Water Quality in a Chilled Water Loop

Issue:

Suitability of water for use in Trane equipment with steel, copper, aluminum materials.

Resolution:

Pueblo Built and Supported Chillers: Trane's Chemistry and Materials Engineering group is frequently called upon to comment on the suitability of water for use in Trane equipment from job sites around the world. This task is approached with great caution because of the dramatic impact water chemistry has upon unit performance and reliability, and because water chemistry is such a complex topic. Normally Trane takes a very conservative view and err on the side of caution.

The questions that need to be answered relative to the water and its interaction with the equipment are:

1. What is the source of the water?
2. Is the water corrosive?
3. Is the water scale forming?
4. Can the water be treated to make it less corrosive or less likely to form scale?

The source of the water can play a significant role in water treatment. If the source is waste water or surface water, water chemistry can vary greatly, and that offers unique challenges for controlling biologicals.

The first three issues that determine the general corrosion and scaling potential of the water are pH, chloride content, and sulfate content. These values should be as follows:

pH: 7.5 to 9.0
Chloride: 125 mg/L (or ppm) maximum
Sulfate: 35 mg/L (or ppm) maximum

If the pH of the water is greater than 9.0 it is likely to be scale forming. Heavy scale reduces heat transfer and may lead to corrosion if the scale flakes off.

If the pH is less than 7.5 the water tends to be corrosive to copper and copper alloys. The fact that water with a pH value of 7.5 is corrosive often surprises people because they understand that a pH of 7.0 is neutral and they believe water needs to be acidic to be corrosive. To a large extent, copper tubing resists aqueous corrosion by forming a thin, tenacious layer of copper oxide on the water side surface, and this copper oxide film is what actually protects the copper from aqueous corrosion. For this oxide film to provide good coverage of the surface, the pH of the water needs to be above 7.5. With water less than 7.5 pH, the oxide film can be dissolved in some areas and precipitate pitting corrosion that can locally penetrate the tube wall. Pitting attack is driven by localized galvanic cells set up in areas without the oxide film that are adjacent to areas with the oxide film.

Waters with chloride contents greater than 125 mg/L, or with sulfate contents greater than 35mg/L, can be corrosive to copper and copper alloys because these elements are able to penetrate the oxide film and attack the copper. There are many areas in the world where water exceeding these limits are used successfully in HVAC applications. In coastal areas of the southeastern US, for example, it would be rare to see chloride contents less than 300 mg/l and in other areas (Oklahoma, for example) well waters with over 100 mg/l sulfate are common. Our rules of thumb are simply guidelines that indicate to us that we should consider the potential for problems and investigate the situation thoroughly.

Beyond pH, chloride, and sulfate we look at water conductivity, dissolved and undissolved solids, and alkalinity. With these items it gets more difficult to apply rules of thumb because the interaction of these items with each other, as well as with pH, gets more complex. Hence we are reluctant to publish acceptable or unacceptable values for these items.

If the water chemistry exceeds the guidelines indicated, a water treatment company should be contacted to treat the water to bring it within the guidelines or to provide appropriate corrosion inhibitors.

In addition to these chemical characteristics of the water, bacteria, algae, and fungi can also be present in water systems. These "biologicals" can also cause operating, maintenance, and health problems. As such, the water system must also be treated to prevent or minimize the growth of biologicals.

Another contributor to scaling is the hardness of water.  The amount of calcium and magnesium make up the total hardness of water. The value for total hardness is as follows: 

Calcium and Magnesium (total hardness): 350 mg/L (or ppm) maximum

Notes:

If aluminum material is in contact with water, reference this document

Suitability of Water

Changes:

Other Question/Problem/Cause Statements:

• Water quality guidelines.
• Water treatment guidelines.
• Water quality or water treatment in heat exchangers.
• Do I need to treat my water?
• Does my system require water treatment?
• What are Trane's guidelines for water treatment?
• What are the condenser water side requirements?
• What are the evaporator water side requirements?
• Do we have a condenser water treatment specification?
• Suitability of water for use in Trane equipment
• Do I need to treat my water?
• Does my system require water treatment?
• What are Trane's guidelines for water treatment?
• What are the condenser water side requirements?
• What are the evaporator water side requirements?
• Do we have a condenser water treatment specification?
• What are the minimum water quality requirements for Trane chillers?
• Water Quality?
• Chilled Solution Quality?
• Condenser Water Quality?
• Condenser Solution Quality?
• Water Treatment?

Facts:

• Air-Cooled Chiller (CGAA)
• Air-Cooled Chiller (CGA, 10-15 Ton)
• Air-Cooled Chiller - Intellipak (CGAF)
• Air-Cooled Chiller (PBL) (CGAD, CGAE)
• Air-Cooled Chiller (Pueblo) (CGAM 20-130 Ton)
• Air-Cooled Chiller (Pueblo) (RTAA 130-400 Ton)
• Air-Cooled Chiller (Pueblo) (RTAA 70-125 Ton)
• Air-Cooled Chiller (Pueblo) (RTAC)
• Air-Cooled Chiller (Pueblo) (RTAE)
• Compressor Chiller (Pueblo) (CCAF)
• Compressor Chiller (Pueblo) (RTUA)
• Compressor Chiller (Pueblo) (RTUD 70-200 Ton)
• Water-Cooled Chiller (Pueblo) (CGWF)
• Water-Cooled Chiller (Pueblo) (RTHA)
• Water-Cooled Chiller (Pueblo) (RTHB)
• Water-Cooled Chiller (Pueblo) (RTHC)
• Water-Cooled Chiller (Pueblo) (RTHD)
• Water-Cooled Chiller (Pueblo) (RTWA)
• Water-Cooled Chiller (Pueblo) (RTWD)
• Air-Cooled Chiller (Pueblo) (RTAF)

Document Identifier: DOC-107281