As a general rule of thumb, the addition of defoamers will help suppress foaming in cooling towers and boilers, but what is the working mechanism and why is foaming an issue?
It is well known in the operation of steam boilers or cooling towers that water, even though initially will show very little tendency to foam, will when the amount of total solids approaches a relatively high concentration and develop a tendency to foam. When this occurs, a considerable amount of water is physically carried out of boilers with the steam, thus appearing in the steam lines and in the eventual condensate. Foaming of water in a steam boiler can also create serious issues due to the difficulty in maintaining the proper water level. In cases of severe foaming, the steam-generating zone may become dangerously filled with foam which can lead to the failure of boiler tubes from overheating. Cooling towers, by design and operation, are susceptible to foaming. Cooling tower foaming presents not only an operational nuisance but can also indicate system operation and water chemistry issues.
Picture 1: Foam present in a cooling tower basin
Simply put, foaming occurs when the surface tension of water is affected, typically as a result of the production of froth or unbroken bubbles. In boilers, foam develops when steam bubbles form rapidly on the surface, faster than the rate of bubble displacement. The excess of dissolved salts can bring about the tendency for foaming. Of the salts commonly found in operating boilers, sodium phosphate (NaPO4) and sodium hydroxide (NaOH) appear to have the greatest foaming potential – each having about one-and-a-half times the effect of an equal quantity of sodium chloride (NaCl). For this reason, an excessive amount of alkalinity and phosphate are undesirable in boiler waters. Foaming is a known issue where colloidal material such as oils or animal grease enters boiler systems.
In cooling towers, foaming may result from overfeeding chemicals, like the excessive addition of a biocidal treatment. Suspended solids possibly windblown from ambient sources in conjunction with over-cycling will also present these issues. Several other parameters such as microbiological activity, high alkalinity, surfactant, and process contamination can all contribute.
Antifoam and Defoamers
If foaming is a persistent issue, it may be of benefit to reevaluate concentration cycles or water chemistries being utilized. For increased stability, improving control equipment and filtration can present viable options. If all else fails, the addition of defoamers or antifoams can be effective. A takeaway from researching this topic is that “antifoam” refers to products that proactively prevent foaming while “defoamers” quell existing foam. It may be the case that foaming does not occur on a continuous basis and that the addition of a defoamer would be sufficient. Personally speaking, I find it handy to always have some defoamer available in the event of a disruption. When antifoams are added in small quantities, foam formation is prevented. Antifoams work by changing the characteristics of the liquid to prevent foam from building up. They are able to displace the surfactant molecules in the foam lamella, meaning the monomolecular layers are less elastic and break down more easily. Combined antifoam and defoaming agents may contain solid particles like waxes, paraffin, or silicas which disperse onto the foam lamella and cause the bubbles to burst, allowing the trapped air to escape and preventing the continued build-up of the foam. There are many useful materials for this purpose: alcohols, amids, amines, glycols, silicones, and even vegetable oils. Some antifoams, however, are not useful because of their unstableness under boiler operating conditions such as caster oil. There has been some success with the addition of a small quantity (1% - 2%) of triamylamine or other soluble amines in boiler applications which can also work synergistically with boiler chemistries.
Picture 2: Mechanism of defoaming through a surface-active agent such as oil or hydrophilic silicone
Conventional physical methods for defoaming include thermal, electrical, and mechanical foam breakers. Thermal methods consist of heating and cooling the foam, producing the expansion and compression of the bubbles that result in their destruction. The application of thermal and electrical methods in industrial plants, however, has been very limited because of the practical difficulties, the energy consumption, and the effects of high temperatures on the product. Mechanical methods, such as rotary devices, cyclones, jet streams of liquid or air, and vacuum chambers, are more popular but oftentimes not economical.
In most cases, foaming can be avoided by proper control of dissolved and suspended solid concentrations. In cooling tower applications, ambient process contamination or microbiological activity may need to be addressed if foaming is persistent. The working mechanisms of chemical control via antifoam/defoamers are vast, from silicone compounds to vegetable oils. It is important to mention that an antifoam that works well in one application may be ineffective or even promote foaming in another. Also, an antifoam that has worked satisfactorily in the past may gradually lose its effectiveness, especially if continually being overfed. With the proper chemical program aligned with effective antifoams, there is really no need for facilities to put up with the inconvenience or expense caused by foaming.
List of Resources:
 Bird, Paul G., Jacoby, Arthur L. 1947. Prevention of foaming in steam boilers. United States. NAT ALUMINATE CORP.2428776 https://www.freepatentsonline.com/2428776.html
 Kohl, Arthur “Antifoams.” Chemical Engineering (Fifth Edition_, vol. 2, 2002. Accessed 11 July 2021.
 Why Is There Foam in a Cooling Tower? Is It a Problem or Good News?. 17 May 2016, www.amsainc.com/why-is-there-foam-in-a-cooling-tower/. Accessed 11 July 2021.
 Bird, Paul and Arthur Jacoby. Method for Controlling Boiler Water Foaming - Calgon Corporation. 14 Oct. 1947, www.freepatentsonline.com/4346017.html. Accessed 11 July 2021.