.71 for class - R Peripheral Gasket: The minimum width of the peripheral ring gasket for external joints shall be 10 mm for shell sizes up to 584 mm and 12 mm for all larger shell sizes. Pass Partition Gasket: The min. width of the gasket web for the pass partition of the channel shall not be less than 6.4 mm for shell sizes up to 584 mm and 9.5 mm for all larger shell sizes. The gasket joint shall be confined type Shell and Head design is done as per selected Pressure Vessels Design Code such as ASME, EN, or AD The most widely used design code across the world is ASME Sect. VIII Div.1 & 2 Body / Girth Flange Design as per Appendix -2 of ASME Sect. VIII Div.1 The tube sheet design is Mandatory as per UHX of ASME Sect. VIII Div.1 The tube sheet is designed for the following three cases. Tube side pressure (Pt) acting and Shell side pressure (Ps) is Zero Shell side pressure (Ps) acting and Tube side pressure (Pt) is Zero Shell side pressure (Ps) acting and Tube side pressure (Pt) acting Please consider the effect of Vacuum in the above load cases Tube sheet Design formula based on the theory of Flat Plates Shell and tube Heat Exchanger Material of Construction The following materials are the most common as Shell & Tube Heat Exchanger MOC. Carbon steel and Cladding Plates Stainless Steel Duplex Stainless steel Tubes - Carbon steel, Stainless steel, Duplex stainless steel, Exotic material such as copper, Inconel, Titanium Maintenance of Shell and Tube Heat exchangers Depending on user experience and manufacturer guidelines, shell and tube heat exchangers should be inspected at regular intervals. A shell and tube heat exchanger can fail by one or more of the following factors: Improper design. Excessive fouling. Air or gas binding resulting from improper piping installation or lack of suitable vents. Excessive clearances between the baffles and shell and/or tubes, due to corrosion. Operating conditions differ from design conditions. Maldistribution of flow in the unit, etc Following preventive maintenance steps at regular intervals can reduce the risk of equipment failure. Following maintenance steps can be followed to enhance shell and tube heat exchanger performance: Cleaning periodically to avoid fouling Inspection of tubes Gasket replacement Repairing leaks if detected during inspection. Application of Shell and Tube Heat Exchangers Shell & Tube Heat Exchangers find their application in the following Industries- Refinery and Petrochemical Fertilizer Oil and Gas Chemical Power Plants Few more resources for You.. 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