How to Detect and Quantify an Internal Shell-Side Bypass Stream Using Thermal Imaging?

Inside the black sealed shell of a PTFE heat exchanger, an unseen internal leak is stealing performance. Shell-side fluid is bypassing the tube bundle through a failing baffle interface or damaged seal and not following the desired flow direction. The exchanger is still running, the pressures are normal, and there's no leak outside." However, a fraction of the process fluid short-circuits directly to the exit, and therefore the thermal efficiency silently decreases. Fortunately, this covert detour leaves a thermal trace on the shell's outside, a discreet heat pattern that an astute infrared camera can detect without disturbing a single bolt.

Thermal imaging has become known as one of the most effective non-invasive diagnostic techniques to discover shell side bypass problems in PTFE heat exchangers under live process circumstances.

Understanding Bypass Flow on the Shell Side
In a correctly operating shell and tube exchanger the shell-side fluid is pushed to flow across the tube bundle in a controlled pattern set up by baffles and sealing systems.

This crossflow motion is important since it:

Maximizes the transmission of heat

Prefers turbulence

Removes standing water

Increases thermal efficiency

Ensures shell is used uniformly

As the bypass develops, some fraction of the shell side fluid is completely bypassing the intended flow channel.

The fluid does not travel over the PTFE tubes but exits via

Baffle seals damaged

Gaps between the baffles and the shell wall

Failed partition plates

Internal components are corroded

Bundle interfaces not seated properly

The leaking stream rushes towards the outlet with little meaningful heat transfer.

How Thermal Imaging Works
A bypass stream forms a local temperature anomaly in the shell of the exchanger.

The bypassed fluid is still at a very different temperature to the shell environment, and hence the steel shell wall above the rogue stream has a different surface temperature distribution.

The thermal camera sees the ghost of the rogue current, painted in heat on the skin of the shell.

This temperature disparity is more apparent during a shift in the controlled process temperature.

Create a Temperature Step Change
The best way of diagnosis is to induce, on purpose, a fast change of temperature at the shell-side inlet.

Here are several examples:

Cold water versus warm water

Hotter process fluid introduced

Temperature short-term adjustment of the cooling water supply

Changing recirculation loop parameters

The purpose is to create a thermal front propagating in the heat exchanger.

The significance of controlled temperature variations
The temperature change should be sufficient to produce a distinct thermal contrast on the outside of the shell.

The temperature transition must not be too aggressive however.

PTFE tubing systems have a lower tolerance to rapid heat shock than metallic tubing systems. Excessive temperature changes can result in extra mechanical stresses on the exchanger bundle.

So for safe diagnostics it is preferable to have a moderate yet distinct temperature step.

Conducting the Thermal Scan
A high-resolution infrared camera constantly scans the surface of the outer shell once the temperature change is commenced.

Healthy Heat Exchanger Thermal Pattern
As a rule, a normally working exchanger shows:

A constant temperature gradient

Slow thermal transitions

Uniform heat dispersion along the shell length

Even Thermal decline from inlet to out let

The temperature of the shell surface changes slowly during typical heat conduction through the bundle.

Bypass Stream Thermal Signature
A bypass on the shellside tells a very different tale.

The thermographic scan shows a different picture:

A continuous hot strip

A chilly channel down the shell

A confined thermal patch.

A temperature pattern that is not symmetric

Rapid thermal propagation to the outflow region

This anomaly follows the route of the bypassing fluid as it is diverted around the tube bundle.

In severe circumstances the thermal signature may go almost straight from entrance to output.

Distinguishing Between Localized and General Leakage
A key benefit of thermal imaging shell side bypass detection PTFE approaches is the capacity to discriminate between different failure types.

Generalized Leak Through Baffle
Large scale internal leaking appears as:

Temperature anomalies are widespread

Thermal smearing, wide

Lower thermal uniformity overall

Many overlapping hot or cold spots

This pattern may be due to the age of the seals or overall degradation of the internals in the exchanger.

Localized Bypass of Failure
Usually a single failing seal or damaged partition will cause:

A thin thermal line

A well defined channel .

Cold hot areas of concentration

Various directional flow signatures

This localized behavior allows maintenance staff to greatly reduce the size of the inspection area prior to shutdown.

4.4. Quantifying Bypass Severity
Thermal imaging is more than simply finding a flaw. It can also get an estimated severity estimation in a controlled environment.

Qualitative Assessment
In many industrial scenarios a qualitative image is adequate to confirm:

Bypass flow existence

Location of bypass (approx)

Leakage relative intensity

Deterioration progression with time

This information is frequently sufficient to justify a planned maintenance activity.

Thermal Analysis, Quantitative
More sophisticated analysis may estimate:

Relative bypass flow fraction;

Temperature difference magnitude

Degree of heat transfer deterioration

Thermal velocity of propagation

Several factors are required for quantitative measurements:

A calibrated infrared camera

Known shell wall absorptivities

Steady environmental conditions

Uniform viewing angles

Process controlled temperatures

Emissivity correction is required for temperature readings, but the visual patterns are still relevant for diagnosis.

Advantages over conventional inspection methods
Routine shell-side inspection usually needs:

Exchanger is shut off

Isolation and drainage

removing bundle

Blind dismantling

Prolonged outage

Thermal imaging avoids a lot of these interruptions.

Advantages of non invasive diagnosis
Main operational advantages are:

Online inspection function

No stoppage of production

Fast defect detection

Reduced uncertainty in maintenance plans

Early failure detection

Enhanced repair targeting

Instead of haphazardly pulling down an exchanger, a maintenance crew can plan specific repair activities for the next scheduled outage.

Factors Influencing Image Quality
Several practical issues affect the clarity of thermal bypass signatures.

Status of Shell Surface
The paint, oxidation, moisture and insulation state affect the infrared emissivity behavior.

Airflow ambient
High air flow rates over the shell can cause distortion of the external surface temperatures.

Removing Insulation
It is often necessary to remove temporary shielding so that the bare shell surface can be scanned accurately.

Time of thermal stabilization
Enough time must pass for the temperature front to propagate through the shell and produce a visible contrast on the outside.

Applications in PTFE Heat Exchanger Systems
Thermal imaging diagnostics are particularly useful for PTFE exchangers, as aggressive invasive inspection procedures can't always be applied to the polymer tubes.

Common uses include:

Chemical process equipment

Cooling circuits acid.

Solvent recovery heat exchangers

Surface finishing machines

Semiconductor process cooling

Thermal systems in pharmaceutics

In particular, in these conditions the uptime has to be maintained and hidden heat losses have to be diagnosed.

Conclusion
Thermal imaging provides a powerful non-contact, non-invasive tool to reveal concealed shell-side bypass streams in PTFE heat exchangers. Thermal patterns on the shell exterior can be used to view bypass flow pathways rapidly and accurately without the need to open the equipment by applying a controlled temperature step change.

PTFE In the wider field of shell side bypass detection in thermal imaging, this method provides rapid visual confirmation of internal leakage, pinpoints specific regions of failure and allows for assessment of the level of thermal performance decline. Clear hot or cold stripes on the shell surface are typically a good indication of exactly where the fluid is escaping effective flow control.

Thermal imaging is becoming more and more an operational X-ray vision for process equipment with the ongoing development of industrial diagnostic technologies. In many circumstances, the most useful troubleshooting tools are those that can diagnose the condition without first having to shut the process down.