How to Choose the Correct Tube Diameter for a PTFE Exchanger Handling a Fibrous Slurry?

A paper mill or a textile industry sends out a waste stream loaded with long, stiff, stringy fibres. Run this through a conventional PTFE heat exchanger with narrow 6mm tubes and you have the recipe for a fast, total and annoying plug. The strands will reach over the entrance to the tube, creating a tangled mat and the heat transfer will crash. The first, and most important, step in the construction of the exchanger is to choose a large enough tube so that a wad of the worst-case fibres can travel straight through without snagging. Selecting the right tube diameter PTFE exchanger for fibrous slurries is the first and most critical step to a dependable, maintainable cooling or heating system.

The Logic of Design: Sacrificing Thermal Efficiency for Flow Assurance
Why Small Diameters Are Generally Recommended
The principal thermal advantage of a PTFE tube is its huge surface area to volume ratio, which is best attained with a small diameter. Narrow tubes (4-6 mm inner diameter, for example) pack more heat transfer area into a given shell volume, cost less material and induce turbulent flow at lower velocities. It is the best design for clean fluids. But for a fibrous slurry this heat logic does not apply if the tubes get plugged. A blocked tube does not carry heat.

The Golden Rule: Tube Diameter Greater Than Fibre Length
The engineer must select a tube with an inner diameter that is much greater than the longest fibres in the slurry. A good safe place to start is a 10mm or 12mm inner diameter (ID) tube. For very long fibres (e.g., >10 mm) diameters of 15 to 20 mm may be necessary. This will obviously lead to a bigger exchanger for the same heat duty and a lower shell side velocity which may need additional baffles or a higher total flow rate. The trade off is obvious, a slightly larger, less thermally efficient exchanger that actually works vs a small, thermally efficient one that is always plugged.

To keep the heat flowing, the fibre clumps must be able to travel along a large open highway; a narrow, twisting channel will just become a clogged, unusable conduit. There still needs to be a strainer or screen upstream to remove the larger debris, but the tube must be able to pass any fibres that come through.

Technical Aspects for Reliable Operation
Speed on the Tube Side
The tube side velocity also needs to be kept high – normally above 2 m/s – to prevent the fibres settling or sticking to the tube wall. At low velocities, the accumulation of fibrous materials at the bottom of horizontal tubes is caused by gravity, thereby gradually constructing a bed which obstructs the flow. Higher velocities (up to 3–4 m/s, limited by pressure drop) keep the fibres in suspension and help flushing out any transient build-ups through the exchanger. This velocity requirement also affects the choice of tube diameter, because larger tubes require a greater flow rate to produce the same velocity, and this can increase the power required for pumping.

Tube Arrangement and Shell-Side Aspects
In the case of fibrous slurries on the tube side, clogging is less necessary for the shell side design but access for cleaning is still important. The square tube layout is chosen over triangular pitch because it provides straight unobstructed passageways between tube rows and allows mechanical cleaning of the shell side in case of fouling. But tube diameter is still the major protection against plugging. Regardless of how perfect the architecture, if the bore of the tube is too small, no amount of shell side baffling or cleaning access will prevent tube obstruction.

Other design aspects
Tube entry geometry: A smooth radius or flare at the intake prevents the fibres from snagging on sharp edges. Tubes shall be flush with the tubesheet face with no lips protruding.

Tube length: A shorter tube (e.g. 1-2 m) is easier to rod out if a blockage occurs. For very long tubes (> 3 m) mechanical cleaning is not practical.

Reversibility The exchanger should be constructed for flow reversal (i.e. able to take flow in either direction) so that temporary blockages can be cleared by back-flushing.

Practical Guidelines for Selection
Maximum fibre lengthMinimum recommended tube IDTypical use < 3 mm 6-8 mm Fine paper pulp, textile finishes
10 mm 3–6 mm Biological waste, normal white water from paper-making industry
12-15 mm 6-10 mm Coarse pulp, shredded waste, occasional food slurries > 10 mm 15-20 mm or bespoke Rag content, long synthetic fibres
The numbers are for a dilute slurry (< 5% fibres by weight). Larger diameters and lower velocities are necessary at greater concentrations to avoid bridging.

The inevitable trade-off: Size vs. clogging
The tube diameter directly affects the size and cost of the exchanger. The surface area per unit length is proportional to diameter, so to get the same surface area for a given heat duty, a 10 mm ID tube needs about 44% more tubes (or longer tubes) than a 12 mm ID tube. This requires an increase in the shell diameter and therefore in material and production expenses. However, the expense of regular downtime to clean a plugged exchanger is nearly always more than the upfront capital premium for a bigger, unpluggable type. The size of the hole is generally the most crucial design aspect in solid-to-solid battles.

Conclusion: Winning the War against Plugging
The simple bold decision of a large-bore tube wins the war against plugging in a PTFE exchanger handling a fibrous slurry. Tube inlet bridging avoided with inner diameter of 10 mm or more, sufficient tube-side velocity (> 2 m/s) and upstream straining. This alternative is a dependable, maintainable and long life heat transfer solution, but does surrender some heat transfer efficiency and increases the size of the exchanger. In the case of solids, the hole size is frequently the most essential design element.