A heat exchanger that is underperforming is rarely announcing a simple problem. It is telling you something about the condition of the tube bundle inside it, and the answer to what that means for your facility depends on information that most facility managers and contractors do not think to gather until the situation is already urgent.
Tube bundle replacement is one of the most common heat exchanger service decisions in commercial and industrial facilities. It is also one of the most frequently delayed, because the path from “the heat exchanger is not performing the way it used to” to “here is what to do about it” involves decisions that feel complicated when they are not. This article makes that path straightforward.
What a tube bundle is and why it degrades
A tube bundle is the internal heat transfer assembly of a shell and tube heat exchanger. It consists of a series of tubes arranged within the shell of the exchanger, through which one fluid flows while a second fluid flows around the outside of the tubes in the shell. Heat transfers through the tube wall between the two fluid streams. The tube bundle is what does the work, and it is what degrades over time.
Degradation takes several forms. Fouling is the accumulation of scale, biological growth, or process deposits on the tube surfaces that reduces heat transfer efficiency without causing structural failure. Corrosion is the deterioration of the tube material itself, driven by the chemistry of the fluids in contact with the tube walls, the temperature conditions, and the material compatibility of the tubes with the application. Erosion occurs where high-velocity fluid flow or particulates in the fluid stream wear away the tube material over time. Mechanical damage from water hammer, thermal cycling, or improper installation can cause tube deformation or cracking at tube-to-tubesheet joints.
Any of these mechanisms, alone or in combination, reduces the performance of the heat exchanger and eventually leads to the point where a decision needs to be made.
When to repair and when to replace
The repair versus replace decision is not always obvious from the outside, and the answer depends on more than just the number of failed tubes. Here is how to think through it.
Repair, specifically retubing or plugging failed tubes, makes sense when the failure is isolated to a small number of tubes, the shell and tube sheets are in good structural condition, the tube material is still appropriate for the current application, and the cost of retubing is meaningfully less than the cost of a new bundle or a new exchanger. A heat exchanger that has been retubed once or twice in good water conditions and with proper maintenance can provide many additional years of reliable service.
Replacement makes more sense when the number of failed or degraded tubes represents a significant portion of the total tube count, when the tube sheets show corrosion or mechanical damage that would compromise the integrity of a retubed bundle, when the current tube material is not well suited to the application chemistry and the original failure was material-related, or when the performance of the exchanger has declined to the point where the facility’s process or comfort requirements can no longer be met even with the remaining good tubes in service.
There is also a third scenario that is worth naming explicitly. Sometimes the heat exchanger itself is performing as designed but the system around it has changed. Higher flow rates, different fluid chemistry, a process that now requires more capacity than the original exchanger was sized for. In those cases the question is not repair versus replace for the existing bundle. It is whether a new bundle with different specifications can solve the problem, or whether a new exchanger entirely is the right answer.
“A tube bundle that has failed is not always the story. Sometimes it is the system that has changed around it. Understanding which situation you are in changes everything about the right response.”
What information you need before calling a fabricator
This is where most tube bundle replacement conversations go wrong. A contractor or facility manager calls a fabricator with the exchanger model number and a delivery date. The fabricator builds what the nameplate describes. The replacement bundle arrives, goes in, and the exchanger performs the same way the old one did, which is to say not well enough, because nobody asked why the original bundle failed.
Before a tube bundle replacement is ordered, the following information needs to be confirmed.
The physical dimensions. Outside tube diameter, tube wall thickness, tube length, tube pitch and pattern, number of tubes, and shell diameter. These need to come from field measurements of the actual exchanger, not from the original specification or the nameplate, which may not reflect modifications made over the life of the unit.
The tube material. Copper, stainless steel, cupronickel, admiralty brass, titanium, and other alloys are all used in tube bundles depending on the application. The replacement material should be appropriate for the current fluid chemistry on both sides of the exchanger. If the original failure was corrosion-related, replacing with the same material will produce the same failure on the same timeline. This is the conversation that needs to happen before the order is placed.
The tube sheet condition. The tube sheets are the end plates that the tubes are rolled or welded into. If the tube sheets are corroded or mechanically damaged, a new tube bundle installed into compromised tube sheets will not seal correctly and will fail prematurely. The tube sheet condition needs to be assessed before the replacement bundle is specified.
The application parameters. Fluid types on both sides, flow rates, inlet temperatures, outlet temperature requirements, and operating pressure. These confirm whether the original bundle design is still appropriate for the current application or whether the replacement should be specified differently.
How HX Coils approaches a tube bundle replacement
HX Coils reviews every application before fabrication begins. For tube bundle replacements that review covers the physical dimensions, the tube material selection relative to the current application chemistry, the tube sheet condition if we can get that information, and the operating parameters to confirm the replacement bundle will meet the current performance requirements.
We can replicate any tube bundle from any manufacturer. We can also specify a replacement in a different material or with modified geometry if the application review indicates that a direct replacement is not the right answer. That review happens before fabrication, not after the replacement bundle arrives on site and does not solve the problem.
The lead time for a tube bundle replacement depends on configuration complexity, material selection, and shop load at the time the order is placed. Standard configurations in copper or stainless steel can move quickly. Specialty materials extend the timeline. The right time to start the conversation is before the situation is urgent, because the options available in a planned replacement are different from the options available in an emergency.
HX Coils manufactures custom tube bundles and replacement coils for commercial and industrial heat exchanger applications across the Mid-Atlantic region. If you have a heat exchanger that is underperforming or a tube bundle replacement coming up, reach out before the order is urgent and we will review the application and tell you exactly what the right replacement looks like.
References
1. Tubular Exchanger Manufacturers Association. TEMA Standards for Shell and Tube Heat Exchangers. Covers design, fabrication, and materials standards for shell and tube heat exchangers. tema.org
2. ASME. Boiler and Pressure Vessel Code, Section VIII, Division 1. Governs the design and fabrication of pressure vessels including shell and tube heat exchangers. asme.org
3. Heat Transfer Research Inc. Fouling in Heat Exchangers. Covers fouling mechanisms, effects on performance, and mitigation strategies. htri.net
4. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Heat Exchangers. Covers selection, application, and maintenance of heat exchangers in commercial HVAC systems. ashrae.org
