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 are among the most effective means of heat exchange. Shell and tube heat exchanger is a device where two working fluids exchange heats 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 range, Shell and tube heat exchangers are compact in design, easy in construction and maintenance 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 the heats 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.
The above figure shows both inlet and outlet nozzle in the front header of the channel side. That means this exchanger consists of even number of tube passes. However, there can be odd number of tube passes. On that situation, the channel side outlet nozzle will be on the read header. Increasing the number of tube passes increase the heat transfer co-efficient.
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 / section one is shell side and other is channel/tube side
- Shell side section consists of the following components: Shell, Cover, Body Flange, Nozzles, Saddle support.
- Channel / Tube side section consists of the following components: Channel, Cover, Body Flange, Nozzles, Tube Sheet and Tubes (Tube Bundle)
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
- 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.
Tube Pattern inside Shell & Tube Heat exchanger
Normally tubes inside the exchanger are of 0.5″ to 2″ in sizes and arranged in triangular or square pattern as shown in Fig. 4
The tube shall be placed with a minimum centre to centre 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 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 Exchanger
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 inch)
- 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 tubesheet is fixed in the shell by welding and hence the term fixed tube sheet exchanger applies. This simple and economical construction allows cleaning of the tube bores by mechanical or chemical means. An expansion bellow is installed in the shell when there is a large temperature differences between the shell and tube materials. Refer to Fig. 5 for an example of 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 floating head heat exchanger.
Stationary Tube sheet with removable tube bundle
Fig. 7 shows an example of stationary tube sheet with removable tube bundle.
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.
Based on the number of times the tube-side/shell-side flows pass through the exchanger, 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 are provided in Fig. 9 below:
Depending on the application of shell and tube heat exchangers, they are known as various types as listed below:
- Preheaters, etc.
Stack Types of Configuration
In this, two or three heat exchangers placed one above other. This is termed as 1 shell in parallel and 2 or 3 Shells in series. Refer Fig. 10.
Codes and Standards for Shell and Tube Heat Exchanger
The following codes and standards govern the design of shell and tube heat exchanger.
- 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 18.104.22.168 and DEP 31.21.01.30
Design of Shell and Tube Heat exchangers
Process Design of Shell and tube Heat exchanger
Design of Shell and tube heat exchanger is a trial and error iterative process. In recent time, the 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 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 co-efficient
- 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 required heat transfer area
- Comparison of 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 Parameters||Fluid Allocation-Shell Side||Fluid Allocation-Tube Side|
|High Pressure Fluid Stream||X|
|High fouling fluid stream||X|
|More Viscous fluid||X|
|Lower Flow Rate Fluid||X|
|Fluid with low heat transfer co-efficient||X|
Fluid Velocity inside Shell and Tube
High fluid velocities increases heat transfer coefficients and reduces fouling but causes erosion and increases pressure drop. So velocity selected should be just enough to prevent settling of suspended soilds. Typical fluid velocities considered for the design of shell and tube heat exchangers are given the following table (Table-2):
|Fluid Types||Fluid Velocity-Shell Side||Fluid Velocity-Tube Side|
|Liquid||0.3 to 1 m/s||1 to 2 m/s|
|Gas /Vapor (Vacuum Pressure)||50 to 70 m/s||50 to 70 m/s|
|Gas /Vapor (Atmospheric Pressure)||10 to 30 m/s||10 to 30 m/s|
|Gas /Vapor (High Pressure)||5 to 10 m/s||5 to 10 m/s|
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 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 consist of calculation of shell thickness, flange thickness, etc. Various codes like ASME Sec VIII, PD 5500, TEMA, etc provides guidelines for mechanical design. The following design guidelines can be followed:
- Minimum Shell Thickness (Fig. 11) as per TEMA for class – R
- 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 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. Gasket joint shall be confined type
- Shell and Head design is done as per selected Pressure vessel 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 above load cases
- Tube sheet Design formula based on the theory of Flat Plates
Shell and tube Heat Exchanger Material of Construction
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 differing 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
- Oil and Gas
- Power Plants
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