Guidelines for Water Treatment of Enhanced Tubes
In the first part of this two-part series, I outlined the 10 steps to protect superenhanced tubes. In this column, I will take a closer look at the use of filters and provide specific water treatment guidelines to provide good protection of enhanced and superenhanced condenser tubes.
The Role of FiltersThe use of filters is a major benefit to keep suspended materials at a minimum. These filters need not filter 100% of the water unless a heavy deposition condition is constantly present (e.g., dirty makeup water or severe airborne dust or deposits generated within the cooling system). Usually, filtering of 3 to 5% of the circulating cooling water is common for smooth copper condenser tubes.
With enhanced tubes, however, 5 to 10% of the circulating water should be filtered during operation. (Just a helpful note: Don't backwash filters with cooling water -- use makeup water for backwash to save cooling water chemicals and prevent wide swings in water chemistry). Another guideline for sizing filters is to filter the entire cooling water system at least 10 to 15 times per day.
In addition to filters, consider other beneficial methods such as ultraviolet (UV) for bio-control because it does not impact on the enhanced tubes.
Cooling Water Chemistry MattersTable 1 identifies the guidelines needed to provide good protection of enhanced and superenhanced copper condenser tubes. These guidelines reflect desirable levels of water chemistry and results of the water treatment program.
Certainly, site-specific considerations may deviate from these guidelines. However, the desirable results are the protection of the enhanced copper tubes and a life expectancy of 10 to 20 years without failures. Monitoring is the only way to determine if protection is being obtained. Certainly, detailed visual and metallographic inspection and examination of the actual enhanced tubes is the ultimate evaluation. This in-place inspection should include the use of fiber optics along with video recording. Samples of deposits should be removed from the tubes for analysis. Removal of an enhanced tube (or several) and a metallographical examination, along with a water chemistry evaluation, is the most positive method to determine not only tube condition but also the cause of corrosion and/or failure.
Now I will take a closer look at the guidelines listed in Table 1.
"Clean" cooling water relative to suspended materials can best be described as less than 10 nephalometric turbidity units (ntu). Drinking water usually is less than 5 ntus. This also can relate to suspended solids of less than 5 mg/l; drinking water usually is less than 3 mg/l.
Scale control should best be done via a "solubilizer," a chemical that keeps scale soluble. Acid polymers and phosphonates act in this manner. Crystal modifier treatments produce suspended solids. They can be used effectively but must be removed via filtration or suspended and removed from the system by the bleed-off.
Indication of concentrated cooling water scaling tendency (with inhibitors) should be at PSI of 4.5 to 6.5. (For those who need more on this, the PSI method can be supplied by the author.) Deposition on heat transfer surfaces should be absolutely none. A test unit often is available from the water treatment service company (or from the author).
Total bacteria and sessile bacterial levels should be low. Normally, total bacteria levels are low on water samples, and if kept at low levels (103 to 104 cfu/ml), implies that sessile microorganisms are low, but this may be misleading. Total bacteria testing methods are commonly available and frequently utilized. However, the sessile microorganism level is most important because these microorganisms "stick" to surfaces, including the enhanced tubes, causing corrosion and deposits. Unfortunately, no standard method for measuring sessile microorganism concentrations is available. Removing surface deposits from a given area can provide a guideline. This should show less than 100 cfu per 9 in2 area.
Because strong oxidants can initiate copper corrosion, maximum chlorine and bromine (halogen) levels should be no more than 1.0 mg/l as free and maximum 2.0 mg/l as combined form.
Ozone is known to increase copper corrosion -- often 10 times greater than halogens -- thus a low (0.1 mg/l or less) level should be the maximum at the enhanced tube surface.
Now, corrosion control is critical. It can be measured by several common techniques, including corrosion coupons and linear polarization (LP). Coupons are inexpensive but can be misleading in that they only provide an average corrosion rate for their exposure period (usually, 30 to 90 days). On the plus side, the coupons can show if pitting is occurring and special coupons can be made from enhanced tubes.
On the negative side, coupons often are too slow to indicate serious corrosion. By contrast, linear polarization almost instantaneously provides the corrosion rate, but it is not as dependable for indicating pitting corrosion. For best results, both techniques should be used.
The corrosion rates must be kept low for enhanced copper tubes -- even less than smooth wall copper tubes. Rates of 0.1 mils per year (mpy) or less are desirable with absolutely no pitting attack. The corrosion rates for mild and stainless steel are shown to be sure that steel corrosion products are minimized, to reduce their potential for deposition on the enhanced tubes. Again, site-specific conditions must be evaluated.
Can enhanced copper tubes be used successfully and be protected? Certainly, but they take more operator attention, better water treatment, improved water quality and greater attention to chiller operations. New chiller design requires newer water treatment approaches.
In my next column, I will address the use of mild steel chiller tubes when ammonia refrigerant is utilized. Mild steel is even more susceptible to rapid, serious corrosion and failure and requires much better water treatments.
Here's to better cooling and water treatment!