SELECTION OF SHELL AND TUBE HEAT EXCHANGER (STHE)GAUTHAM SPROCESS ENGINEER

Principal Components of STHE

Shell Shell cover Tubes Channel Channel cover Tubesheet Baffles and nozzles

Other Components

Tie-in rods and spacers Pass partition plates / Channel partitions Impingement plates/rods Longitudinal baffles Sealing strips and Supports

SCHEMATIC

TEMA DESIGNATIONS

TEMA DESIGNATIONS

SHELL SIDE OR TUBE SIDE?

The General method of Selection is based on the necessity to reduce cost and make maintenance easy

PARAMETER SHELL SIDE TUBE SIDEMechanical Cleaning Fouling fluids Pressure High PressureViscosity High Viscous Fluids

(Doubt)Corrosion Very corrosive fluidsMetallurgy Expensive metallurgy

Straight Tube and Fixed Tubesheets

Examples such as BEM, AEM, NEN, etc., This TEMA type is the simplest design and is constructed without packed

or gasketed joints on the shell side. The tubesheet is welded to the shell and the heads are bolted to the

tubesheet On the NEN heat exchanger, the shell and the head is welded to the

tubesheet. Typically, a cover plate design is provided to facilitate tube cleaning.

NEN is the lowest cost TEMA design per square foot of heat transfer surface.

Straight Tube and Fixed Tubesheets

ADVANTAGES LIMITATIONS APPLICATIONS

Less costly than removable bundle designs

Shell side can be cleaned only by chemical methods

Oil Coolers, Liquid to Liquid, Vapor condensers,reboilers, gas coolers

Provides maximum amount of surface

No provision to allow for differential thermal expansion, must use an expansion joint

Provides for single and multiple tube passes to assure proper velocity

Removable Bundle, Externally Sealed Floating Tubesheet Example such as, AEW, BEW This design allows for the removal, inspection and cleaning of the shell

circuit and shell interior. Special floating tubesheet prevents intermixing of fluids. Maximum surface for a given shell diameter for removable bundle

design Tubes can be cleaned in AEW models without removing piping. Packing materials produce limits on design pressure and temperature

Removable Bundle, Externally Sealed Floating TubesheetADVANTAGES LIMITATIONS APPLICATIONS

Floating tubesheet allows for differential thermalexpansion between the shell and the tube bundle.

Fluids in both the shell and tube circuits must be non-volatile, non-toxic

Intercoolers and after coolers, air inside the tubes

Shell circuit can be steam or mechanically cleaned

Tube side passes limited to single or two pass design

Jacket water coolers or other high differentialtemperature duty

The tube bundle can be repaired or replaced without disturbing shell pipe

All tubes are attached to two tubesheets. Tubes cannot expand independently so that large thermal shock applications should beavoided

Removable Bundle, Outside Packed Head, Example such as, BEP, AEP, etc., This design allows for the easy removal, inspection and cleaning of the

shell circuit and shell interior without removing the floating head cover. Special floating tubesheet prevents intermixing of fluids. In most cases, straight tube removable design is more costly than U-

tube designs. On AEP design, tubes can be serviced without disturbing tubeside piping Less costly than TEMA type BES or BET designs

Removable Bundle, Outside Packed HeadADVANTAGES LIMITATIONS

Floating tubesheet allows for differential thermal expansion between the shell and the tube bundle.

Shell fluids limited to non volatile, non toxic materials

Shell circuit can be inspected and steam cleaned. If the tube bundle has a square tube pitch, tubes can be mechanically cleaned by passing a brush between rowsof tubes.

All tubes are attached to two tubesheets. Tubes cannot expand independently so that large thermal shock applications should be avoided

The tube bundle can be repaired or replaced without disturbing shell piping

Packing limits shell side design temperature and pressure

Removable Bundle, Internal Split Ring Floating Head Example such as, AES, BES, Ideal for applications requiring frequent tube bundle removal for

inspection and cleaning Uses straight-tube design suitable for large differential temperatures

between the shell and tube fluids More forgiving to thermal shock than AEW or BEW designs. Suitable for cooling volatile or toxic fluids. Higher surface per given shell and tube diameter than “pull-through”

designs such as AET, BET, etc.

Removable Bundle, Internal Split Ring Floating HeadADVANTAGES LIMITATIONS

Floating head design allows for differential thermal expansion between the shell and the tube bundle.

Shell cover, split ring and floating head cover must be removed to remove the tube bundle, results in higher maintenance cost than pull-through

Provides multi-pass tube circuit arrangement

More costly per square foot of surface than fixed tubesheet or U-tube designs

Shell circuit can be inspected and steam cleaned. If it has a square tube layout, tubes can be mechanically cleaned

Removable Bundle, Pull-Through Floating Head, Example such as, AET, BET Ideal for applications requiring frequent tube bundle removal for

inspection and cleaning as the floating head is bolted directly to the floating tubesheet. This prevents having to remove the floating head in order to pull the tube bundle

Removable Bundle, Pull-Through Floating Head,ADVANTAGES LIMITATIONS

Floating head design allows for differential thermal expansion between the shell and the tube bundle.

For a given set of conditions, this TEMA style is the most expensive design

Shell circuit can be inspected and steam or mechanically cleaned

Less surface per given shell and tube diameter than other removable designs

Provides large bundle entrance area for proper fluid distribution

Provides multi-pass tube circuit arrangement.

Removable Bundle, U-Tube

Example such as, BEU, AEU, Especially suitable for severe performance requirements with maximum

thermal expansion capability. Because each tube can expand and contract independently, this design is suitable for larger thermal shock applications.

While the AEM and AEW are the least expensive, U-tube bundles are an economical TEMA design.

Removable Bundle, U-TubeADVANTAGES LIMITATIONS

U-tube design allows for differential thermal expansion between the shell and the tube bundle as well as forindividual tubes

Draining of tube circuit is difficult when mounted with the vertical position with the head side up.

Shell circuit can be inspected and steam or mechanically cleaned

Because of u-bend, tubes can be cleaned only by chemical means

Less costly than floating head or packed floating head designs

Because of U-tube nesting, individual tubes are difficult to replace

Provides multi-pass tube circuit arrangement.

No single tube pass or true countercurrent flow is possible

Bundle can be removed from one end for cleaning or replacement

Tube wall thickness at the U-bend is thinner than at straight portion of tubes

Discussion

What type of exchanger do we select for a cost-wise economical design?1. No Fouling/ Fouling condition2. Non-Corrosive/ Corrosive condition3. High differential thermal gradients4. Combination of any 2 5. All the above expect 4

Front Head Selection

QUIZ

TYPE B TYPE C TYPE N

Fixed Tubesheet

Shell side fluid is non-fouling or the fouling can be chemically cleaned. The Mean temperature differential between shell and tube wall must be

less than 50 Deg C. Otherwise, expansion bellow is required. Stress caused by differential expansion between the shell and the tube

should not exceed the design stress limits considering winter and start up conditions

U-Tube Bundle

The U-tube is limited to applications where the tube side fluid is non-fouling; any fouling fluid must be routed through shell side only. In this respect, tube side mechanical cleaning is considered possible, if the centre to centre distance between the parallel legs of the U-tube is at least 150mm. However, this later option may be used only if required by specific process requirements.

Horizontal U-Tube should be used when condensing fluid in the tube side.

Floating head

Split ring (S type), Pull through (T type), Externally sealed tubesheet (W Type) and Outside packed (P Type)

Type S and Type T are the common types of Floating head Floating head type or U-tube type heat exchanger should be selected if

flexibility is required to avoid overstressing. The maximum shell diameter is based upon tube-bundle removal requirements and is limited by crane capacities. Thus, floating head heat exchangers are often limited to a Shell ID of 1400 to 1500 mm.

In S-type as there is an internal joint at the floating head, a careful design is necessary to avoid leakage of one fluid to the other.

Shell Selection

QUIZ

TYPE E TYPE J TYPE G

E Shell

Commonly used shell type Its limitations are shell side

pressure drop or problem due to flow-induced vibrations

Shell types such as Type J, Type X ,Type H can be used as alternatives.

Special case of E type shell where either entry or exit of the shell side fluid can be split into two parts to reduce pressure drop or flow induced vibrations in the shell side.

J Shell

G Shell

G shells have a longitudinal baffle axially

“G” shells are used for a maximum tube length of twice the maximum unsupported span as per TEMA as there is one full support plate in the tube bundle.

It is used for horizontal thermosyphon reboilers in order to reduce the pressure drop as well as to avoid flow mal-distribution

H shells have a longitudinal baffle axially

“H” shells are used for a maximum tube length of four times of the maximum unsupported span due to presence of 2 full support plates in the tube bundle.

It is used for horizontal thermosyphon reboilers in order to reduce the pressure drop as well as to avoid flow mal-distribution

H Shell

F Shell

This shell is used when there is a temperature cross i.e., when the outlet temperature of cold stream is higher than the outlet temperature of the hot stream.

“F” shells are prone to leakage across the longitudinal baffle in removable bundle exchangers and hence their use is generally not recommended.

If “F” shell is employed in a removable type exchanger, the shell side pressure drop should be limited to 0.35 kg/cm2g.

multiple shells in series are to be employed when more number of tube passes is required.

This type of shell is called kettle construction where there is an enlarged shell above the tube bundle for the disengagement of the vapor from the boiling liquid.

The kettle type is a special application of the U-tube type and pull through type of construction

K Shell

Rear End Selection

QUIZ

TYPE T TYPE S TYPE W

Rear End Selection

Rear End selection depends on a number of factors such as cost, maintenance, Fluid characteristics, application, etc.

Normally, Rear End “Type M” is used for “Type A” (Bonnet Type) Front End but for heat exchanger with “Type A” front end stationary head and an odd number of tube passes, “Type L” shall be selected.

Rear end head Type T shall be used for a kettle type exchanger with floating head.

TYPE M TYPE L TYPE T

Appendix – A (Baffle Design)

Longitudinal baffles, cross baffles and support baffles are the 3 types of baffles.

Commonly used baffle is single segmental but if it is necessary to reduce the shell side pressure drop then double segmental baffle is used

To reduce flow induced vibrations on the shell side, the no tube in windows design with single segmental baffle can be used.

Optimum baffle spacing is between 0.3 to 0.6 of Shell ID. The minimum baffle spacing as per TEMA is one fifth of the shell inside diameter or 4” whichever is lower

Appendix-B (Tube layout and Pitch)

Triangular, rotated triangular, square and rotated square are different tube layouts

Triangular or rotated triangular tube layouts is commonly used as it can accommodate more number of tubes but it is limited to clean services

Square and rotated square tube layouts is used for dirty shell side service and when mechanical cleaning of shell side is necessary

The minimum tube pitch should be 1.25 times tube OD. For square and rotated square layouts, a minimum cleaning lane of 6mm

should be provided.

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