How Do Ball Turbulators (Packed Beds) Operate in PTFE Tubes for High-Fouling Services?

Standard turbulators tend to clog with slurries or other heavily fouling liquids. The tube is filled with a bed of small movable spheres that offers a self-cleaning mode, with heat transfer enhancement and mechanical descaling.

Fouling of Heat Exchanger Tubes – The Problem
Heat transfer surfaces are prone to fouling in many chemical, pharmaceutical and bioprocessing applications . Fouling is the deposition of substances such salts, organic residues, polymerisation byproducts or biological films. Thermal resistance increases significantly with the formation of a fouling layer, and cleaning shutdowns are needed often. Conventional augmentation methods such as twisted tapes or wire coils, while helpful for clean fluids, tend to aggravate the problem because they give crevices for deposits to attach and proliferate.

For PTFE tubing, which has been selected for corrosion resistance and chemical inertness, fouling can be particularly difficult because PTFE's hydrophobic, low-friction surface decreases, but does not prevent, adhesion. For highly fouled services, a more aggressive, self-cleaning solution is needed.

How a Packed Bed of Balls Works
A ball turbulator is a packed bed of little loose spheres filling the tube. Usually the balls are formed of PTFE itself or with ceramic materials such as alumina or zirconia, which are fully chemically compatible with the process fluid. Diameters are usually 3–8 mm, designed to produce a bed with around 35–40% void percentage. The bed is loosely packed (balls are not fused or glued together) and held in the tube by screens or perforated end caps.

The process fluid runs through the tube, and is forced to weave through the tortuous interstitial spaces between the balls. This results in very high local velocities and strong turbulence, at low global flow rates. More crucially, the balls themselves are not at rest. They are jostled and rotated by the fluid motion and occasionally bump into the inner wall of the PTFE tube. The hits are low energy and physically scour off any deposit that starts to build keeping the wall clean.

In a packed bed PTFE tube design with a ball turbulator, the heat transfer process is twofold. First the chaotic flow path disrupts the thermal barrier layer far more efficiently than any static insert. Secondly, the incessant wall contact to the rotating balls provides an additional direct conductive path and momentum transfer, thereby substantially enhancing the heat transfer coefficient.

Performance characteristics Heat transfer vs pressure loss
Ball turbulators can have very high heat transfer coefficients, frequently 2 to 4 times those of an empty tube for the same flow circumstances. The augmentation of the Nusselt number is comparable to that of a tightly twisted tape insert but with the important added benefit of continual wall cleaning.

Pressure drop is a major trade-off. The packed bed causes large flow resistance. Pressure decreases are typically 5 to 15 times that of an empty tube depending on ball diameter, packing density and flow velocity. Hence the technique is only used for those services where fouling is the main constraint on plant availability. In such circumstances, the added pumping energy is justified by the removal of unscheduled cleaning shutdowns, decreased maintenance labour and steady thermal performance over long run lengths.

The benefit of months of operation without tube cleaning in specialised applications such as heating of viscous polymer slurries or recovering heat from crystallising solutions or processing biological broths containing high cell debris content offsets the higher energy consumption. Ball turbulators are rarely utilised in clean fluid service since the simpler upgrades with lower pressure drops are more inexpensive.

Fluidisation resistance and bed retention
The balls must stay in the tube, not be pushed out by the flow. At low flow velocities the bed is lightly packed and does not move other than locally. But when the velocity increases the drag force on the balls can make the whole bed expand and begin to fluidise, or act like a fluidised bed with the balls suspended in the flow.

Complete fluidisation is normally undesirable in a heat exchanger tube, since then the balls will strike the walls less often and can be blown out of the end of the tube. Thus, the bed is generally bounded by a retaining screen at the outflow. Further, the tube length is such that the overall pressure decrease across the bed keeps the surface velocity below the minimum fluidisation velocity (Umf). For PTFE balls in water-like liquids Umf is attained at velocities of 0.5-1.0 m/s depending on ball size and density. For ceramic balls (greater density) Umf appears at higher velocities. Umf is just below where designers commonly work, where the balls are in a "packed but agitated" state-- spinning and rubbing against each other and the wall, but not suspended.

Choice of Material for the Balls
The obvious answer of course is PTFE balls for PTFE tubes, to keep the same chemical inertness and non-stick qualities. They have a density of around 2.2 g/cm3, much higher than water or organic solvents, and so remain settled in the tube. The PTFE balls are so soft that they will not damage the tube wall even after millions of collisions.

Ceramic balls (e.g. 99.5% alumina, density ~3.9 g/cm3) are employed where a higher ball weight is required to boost the impact force for tough deposits, or where the process temperature surpasses the 110°C limit of PTFE (ceramics operate up to 300°C). Ceramics are also more tough, providing a more vigorous cleaning action. But care must be taken that the ceramic substance does not abrade the PTFE tube wall over long periods of time. In practice, use of PTFE balls for most chemical services and ceramic balls for high temperature or particularly tenacious fouling is usual.

Designing for Practice
In the design of a ball turbulator packed bed for a PTFE tube, the following parameters are optimised:

Ball diameter: 3-5 mm for small tubes (10-20 mm I.D.); 6-8 mm for larger tubes (25-40 mm I.D.). The smaller balls have more contact points with the wall but have larger pressure drop.

Bed length: 10-30 tube diameters typically. Longer beds mean more transfer of heat but the loss in pressure increases correspondingly.

Void fraction: ~ 0.35-0.40 for homogeneous spheres in random close-packing. This can be adjusted by mixing different size balls but is rarely done.

Retention method: Perforated discs of PTFE or PFA at both ends, hole diameter less than ball diameter. The input disc prevents balls from being ejected by flow surges.

The scouring action is particularly effective in eliminating soft, gelatinous or crystalline fouling deposits. Ball turbulators are not as effective for hard, baked-on coke or scale, because the low-energy blows cannot dislodge tightly adherent deposits. Such instances nevertheless require periodic chemical cleaning.

Comparison of Other Enhancement Techniques in Fouling Services
Method Enhancement of heat transferIncrease in Pressure Drop Mitigation of Fouling
Empty pipe, smoothBad Baseline Baseline
Twisted tape insert 100-150% 200-600%None (often makes worse)
Corrugated tube 20–60% 30–100%Minor (decrease adhesion)
Ball Turbulator (Packed Bed) 100-300% 500-1500%Excellent (ongoing cleansing)
Conclusion 
Ball turbulators are a niche but very efficient option for improving heat transfer on severely fouling services. These loosely packed beds of small PTFE or ceramic balls in PTFE tubes are installed in the process stream. By establishing complex, turbulent flow patterns and continuously washing the tube wall by gentle impacts, they keep heat transfer surfaces clean and enable long periods of uninterrupted operation. The large pressure drop penalty restricts their use to situations where fouling would normally cause repeated shutdowns. Different improvement methods suit different operational issues, however for high fouling, high value processes the ball turbulator packed bed delivers an unparalleled combination of thermal performance and self-cleaning reliability.