How Does the Use of Twisted Tape Inserts Made of Conductive Material (e.g., Metal) Affect Performance vs. PTFE Inserts?

It is a simple PTFE twisted tape that promotes heat transfer by swirling the flow. A metal tape does the same but it also transmits heat from the tube wall into itself, working essentially as an internal fin. This leads to a convective enhancement over the conductive one.

Two Mechanisms of Twisted Tape Insert
Any twisted tape insert in any material produces a helical flow channel within the tube. The generated swirl produces centrifugal forces that push the fluid toward the tube wall resulting in a thinning of the thermal boundary layer and an increase in the convective heat transfer coefficient. This swirling mechanism alone can increase the Nusselt number by 100-150% over an empty tube depending on the pitch ratio.

A further fin effect is present if the tape itself is a thermally conductive material (e.g. stainless steel, Hastelloy, titanium, copper alloys). The tape is in direct contact with the tube wall (usually spring fit or mild interference fit). heat flows into the tape from the hot wall and the complete tape body is heated. The tape then delivers the stored energy to the passing fluid from both broad surfaces. This gives the tube cross-section an extra heat transfer area, like a sort of internal longitudinal fin.

Performance of Conductive and Non‑Conductive Tape
A twisted tape of a non-conducting material such as a PTFE or a PFA can provide solely the convective swirl enhancement. PTFE has a very poor thermal conductivity (approx. 0.25 W/m.K) hence there is hardly any heat transmission from the wall to the tape. The tape remains at about local fluid temperature and contributes no fin impact.

On the other hand, a metal twisted tape might have a thermal conductivity on the order of magnitude higher. Stainless steel (~15 W/m·K), Hastelloy (~10 W/m·K), or copper (~400 W/m·K). With good thermal contact maintained between the tape and the tube wall the conductive advantage greatly enhances the overall heat transfer. In the PTFE tube arrangement with conductive twisted tape insert, the swirl plus fin effect usually provides an extra heat transfer boost of 20-30% over the non-conductive tape under the same flow circumstances.

For a PTFE twisted tape, the Nusselt number increases by 120% (2.2 times the value in an empty tube), but a well-designed metal tape in the same geometry might boost it by 150-170% (2.5-2.7 times). The actual gain relies on the thickness of tape, the thermal contact conductance and the fluid characteristics. The conductive advantage is greatest when the film coefficient on the tube wall is already large, such that the fin effect is not limited by the wall-tape interface resistance.

The Bottom Line: Good Thermal Contact
The metal tape should be in intimate thermal contact with the inner wall of the PTFE tube in order to achieve the fin effect. This is generally done by making the tape width a little larger than the tube inner diameter so that it is inserted with a spring fit. The intrinsic flexibility of the metal tape (or modest oversizing twist) pushes the tape edges against the tube wall. However, a too-tight fit can damage the PTFE tubing or make insertion impossible. A fit that is too loose creates an air gap, which is a good thermal insulator and negates the fin effect.

In practice, metal twisted tapes for PTFE tubes are commonly manufactured with a width of 0.95 to 1.00 of the tube ID, using the helical shape to provide point contacts rather than full line contact. The real contact area is limited and the fin effectiveness is decreased. But even when the contact is only partial, the conductivity is increased a little, since the tape takes up heat at the contact sites and carries it along its length. More sophisticated designs employ a thicker tape or a folded edge to improve the contact area.

Corrosion and Contamination Risk
One major risk to control is galvanic corrosion and metal contamination. Even when the metal tape itself is selected for compatibility with the process fluid (e.g., Hastelloy C-276 for aggressive chlorides, or titanium for oxidising acids), the introduction of a metal component within a PTFE tube presents two potential failure scenarios.

First, if the process fluid corrodes the metal at the operating temperature, the tape will corrode slowly. Corrosion products (metal ions or particles) can seep into the fluid and contaminate the product. This is inappropriate in pharmaceutical, food or ultra-pure chemical applications. The whole point of a PTFE tube is usually to avoid contact with the metal at all. The use of a metal tape is against that purpose.

Secondly, even if the tape is itself corrosion resistant, a galvanic couple can still be created if any other metallic component is present in the system (e.g. a metal tube sheet, a pump impeller, or a downstream valve). As long as it doesn't touch any exterior metal items, the tape is electrically insulated from them - the PTFE tube is an electrical insulator. However, if the tape comes into contact with a metal end fitting, or if the fluid used is electrically conductive (e.g. aqueous electrolyte), a galvanic circuit may be constructed through the fluid. The more noble metal ( e.g. titanium ) might promote corrosion of the less noble ( e.g. carbon steel ) elsewhere in the system .

When to Use Metal vs. PTFE Twisted Tape
Depending on the priorities of the application, the choice is between a conductive metal tape or a non-conductive PTFE tape.

When to use metal twisted tape:

The process fluid is non-corrosive to the metal selected (e.g. deionised water, mild organic solvents, neutral brines).

Need maximum heat transmission per unit length, pumping power already constrained.

Fluid compatibility is allowed and product contamination is not an issue (e.g. non-pharmaceutical industrial chemistry).

Good heat contact is possible without damage to the PTFE tube.

Select PTFE or PFA Twisted Tape for:

The process fluid is very corrosive (strong acids, alkalis, halogens, oxidising agents).

Avoidance of contamination with metal ions (pharmaceutical API synthesis, semiconductor cooling, food processing).

The working temperature is beyond the corrosion limit of available alloys but below 110°C (PTFE) or 260°C (PFA).

The simplicity and proven chemical inertness outweigh the 20-30% performance improvement of a metal tape.

For extremely corrosive services, despite the loss of the fin effect, PTFE or PFA tapes are utilised as the reliability and purity they provide are non-negotiable.

Conductive and Non-Conductive Tapes Performance Summary
Parameter PTFE (Non-Conductive) Tape Metal (Conductive) Tape
Mechanism of heat transferJust swirl Swirl + fin impact
Typical Nu augmentation (over empty tube)100‑150% 130‑180% 
Extra gain from conductivity - +20-30 % relative to PTFE tape
Thermal contact requirement Not applicable (no fin impact)Critical - spring fit essential
Corrosion risk None (inert to all chemicals up to 110°C) Dependent on fluid-metal compatibility
Risk of metal contaminationNone possible (corrosion or wear)
Maximum temperature 110°C (PTFE); 260°C (PFA) Alloy dependant (e.g. stainless 400°C)
Cost Moderate (extruded PTFE)Higher (metal machined or stamped)
A Working Example: Heating of an Aqueous Neutral Stream
Suppose a PTFE heat exchanger is used to heat a neutral, chloride-free stream of water from 20 °C to 60 °C. The swirl alone would improve the heat transmission with a PTFE twisted tape insert. The fin effect can be introduced by replacing it with a twisted tape in stainless steel 316, suitably fitted to meet the tube wall. The overall heat transfer coefficient can be increased from 150 W/m²·K to 190 W/m²·K (27%). The fluid is non-corrosive to stainless steel at 60°C and does not produce considerable metal leaching. The performance increase permits a reduced heat exchanger or a lower flow rate for the same duty.

In the same application but a slightly acidic brine (pH 4, 1000 ppm chloride) stainless steel 316 would be subject to pitting corrosion. The cost would be very high, requiring a Hastelloy C-276 tape. Another option is to go with the cheaper performance of the PTFE tape that avoids the corrosion issue altogether.

Summary
By applying metal twisted tapes on the plastic tapes, a conductive fin effect may be achieved, which can improve the heat transfer performance by about 20-30%. However, this benefit is realised only if good thermal contact with the tube wall is maintained and more importantly if the metal is totally compatible with the process fluid. For delicate operations such as pharmaceutical synthesis or high-purity chemical processing, the heat benefit is eclipsed by any risk of corrosion or metal contamination. The material selection for enhancement devices should be as careful as for the exchanger itself; for many corrosive or purity sensitive services, the inertness of a PTFE or PFA tape remains the superior choice despite the absence of the fin effect.