Twisted Tube® heat exchangers are efficient but present challenges for inspection due to their unique tube geometry, affecting signal quality when using standard eddy current probes. The Twistec™ probe has been developed to adapt to these geometric changes, offering enhanced inspection accuracy by minimizing liftoff and noise. This probe allows for clearer defect identification and improved results compared to traditional methods.

1. Twisted Tube® Heat Exchanger
Inspection with Eddy Currents
Application Note

2. Twisted Tube® Heat Exchangers
Heat exchangers are essential in many industries. A unique
type, the Twisted Tube® heat exchanger (HX), offers:
• Highest efficiency among HX
• Challenge for periodic inspections because of tube profile
2 © Eddyfi NDT Inc. 2009–2015

3. The Challenge
A twisted tube is composed of:
• 2 straight, circular sections near tubesheets
• 1 length of helical, oval tube
The helix creates a small, circular central passage along
the length of the tube where
traditional bobbin probes can
be pushed and pulled.
3 © Eddyfi NDT Inc. 2009–2015

4. The Challenge
Poor signal-to-noise ratios lead to suboptimal
performances with standard ECT probes.
• Because of liftoff created by tube’s geometry, similar-sized
flaws:
• Close to crests = small distorted
signals
• Close to dips = large
distorted signals
• Results: Lower PoD, less
accurate sizing
4 © Eddyfi NDT Inc. 2009–2015

5. The Challenge
• Common sizing method to determine flaw depth are
inaccurate because:
• Reduced sensitivity
• Large tube signal variations from flaw locations (crest/dip)
• IRIS also affected by non-linear geometry
• Reflecting echoes not perpendicular to inspected wall. Not an alternative.
• Tubes in Twisted Tube® HX come in variety of:
• Pitches
• Diameters (typical: 19.1–25.4 mm [0.75–1 in])
• Non-ferromagnetic and ferromagnetic materials
5 © Eddyfi NDT Inc. 2009–2015

6. The Solution
A probe that:
• Fits most Twisted Tube® HX
• Dynamically adjusts to changing geometry to minimize liftoff and optimize
inspection results
Result: Patent-pending Twistec™ probe
Designed with:
• Rotating head
• Equipped with two “feelers” that press down the coils
they contain against tube’s inner surface.
• Pair of oval coils matching helix pitch
of twisted tube.
6 © Eddyfi NDT Inc. 2009–2015

7. The Solution
Twistec probe remains close to surface under test for
more precise inspection data
• Feelers compress to fit through circular section at the tubesheet
Pulling speed:
• Approximately 0.3 m/s (12 in/s)
Life:
• 1500 tubes (60 to 80 tubes/h)
Results allow defects to be clearly identified. Twistec
probe signals are less noisy than those from a conventional
bobbin probe.
7 © Eddyfi NDT Inc. 2009–2015

8. The Solution
8 © Eddyfi NDT Inc. 2009–2015
Bobbin Probe Results Twistec Probe Results
Crest detection

9. The Benefits
• Minimized liftoff
• Minimized noise
• Optimized results
• Inner/outer-diameter defect distinction
• Determine whether defects are in crests or dips
• Easier-to-analyze scan results
• More accurate scan results
• Longer asset life
9 © Eddyfi NDT Inc. 2009–2015

10. www.eddyfi.com