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. Shell and Tube Heat exchangers can be used for wide temperature and pressure range.
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 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 the channel side both inlet and outlet nozzle in the front header. That means the there are even number of tube passes in this example. However, there can be off 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.
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.
Type of Shell and Tube Heat Exchanger
TEMA Exchanger Type
- R – Refinery and Petrochemical Application
- C – General Process Application
- B – Chemical Process Application
TEMA Heat Exchanger applicable Criteria
- Inside diameter 2500 mm
- Product of nominal diameter (mm) and design pressure (kPa) of 5 x 106
- design pressure of 3000 psig
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 Removable Bundle
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.
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. 8.
Design codes used for Heat exchangers
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
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
Design of Shell and Tube Heat exchangers
- Minimum Shell Thickness (Fig. 9) 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
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
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
Typical applications of shell & Tube Heat Exchangers are in the form of the following equipment:
- Preheaters, etc.
Few more resources for You..
Shell & Tube Heat Exchanger Piping: A brief Presentation
An article on Plate Heat Exchanger with Steam
A typical Check List for Reviewing of Shell & Tube Heat Exchanger Drawings
A brief presentation on Air Cooled Heat Exchangers
Basic Considerations for Equipment and Piping Layout of Air Cooled Heat Exchanger Piping
Reboiler Exchanger and System Type Selection