Carbon Steel Tubes in Chillers
The movement from halogen-based to nonhalogen-based refrigerants results in a choice of either ammonia or nonhalogen-based products. If your choice is ammonia, copper alloy tubes cannot be used. Carbon steel, stainless steel or titanium are the only choices.
Often, carbon steel tubes are selected due to costs. If this is done, the condenser's water treatment requirements are considerably different than with copper tubes. It requires special startup and routine water treatment chemicals and procedures. If not implemented, severe corrosion and early carbon steel tube failure likely will occur.
Figure 1 is an example of severe corrosion of a carbon steel tube. This tube was removed from a retrofitted chiller previously using copper-alloy condenser tubes. The retrofit was due to changing from halogen-based refrigerant to ammonia. Failure of the carbon steel tubes occurred in less than one year after the retrofit. The cooling tower water passed through the tube, providing cooling to the warm ammonia refrigerant on the outside of the tube.
Looking closely at the cleaned internal surfaces reveals two major problems:
- Penetration via a pit caused by corrosion.
- An incomplete weld appearing as a crevice along the length of the internal tube surface.
So, what happened to cause this early failure in this chiller? I'll address the two problems separately and collectively.
The penetration of the tube due to cooling water corrosion requires a brief review of materials of construction along with the common cooling water treatment program objectives and chemicals.
For many years, chillers have utilized copper condenser tubes due to excellent heat transfer and good corrosion resistance. Copper tubes normally do not corrode rapidly, except as noted in the last two articles on copper enhanced and super enhanced condenser tubes. By contrast, mild steel rusts (corrodes) rapidly in cooling water unless special attention is taken to prevent this corrosion.
Ideally, mild steel can corrode uniformly but slowly across the entire surface, or it can corrode isolated spots -- which is not desirable -- on the metal surface. These rapidly corroding spots can be caused by tube metal imperfections (for example, cracks, stresses, etc.), the corrosive nature of the cooling water, ineffective mild steel corrosion control treatment or ineffective startup procedures.
Thus, for several years, chillers with copper condenser tubes utilized either no or marginal amounts of mild steel corrosion inhibitors in the cooling tower water. Even low levels of microbiocides were utilized unless Legionella bacteria control was being practiced. So, when mild steel tubes are in a new chiller or installed in a chiller that previously used copper tubes, the cooling water treatment program needs to be upgraded to excellent mild steel corrosion control.
Let me go back to the case history I mentioned at the beginning of this column. What caused the mild steel tube perforation to occur within just one year? As the tube in figure 1 illustrates, the pit and perforation was due to an ineffective mild steel corrosion control water treatment program. Also, an incomplete weld created a partially perforated tube wall and a crevice for corrosive bacteria to thrive.
The lessons learned here are that when mild steel condenser tubes are utilized, one must be sure that the water treatment program incorporates an excellent mild steel corrosion inhibitor and that facility operations maintains excellent water quality controls. Also, one needs to be sure that the microbiological treatment is effective on mild steel corrosive microorganisms to reduce the potential for microbiologically induced corrosion (MIC). Yet another lesson learned is to be sure the mild steel condenser tubing has complete welds to prevent crevices that can initiate corrosion.
But how can this be done? Should the mild steel tubes be inspected before installation? Ideally, yes. The use of a fiber optics boroscope may reveal the incomplete welds if done carefully. Otherwise, cutting a "typical" tube in half and examining it carefully can reveal an incomplete weld. But it generally is not practical or economical to destroy too many tubes, so this approach still might not reveal incomplete welds. Certainly specify that the mild steel tubes have complete welds or specify seamless mild steel tubes, which are somewhat more costly. One other alternative is to specify stainless steel rather than mild steel tubes. Copper alloys cannot be used with ammonia refrigerant.
The other important consideration with mild steel tubes is the chiller startup, both initially and after being shut down for more than a week.
During fabrication of the chiller, the presence of lubricants, oils, rolling compounds and mild rusting of the tube internally and externally most likely occurred. Water flushing can remove some of these contaminants and even create more rusting, but the tube surfaces will not be clean. This means the water-borne mild steel corrosion inhibitors will not be able to react with the steel surfaces due to this barrier. The outcome is more corrosion, resulting in more corrosion product deposits. The regular cooling water treatment program -- even if it contains excellent mild steel corrosion inhibitors -- will not be able to reach the surface and stop corrosion, thus causing early penetration and failure.
What needs to be done is to clean and initially protect the mild steel tube surface. This can be accomplished by a specific procedure that incorporates cleaning agents to remove oil and rust on the surfaces. This is followed by initially protecting the clean steel surface, which should be continued via the regular cooling water treatment program utilizing excellent mild steel corrosion inhibitors.
This discussion about the use of mild steel condenser tubes might indicate that they should not be used in chillers with ammonia refrigerant. This is not true -- they can and are successfully utilized. The startup, use and effective control of water treatment programs and procedures are essential for protecting the mild steel tubes. In other industries, mild steel tubes are protected and last for 10 to 15 years before replacement is necessary. Yes, they are and can be successful.
Note: A discussion of this procedure and chemicals are in the Puckorius & Associates Inc., publication Volume 5, Number 2, Cooling Water Systems -- Passivation, Part 1, Heat Exchangers. To order, call Puckorius & Associates.