What is a Shell and Tube (S&T) Heat Exchanger?
A shell and tube heat exchanger (S&T) is a type of heat exchanging device constructed using a large cylindrical enclosure, or shell, that has bundles of perfectly spaced tubing compacted in its interior. Shell and tube heat exchangers are the most common form of heat exchange design. They are classified according to their properties, tubing type, and other characteristics. The use and popularity of shell and tube heat exchangers is due to the simplicity of their design and exceptionally efficient heat exchange rate. The process of a shell and tube heat exchanger involves the use of a liquid or steam that flows into the shell to heat the tubes.
In this article, we will study in detail about how to do piping arrangements around Shell and Tube Heat Exchanger. How to set location and elevation, Best practices to follow etc.
- Types for S&T Exchangers
- Different Parts of Heat Exchangers
- Location for S&T Exchangers
- Elevation for S&T Exchangers
- Maintenance & Access Space Consideration
- Nozzle Orientations for S&T Exchangers
- Study of Structure for S&T Exchangers
- Piping Arrangements Around S&T Exchangers
- Piping Support Arrangement for S&T Exchangers
1. Types for S&T Exchangers
There are mainly four types of S&T Exchangers based on the construction of the exchangers. These are as follows:
- Floating Head Heat Exchanger
- Fixed Tube Sheet Heat Exchanger
- U- Tube Heat Exchanger
- Kettle Type Heat Exchanger
Floating Head Heat Exchanger
In the floating head exchanger, the tube sheet is not welded to the shell at the rear header end but is permitted to move or float. The tube sheet at the front header (fluid inlet end on the tube side) has a larger diameter than the shell and is sealed in a manner similar to that applied in the fixed tube sheet design. The tube sheet at the rear header end of the shell is somewhat smaller in diameter than the shell and allows the bundle to be drawn through the shell.
The application of a floating head means that thermal expansion is allowed, and the tube bundle can be eliminated for cleaning purposes. Several types of rear headers can be utilized, but the S-Type Rear Head is the most common. The floating head heat exchanger is suitable for performing the precise services related to high temperatures and pressures but is more expensive than the equivalent fixed tube sheet exchanger (typically 25% for carbon steel construction).
Fixed Tube Sheet Heat Exchanger
A fixed tube sheet heat exchanger has straight tubes secured at both ends to the stationary tube sheets that are welded to the shell. A straight tube exchanger has a simple design and construction and is the least expensive type of heat exchanger. It is unable to accommodate large temperature variances between the fluids, which can be fixed by adding an expansion joint. An advantage of the fixed tube sheet heat exchanger is how easy it is to clean and maintain.
U-Tube or Hair-pin Type Exchangers
The name of the U tube shell and tube heat exchanger can be seen in the configuration of the tubes. The inlet and outlet valves for the tubes are located at one end of the heat exchanger. Fluids enter at the top of the tube sheet and exit through the lower half. The floating tubes at the U turn end of the exchanger can expand, which gives a U tube heat exchanger the ability to handle high temperature variances. The inlet and outlet valves for a U tube shell and tube heat exchanger vary according to the design of the heat exchanger. In the diagram below, the shell fluid inlet is on the top left and its exit is on the bottom right.
Kettle Type Heat Exchanger
Kettle Type Heat Exchangers are horizontal placed evaporators, which can be used for various applications as well as for various sizes. By using U-bend pipes, free expansion of the pipes can be created and the internal separation area is realized by a larger size of the shell diameter. The larger shell diameter also allows for internal fluid circulation, increasing the heat transfer coefficient. The heating medium on the tube side can be steam as well as, for example, thermal oil. The variation in the load is controlled by the flow on the tube side or the steam pressure.
There may be a retaining wall or overflow weir separating the tube bundle from the reboiler section where the remaining liquid is withdrawn while the tube bundle remains covered with liquid. Kettle Type Reboilers are reliable and easy to maintain.
2. Different Parts of S&T Exchangers
The different parts of the S&T Exchangers include the following
Tube Bundle
This set consists of the tubes, tube sheets, baffles, and tie rods to retain the bundle together.
Shell
The shell contains a tube bundle; one of the working fluids flows in the shell over the tubes.
Front Header or Channel
The front header also called the stationary header, is where the fluid enters the tube side of the exchanger.
Rear Header or Channel
The rear header is where the tube side fluid leaves the heat exchanger or in heat exchangers with multiple tube side passes, where it is returned to the front header.
Nozzles
Nozzles are the flange connections protruding from shell or head or channel side through which the fluid comes in our out of the exchangers via a connecting piping flanges.
The specifications and standards for STHEs have been established by the Tubular Exchanger Manufacturers Association (TEMA).
3. Location for S&T Exchangers
Heat Exchangers are located within the conventional process unit plot area, close to related equipment, to support economic pipe runs, flexibility, process requirements, and operator and maintenance access. Horizontal shell and tube exchangers should be positioned so that the channel end faces the auxiliary road or maintenance access way for tube bundle removal with adequate space provided at the front end of the exchanger for bennet removal. See figure below.
(a) Floating Head Type, U Tube Type, and Fixed Tube Sheet Type (1 pass)
More than one heat exchangers shall be arranged by keeping the channel nozzles straight in line. This is not only for making the appearance better, but for making the connection of the pipes easier. This has an advantage particularly when a cooler has its cooling water fed in parallel.
(b) Fixed Tube Sheet Type (2 pass)
Fixed tube sheet type can be arranged either side as there is no requirement of bundle pulling. We have to secure maintenance space at each side of the heat exchanger. Heat exchanger shall, therefore, be arranged at a best position in terms of piping design. For the 2 pass type, heat exchanger shall be arranged to have the channel head, which has dual nozzles, come to the pipe way side in terms of better piping connection
4. Elevation for S&T Exchangers
Height of the base when heat exchanger is installed on floor and height of the saddle when this is installed on platform are determined from the dimension of a drain installed on a line that is leading to bottom nozzle of the heat exchanger.
H (Height of base of heat exchanger) = W + X + Y + Z
W = Flange and Elbow Y = Dimension of Drain
X = Half Diameter of Pipe Z = Space between Drain and Floor (min. 150)
But, since the height at which heat exchanger is installed must be grouped and be at the same level, value in Figure 3-1 shall use a basic height as an example. Also, if system is capable to keep itself warm or cool, it should be considered that operation is not affected.
Note 1. Dimension “A” (height of saddle) and dimension “B” (nozzle project) shall be the same.
Note 2. Dimension of drain valve shall be a gate valve SW of API 602 (Type A is SW-SCRD).
Height of the heat exchanger placed on the stand must be determined in consideration of the shape of pipes and the operability of valves. But, when the pipe from the bottom nozzle of the tube side or the shell side goes directly through the platform, heat exchanger must be placed at a height minimum of 200mm from the crossbeam. That is to provide a minimum of 200mm distance piece between the heat exchanger and the saddle.
Reasons for above :
(a) To secure bolting space of the nozzle flange.
(b) If the distance piece is not installed, set bolt of the heat exchanger hits the web of the crossbeam as shown in Figure below and can’t be fixed. (when the crossbeam is either H-shape or I-shape).
5. Maintenance & Access Space Consideration
(1) Space for removing and attaching channel cover, channel head and shell cover.
- Obtain spaces “1” through “8” in figure below as much as possible for the purpose of maintenance. Spaces “6” through “8” are not required for U-Tube type heat exchangers.
- If the pipe can be removed by making providing break flanges in piping, spaces “1” through “7” in below figure can be used for piping. If piping cannot be removed, then do not run pipes on these spaces.
- If there is no choice then any one of the side spaces “1/2” or “3/4” , or either “6” or “7” may be used for a space for piping arrangements. In this way, we clear one side of exchanger for maintenance space.
(2) Space for using lifting lug for removing and attaching channel cover, channel head and shell cover.
- As shown in figure below, for the heat exchanger placed on the floor, channel cover, channel head and shell cover will be lifted with a crane using a lifting lug.
- Heat exchanger placed on the platform, those will be lifted with a trolley beam using a lifting lug for removing or attaching. Position of pipes must be determined by avoiding any interference with lifting operation.
- Since U tube type heat exchanger does not have a shell cover, removing work is not required.
- To keep balance of the channel head, position of lifting lug is slightly moved to the flange side.
6. Nozzle Orientations for S&T Exchangers
In order to optimize the piping arrangements, there are some cases where possible relocation of nozzles can be done without compromising the heat exchanger basic operation. Accordingly, basic concept on the position of heat exchanger nozzle and examples of nozzle relocation for possible piping arrangement optimization are described below. Also, related to the nozzle relocation, it should be verified from respective system design personnel or process design engineer that there is no impact on the function of heat exchanger.
Example of Relocating the Nozzle of Heat Exchanger:
(1) Change the entering and exiting of the fluid without changing the nozzle location.
In case of Figure shown below, example on the right has a piping arrangement that is more effectively designed than the example on the left. This example includes a case that would exchange the fluid between tube side and shell side. (Frequently seen in situation where tube side and shell side have the same type of fluid.)
(2) Move the location of the nozzle.
As shown in Figure below, shell side nozzle on the heat exchanger, which was located at the position where entrance is close to the channel head and exit is close to the shell cover, can be reversed as is shown on the right. This is a relocation of nozzle to save the use of high-grade pipe and reduce the pressure drop to a minimum as possible.
(3) Make the nozzle as elbow nozzle.
Nozzle of the heat exchanger is normally made of straight pipe and flange, but can be changed to that made of elbow and flange. This is to make easier access to valve and meter by lowering the position of the heat exchanger and the level of alignment.
(4) Install the nozzle by giving an angle
Horizontally rotated elbow and flange of the nozzle to give advantage of obtaining piping route and maintenance space.
(5) Move the nozzle to a symmetric position
7. Study of Structure for S&T Exchangers
Size of Platform
Size of platform shall basically be determined in accordance with Figure below. The below is the basic requirements for determining platform size for Heat Exchanger. Read is also advised to refer client or project specification related to equipment arrangements.
Location of Pillar or Column
Position of the Pillar for longitudinal direction of the heat exchanger is to be on one of the lines (“1″– “6”) shown in Figure below.
Position of beam for vertical direction of heat exchanger is determined by setting the position of the pillar for longitudinal direction of heat exchanger. Therefore, following are to be noted in determining the pillar position for longitudinal direction.
Position of beam for vertical direction of heat exchanger is determined by setting the position of the pillar for longitudinal direction of heat exchanger. Therefore, following are to be noted in determining the pillar position for longitudinal direction.
(1) Full consideration shall be given not to affect the arrangement of pipe that go through the platform.
(2) Keep the maximum overhang at 2500mm and obtain dimensions A and B described above. For “3” and “4” of the pillar positions shown in figure above, there are many cases where overhang of the platform exceeds 2500mm by securing dimensions A and B specified above. Careful review should, therefore, needed in employing “3” and “4”.
(3) If a drum is installed on the same platform, saddle position of the drum and saddle position of the heat exchanger shall be in the same position to share the same beam.
(4) Pillar positions on line “1” and line “6” in Figure above are basically not employed unless they are installed on a pipe rack, since the steel frame will be wasted.
Staircase
Staircase shall normally be installed parallel to the heat exchanger, and shall be walked up from the maintenance area side. See Figures below, for examples of staircase arrangement.
Trolley Beam
When installing a trolley beam for maintenance of a heat exchanger placed on a structure or platform, height and size of the trolley beam shall be determined in accordance with below Figures.
8. Study of Piping Arrangements Around S&T Exchangers
Key points in study and placement of piping arrangement around S&T Exchangers are as follows:
(1) Arrange piping in such a way that there is no interference with lifting lug during maintenance and cleaning of the exchangers.
(2) Block valves of the exhangers should be placed as close as possible to the exchanger nozzles. Preferably, these should be placed directly onto the exchanger nozzles.
(3) Piping can’t be done over the center line since it causes problem in using the lifting lug of the shell cover. However, there is no problem if it curves to right or left before the lifting lug.
(4) Since piping gets in the way and the channel cannot be moved forward by being hung when removing the channel head, this type of piping configuration should be avoided. If this cannot be avoided, piping of channel head bottom nozzle must be made removable. See figure below.
(5) Piping has to be removable to use a lifting lug during maintenance.
(6) When connected to underground piping, flange will be installed to allow separation from ground
piping.
(7) Beware not to hit the beam when connected under the stand.
(8) Obtain more than 100mm for bolting space in the nut side of flange bolting.
(9) Beware not to have the piping hit the base or the distance piece.
(10) When connecting to ground piping, flexibility should be given to absorb unequal subsidence, or make it able to make adjustment for connection work with ground piping, in another word, to absorb gap in dimensions caused during the ground piping work.
(11) Since lifting lug of the shell cover is used during maintenance, piping cannot be placed over the center of heat exchanger, but if 90 or 45 degrees elbow is used not to hit the wire rope and the hook used for lifting, as shown below, piping can be placed at a minimum dimension away from the center.
(12) The free space at he side of the horizontal shells can be used for placement of control and manual valves etc.
(13) For the pipes on the bottom nozzle of shell or channel side, rigid pipe supports are avoided normally. see figure below.
(14) Run pipes at typical common elevations with head room clearance normally 2100mm to 2500mm high from ground. This eases us in supporting pipes on common pipe supports and to suite the pipe rack elevations.
Figure below shows typical piping arrangements example around S&T Exchangers
9. Piping Support Arrangement for S&T Exchangers
Piping routed around the heat exchanger and close to the shell or channel head, we use stanchion post as supporting arrangements for such pipes. Below figure shows a standard to be used in determining the position where pipe stanchion is installed. This standard shall be complied with unless there is problem.
When a support is installed on an elbow part of the beginning of the overhead piping that is connected to a bottom nozzle of the heat exchanger (channel side or shell side), following are to be thoroughly reviewed.
(a) Since the downward extension of heat exchanger nozzle and piping is restricted by the support, resulting in excessive increase of thermal stress between the nozzle and the support, it is not suitable for support installation.
(b) It is difficult to insert a spectacle blind plate in the nozzle flange area during maintenance (water pressure test).
Regarding support from structure, consider support arrangements in such a way as to allow a spectacle blind plate to be inserted in the nozzle flange area during maintenance (water pressure test).
Conclusion:
Shell and Tube Heat Exchanger is one of the major components in any Process or Oil & Gas Plants. Exchanger piping must be routed in such a manner that it meets economy, flexibility, support, and operation and maintenance access requirements. This article discussed about all these important topics while considering piping and structure arrangements for S&T Exchangers. We tried to be as to the point as possible and also as elaborative to understand and grasp the piping concepts applicable for S&T Exchangers Piping. Any comments or suggestions for improvement will add value to our work and will be appreciated.
