Why do heat exchangers require cleaning?
Scale induced fouling in many water-cooled heat exchangers is a longstanding problem that impacts 97% of plants. Biofilm and scale accumulate at varying rates based on the quality of source water. In turn, this reduces the effectiveness of the heat exchanger requiring cleaning maintenance.
The degradation of heat transfer increases the cost of energy (electricity) and may negatively influence process chemistry in some situations. Organizations looking to strengthen their operational best practices can begin by following a set of best practices related to heat exchanger maintenance.
Removing Scale from Heat Exchangers
The most common mineral deposit on the water side of heat exchangers is calcium carbonate (CaCO3). Raw water sources contain dissolved concentrations of calcium (Ca2+) and bicarbonate (HCO3–). This is also the compound known as lime scale that forms in the hot water piping, faucets, and showerheads of many home plumbing systems.
The traditional approach to removing calcium carbonate from heat exchangers has been to use chemicals that attack the CaCO3 as any solution with moderate acidity will dissolve the compound.
Use the right chemistry
Facilities Maintenance teams who are unfamiliar with chemistry often run into trouble by selecting off-the-shelf muriatic acid when formulating cleaning solutions. Muriatic acid, also known as hydrochloric acid (HCl), will easily remove calcium carbonate, however the chloride ion may cause serious pitting and stress corrosion cracking of stainless steel. Repeated use of HCl lowers pH, potentially damaging carbon steel piping and other equipment and can lead to heat exchanger failures.
A better option for cleaning heat exchangers with a chemical treatment are sulfamic and citric acids. The cost is not that much greater than HCI and these compounds not only remove calcium carbonate but also accumulated iron oxides. Their efficacy is improved by heating the solution to approximately 100°F and circulating it for a several hours (usually for no more than eight hours) to remove CaCO3 and iron.
A note of caution
Understanding the chemical properties, specifically how calcium carbonate reacts with acid, is important for the safety of maintenance crews. When acid is introduced to CaCO3 carbon dioxide is released which may cause the solution to foam. At the beginning of the procedure, it is important to monitor pump operation to ensure the pump maintains prime.
Still more chemistry
Depending on source water composition, other types of deposits may occur. Calcium sulfate (CaSO4) is a compound derived from the salt of a strong acid and will not dissolve in acids like calcium carbonate. This requires an alternate method that attacks the cationic portion of the deposit such as the calcium ions. The downside of this procedure is the risk of damaging the base metals within the heat exchanger.
Disposing of chemicals and environmental considerations
Compliance with discharge permits generally requires running spent solutions through a wastewater plant for proper conditioning before being discharged. Careful considerations should be made for minimizing the environmental impact of heat exchanger cleaning and maintenance.
Embracing New Technologies
Facilities operators rightfully place a priority on maintaining reliability within heat exchangers and the cooling plants they serve. This can lead to a hesitancy in adopting new technologies, despite their effectiveness. For example, at one point in time it was common practice to light homes with kerosene despite the comparably safer alternative, electricity.
The Helios TCS is a permanently installed, closed-loop system that utilizes cleaning balls that are circulated through heat exchanger tubes and continually remove scale. Cleaning balls can be replaced and are stored in an integrated ball-trap between cycles which are programmed on an integrated controller.
The Helios TCS has run millions of cycles with zero process interruptions due to failure in the system. While the system requires a capital investment, it offers a fast payback and eliminates chiller fouling related maintenance. Our use case examples provide deeper insights into how this technology has benefitted universities, hospitals, refineries, and power generators.