Why Do PTFE Heat Exchangers Develop Bypass Leakage and How Is It Corrected?

Bypass leakage is not a leak to the outside environment, but an internal short-cut-shell-side fluid that flows around the tube bundle instead of through it. This internal bypass is a loss of heat transfer surface and a reduction in exchanger efficacy but is not visible from the outside. It is important to know the routes and causes of bypass leakage PTFE heat exchanger problems to diagnose performance loss and plan successful repairs. This article discusses the common leakage channels, causes of leakage, and repair steps to restore thermal performance.

Bypass Leakage: What it Is and Why it Matters?
In a shell-and-tube heat exchanger, the shell-side fluid is supposed to flow across the tube bundle, directed by baffles. Bypass leakage happens when some of that fluid doesn't touch the tubes, because it flows down a shorter, lower resistance path. The fluid departs the exchanger with a small or zero temperature change, which decreases the overall heat transfer rate.

Bypass leakage is harmful, especially for PTFE heat exchangers that already have lower thermal conductivity than metal units. The bypass flow of 20% can reduce the effective heat transfer coefficient by 25–35% because the bypassed fluid does not take part in the heat exchange. The problem is internal and cannot be found by external visual inspection or pressure testing, only by thermal performance monitoring or internal examination.

PTFE Heat Exchanger Primary Bypass Leakage Paths
The shell-side fluid can bypass the tube bundle in three different ways.

1. Bundle-to-Shell Clearance (Annular Bypass)
There is always a space between the outermost tubes and the inner wall of the shell. This annular gap is required for inserting and removing the tube bundle. This gap is filled in a well constructed exchanger with sealing strips (sometimes called bypass seals) or dummy tubes which restrict the flow. The sealing strips are often flat steel bars or PTFE-filled strips affixed to the baffles, projecting outward and contacting the shell wall.

When sealing strips are missing, worn or torn, fluid on the shell side flows easily through the annular gap without entering the tube bundle. This is the major bypass road.

2. Baffle to shell clearance
The baffles do not fit tightly in the shell. A little clearance (usually 1–3 mm) is provided to enable assembly and thermal expansion. Fluid may run over the edges of each baffle, especially the top and bottom where the baffle cut occurs. The leakage in new exchangers is modest (2-5% of the total flow). If, however baffles become damaged, twisted, corroded or eroded, clearance will increase and bypass flow will increase considerably.

Water hammer (steam hammer), foreign items transported in the shell side fluid, and corrosion of metal baffle components can cause damage to PTFE heat exchanger baffles (PTFE tubes themselves do not corrode, but the metal baffles and support plates may).

3. Tube to Baffle Distance
Each baffle has a hole through which a PTFE tube flows. To permit movement of the flexible PTFE tube and prevent binding during thermal expansion a space of 0.2 to 0.5 mm is necessary. These little annular holes may allow fluid to escape. Tube to baffle leakage is generally a minor contributor to bypass in most designs (usually <5% of total bypass) due of the pressure drop across each baffle forcing flow down the many tube holes. However, the cumulative effect can be significant in exchangers with many baffles and low-viscosity fluids.

Reasons for PTFE Bypass Leakage Heat Exchangers
Internal examination usually reveals one or more of the following conditions:

Worn or Missing Weather Stripping
Sealing strips are fastened to baffles with screws, clips or welding. Screws can loosen over time, clips can fail, or the strips themselves can corrode or degrade. In certain field-repaired bundles the sealing strips may not have been re-installed following re-tubing. It is a regular discovery that sealing strips are only present on a few of the baffles, or are completely absent.

Damaged baffles,
Baffles can be broken by:

Steam hammer –Repeated shock waves can bend or shatter metal baffles near the tube openings.

Corrosion - Steel baffles can thin down and lose shape in corrosive usage (even with PTFE tubing) .

Mechanical Impact - Baffles could be bent during bundle insertion or removal if not properly aligned.

No longer fitting snugly against the shell, a bent baffle creates a considerable clearance. A broken baffle may have parts missing that will allow for flow.

Large Shell Clearance from Original Design
Some of the low-cost PTFE heat exchangers are constructed with excessive bundle-to-shell clearances to facilitate manufacture. There can be a gap of 10 mm or more. Sealing strips are missing or too small. In these situations, bypass leakage is a design feature and cannot be completely eliminated without a major redesign.

How to Diagnose Bypass Leakage Without Opening the Exchanger
If bypass leakage is suspected:

The heat transfer rate, computed from inlet/outlet temperatures and flow rates, is much lower than the design figure, even though the tubes are clean and there are no exterior leaks.

The shell side pressure drop is smaller than historical readings at the same flowrate. Less pressure drop means the fluid is flowing through a channel of less resistance (bypass).

The temperature change of the shell-side fluid (ΔT) is not as expected and the tube-side output temperature is similarly off target.

First, it is vital to rule out alternative explanations for poor performance: fouling, trapped non-condensables, or lower flow rates. If those disappear then it is probably bypass leaking.

Bypass Leakage Corrective Actions
The proper repair is dependent on the severity and accessibility of the tube bundle.

1. Replacing sealing strips (most common fix)
For exchangers with removable tube bundles, sealing strips can be inserted or removed during a planned shutdown. The process is:

The shell cover is removed and the tube bundle pulled out.

Baffles are inspected. Clean debris and corrosion from the grooves of the existing sealing strip.

New seal strips (often PTFE impregnated fabric, metal spring strips or solid PTFE bars) are cut to the required length and fitted into the slots on each baffle. The strips are to project out to contact the inner wall of the shell with a small interference fit (0.2–0.5 mm compression).

The strips are held in place by screws or by peening the edges of the baffle (for metal strips). Some designs hold the strips by the pressure of the shell against the baffle.

The bundle is reinstalled and the exchanger is tested.

This repair normally recovers 90-100% of the lost performance if the baffles are not damaged.

2. Repair or Replacement of baffle
Sealing strips alone cannot fix baffles that are bent, fractured or corroded. Damaged baffles should be fixed or replaced. For PTFE heat exchangers, baffles are generally made of metal (stainless steel or carbon steel) which can be:

Straightened -- Small bends can be pushed flat in a hydraulic press.

Welded - Cracks are welded up and ground smooth. Care is taken not to distort the baffle.

Replaced – Severely damaged baffles are cut from the bundle and new baffles are threaded onto the PTFE tubes. This is a labor-intensive operation, often performed during a full retubing.

After baffle repair, new sealing strips are installed.

3. Shell-Side Flow Adjustment (Temporary Mitigation)
For situations where immediate disassembly is not possible, operating adjustments can partially compensate for bypass leakage. Increasing the shell-side flow rate raises the velocity through both the tube bundle and the bypass gaps. Because the pressure drop increases with the square of velocity, a higher flow rate may improve the proportion of fluid that actually crosses the tubes. However, this does not fix the root cause and increases pumping energy. It is considered a temporary measure only.

4. Redesign for Severe Clearance Issues
In exchangers with inherently oversized shell clearance (poor original design), retrofitting with thicker sealing strips or installing dummy tubes in the annular space may be attempted. Dummy tubes are short lengths of PTFE tubing that are placed in the outermost tube holes but are sealed at the tubesheet ends so that no fluid flows through them. They occupy space and block bypass. This approach requires drilling additional holes in the baffles and tubesheets and is usually done by a specialized repair shop.

Cross-Sectional Diagram Description (Text Representation)
The following description illustrates the bypass paths in a typical PTFE shell-and-tube heat exchanger as viewed in a transverse cross-section:

The outer circle represents the shell inner wall.

Inside, a circular array of small circles represents the PTFE tubes.

The gap between the outermost tube circles and the shell inner wall is the bundle-to-shell clearance (annular bypass path). In a well-designed exchanger, sealing strips (depicted as small rectangles attached to the baffle) protrude into this gap, nearly touching the shell wall.

A baffle is shown as a horizontal line crossing the shell interior, with cutouts (the baffle window) at the top and bottom. Fluid that flows around the baffle edges-through the baffle-to-shell clearance-is shown as arrows passing between the baffle tip and the shell wall.

Small arrows between individual tubes and the baffle represent tube-to-baffle leakage.

The sealing strips block the annular bypass. When missing, large arrows indicate unobstructed flow through the annular gap.

Prevention of Bypass Leakage
Bypass leakage is best prevented during design and initial assembly:

Specify TEMA class R or B construction – These standards define maximum clearances and require sealing strips for shell diameters above a certain size.

Use double sealing strips – Two rows of sealing strips per baffle provide redundancy.

Inspect sealing strips during retubing – Any missing or damaged strips are replaced before reassembly.

Avoid water hammer – As discussed in related guides, steam hammer damages baffles and loosens sealing strip attachments.

A periodic internal inspection (every 3–5 years) should include checking the tightness of sealing strips and the condition of baffles. If bypass leakage is suspected earlier, a pressure drop test at a known flow rate can be compared to baseline data.

Conclusion
Bypass leakage is a design and maintenance issue that degrades PTFE heat exchanger performance silently. The primary paths are the bundle-to-shell clearance (annular bypass) and baffle-to-shell clearance. Missing or worn sealing strips and damaged baffles are the most common causes. Internal inspection often reveals these problems. Corrective actions include replacing sealing strips, repairing or replacing damaged baffles, and, as a temporary measure, increasing shell-side flow. For severe clearance issues, dummy tubes or redesign may be required. A bypass leakage PTFE heat exchanger problem can reduce heat transfer by 10–30%, but most cases are fully correctable during a planned outage. Proper fabrication with adequate sealing strips and periodic internal inspection preserve thermal efficiency and extend service life.