Selection of the proper water treatment program involves specific steps to be not only effective but cost effective. These steps are:
- Determine all of the different metals within the entire cooling water system.
- Determine the design of all cooling tower water-contacted equipment.
- Define system operation.
- Identify water quality expected.
- Identify restrictions on chemical selection.
- Determine protection desired.
Now, and only now, can the cooling water treatment program be selected.
Let’s briefly review each of these steps so that the cooling system owner has sufficient knowledge to determine if his chemical treatment supplier will recommend the most cost-effective program.
Step 1: Determine Metals in SystemThis will identify those metals that need to be protected from corrosion. Start with the most important component and list the specific metal.
- Chiller tubes (condenser) -- copper, stainless steel, galvanized steel, mild steel.
- Condenser tube sheet -- mild steel, galvanized steel, stainless steel, copper alloy.
- Cooling tower -- galvanized steel, stainless steel.
- Cooling water lines -- mild steel, copper, stainless steel.
Step 2: Determine Design of Water-Contact EquipmentThe design of cooling tower water-contacted equipment will determine where and if deposits/corrosion/fouling will likely occur.
- Chiller tubes (condenser) -- smooth, enhanced or superenhanced.
- Plate-and-frame heat exchanger.
- Cooling water heat exchanger -- water in tubes, water outside tubes.
- Tubesheet -- coated, not coated.
- Water box -- coated, not coated, type of sacrificial anodes.
- Cooling tower -- crossflow, counterflow, fill type (splash or film).
- Fill materials of construction -- PVC plastic, stainless steel, wood.
- Tower structure -- wood type and whether it is treated, fiberglass, concrete.
Step 3: Define OperationKnowledge of the operation of the cooling water system is needed to determine where, when and if corrosion, deposits and microbiological problems might first occur.
- Maximum cooling water temperature?
- Minimum cooling water velocity through tubes? Continuous or periodic water flow?
- How often is water stagnant? How long?
- Is pH control desired?
- Is bleed controlled?
Step 4: Water Quality ExpectedDefine the cooling tower water quality expected (no treatment) to identify the levels of water minerals that need to be controlled by the water treatment program. Criteria to be considered includes conductivity, calcium hardness, total alkalinity, silica and chlorides.
Step 5: Identify RestrictionsIdentification of restrictions on chemicals or pH is needed to determine which chemicals can be utilized.
- Metals (e.g., zinc, molybdate).
- Nutrients (e.g., phosphates, nitrites).
- pH (i.e., maximum/minimum).
- Conductivity (maximum).
- Halogen (e.g., chlorine, bromine, ozone).
- Nonoxidizing biocides.
- Limits of treatment chemicals.
Step 6: Define Protection DesiredDefining the protection levels desired (or required) is one of the most important designations the owner/operator needs to identify to the water treatment chemical supplier. What are the needs for efficiency and life expectancy?
- Corrosion rates (mils per year) -- copper, mild steel, stainless steel, galvanized steel or other metals.
- Deposits (little or none) -- on condenser tubes, in cooling tower film fill or circulating lines.
- Microbiological control -- for corrosion, deposits (algae/slime) and pathogens (Legionella).
It sounds difficult, but actually, it is not. So do your homework, and then I’ll have more on selection of the proper water treatment program in our next column. Remember, good cooling system protection is a must!