Design Guide For Heat Exchanger Piping

1.0 GENERAL


The heat exchangers are widely used in most process plants. The main application of heat exchangers is to maintain the heat balance by addition or removal by exchange between streams of different operating temperatures.

2.0 TYPES

The most common heat exchangers used in process plants are :

a)        Shell & Tube Exchanger

b)        Plat type Exchanger

c)        Spiral Exchanger

d)        Double Pipe Exchanger

e)        Air Cooler Exchanger

The above types are  identified in figure 1.

3.0 DEFINITIONS


The type of  heat exchangers are defined below:

a)    Shell & Tube Exchanger

Shell & tube exchangers are elongated steel cylindrical vessels containing bundles of parallel tubes. Liquid passes through inside of the shell over the exterior side of the tubes, causing necessary exchange of heat between the two liquids.  Figure 2 illustrates an exchanger with two passes on tube side and one on shell side.  Exchangers are provided in various combinations eg. Figure 3 shows U tube, fixed tube and kettle arrangement.

b) Plate Exchangers

Plate exchangers are generally used in low pressure, low temperature applications and are made up of end covers, carrying bars, inlet & outlet nozzles, plates & gaskets. The exchanger plates having spacing between them for liquid flow. Figure 4 illustrates the plate exchanger configuration.

c) Spiral Exchanger

Spiral heat exchangers are of circular construction consisting of an assembly of two long strips of plate wrapped to form a pair of concentric spiral passages. Alternate edges of the passages are closed so that liquid flows through continuous channels. Figure 5 illustrates the spiral exchanger configuration.

d) Double Pipe Exchanger

The Double Pipe exchanger is used when one liquid has a greater resistance to heat flow than another r when the surface area is small. As indicated in Figure 7, the double pipe exchanger consists of a pipe within a pipe ; both pipes have a return bend at one end.

e)  Air Cooler Exchanger


An air cooler consists of fin-tube bundles with a header box attached to each end, supported horizontally by a steel frame or structure. The cooling medium is air instead of a liquid.

4.0  CONSIDERATION FROM EQUIPMENT LAYOUT OF VIEW

Heat exchangers are located within the conventional process unit plot area, close to related equipment, to support economic pipe runs, flexibility , process requirement and operator & maintenance access. Support of the equipment (eg for air coolers or vertical reboilers)  can also influence heat exchanger location.

Figure 8 depicts a typical plot plan with several heat exchanger applications. Horizontal shell & 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 bonnet removal. This is indicated in figure 9.

Exchangers can be located as single items , in pairs (this is the most common installation) , or in larger groups when no intermediate control is required between the shell & tube streams. The single and paired installation is shown in figure 10. Paired exchangers may operate in series , parallel or dissimilar services; grouped exchangers operate only in series or parallel. Fig. 11 shows samples of parallel and series exchanger installations.

Paired or grouped exchanger should be spaced to allow a minimum 450mm between the outside of adjacent channel or bonnet flanges to facilitate access to flange bolts for maintenance. Space should be provided on either side of paired exchangers and at both end s of grouped exchangers for control & operator access. Fig 12 illustrates a structure mounted installation & its required access areas.

Horizontal exchangers may be stacked to a preferred maximum top shell centerline of 3600 mm from grade or platform as indicated in fig.13. Stacking of exchangers above this height may require platforming for access to channel and bonnet flanges and fixed handling devices.

Horizontal shell & tube exchangers may be located at grade or elevated in steel or concrete structures when process requirements or space availability dictate. Fig 14 indicates support of horizontal exchangers is by saddles attached to concrete piers.

If process requirements permit, shell & tube exchangers can also be mounted in a vertical position, supported by lugs and tower nozzles in a tower supported installation (as shown in fig.15), and  within concrete or steel structures ( as shown in fig.16). The same considerations for maintenance , control and operator access should be given for vertical installations as are for horizontal installations.

Tube bundle removal for Tower supported vertical reboilers , not requiring springs, is indicated in figure 16a.

  1. the layout designer must set the reboiler support lug elevation 25 mm above the platform steel, and not at the steel elevation.
  2. Before maintenance,  the 25 mm gap will be shimmed, thereby enabling the reboiler load to be carried by the platform steel during maintenance turnaround.
  3. The nozzle and channel flange will be unbolted and the channel section removed.
  4. The tube bundle is then ready to be removed.

Spiral & plate exchnagers can operate in series or parallel , but because of their configuration and maintenance requirements, it is preferable to position them as single items. Space is provided at both arrangements for control & operator access , with enough room allowed at the spiral exchanger to swing the cover plates open , as shown in fig.17 and at the plate exchanger to remove the individual plates , as shown in fig. 18.

Air coolers are located adjacent to the equipment that they serve for piping flexibility & maintenance. They can be supported from grade, at the top of structure, or above piperacks , which is the most common installation.

5.0 CONSIDERATIONS FROM PIPING POINT OF VIEW

Nozzle orientation & location can affect the piping configurations at most exchanger arrangements. A decision by the piping designer to relocate the exchanger nozzles can often produce a neat and cost effective arrangement.

Although the piping designer does not have the freedom to independently relocate the exchanger  nozzles , suggested alternate nozzle locations can be made to the exchanger engineer in the interest of improving piping arrangements , for example, alternative B is fig. 19 highlights an improved arrangement by relocating exchanger nozzles provided is acceptable from process point of view. Fig. 20 shows allowable nozzle configurations. Elbow nozzles (see fig. 21) are especially helpful in reducing the height of large stacked exchangers.

Exchanger Piping must be routed in such a manner that it meets economy., flexibility, support and operation and maintenance access requirements. Piping at shell & tube exchangers is positioned to allow adequate space for removal of channel heads and shell covers. The free space at the side of horizontal shells can be used for placement of controls. Piping is elevated a minimum distance from grade or platform to provide operator headroom clearances,  to offer ease of support, and to meet designed pipe rack elevations.

Piping connected to channel head nozzles should be furnished with break flanges to facilitate the removal of channel head. Large diameter or more expensive pipe can not be set to accommodate smaller or less expensive piping

Piping at spiral and plate exchangers is also positioned to allow the opening of covers and the removal of plates. Control at the spiral exchanger are located on the ends of the unit, clear of the cover plate swing area , and at the front and on one side for the plate exchanger. Piping is elevated similar to shell & tube exchanger. Piping attached to cover plate nozzles of the spiral units is furnished with break flanges.

Figures 22 to 29 indicates various piping configurations for heat exchangers.

The internals of heat exchangers requires periodic cleaning and repair. It is important  that exchangers are positioned in a such a way as to facilitate access to their internal parts.

For shell & tube exchangers, the tubes & interior of the shell can be cleaned in place with high pressure steam or water and rodding devices.  If the design of the exchangers permits, the tube bundle can also be removed for repair & cleaning. Tube bundles and head and shell covers can be removed by built in fixed handling devices (davits, pulling posts), fixed structures with trolley beams, or by mobile equipment (eg. Crane). Figure 30 to 32 gives examples of tube bundle removal equipment.

Areas for tube maintenance and bundle pulling shall be shown on plot plan and may extend over access ways within a unit area or over peripheral roads which are not required for access to other  plants. The extent of the pulling area shall be head length + tube length + 1500mm preferable. ( but 1000 mm minimum.)

Equipment such as condensers, coolers etc shall be located in such a manner that the tubes will drain clear by gravity and shall only be sloped where shown on P&I diagrams. Where this slope will prevent the tubes draining by gravity consideration must be given to the means of removing any liquids.

6.0  KEY FACTORS TO BE CONSIDERED DURING DESIGN

  1. Exchanger Piping must be routed in such a manner that it meets economy., flexibility, support and operation and maintenance access requirements.
  2. Piping connected to channel head nozzles should be furnished with break flanges to facilitate the removal of channel head.
  3. Piping attached to cover plate nozzles of the spiral units is furnished with break flanges.
  4. Large diameter or more expensive pipe can not be set to accommodate smaller or less expensive piping.
  5. The internals of heat exchangers requires periodic cleaning and repair. It is important that exchangers are positioned in a such a way as to facilitate access to their internal parts.
  6. Due consideration shall be given to stress analysis for deciding the fixed saddle location of heat exchangers.
Types of Heat Exchangers

Fig 1 Types of Heat Exchangers

Shell & Tube Exchanger

Figure 2
Shell & Tube Exchanger

Shell & Tube Exchanger Arrangements

Figure 3
Typ. Shell & Tube Exchanger Arrangements

Plate Exchanger

Figure 4
Plate Exchanger

Spiral Exchanger

Figure 5
Spiral Exchanger

Double Pipe Exchanger

Figure 7
Double Pipe Exchanger

Plot plan of exchanger Application

Figure 8
Typical Plot plan of exchanger Application

Exchanger Orientation

Figure 9
Exchanger Orientation

Single  & Paired Exchangers

Figure 10
Single & Paired Exchangers

Parallel & Series Exchanger Installation

Figure 11
Parallel & Series Exchanger Installation

Structure Mounted Exchanger Installation

Figure 12
Structure Mounted Exchanger Installation

Stacked Exchanger Installation

Figure 13
Stacked Exchanger Installation

Horizontal Exchanger Supports

Figure 14
Typical Horizontal Exchanger Supports

Tower Supported Vertical Installation

Figure 15
Tower Supported Vertical Installation

Structure Supported Vertical Installation

Figure 16
Structure Supported Vertical Installation

Tube Bundle Maintenance

Figure 16a
Tube Bundle Maintenance

Control & Operator Access in a Spiral Exchanger Arrangement

Figure 17
Control & Operator Access in a Spiral Exchanger Arrangement

Figure 18 Control & Operator Access in a Plate Exchanger Arrangement

Figure 18
Control & Operator Access in a Plate Exchanger Arrangement

Figure 19 Alternative arrangement for locating exchanger nozzles

Figure 19
Alternative arrangement for locating exchanger nozzles

Figure 20 Allowable Nozzle Configurations

Figure 20
Allowable Nozzle Configurations

Figure 21 Effect of Elbow Nozzles on Stacked Exchangers

Figure 21
Effect of Elbow Nozzles on Stacked Exchangers

Figure 22 Piping Arrangement for Horizontal Shell & Tube Exchangers

Figure 22
Piping Arrangement for Horizontal Shell & Tube Exchangers

Figure 23 Piping Arrangement for Underground Cooling water system

Figure 23
Piping Arrangement for Underground Cooling water system

Figure 24 Sample Piping Economy Arrangement

Figure 24
Sample Piping Economy Arrangement

Figure 25 Supports for a Piping Arrangement

Figure 25
Supports for a Piping Arrangement

Figure 26 Piping Arrangement for High Temperature, High Pressure Feed Exchangers

Figure 26
Piping Arrangement for High Temperature, High Pressure Feed Exchangers

Figure 27 Piping Arrangement for Feed Exchangers

Figure 27
Piping Arrangement for Feed Exchangers

Figure 28 Piping Arrangement for Spiral Exchangers

Figure 28
Piping Arrangement for Spiral Exchangers

Figure 29 Piping Arrangement for Plate Exchangers

Figure 29
Piping Arrangement for Plate Exchangers

Figure 30 Bundle Pulling

Figure 30
Bundle Pulling

Figure 31 Fixed Structure with a Trolley Beam

Figure 31
Fixed Structure with a Trolley Beam

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