A PTFE shell‑and‑tube heat exchanger is in operation, and a small, internal tube leak is suspected. The traditional approach is to shut down the process, drain the unit, and perform a pressure test. But a sensitive, handheld thermal imaging camera, aimed at the outside of the metal shell, can often see the invisible, thermal ghost of a leak while the exchanger is still running. The cold fluid escaping from a pinhole inside a tube creates a localized, abnormally cold patch on the external shell wall, a telltale cool spot in a sea of otherwise uniform warmth.

How Thermal Imaging Reveals a Hidden Tube Leak

The principle behind thermal imaging leak detection PTFE exchanger diagnosis is straightforward. In a typical shell‑and‑tube heat exchanger, two fluids are separated by the tube walls. The tube side carries one fluid (often the hotter or colder process stream), and the shell side carries the other fluid. The external shell surface normally exhibits a smooth, relatively uniform temperature profile, with minor variations caused by baffle positions, inlet/outlet nozzles, and natural convection.

When a single PTFE tube develops a pinhole leak or a crack, the fluid inside that tube escapes into the shell side. If the tube‑side fluid is at a higher pressure (which is common to prevent shell‑side contamination), the leaking fluid jets out of the hole and impinges on adjacent tubes or directly onto the inside wall of the shell. This localized fluid stream has a temperature that is different from the bulk shell‑side fluid. The temperature difference can be as small as 2–5°C or as large as 20–30°C, depending on the process.

The metal shell wall conducts heat very efficiently. The area where the leaking fluid touches the inner shell surface quickly assumes a temperature close to that of the leaking fluid. From the outside, a sensitive thermal imaging camera (also called an infrared or IR camera) detects this as a localized, persistent cold spot (or hot spot, if the leaking fluid is hotter than the shell‑side fluid). The spot is typically small-just a few centimeters in diameter-and its exact location corresponds to the point where the jet impinges on the shell interior.

Performing the Thermal Scan: A Step‑by‑Step Field Procedure

Thermal imaging is a non‑invasive, rapid technique that can be performed while the exchanger remains in full operation, without draining any fluid or de‑pressuring the system. The following procedure is recommended:

1. Prepare the Exchanger Surface

The external shell surface must be clean, dry, and free of insulation (at least in the area to be scanned). Insulation can be temporarily removed from a strip or a small window. Any dirt, oil, or water on the shell surface can create false thermal reflections or obscure the true temperature. A quick wipe with a dry cloth is sufficient.

2. Select the Right Thermal Camera

A high‑resolution thermal camera with a thermal sensitivity (NETD) of ≤0.05°C (50 mK) is recommended. Lower‑cost cameras (e.g., 0.10°C sensitivity) may still detect larger leaks but will miss subtle temperature differences. The camera should have a field of view suitable for the exchanger size; a standard 24° lens is adequate for most shell diameters (300–1000 mm). Emissivity should be set to 0.95 for painted or lightly rusted carbon steel, or 0.85 for shiny stainless steel.

3. Scan the Entire Shell Exterior

The operator stands at a safe distance and slowly pans the camera along the length of the shell, covering the full circumference if possible. Multiple passes are made, focusing on areas near baffle cuts and tube sheets, because leak jets often impinge at those locations. The scan should take 5–10 minutes for a typical 3‑meter long exchanger.

4. Identify Localized Thermal Anomalies

On a normally operating, healthy exchanger, the external shell temperature appears as a gradual gradient-cooler near the inlet of a cooling fluid, warmer near the outlet. A tube leak creates a sharp, localized, circular or oval cold spot that does not follow the natural gradient. The spot typically has:

A temperature that is 1–5°C (or more) different from the surrounding shell temperature at the same axial position.

A well‑defined boundary, often with a concentric "halo" of intermediate temperature.

A fixed position that does not move when process flows change (unlike temporary turbulence effects).

The thermal camera sees the ghostly, cold signature of the hidden leak, painted as a dark, cool bruise on the warm skin of the steel shell.

5. Document and Compare

The thermal image is saved. If a suspected cold spot is found, its location is marked on the shell with a piece of chalk or a permanent marker. The same location is scanned again 15–30 minutes later. If the cold spot persists and remains at the same location, the diagnosis is strengthened. Transient thermal anomalies (e.g., from a passing steam bubble or a local draught) will disappear quickly.

6. Confirm with a Targeted Pressure Test

Thermal imaging is a screening tool. It does not replace a pressure test. However, it provides a precise location for the suspected leak. During a scheduled shutdown, the shell is drained, the suspect area is noted, and the corresponding tube(s) can be individually pressure‑tested (e.g., by plugging one end and pressurizing with air while submerged in water or using a soap solution). The thermal image guides the technician directly to the leaking tube or at least to a small group of tubes, saving hours of blind testing.

When Does Thermal Imaging Work Best?

The technique is not universally applicable. It works best under the following conditions:

Significant temperature difference between tube‑side and shell‑side fluids. A difference of at least 5°C (preferably >10°C) creates a clear thermal contrast. If both fluids are at nearly the same temperature, the leak's thermal signature is invisible.

Tube‑side fluid at higher pressure. This ensures that the leaking fluid jets out with enough velocity to reach the shell wall. If the shell side is at higher pressure, the leak will be inward (shell fluid into the tube), and the thermal signature may be on the tube sheet face rather than the shell exterior.

Clean, uninsulated shell surface. Insulation acts as a thermal blanket, smoothing out localized temperature differences. The insulation must be removed from a strip. However, some thin, low‑density insulation materials (e.g., mineral wool) still allow enough thermal conduction to the surface to detect large leaks.

Steady‑state operation. The exchanger should be at normal operating conditions for at least 30 minutes before scanning. Startup transients produce many thermal anomalies that can be mistaken for leaks.

Practical Examples and Limitations

Example 1: Cooling of Hot Brine with Cooling Water

A PTFE exchanger cools hot brine (85°C) on the tube side with cooling water (25°C) on the shell side. A tube develops a leak. The 25°C cooling water leaks into the 85°C brine, but the leak direction is inward (shell to tube). The shell exterior remains at an elevated temperature because the cooling water is not impinging on the shell wall. Thermal imaging of the shell exterior would not detect this leak because the leaking fluid does not touch the shell. Instead, the operator would need to scan the tube sheet face or the piping connected to the tube side. This highlights the importance of understanding flow direction and pressure differential.

Example 2: Condenser with Cold Tube Side

A PTFE exchanger condenses a hot vapor (120°C) on the shell side, with cold cooling water (30°C) on the tube side at higher pressure. A tube leaks, and cold water jets out of the pinhole, hitting the interior of the shell. The external shell surface shows a distinct, persistent cold spot, 5°C below the surrounding shell temperature. This is a perfect application for thermal imaging.

Example 3: Small Leak, Large Exchanger

A very small leak (e.g., a pinhole < 0.5 mm) may produce only a tiny flow rate. The jet may not reach the shell wall; instead, it might be entrained in the shell‑side flow and evaporate or mix before impingement. In such cases, no detectable thermal anomaly appears on the shell. The technique has a sensitivity limit; very small leaks may still require a conventional pressure test for detection.

Advantages Over Traditional Methods

For plants that already own a thermal imaging camera (used for electrical inspections, refractory surveys, or other predictive maintenance), the incremental cost of using it for leak detection is zero. The time saved by avoiding an unnecessary shutdown-or by drastically reducing the duration of a shutdown for targeted repair-quickly justifies the investment.

Best Practices and Common Pitfalls

Do not rely on a single scan. Ambient wind, solar loading, and nearby hot surfaces can create false cool spots. Scan at different times of the day or from different angles.

Mark the suspected location immediately. Use a non‑permanent marker on the shell. Take a visible‑light photograph alongside the thermal image to correlate.

Consider the baffle geometry. Leak jets often impinge on the shell directly opposite a baffle cut, because the flow path is forced around the baffle edge. Pay special attention to these areas.

Use differential temperature analysis. Many thermal cameras can display the temperature difference between a reference point and the suspect spot. A differential of 2°C or more that is stable over time is a strong indicator.

Correlate with process data. A slow rise in shell‑side conductivity, a drop in tube‑side pressure, or a change in outlet temperatures can support the thermal imaging finding.

Conclusion: Seeing the Invisible Heat

Thermal imaging is a powerful, non‑invasive, and predictive diagnostic tool that can spot an internal leak in a PTFE exchanger from the outside, while the process is still running. By identifying the characteristic cold spot caused by a leaking fluid jet impinging on the shell wall, a skilled operator can locate the approximate tube or tube region requiring attention. This information guides a targeted, low‑downtime repair during a planned shutdown, rather than a blind, time‑consuming search. The most powerful diagnostic eyes are the ones that can see the invisible heat. For maintenance teams responsible for PTFE heat exchangers in aggressive chemical or geothermal service, adding thermal imaging to the troubleshooting toolkit turns a mysterious, gradual performance loss into a visible, actionable thermal map of the hidden failure.