O&M

Condenser tube coatings – Some questions

The latest in Brad Buecker’s series on condenser performance monitoring.
Condenser tube coatings – Some questions
(A GE surface condenser. Source: GE Vernova.)

By Brad Buecker – Buecker & Associates, LLC

Steam surface condensers are still a critical component at many power plants and co-generation
facilities. Maintaining tube cleanliness is vital to maximize heat transfer. Table 1 illustrates the
thermal conductivities of two common heat exchanger tube materials and also three common
waterside foulants, and in particular, bio-slime and calcium carbonate.

*Data extracted from Reference 1
The data shows the strong insulating properties of deposits. A primary focus of this article is
purported new technology for lowering the potential for CaCO3 deposition.

Issues with calcium carbonate scale formation

Probably from the time humans began heating water for cooking, hygienic, and other purposes
they became acquainted with calcium carbonate scale formation. Two of the most common ions
in fresh waters are calcium (Ca2+) and bicarbonate alkalinity (HCO3–). In combination, these
ions have a low solubility, and when heated the solubility decreases even further.

Ca2+ + 2HCO3– + heat → CaCO3↓ + CO2↑ + H2O

In untreated waters, CaCO3 (calcium carbonate) is usually the predominant scaling compound.
CaCO3 is the deposit that forms in home hot water piping and showerheads, and which is
incorrectly referred to as “lime scale.” (Lime is calcium oxide (CaO) or in its hydrated form,
Ca(OH)2) Furthermore, many thousands of cooling water systems have a cooling tower at the
heart of the process. Heat transfer primarily occurs from evaporation of a fraction of the
recirculating water. Evaporation causes the water to “cycle up” in concentration, which in turn
significantly increases the potential for scale formation without proper treatment.

A very effective solution to reduce CaCO3 scaling potential is sulfuric acid injection to the
makeup or circulating water to convert bicarbonate alkalinity into carbon dioxide that escapes
from solution.

HCO3–(aq) + H2SO4 → HSO42-(aq) + H2CO3(aq)

H2CO3(aq) ⇌ CO2↑ + H2O

In the middle of the last century a hugely popular treatment program for open-recirculating
systems consisted of sulfuric acid feed for scale control (to establish a pH range of 6.5-7.0), and
use of disodium chromate (Na2Cr2O7) for corrosion control. This latter compound provides
chromate ions (CrO42-) that react with carbon steel to form a pseudo stainless-steel layer which
passivates the metal surface and inhibits corrosion.

In the 1970s and 1980s dawning recognition of hexavalent chromium (Cr6+) toxicity led to a ban on
chromium discharge to the environment that essentially eliminated chromate treatment for open-recirculating cooling water systems. The replacement programs utilized inorganic and organic
phosphate compounds, with supplemental or inclusive polymer chemistry becoming much more
common in recent times.2,3

But articles and reports continue to appear about the possibility of waterside tube coatings to
minimize scale formation in condensers, with a piece last month from Power Engineering’s editor,
Kevin Clark, posted on the PE website.4 These coatings are designed to inhibit scale formation. It
seems though that a number of questions must be answered before coatings technology becomes
viable.

Coating questions

Having worked with condensers for many years and recently authoring two articles on the subject
for Power Engineering,5,6 several questions/comments came to mind regarding Reference 2. Perhaps many of these questions could be answered from pilot testing. They include:

  • The authors focus on the coating’s potential to inhibit calcium carbonate scaling. The
    question that obviously comes to mind is, “How much would this coating (or any other
    coating for that matter) impede heat transfer?” If the thermal conductivity is not
    significantly greater than the mineral deposits the coating inhibits, it becomes difficult to see
    the benefits.
  • What is a typical coating thickness?
  • How is the coating applied?
  • What surface preparation is needed to ensure strong coating attachment?
  • What process is needed to impart the ridges (highlighted in Reference 2) on the coating
    surface?
    o Might these ridges offer more sites for settling of microorganisms and subsequent
    microbial fouling?
  • What are the curing methods and cure times for the coating?
  • What safety precautions are necessary for coating applications?
    o Necessary personal protective equipment
    o Confined space issues
    o Potential release of volatile organic carbon (VOC) compounds
    o Disposal of application waste materials
  • How resistant is the coating to normal mechanical wear, oxidizing biocides, and
    temperature?
  • Consider the potential situation of a condenser that becomes fouled and requires mechanical
    cleaning with either tube scrapers or brushes. How much damage would this do to the
    coating?

Conclusion

Having lived all my life and attended college in three of the states that border Missouri, I guess I
have a bit of a “show me” attitude at times. From the questions above, the most important still
seems to be the heat transfer issue. It is hard to imagine that any coating would have a heat transfer
coefficient close to those of typical tube metals. Also, other cooling system components require
corrosion and deposition protection that come with good chemistry programs. Finally, even if
coatings prove to be successful at inhibiting scale formation, microbial control issues will surely
remain and will continue to require well-designed and operated biocide feed systems.

References

  1. “Thermal Conductivities of Biofilm, Metals, and Scale”; ChemTreat Technical Bulletin #164, June 2013.
  2. B. Buecker (Tech. Ed.), “Water Essentials Handbook”; 2023. ChemTreat, Inc., Glen Allen, VA.  Currently being released in digital format at www.chemtreat.com.
  3. Buecker, B. and P. Kalakodimi, PhD., “Current Concepts in Cooling Water Chemistry”; pre-conference seminar to the 41st Annual Electric Utility Chemistry Workshop, June 6-8, 2023, Champaign, Illinois.
  4. K. Clark, “Researchers say hydrogel coating a possible solution in the fight against scaling”; Power Engineering, www.power-eng.com, February 23, 2024.
  5. B. Buecker, “Condenser Performance Monitoring – Part 1”; Power Engineering, August 2023.
  6. B. Buecker, “Condenser Performance Monitoring – Part 2”; Power Engineering, December 2023.

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing.  Most recently he served as Senior Technical Publicist with ChemTreat, Inc.  He has many years of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station.  Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry.  He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control.  He is a member of the ACS, AIChE, AIST, ASME, AWT, CTI, and the Electric Utility Chemistry Workshop planning committee.  He is active with the International Water Conference and Power-Gen International.  He may be reached at [email protected].