The Effects of Time, Temperature, and Concentration in Chemical Reactions
In recent articles, we examined the importance of determining and controlling cycles of concentration in cooling water and boiler systems. We learned that at high cycles of concentration, treatment programs can break down and fail to protect the systems. To address this problem, it is important to keep in mind that chemical reactions in these systems are governed by three main factors: time, temperature, and concentration. Exceeding any one of these factors can lead to problems. Therefore, maximum cycles allowed in systems need to be determined and then met.
Time: For cooling water systems, time is often represented by a system’s Holding Time Index (HTI). This is the quantitative determination of the half-life of water, as well as any substance, in a cooling water system. In a cooling tower, the treatment chemicals will break down over time, especially when they are exposed to oxidizing biocides and other stresses, like heat. I had two similar cooling tower systems on the same make-up water, treating almost identical processes. One system was not a problem to treat, while the second system formed scale in heat exchangers. The only difference was that the first system had a standard-sized basin and the second system had a much larger basin than needed. [The engineers for the company decided the larger basin would protect the system better.] The larger basin extended the time the water was in the system (HTI) two-fold. The result: The treatment program in the second system broke down and failed.
In boilers, time is best represented by the cycles of concentration (CoC). (You could do an HTI for a boiler system, but this is rarely considered.) The blowdown is limited in an effort to mainly save money in fuel costs, but once you get beyond 50 CoC the fuel savings are marginal. I knew of a boiler system makeup which was changed from softened water (TDS ~200 ppm) to demineralized water (TDS <1 ppm); however, the conductivity limits were not changed on the blowdown settings. The result: The boiler cycled up so much that blowdown was almost nonexistent, the polymer treatment chemical failed from being in the boiler too long, and tubes failed due to scaling. [Note: Even though you may not see “hardness” in a feedwater, hardness is still there in trace (ppb) levels.]
Temperature: A generality (but not an absolute) is that chemical reactions double with every 15°F increase in temperature. This is why temperature is an important consideration when designing a cooling tower treatment program. As the cooling water absorbs heat (usually 10°F‒20°F), the water saturation index (LSI or RSI) changes. Additionally, “surface temperatures” in heat transfer equipment will cause localized changes in scaling tendencies.
In boiler house systems, the cooling water temperatures in lube oil coolers and the compressor’s coolers can be much higher than temperatures experienced in the condensers. These auxiliary pieces of equipment can see scale and fouling before the condenser sees scale. For boiler waters, we see scale forming much more quickly on the hot side of the boiler tube than on the cold side.
Concentration: Cooling systems with a low level of minerals in the make-up water can handle higher CoC than systems with a high mineral concentration. High concentrations of calcium and carbonate alkalinity lead to scale formation, and high concentrations of minerals, like chlorides, can accelerate corrosion. Concentration is a major factor in the saturation index calculation (LSI/RSI).
I would like to leave you with this analogy: My family lives in Baltimore, and my wife decides she wants to visit friends and family in California during the school summer break. We pack the two boys, along with enough luggage to carry two weeks of clothes for all, into the family SUV for a trip across country. We immediately have “concentration” — four people and tons of luggage in an SUV. As we get into the second day, we start to feel the effect of so much “time” spent on the road. As we enter St. Louis on our third day, the air conditioning fails in the SUV. We now have “temperature”! Is there an adverse reaction? You bet!
Chemical reactions are affected by time, temperature, and concentration. If a system is not behaving properly, keep in mind that it just might be because one of these factors is out of sync.
