Cooling towers and boilers will constantly need to add makeup water both to replace the evaporative loss that comes from heat transfer as well as to replace “bleed” or “blowdown” that purges impurities that have increased in concentration. Getting makeup water right is vital for operational uptime and efficiency. In this article, we’ll take a look at what these risks are, why they are present, how your facility team can monitor them, and what treatment solutions are appropriate to manage them. At EAI, we believe that proactively managing cooling tower makeup water through pretreatment can often be a worthy investment – although not every water treater will tell you this. 

Makeup water quality is the start of a positive feedback loop in a cooling tower. The higher the quality of makeup water, the less the tower needs to blowdown. The less it blows down, the less makeup water it needs, all else being equal. 

The Ever-Increasing Concentration of Impurities

Water comes from many sources and can widely vary in quality. The quality of the water will have a direct effect on the system it’s being used in and based on the process may pose serious risk. In systems which boil water to create steam, or evaporate water for cooling, water quality can make or break the system. The higher the purity of water is the closer it approaches pure H₂O and does not include any other compounds or chemicals. Common industrial water sources include municipal water supplies or ground water and though they are normally potable, are not pure. When water boils or evaporates, only water molecules (and anything with lower boiling points or vapor pressure) will change phases from liquid to gas. Any impurities such as minerals in the water are left behind in the liquid. As more water vaporizes, the concentration of impurities in the liquid water increases. Without replacing the removed water, the remaining water quality will become increasingly worse.

Why are High Concentrations of Impurities an Issue?

Water impurities often come in the form of minerals such as magnesium and calcium ions but can be wide ranging from inorganic and organic compounds (silica is another major culprit). As water is removed and the concentration of impurities such as ions increase, they are more likely to precipitate or come out of suspension within water. As these compounds precipitate they tend to accumulate as a hard shell on the surface of pipes, containment vessels, valves, and most importantly, heat exchange elements. Any accumulation on the heat exchange elements will decrease the efficiency of heat transfer. This means that in a cooling tower, for instance, your tower will have to run harder and harder to provide the required level of cooling. This can waste tens of thousands of dollars per year in utility costs if not dealt with.

Removing Scale Is Painful

Beyond simply reducing heat transfer efficiency, reacting to scaling events, or not reacting to them, introduces many secondary problems. Removing scale from a heating or cooling system is an expensive, time, and labor-intensive process. Cooling towers and loops will need to be drained and placed offline for several days as special descaler chemicals, pressure washing, and manual chiseling are employed to attempt to reverse the damage. This is rarely 100% effective at removing buildup. In cases where scale passes the point of no return, entire cooling towers and chillers may need to be replaced far earlier than planned.

 

The Important Role of High Quality Makeup Water

While adding makeup water is the short-term tactic for preventing scale precipitation and accumulation, it is also the underlying source of the ingredients that cause scale. There are two directions that can be taken with makeup water, one is reactive and one is proactive. The reactive response is to bleed and make up frequently (wasting water) and employing traditional scale inhibitor chemicals or non-chemical devices to prevent hardness and other contaminants from precipitating in places they shouldn’t. The proactive response is not only providing make up water when it’s required but also preventing build up in the first place by pre-treating the makeup water with softeners, ion exchange resins, or even reverse osmosis in certain instances.  The makeup water must have a lower concentration of scale-forming impurities than the water within the vessel if it is to make a positive difference (and the lower, the better). Because of this, it is critical to monitor the quality of the makeup water and take action to improve the water quality when necessary. The ideal makeup water quality for each facility may vary, but there will be a target pH, generally low hardness/conductivity, and low enough levels of other scale-forming contaminants such as silica to avoid causing new problems once hardness is removed. Once this is ensured, you can confidently run your tower longer and longer without blowing down knowing that you will not be causing a scaling event.  

Testing for Water Quality

Water quality parameters will vary based on the process but similar principals will apply to all systems which require makeup water. As discussed so far, the main principle is that makeup water must be as close to “perfect” as possible – meaning as low impurities as possible so you are starting at an advantage before boiling or evaporation concentrates those impurities. The more pure the water, the more efficient the makeup water will be in increasing equipment longevity and maintaining the function of the process. The following water quality parameters should always be measured in the makeup water:

Total Hardness:  The  #1 most important thing to measure. This is the measurement of hardness-related ions in the water, particularly Ca2+ and Mg2+. Typically recorded as  the amount of calcium carbonate (CaCO3) equivalent in parts per million (ppm).

pH: This is a measurement of the concentration of hydrogen ions in water which gauges the acidic or basic nature of the water, on a scale of 1-14.

Alkalinity: Is the measurement of a water’s ability to resist pH change but is also the amount of dissolved alkali in water. Hydroxyl Alkalinity is necessary for corrosion control and scale prevention.

Dissolved Solids: Any minerals, salts, metals, cations, or anions that are dissolved in water. Total dissolved solids (TDS) are the amount of organic and inorganic materials dissolved in a given volume of water. In industrial settings, this is often expressed through conductivity, measured in micro ohms either through a hand held meter or a sensor read through the controller.

If you are having additional problems controlling scale, corrosion, biological growth or other fouling, then you should dig deeper with the following parameters:

Turbidity: The cloudiness or opaqueness of water, typical symptom of suspended solids and impurities

Dissolved Iron: Mainly present as Fe(OH)2+ (aq) under acidic and neutral, oxygen-rich conditions

Dissolved Gasses:  Gasses that have been incorporated into a liquid. The gas molecules are attracted to the water molecules, which causes the gas molecules to become surrounded by water molecules

Regardless of the specific process, it is critical to understand the water quality of your system at any given time. Second to water quality is understanding the rate at which you need makeup water. Providing high quality water is still inefficient if the required volume is insufficient.

Investing in High Quality Makeup Water

 The most common pretreatment technology employed to improve makeup water quality is water softeners. As towers run longer without blowing down, the facility will save money by using less makeup water and also significantly less chemicals such as scale inhibitors (since less is being flushed down the drain).  While water softeners for cooling towers can often have payback periods of below two years, they do require additional considerations to operate safely and effectively. Specialized ion exchanges can also be used to achieve even higher makeup water quality (and thus  water efficiency gains), but the return-on-investment is less certain than softeners.

Another easy investment you can make to better manage makeup water quality,  apart from pretreatment, is in control automation. Bleed points are generally set to a specific conductivity level for the recirculating water. However, the conductivity of the makeup water can show significant variance, even daily, in many municipal water systems and may not be related to hardness – our enemy #1. For example, if city water conductivity spikes, it may be appropriate to adjust bleed set points to a higher conductivity to avoid wasting water and treatment chemicals. By adding a conductivity sensor on the incoming water, several modern controller models can automatically adjust the bleed set point to avoid bleeding off at a set point that is no longer appropriate. This type of control can also provide valuable protection in the opposite scenario, when the conductivity of the city water decreases, but other scale-forming water quality parameters such as hardness and alkalinity do not change to the same degree.

 A full review of your treatment process may be required to ensure the performance and longevity of industrial process equipment where scale can be a concern. Contact us today for a free consultation.