Tube corrosion can reduce the cooling-water-to-refrigerant heat transfer efficiency. Find out how to maximize the service life of an enhanced tube condenser bundle to help boost your company's bottom line as well as improve overall efficiency.

Enhanced copper condenser tubes are designed to improve the cooling-water-to-refrigerant heat transfer efficiency. For many years, tube manufacturers have machined fins on the refrigerant side to increase surface area. Copper condenser tubes enhanced with refrigerant-side fins on the outside work well to improve chiller efficiency and raise few corrosion concerns. However, chiller manufacturers also produce enhanced tubes that are spiraled on the inside (condenser-water-side) surface as well as on the outside. These internal helical ridges act to induce turbulent water flow and increase surface area, both of which improve heat transfer. Machine rifling or spiraling of the internal tube surfaces can contribute to early corrosion because contaminants often are found in open cooling water systems.

Premature corrosion or failure of internally enhanced tubes is related to their design and fabrication. Generally, the internally enhanced tubes are susceptible to deposition, under-deposit corrosion and microb iologically influenced corrosion because of their increased surface area and small helical grooves.

Water stagnation or low flow conditions are particularly troublesome for these tubes because these conditions allow suspended solids to drop out of so l ution and deposit on the tube surfaces. Once deposits have formed, a mechanism for chemical-concentration-cell corrosion or differential-aeration-cell corrosion immediately is established. Deposits also facilitate biological growth and subsequent microb io logically influenced corrosion.

Even though the internally enhanced tubes are designed to create turbulent water flow and increase efficiency, the increase in localized water turbulence also can affect the protective, microscopic film barrier on th e h elical ridges. Copper corrosion inhibitors (azoles such as TTA and BZT) protect tubes by forming a very thin protective film on the metal surface. If this film is disrupted by turbulent water flow and not rapidly replaced by free azole inhibitor in th e wa ter, corrosion will ensue.

Tube fabrication methods also must be considered when diagnosing the premature failure of the machine rifled tubes. First, the cold working (such as rifling) of any metal component will distort and fragment its grain structure and produce significant residual stresses. Second, the rifling process also can lead to many scratches and fissures on the tube surfaces. Both of these factors result in "built-in" corrosion sites.

High efficiency chiller tubes have a greater tendency for corrosion (pitting) and deposition. The following practices can help maximize the service life of an enhanced tube condenser bundle.

Specifications. The high residual stresses of the co ld-worked, machine-rifled tubes can be relieved by annealing (heat treating) the tubes. Annealing will improve the corrosion resistance and general toughness of the metal after cold working. When replacing or installing a new condenser bundle, specificat i ons should require the proper annealing of the enhanced tubes.

System Cleanliness. Frequent manual cleaning and side-stream filtration are imperative for the proper care of enhanced chiller tubes. When the tubes are brushed, the brush type is c ritical. Manufacturers' specifications on brush type must be followed. After brushing, immediately re-passivate the tubes with 20+ ppm azole. Remember, the rifled grooves and any surface scratches are magnets for silt and deposit accumulations.

Biological Control. An alternating biocide program must be implemented to control biofilm formation and general micro-organism growth. Biocides should be dosed near their maximum legal levels to ensure biological control. In particular, the stressed areas on the enhanced tube surface (from cold working) are very susceptible to oxidizing biocides. A free chlorine residual that exceeds 0.3 ppm during idle operation (stagnation) can be particularly damaging to the passive film created by the azoles.

Chemical Inhibitors. The azole copper corrosion inhibitor levels should be tested independently of the normal cooling product control test (i.e., ortho-PO4, molybdate, phosphonate). Detectable azole concentration levels such as BZT or TTA always must be present. As a general rule of thumb, the azole concentration in the recirculating water should be at least 2.0 ppm, or
(Total Copper residual x 2) + 2 ppm
whichever is higher. If the cooling water formulation does not contribute the necessary levels, additional azole must be fed into the system.

Operation. When the chiller is in standby or lay-up mode with stagnant tower water in the condenser tubes, the potential for corrosion and deposition is at its highest. Treated water must be circulated through the unit for a minimum of 30 minutes each day to help maintain passivation of the tube metal.

High-efficiency condenser tubes demand special care. To help protect and maximize the service life of these capital investments, it is important to be aware of the potential hazards. The cost of chemical treatment, filtration and manual cleaning is insignificant when compared to the capital cost of replacing condenser tubes or entire chillers.

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