Do All Closed Chilled Water Systems Need Water Treatment?
As a consultant, I listen to those persons and ask questions to understand what conditions are found in those untreated closed chilled water systems. I ask about the water quality, the materials contacted by the water, and details on system operation.
A typical basic system is shown in the figure 1. A basic closed chilled water system often utilizes mild steel piping and copper tubes in the chiller and air-handling coils. There also are a mild steel water box and tubesheet in the chiller. Water quality is usually city water, at times softened to remove hardness minerals. Operation usually is continuous chill water flow, but it may be stagnant during evening and weekends as well as in unoccupied areas.
There are many variations in design. Some examples are large chilled water storage vessels in thermal storage systems, ice machines and clean room coils. Construction materials can be cement tanks, aluminum tubing and/or piping, galvanized steel, and stainless steel plate-and-frame heat exchangers when in free-cooling operation.
With all these different metals, water treatment is quite common -- so how can a chilled water system remain in very good condition without any water treatment?
Here is what we found: These systems were fabricated almost entirely of stainless steel but occasionally used titanium plate-and-frame heat exchangers. Pumps were even stainless. The water used to makeup the system was high purity reverse osmosis (RO) product water. This water is free of minerals, microbes and nutrients and thus will not cause deposits or enable microbiological growth.
However, oxygen can be present in these systems. As oxygen does not attack stainless steels or titanium, it actually is the best corrosion inhibitor for these metals.
Some of these systems use ultraviolet (UV) to ensure that no microbiological growth occurs in the water. Some even have filters to remove any particulate matter from the water, possibly from startup.
Yes, these chilled water systems do not use any water treatment chemicals at all, and the systems remain trouble-free from the chilled water. So it is true -- some chilled water systems do not need any water treatment.
But beware if the chilled water system contains mild steel, copper alloys, aluminum or galvanized steel and uses makeup water that contains minerals and microbes. If your system has any of these characteristics, water treatment is absolutely necessary if you want to protect your system for many years. Also, you need to constantly maintain the water treatment and monitor the results for corrosion, deposits and microbiological control to ensure that protection is continuing.
Another important consideration, particularly in newly constructed closed cooling, chilled and even hot water systems, is to initially clean and passivate (protect) them prior to startup. Too often, a water flush or even a mild cleaning does not remove the rust, dirt and microbes from the system due to construction and prior to initial startup. This leads to corrosion, deposits and microbio fouling and/or corrosion. If it is a nitrite- or phosphate-based program, the water treatment can lead to even more corrosion and deposits. So, cleaning is necessary prior to initial water treatment. This might mean even an acid cleaning to ensure clean surfaces. The initial cleaning should be followed immediately by protection with an effective water treatment program that is maintained and monitored. These steps will provide long life and efficient closed system protection. I'll have more on the specifics of these procedures in future articles.
Well, now you can say some closed chilled water systems do not need any water treatment -- "if" -- and that "if" is the key to determining if your system needs water treatment. PCE