The Field Bend That Kinked
A field technician routing a PTFE heat exchanger tube around a tank support beam bends the tube by hand to a radius that looks reasonable. The tube appears to accept the bend without cracking. Three months later, the tube at that bend has flattened into an oval cross-section. Steam flow is restricted. The tube must be replaced.
The error was bending PTFE below its minimum bend radius. Unlike metal tubing, which yields plastically at a well-defined bend angle, PTFE can be bent to surprisingly tight radii without immediate visible damage. The polymer chains reorient. The tube looks acceptable. But the outer wall of the bend is in tension beyond the material's elastic limit. Over time at elevated temperature, the stretched polymer creeps, the wall thins, and the tube ovalizes.
The minimum bend radius for PTFE heat exchanger tubing is not the radius at which the tube kinks immediately. It is the radius at which the long-term creep and ovalization remain within acceptable limits for the service life.
The Elastic vs. Plastic Bend Regime
When a tube is bent, the outer wall stretches in tension and the inner wall compresses. The strain at the outer surface is: ε = r / R, where r is the tube outer radius and R is the bend centerline radius.
PTFE has an elastic limit strain of approximately 2-3% at room temperature and 1-2% at 120°C. Bending to strains below this limit produces fully elastic deformation-the tube returns to straight when released. Bending to strains above this limit produces permanent deformation. The polymer chains have been stretched beyond their elastic recovery capacity.
The minimum safe bend radius is calculated to keep the outer fiber strain within the elastic limit at the maximum operating temperature. This ensures that the bend does not progressively ovalize over time.
For 10mm OD PTFE tubing, the tube outer radius r is 5mm. To limit strain to 2% at 120°C, the minimum bend radius R = r / ε = 5 / 0.02 = 250mm. For 8mm OD tubing, r = 4mm, and R = 4 / 0.02 = 200mm.
Table 1: Recommended Minimum Bend Radii for PTFE Heat Exchanger Tubing
| Tube OD (mm) | Wall Thickness (mm) | Minimum Bend Radius at 20°C (mm) | Minimum Bend Radius at 120°C (mm) | Multiplier of Tube OD (at 120°C) | Notes |
|---|---|---|---|---|---|
| 6 | 0.8 | 90 | 150 | 25× | Tight bends require mandrel support during bending |
| 8 | 1.0 | 120 | 200 | 25× | Standard field bend radius |
| 10 | 1.0 | 150 | 250 | 25× | Use bending spring or form for consistency |
| 10 | 1.2 | 160 | 260 | 26× | Thicker wall slightly increases required radius |
| 12 | 1.0 | 180 | 300 | 25× | Larger diameter naturally requires larger radius |
| 12 | 1.5 | 200 | 320 | 27× | Thick wall increases required radius |
The Thermal Recovery Effect
PTFE has a useful property for field bending: thermal recovery. If a tube is bent cold to a radius slightly tighter than the recommended minimum, heating it to 120-150°C allows the polymer chains to relax. The internal stress from bending partially dissipates. The tube retains its bent shape but with reduced residual stress.
This technique allows field bends at radii approximately 20-30% tighter than the cold-bend minimum, provided the tube is subsequently heat-set by circulating steam through it at low pressure before putting it into full service. The heat-setting procedure should be documented in the installation instructions.
Bending Tool Requirements
Unsupported hand bending produces inconsistent radii and risks local kinking. A bending spring-a coil spring that slips over the tube OD-provides external support and prevents ovalization during bending. A bending form-a shaped block with the desired radius-provides a consistent bend geometry.
For field installations where multiple identical bends are required, a simple bending fixture is justified. The fixture ensures every bend meets the minimum radius requirement and produces a consistent pressure drop across identical tube circuits.
Summary
The minimum bend radius for PTFE heat exchanger tubing is 25 times the tube outer diameter for cold bending at 20°C and 25-27 times for bending at 120°C. This keeps the outer fiber strain within the 2% elastic limit, preventing long-term creep ovalization and flow restriction.
Thermal recovery after cold bending allows slightly tighter radii with a controlled heat-setting procedure. Bending springs or forms ensure consistent, kink-free bends during field installation.
Engineering specifications for PTFE tube bending procedures and acceptance criteria are available upon submission of tube dimensions, operating temperature, installation space constraints, and required bend geometries.
