Poor water quality is draining your process cooling efficiency. How can you regain this lost ground? You guessed it, with filtration!
Process cooling engineers are constantly striving for improved efficiencies — and for good reason. Most process cooling systems consume approximately 60 percent of a building’s energy, and seemingly modest gains in efficiency can yield both significant and measurable effects.
Process system designers typically focus on optimizing elements of the actual cooling system. For example; engineers have re-designed the cooling tower fan blades, upgraded the current chiller tube technology, and introduced many other innovations that have yielded improved operating efficiencies. These optimizations have caused notable and important gains in efficiency among the industry.
Water quality is one important aspect of cooling systems that is many times overlooked. Many incorrectly assume that once water reaches the array of systems, the quality will be under control. While this is true to a certain degree, what happens to the wide range of suspended solids? The substances picked up by water flow after it enters the system. Such substances include:
- Corroded metal particles
These suspended solids can wreak havoc on a process cooling system and drastically reduce efficiencies.
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 meets metal pipes. Over time this collection of dust and other fine particles in cooling systems form a layer of scale. This scale is not only unpleasant — it is expensive to resolve! In a 3,500-ton chiller operateing year round, a buildup of only 0.01" (300 µm) of scale can result in an increased operating cost of $100,000 per year.
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 noted above. Furthermore, these deposits can result in the fouling of equipment, which can be costly 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 cross-flow microsand filtration system that can capture submicron particles. A microsand filtration system combines a cross-flow conditioner and microsand filtration in the same vessel, allowing 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.
Paper Mill and Pharma Plant
In these examples, two real-world process plants added a cross-flow 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 cross-flow 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 filter 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 a 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. High efficiency cross-flow microsand filtration technology was able to help the plant achieve the desired water quality.
A large American brewery approached a filtration system provider inquiring about using cross-flow 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 from the process water, and the other from the evaporator condensate. The filtration system was required to handle each stream independently or at times combined, which provided a unique challenge for the application engineers.
The differing TSS loads of the two streams of water was 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 cross-flow microsand filtration system that met all the above requirements. The system was capable of being activated as needed and was designed to handle flows from 80 to 350 gal/min.
Regardless of the chosen solution, when it comes to gaining and sustaining efficiency, addressing water quality is vital.