How to Protect Enhanced Copper Tubes
Attention, end users - especially those with recently installed chillers that have water-cooled condensers. Has your cooling water treatment program been improved to protect the condenser tubes from corrosion and deposit? If not, look out for possible early failures.
Recently, tube design has been changing. In the past, copper condenser tube surfaces contacted by cooling water had only smooth internal tube surfaces. Today, chiller condenser tubes called "enhanced tubes" are available with lands and grooves similar to rifle barrels. Even superenhanced copper tubes are available that have shallow fins similar to those found on the refrigerant-side tube surfaces (figure 1).
The benefit is that this relatively new design provides considerably improved heat transfer due to greater surface area. The danger is that water treatment must be improved substantially. Otherwise, corrosion and deposits can cause early tube failures (leaks and plugging), even within one year. I have been investigating a number of enhanced and superenhanced condenser tube failures originating on the cooling water (internal) contacted surfaces.
There are 10 specific points that should be addressed to greatly improve enhanced copper tube protection and life expectancy. Briefly, I'll review these points.
Cleaning and PassivationAs soon as possible - ideally, within a few days - after the installation of a new chiller or the retube of an existing chiller with enhanced or superenhanced condenser tubes, perform a cleaning and passivation procedure. The cooling water side of the tubes should be cleaned of oils, greases, dust, dirt, corrosion products, etc., from fabrication, installation and startup. This cleaning should not damage the enhanced tubes or create a condition that will initiate corrosion. A neutral or slightly alkaline pH surfactant/cleaner should be utilized. Avoid any acids or high pH (above 8.5) cleaners.
Immediately after cleaning and rinsing, the tubes should be passivated, which is the establishment of a protective coating or film on all of the tube surfaces. The use of an effective copper corrosion inhibitor such as an azole should be circulated through the tubes at quite high dosages until the tubes are thoroughly filmed and protected.
The next four points demand an effective, continually applied and continually controlled cooling water treatment and water chemistry program.
Elimination of Scale Deposits and Good Sediment Deposit ControlGood scale and suspended solids control is accomplished with a good dispersant and scale-inhibitor program. It may take 50 to 200% higher treatment levels vs. that required for smooth copper tubes to keep enhanced tubes deposit-free. After evaluation of site-specific conditions such as the makeup water composition, air-introduced dust and dirt, and cleanliness of the cooling water system, the specific water treatment program can be established.
Good Sessile Microorganism ControlThis also involves a site-specific evaluation. Sessile microorganisms, which are those that stick to surfaces, include deposit formers, the dreaded sulfate-reducing bacteria and other acid-producing bacteria. The treatment often includes a good biodispersant as well as a good biocide program that involves both an oxidizing and a nonoxidizing product. It should be applied frequently and at dosages needed to ensure good control.
Good Copper Corrosion ControlGood copper corrosion control generally means at least twice the copper-inhibitor level for enhanced tubes vs. smooth copper tubes. It also means that the copper inhibitor should be maintained to ensure constant protective film maintenance.
All of the chemicals used for deposit, corrosion and microbiological control must be compatible and must not interfere with their actions and effectiveness.
Avoid Excessive Oxidizing Biocide LevelsThis is essential particularly if a chlorine-, chloramine-, bromine- or ozone-producing microbiocide is utilized. Excessive free chlorine/bromine oxidant levels are considered to be levels above 1.0 mg/l; combined halogen should be less than 2.0 mg/l. However, the use of oxidant-resistant copper-corrosion inhibitors can minimize or prevent corrosion of enhanced tubes. Ozone must be less than 0.1 mg/l. Do not slug feed oxidizing biocides that can cause high oxidant levels even for short periods. Also, do not use dry oxidants that may not dissolve completely. They can settle into the grooves of the enhanced tubes and initiate pitting corrosion.
Maintain Water CirculationIf the chiller is shut down, continue to circulate the cooling water through the enhanced tubes. Ideally, circulation should be continuous, but periodic cooling water flow - say one to two hours per day - may be satisfactory. However, usually higher levels of dispersants, corrosion inhibitors and microbiocides are needed to keep the enhanced tubes clean and protected.
Reinitiate Cleaning and Passivation After ShutdownIf cooling water flow is stopped for a week or more, reinitiate the cleaning and passivation procedure at startup. In essence, clean the enhanced tubes (certainly within the grooves) and reestablish the copper protective film. This could include brushing the tubes.
Maintain Good Water ChemistryThis is an often-misunderstood qualification involving a description of the cooling water. It actually means to utilize both chemical and nonchemical products to protect the enhanced tubes and keep them clean and protected. Chemicals, when properly selected and applied, can keep the enhanced tubes free of deposits and well protected. The use of filters is a major benefit to keep suspended materials at a minimum. (More on filters next month.) In addition, consider other beneficial methods such as ultraviolet (UV) for bio-control because it does not impact on the enhanced tubes.
Monitor the SystemRegular evaluations will show whether you are maintaining good corrosion, deposition and microbiological control.
In my next column, I'll look further at cooling water chemistry.
Sidebar1. Perform initial cleaning and passivation
10 Steps to Protect Superenhanced Tubes
2. Eliminate scale deposits
3. Establish good sediment deposit control
4. Establish good sessile microorganism control
5. Ensure improved copper corrosion control
6. Avoid excessive oxidizing biocide levels
7. Maintain cooling water circulation through tubes if the chiller is shut down
8. If cooling water flow is stopped, restart with initial cleaning and passivation procedure
9. Maintain good water chemistry
10. Monitor for corrosion, deposits and biological