12.1 Shell-and-Tube Heat Exchangers: Bundles, Baffles & Floating Heads

11.1 Shell-and-Tube Heat Exchangers: Bundles, Baffles & Floating Heads

Core Trade Concept: Shell-and-tube heat exchangers are the workhorses of chemical plants, power stations, and petroleum refineries. They transfer thermal energy between two isolated fluid streams running at disparate pressures and temperatures. Boilermakers must master Tubular Exchanger Manufacturers Association (TEMA) standards, understand front-end, shell, and rear-end mechanical classifications, execute precision bundle pulling and rigging without damaging delicate tube matrices, and perform rigorous hydrostatic leak testing across complex floating head geometries.

1. TEMA Standards, Classifications & Three-Letter Designations

The design, fabrication tolerances, and mechanical construction of shell-and-tube heat exchangers in North America are governed by TEMA standards (in conjunction with ASME BPVC Section VIII, Division 1 for pressure boundary rules).

                       TEMA CLASSIFICATION HIERARCHY
                       
  +-------------------------------------------------------------------------+
  |                               TEMA R                                    |
  |  * Severe petroleum, refinery, and heavy industrial processing          |
  |  * Maximum wall thickness, larger corrosion allowances (typically 1/8") |
  |  * Heavy-duty bolted joints, reinforced pass partitions, severe fatigue |
  +-------------------------------------------------------------------------+
                                      |
  +-------------------------------------------------------------------------+
  |                               TEMA B                                    |
  |  * Chemical process service with corrosive fluid streams                |
  |  * Focus on corrosion-resistant alloys, titanium, duplex, nickel alloys |
  |  * Intermediate design safety factors tailored to chemical plant safety |
  +-------------------------------------------------------------------------+
                                      |
  +-------------------------------------------------------------------------+
  |                               TEMA C                                    |
  |  * Commercial and general process applications                          |
  |  * Moderate temperatures and pressures; optimized for cost economy      |
  |  * Standard 1/16" corrosion allowance; standard commercial tolerances   |
  +-------------------------------------------------------------------------+

The Standardized Three-Letter TEMA Designation System

Every shell-and-tube heat exchanger is designated by a standardized three-letter code that describes its mechanical anatomy from front to back:

1. First Letter: Front-End Stationary Head Type
2. Second Letter: Shell Type
3. Third Letter: Rear-End Head Type

$\text{TEMA Designation Format: } [\text{Front Head}]\;[\text{Shell}]\;[\text{Rear Head}] \quad (\text{e.g., } \mathbf{AES}, \; \mathbf{BEM}, \; \mathbf{AJU})$

                    TEMA THREE-LETTER NOMENCLATURE MATRIX
                    
   FRONT-END HEAD                SHELL TYPE                 REAR-END HEAD
  +---------------+          +---------------+          +-------------------+
  | A: Channel w/ |          | E: 1-Pass     |          | L: Fixed Tubesheet|
  |    Remov. Cvr |          | F: 2-Pass     |          | M: Fixed Tubesheet|
  | B: Bonnet     | ======>  | G: Split Flow | ======>  | N: Fixed Tubesheet|
  | C: Channel w/ |          | H: Dbl Split  |          | P: Packed Float.  |
  |    Int. Sheet |          | J: Div. Flow  |          | S: Split-Ring Fl. |
  | N: Channel-Sh.|          | K: Kettle Reb.|          | T: Pull-Thru Fl.  |
  | D: Special HP |          | X: Crossflow  |          | U: U-Tube Bundle  |
  +---------------+          +---------------+          | W: Ext. Sealed Fl.|
                                                        +-------------------+

Front-End Stationary Head Classifications

Shell Types & Flow Patterns

• Type E (One-Pass Shell): The most common industrial shell. Fluid enters one end, traverses the length of the bundle across segmental baffles, and exits the opposite end.
• Type F (Two-Pass Shell): Contains an internal longitudinal baffle creating two shell passes in series. Delivers pure counter-current flow when paired with a two-pass tube bundle, but internal leakage past the longitudinal baffle seal will drastically degrade thermal efficiency.
• Type G (Split Flow) & Type H (Double Split Flow): Utilizes longitudinal baffles with central inlet and outlet nozzles to distribute flow, reducing shell-side pressure drop for boiling and condensing services.
• Type J (Divided Flow): Features one central inlet and two outlet nozzles (or two inlets and one central outlet). Splitting the fluid stream cuts shell-side velocity in half and reduces pressure drop to approximately $\frac{1}{8}\text{th}$ ($12.5%$) of an equivalent Type E shell, making it ideal for low-pressure gas cooling and vacuum condensers.
• Type K (Kettle Reboiler): Features an enlarged shell diameter above the tube bundle, creating a vapor-liquid disengagement space. An internal overflow weir maintains the liquid level above the submerged heating bundle while allowing dry vapor to exit the top nozzle and heavy unvaporized bottoms to spill into a collection sump.
• Type X (Crossflow): Fluid enters a full-length distribution plenum and sweeps straight across the tube matrix perpendicular to the tube axis without segmental baffles. Provides the lowest possible shell-side pressure drop.

2. Rear-End Head Mechanisms: Fixed, U-Tube & Floating Heads

Differential thermal expansion is the single greatest mechanical challenge in heat exchanger operation. When hot fluid flows through the tubes while cold fluid circulates in the shell (or vice versa), the tubes expand or contract at a different rate than the shell. The rear-end design governs how this expansion is accommodated.

                      REAR-END HEAD EXPANSION MECHANISMS
                      
   FIXED TUBESHEET (BEM)             U-TUBE BUNDLE (AEU)
  +-------------------------+      +-------------------------+
  | Shell   [ Tubesheet ]   |      | Shell    /=============\ |
  | ========[ Welded to ]   |      | ========( U-Bend Area  ) |
  |         [ Shell ID  ]   |      |          \=============/ |
  +-------------------------+      +-------------------------+
  (Restrained Expansion;           (Unlimited Free Expansion;
   Requires Shell Bellows)          No Rear Tubesheet or Gasket)
   
   SPLIT-RING FLOATING (AES)        PULL-THROUGH FLOATING (AET)
  +-------------------------+      +-------------------------+
  | Shell   +-[Split Ring]-+|      | Shell   +-------------+ |
  | ====\   | [Floating   ]||      | =====\  | [Floating   | |
  |     |   | [Head Cover ]||      |      |  | [Head Flange| |
  +-----+---+--------------++      +------+--+-------------+-+
  (Bundle Pull Requires            (Entire Bundle Pulls Through Shell
   Removing Shell Cover & Ring)     Without Dismantling Floating Head)

Detailed Comparison of Rear-End Types

                  FLOATING HEAD MECHANICS: TYPE S vs. TYPE T
                  
  TYPE S (FLOATING HEAD WITH SPLIT BACKING RING)
  ---------------------------------------------
  Shell ID
  ==============================+
                                |   [Shell Cover]
      [Floating Tubesheet]      |  +-------------+
      +----------------+        +--+             |
      |                |===========| Floating    |
      +----------------+           | Head Cover  |
        | [Split Ring] |           +-------------+
        +--------------+
  * Floating head diameter exceeds shell ID.
  * Boilermakers must unbolt shell cover, unclamp split backing ring,
    and remove floating head cover before pulling the bundle.

  TYPE T (PULL-THROUGH FLOATING HEAD)
  -----------------------------------
  Shell ID
  =======================================================+
                                                         | [Shell Cover]
      [Floating Tubesheet]    [Floating Head Flange]     | +-----------+
      +-----------------------+--------------------+     | |           |
      |                       | Bolted Directly    |=====| |           |
      +-----------------------+--------------------+     | +-----------+
  * Floating head flange outer diameter is smaller than shell ID.
  * Entire bundle pulls out through the shell without dismantling head.
  * Tradeoff: Requires large annular clearance, causing shell-side bypass.

3. Internal Components: Tubesheets, Pass Partitions, Baffles & Vibration

Tubesheets & Tube-to-Tubesheet Joint Mechanics

The tubesheet is a thick circular plate drilled with high-precision hole patterns (triangular $30^\circ/60^\circ$ or square $90^\circ/45^\circ$ pitch) that anchors the tube matrix and separates the shell-side and tube-side fluids.

                   TUBE-TO-TUBESHEET JOINT & GROOVES
                   
       Tube Hole in Tubesheet Wall (ASME / TEMA Standard)
     +--------------------------------------------------------+
     |                                                        |
     |    [Groove 1]     [Groove 2]                           |
     |    (1/8" wide     (1/8" wide                           |
     |    x 1/64" deep)  x 1/64" deep)                        |
     |     +-----+        +-----+                             |
     |     |     |        |     |                             |
     +-----+     +--------+     +-----------------------------+
     ========================================> [Tubesheet Face]
     ----------------------------------------+ (Tube Projection
     [ TUBE WALL EXPANDED INTO GROOVES ]     |  1/16" to 1/8")
     ----------------------------------------+
     +-----+     +--------+     +-----------------------------+
     |     |     |        |     |                             |
     |     +-----+        +-----+                             |
     |                                                        |
     +--------------------------------------------------------+

1. Tubesheet Serrations (Grooves): Under TEMA and ASME Section VIII rules, tube holes in tubesheets are machined with two annular concentric grooves (typically $\frac{1}{8}\text{ in.}$ wide by $\frac{1}{64}\text{ in.}$ deep). When mechanical roller expanders expand the tube wall, the tube metal flows plastically into these grooves, creating a high-strength joint that resists tensile pull-out forces and thermal cycling.
2. Roller Expansion & Wall Reduction: Boilermakers utilize electric, pneumatic, or hydraulic torque-controlled roller expanders.
   • Target Wall Reduction: Carbon steel tubes require $4%\text{ to }8%$ wall reduction; stainless steel and titanium require $3%\text{ to }5%$.
   • Formula: $\text{Target Expanded ID} = \text{Hole Diameter} + (2 \times \text{Nominal Wall Thickness}) - (2 \times \text{Nominal Wall Thickness} \times \%\text{Reduction})$
   • Under-expansion results in joint weepage during hydrotest; over-expansion work-hardens the tube, distorts adjacent tubesheet holes, and crushes the tubesheet ligament.
3. Seal Welding vs. Strength Welding: In severe, cyclic, or lethal service, tubes are welded to the tubesheet face using automated Gas Tungsten Arc Welding (GTAW):
   • Strength Weld: Weld metal carries the full longitudinal pressure load (weld size $\ge$ tube wall thickness).
   • Seal Weld: Small bead deposited solely to prevent fluid weeping across the joint; joint strength is provided by subsequent roller expansion.

Pass Partition Plates & Gasket Ribs

In multi-pass heat exchangers (2-pass, 4-pass, 6-pass), flat steel divider plates called pass partitions are welded inside the channel head. These partitions match machined rebates in the tubesheet and channel cover.

                     PASS PARTITION GASKET DETAIL
                     
              Outer Gasket Rim (Seals Shell/Channel Flange)
                       /====================\
                      ||                    ||
                      ||   Pass Partition   ||
                      ||   Gasket Rib       ||
                      || +----------------+ ||
                      || |                | ||
                      || +----------------+ ||
                      ||                    ||
                       \====================/

• Pass Bypass Hazard: The channel gasket features integral pass partition ribs matching the divider plates. If the pass partition gasket weeps, corrodes, or is improperly torqued, fluid short-circuits (bypasses) directly from the channel inlet to the channel outlet without circulating through the tube bundle, causing an immediate, catastrophic loss of heat transfer.

Baffle Geometry, Baffle Cut & Flow-Induced Vibration

Shell-side baffles serve two vital functions: (1) they direct shell-side fluid across the tube bank at high velocity to maximize convective heat transfer, and (2) they structurally support the tubes against gravity and flow-induced vibration.

                       SEGMENTAL BAFFLE GEOMETRY
                       
        SINGLE SEGMENTAL BAFFLES (20% Cut)      DOUBLE SEGMENTAL BAFFLES
       +---------------------------------+     +-------------------------+
       |         [ Baffle Window ]       |     |   [ Outer Window ]      |
       |         (Fluid Sweeps Over)     |     +=========================+
       | +=============================+ |     |   [ Center Baffle ]     |
       | |  O   O   O   O   O   O   O  | |     |   (O   O   O   O   O)   |
       | |  O   O   O   O   O   O   O  | |     +=========================+
       | |  O   O   O   O   O   O   O  | |     |   [ Outer Window ]      |
       | +=============================+ |     +-------------------------+
       +---------------------------------+     (Reduces Shell Velocity
       (Fluid Sweeps in Zig-Zag Pattern)        & Crossflow Pressure Drop)

• Segmental Baffles & Baffle Cut: A circular plate with a segment sliced off. Baffle cut is defined as the height of the segment removed expressed as a percentage of the shell inside diameter: $\text{Baffle Cut } (\%) = \left( \frac{H_{\text{window}}}{D_{\text{shell}}} \right) \times 100$
  • Standard Range: $15%\text{ to }45%$ (optimal thermal performance occurs between $20%\text{ and }25%$).
  • Small Baffle Cut ($< 15%$): Creates severe pressure drop, excessive pumping costs, and stagnant dead zones behind baffles.
  • Large Baffle Cut ($> 45%$): Causes longitudinal fluid bypass with minimal crossflow, degrading heat transfer.
• Baffle Spacing: TEMA mandates a minimum baffle pitch of $1/5\text{th}$ of shell ID (or $2\text{ inches}$) and a maximum unsupported span based on tube diameter to prevent Flow-Induced Vibration (FIV), vortex shedding, and acoustic resonance that cause tubes to fatigue-crack or saw through each other at mid-span.
• Impingement Plates: When fluid enters the shell inlet nozzle at high velocity (where kinetic energy $\rho v^2 > 1{,}500\text{ lb/(ft}\cdot\text{s}^2)$ for liquids or $> 500$ for gases), droplets will rapidly erode, cavitate, and fatigue the top tube rows. A solid stainless steel impingement plate (or annular vapor distributor belt) is welded under the nozzle to deflect fluid safely into the bundle lanes.

4. Tube Bundle Rigging, Extraction & Insertion

Tube bundles are heavy, flexible, precision-machined assemblies weighing anywhere from $5\text{ to }50+\text{ tons}$. Improper rigging can bend tubes, crush outer tube passes, or gall machined tubesheets.

                      TUBE BUNDLE CRANE RIGGING RULES
                      
   INCORRECT RIGGING: CHOKING BARE TUBES      CORRECT RIGGING: BAFFLE SADDLES
  +-------------------------------------+    +---------------------------------+
  |           /\                        |    |           /\                    |
  |          /  \ (Choker Crushes       |    |          /  \  (Spreader Beam)  |
  |         /    \ Thin Tube Walls)     |    |         /    \                  |
  |        /      \                     |    |        /      \                 |
  |     (==X======X==)                  |    |     [Saddle]  [Saddle]          |
  |     | |||||||||| |                  |    |     | ||||||  |||||| |          |
  |     +------------+                  |    |     +------+--+----+ +          |
  |  [ DENTED / CRUSHED TUBES ]         |    |   [ SUPPORT AT SOLID BAFFLES ]  |
  +-------------------------------------+    +---------------------------------+

Specialized Extraction Equipment

1. Hydraulic Aerial Bundle Extractors (Pullers): Suspended from a crane or mounted on a dedicated self-propelled transport truck. The extractor frame clamps rigidly to the heat exchanger shell flange using heavy steel jaws. A hydraulic pulling carriage engages the tubesheet with pulling pins or hook plates, pulling or pushing the bundle smoothly along greased steel tracks without imposing any pulling load on the crane hook.
2. Manual Come-Along & Winch Pulling: When mechanical pullers cannot access the location, boilermakers use pulling brackets, steel deadmen, and synchronized lever hoists (come-alongs) or air tuggers attached to lifting eyes on the stationary tubesheet. Pulling must be absolutely parallel to the vessel axis to prevent cocking and jamming the bundle in the shell.

Rigging Protocols & Tube Protection

CRITICAL BOILERMAKER RIGGING DIRECTIVE: Under no circumstances should wire rope or synthetic slings be choked directly around bare, unsupported heat exchanger tubes.

• Baffle Support Locations: Rigging slings (wide synthetic nylon or polyester web chokers) must be placed exclusively around solid tubesheet rings or at solid transverse baffle plate locations.
• Rigging Saddles & Softeners: Boilermakers must fit heavy wooden, split-steel, or high-density rubber cradling saddles between the slings and the bundle circumference. These saddles distribute the lifting pressure over a wide arc, preventing sling tension from crushing outer tubes against baffle hole edges.
• Bundle Insertion Alignment: During insertion, the shell interior must be thoroughly cleaned, inspected for burrs, and lubricated with water-soluble lubricant. Long threaded guide pins (drift pins / bullet pins) are installed in tubesheet bolt holes to align the bundle precisely with shell flange bolt holes without damaging gasket faces.

5. Hydrostatic Testing of Heat Exchangers

Hydrostatic testing verifies the mechanical integrity of pressure shells, channel heads, and hundreds (or thousands) of rolled/welded tube joints. Testing is conducted under ASME Section VIII, Division 1 (UG-99) at $1.3 \times \text{MAWP}$ (adjusted for material temperature stress ratios).

                     HYDROTEST CONFIGURATIONS
                     
  1. TUBE-SIDE HYDROTEST
     [Channel Head Pressurized] ===> Inspect Shell Nozzles & Shell Seams
     (Checks Tube Wall Leaks & Tube-to-Tubesheet Joint Leaks From Rear)
     
  2. SHELL-SIDE HYDROTEST (Fixed Tubesheet / U-Tube)
     [Shell Pressurized] ===> Channel Covers Removed
     (Directly Inspects All Front Tube-to-Tubesheet Expanded Joints)
     
  3. FLOATING HEAD SHELL-SIDE HYDROTEST (Type S)
     [Shell Pressurized] ===> Test Ring & Packing Gland Installed at Rear
     (Seals Shell Annulus; Exposes Floating Tube Joints for Inspection)

Step-by-Step Test Sequence for Type S Floating Head Exchangers

On a Type S floating head exchanger, the floating tubesheet sits recessed inside the shell cover. To test the shell side and inspect the floating tube-to-tubesheet joints, boilermakers must utilize a specialized hydrotest ring (test ring) and test gland.

                 FLOATING HEAD TEST RING & GLAND ASSEMBLY
                 
       Shell Flange                     Floating Tubesheet
      +-------------+                  +------------------+
      |             |                  |  Tube Ends       |
      |             |   [ TEST RING ]  |  Exposed For     |
      |   SHELL     |==================|  Inspection      |
      |             |  [ PACKING GLAND]|  (Dry & Clean)   |
      |             |  [ & PACKING ]   +------------------+
      +-------------+  +---------------+  |
             |                 |          |
             +=== Bolted ======+          v
             (Pressurized Shell)     [Visual Leak Check]

1. Step 1: Disassembly: Remove the front channel cover, rear shell cover, split backing ring, and floating head cover.
2. Step 2: Install Test Ring & Gland: Bolt the cylindrical test ring to the rear shell flange. Insert braided PTFE or graphite packing into the annular stuffing box between the test ring and the outer diameter of the floating tubesheet. Bolt the test gland (follower) to compress the packing against the tubesheet rim. This creates a temporary, pressure-tight seal on the shell side while leaving the tube ends open.
3. Step 3: Fill, Vent & Pressurize Shell: Fill the shell with treated water, venting air from high-point vents until a solid stream of water exits. Pressurize the shell side to test pressure ($1.3 \times \text{MAWP}$). Hold for minimum code soak time (typically $10\text{ to }30\text{ minutes}$).
4. Step 4: Inspection: Visually inspect both the stationary and floating tubesheets. Any water weeping around a tube indicates an under-rolled joint or weld defect. Any water running out of the inside of a tube indicates a split or perforated tube wall.
5. Step 5: Tube-Side Hydrotest: Depressurize shell. Reinstall the floating head cover and split ring. Install the channel head. Pressurize the tube side while inspecting shell nozzles and shell body seams.