How to Design a PTFE Exchanger for a Shipboard Application with Severe Pitching and Rolling?

A Heat Exchanger for Continuous Operation
A heat exchanger on a ship is never stationary. The vessel is constantly rolling, pitching, yawing and heaving as it responds to wave loading and varying sea states. These motions place all installed equipment in a constant and multi-directional fatigue environment, and each piece of equipment within becomes a long-duration dynamic stress test.

A normal lightly supported horizontal tube bundle would be quickly damaged under such conditions by repetitive slapping, vibration and support fretting. Therefore a PTFE exchanger appropriate for marine operation must be designed as a structurally integrated system with its interior components constrained and its external parts installed as a single stiff mechanical unit.

A correctly designed PTFE exchanger shipboard pitching rolling device must be considered a ship inside a ship. Every tube and every support structure must be protected for continuous storm type loading.

The exchanger must be a strong, self-contained block, braced inside like the hull of a ship, to withstand the incessant slow-motion earthquake of ocean motion.

Why a special design philosophy is necessary for shipboard motion
Multi-Axial Dynamic Loading Conditions
Unlike land-based installations, marine systems are subjected to complicated motion patterns including:

Roll (side-to-side rotation)

Roll (lateral rotation)

Vertical translation (heave)

Yaw (rotation about a horizontal axis)

Sudden slamming incidents in heavy waves

These movements are controlled by the stability parameters of the vessel (e.g. metacentric height). These determine the roll period and angular acceleration. Large roll angles can generate large inertial forces within the equipment enclosures.

Components of internal exchangers without specific design modification may be subject to:

Frettings and tube vibration

fatigue cracking of baffle

Loose tie rod ends

Bending of support plate

Impact loading of neighbouring tubes

Preferred design is vertical U-tube configuration
Stability Gravity-Assisted Motion
A vertical orientation is especially desired for shipboard applications. In this arrangement the flow of fluid upward guarantees that the shell is always fully flooded and the effect of gravity will help to stabilize the distribution of fluid inside the vessel during vessel motion.

Advantages of vertical orientation include:

Reduction of lateral sloshing effects

Enhanced phase stability

Reduced vapor pocket generation risk

More consistent heat dispersion under tilt circumstances

U-Tube Bundle Removes Fixed Tubesheet Stress
In marine service, it is common to select a U-tube bundle since it permits:

Independent thermal expansion of pipes

Removal of Fixed Tubesheet Stress Concentration

reduced thermal fatigue loads

Better tolerance to structural movement

The mechanical compliance of the floating U-bend segment is advantageous during combined heat and mechanical cycling.

Internal reinforcement against ship movement
Close spacing of tube supports prevents vibration damage.
Under strong roll circumstances, the motion of the ship considerably increases the probability of tube-to-tube interaction. To compensate for this the tube support plates must be spaced closer than in land based exchangers.

Important design effects are:

Smaller tube spacing without support

Tube vibration natural frequency increased

Oscillation roll occurrences of lower amplitude

Transient loading: prevention of tube slap

The supports are close enough that the tubes act as a confined bundle rather than as independent flexible parts.

Sturdy Baffle and Tie Rod Systems
Baffles and tie rods must be designed to have strong resistance to fatigue from cyclic ship motion.

Design requirements:

Enhanced structural thickness of baffles

Locking tie rod sets

Locking devices against loosening

Reinforced weld and attachment point

Selection of vibration resistant fasteners

Marine equipment must be of high integrity and classification societies such as DNV, ABS and Lloyd's Register set severe criteria. Micro-movement can cause fasteners to loosen and mechanical locking or safety wire systems must be used to avoid this.

Equivalent Seismic Mounting for Marine Service
Continuous Seismic Load of Ship Motions
The mounting of shipboard equipment is often considered in a manner comparable to the seismic design of terrestrial structures. The roll acceleration and slamming loads are equal inertial forces that have to be transmitted via the support structure.

Increasing design considerations include:

Peak loads on roll and pitch angles

Dynamic acceleration enhancement

Fatigue loading during the service life of a ship

Shock loading due to wave impact

Heavy Duty Base Frame Construction
The complete exchanger shall be installed on a rigid, ship-classed support frame, designed for distribution of loads into the vessel structure.

Common characteristics are:

Gusset-reinforced structural elements

Frames, steel, welded bases

Anti-vibration mounting pads.

Load distributing base plates

Structural tie-in points to the hull framing

The goal is to prevent local stress concentrations and to provide a uniform transfer of load into the structural grid of the vessel.

Marine Operation Life Fatigue
Long-term Fatigue Loading in Continuous Motion
Even if instantaneous forces are within acceptable limits, cyclic loading over thousands of hours can cause fatigue damage:

Tube supports

Welded joints

Bracket connectors

Tie-rod assemblies

Interfaces mounting

Fatigue life analysis is becoming a key aspect of the design process, in particular for vessels operating in extreme sea states.

Resonance: Why It's Important to Avoid
The inherent frequencies of internal structures should be well separated from the main exciting frequencies of ship motion. Resonance circumstances may take minor motions and amplify them into destructive oscillations.

Design strategies are:

Increased stiffness in the structure

Periods not reduced

Addition of damping interfaces

Optimization of mass distribution

Material & Polymer Selection for PTFE Systems
PTFE Flexibility aids marine motion
PTFE tube materials have a built-in benefit in maritime settings in that they are:

• Excellent chemical resistance

Flexibility under cyclic loads

Resistance to marine atmospheric corrosion

Low surface energy reduces fouling risks

PTFE still needs mechanical constraint however to avoid excessive motion under dynamic loads.

Thermal-mechanical coupling effect
The combination of heat expansion and ship motion results in complex loading circumstances. The U-tube flexibility allows thermal expansion and the structural supports take care of mechanical motion, and hence the system stability during operation.

Compliance with Classification Societies
Required Marine Design Criteria
PTFE exchangers for shipboard applications must satisfy the standards of the classification society which may include:

Structural integrity verification (DNV, ABS, Lloyd's Register)

Approval of vibration and fatigue analysis

Shock load qualification

Requirements for material traceability

Welding and fabrication standards

These requirements specify that equipment can endure the operational life of the vessel in specified environmental circumstances.

Summary
A shipboard PTFE exchanger for severe pitching and rolling is basically a tough, vertically mounted, internally braced thermal machine. Continuous multi-axial fatigue loading is managed by a U-tube structure, closely spaced tube supports, strengthened baffle systems and a thoroughly constructed mounting frame.

The PTFE exchanger shipboard pitching rolling design technique converts the exchanger into a single structural system that can handle the continual motion, occasional slamming loads, and long-term vibrational wear of the maritime environment.