Process cooling engineers are constantly striving for more efficient systems — and for good reason. Most process cooling systems consume approximately 60 percent of a building’s energy, and seemingly modest gains in efficiency can have significant, measurable effects.
System designers typically have focused on optimizing elements of the process cooling systems themselves. For example, engineers have improved fan blades for cooling towers, designed improved tubes for chillers and many other innovations. These optimizations have caused notable and important gains in efficiency.
One important aspect of cooling systems that designers have overlooked, however, is water quality. Many engineers incorrectly assume that once water reaches the systems, the quality will be under control. While this is true to a point, many forget about suspended solids — the substances picked up by water flow after it enters the system. They include:
- Corroded metal particles.
These suspended solids can wreak havoc on a process cooling system and destroy hard-won efficiency.
Dust Particles. Chemical programs are excellent at purifying water before it arrives at the cooling system. However, cooling towers have gaps and vents that allow air and dust into the water stream. These suspended solids can diminish water quality and reduce efficiency. A cooling system with a cooling capacity of 1,300 tons can let in approximately 147 lbs of dust in 90 days.
Additionally, water precipitates calcium carbonate when it comes in contact with metal pipes. This in turn collects in cooling systems and forms scale. This scale is not only unpleasant — it is expensive. In a 3,500-ton chiller operated year round, a buildup of only 0.01" (300 µm) of scale can result in an increased operating cost of $100,000 per year.
Biofilm. Once scale accumulates, biofilm begins to grow in a cooling system. Composed of bacteria and proteins, biofilm can cause illness by fostering Giardia and other dangerous organisms. Additionally, film buildup can cause many of the same efficiency problems as scale collection. A cooling system with a substantial accumulation of biofilm can lose up to 30 percent of its operating efficiency.
Corrosion and Fouling. Biofilm also is a factor that contributes to corrosion. Affected by water temperatures, water velocity, residence time and metallurgy, corrosion is one of the major causes of reduced equipment life. Corrosion also releases sediment into the water stream, which is subsequently deposited to form scale. This scale leads to the efficiency problems already noted. Furthermore, these deposits can result in the fouling of equipment, which can be expensive to repair.
Dealing with Suspended Solids in Process Cooling Systems
Because most suspended solids find their way into the water stream once the water has actually entered the process cooling system, the best solution is a post-hoc removal solution.
Most of these suspended solids (approximately 95 percent, depending on the system) are smaller than 5 µm in size. This means many of them are not caught in traditional filtration systems. Therefore, it is necessary to find a system that can either clump or filter particles.
For example, one might use a chemical coagulant in conjunction with a traditional filter to clump small particles into larger particles and remove them.
Another alternative is a crossflow microsand filtration system that can capture submicron particles. A microsand filtration system combines a crossflow conditioner and microsand filtration in the same vessel, which allows for high filtration efficiency. This approach to filtration also results in less water needed for backwash, making it useful as a side-stream filtration solution for cooling system water.
Real-Life Examples: Paper Mill and Pharma Plant
In these examples, two real-world process plants added a crossflow microsand filtration system to help address water quality in process operations. Coincidentally, both a large American paper mill and a leader in food and pharma approached a filtration system provider inquiring about crossflow microsand filtration technology. Both applications were for pretreatment of a reverse-osmosis membrane filtration system. Designers had evaluated several other technologies, including ultra-filtration and cartridge filters, but the microsand technology offered the filtration performance and footprint the designers were seeking.
Fouling of reverse-osmosis membranes can seriously affect the filtration system. A proper pretreatment can help to extend the life of reverse-osmosis membranes and ensure the filtration system is operating at the designed efficiency. The best available technology for determining the fouling potential of reverse-osmosis inlet water is by measuring the silt density index (SDI).
SDI measurement must be taken prior to designing an RO pretreatment system. Raw-water SDI values averaged close to 8 at the paper mill and just over 4 at the food and pharma plant. The filtration challenge was clear. Microsand filtration was able to help the plants achieve the filtration desired (figure 1).
Real-Life Example: Brewery
A large American brewery approached a filtration system provider inquiring about using crossflow microsand filtration technology to reduce the total suspended solids (TSS) load in water that was being reclaimed from both processes and evaporator condensate.
This project was unique in that there were two different inlet water flows. One came from process water, and the other would come from the evaporator condensate. The filtration system needed to be capable of handling each stream independently or combined, which provided a unique challenge for the filtration engineers.
The differing TSS loads of the two streams of water were also a challenge. From a laser particle count analysis, it could be seen that more than 90 percent of the TSS load was larger than 30 µm in size.
The filtration system provider was able to design and produce a crossflow microsand filtration system that met all the above requirements. The system was able to be activated as needed and was designed to have the capability to handle flows from 80 to 350 gal/min (figure 2).
Regardless of the chosen solution, when it comes to gaining and sustaining efficiency, addressing water quality is vital.