Many plants rely on water for cooling, e.g., to control reaction rates, recover light ends from a distillation column, maintain vacuum systems, and lower the temperatures of product streams. As with hot water (www.chemicalprocessing.com/articles/2011/save-energy-in-water-systems-part1.html), opportunities often exist to cut energy consumption. Here, we'll look at five tips for achieving energy savings
Tip 1: Maintain cooling water quality. Checking the water chemistry can give an idea of the required blowdown for the cooling water system. In one chemical plant the blowdown level was so low that most of its heat exchangers got fouled within a few weeks of cleaning. The well water being used to make up for evaporation losses was the culprit. Once the blowdown level was increased, the fouling rate in the heat exchangers dropped. This allowed the plant to reduce the number of operating pumps for cooling water circulation while achieving the same cooling targets.
Tip 2: Stop circulating cooling water when not needed. In a large chemical plant in Alabama, we found that 30% of its heat exchangers were circulating the tower water even when the process side of the heat exchanger was stopped. So, the plant engineers asked operators to shut off cooling water flow whenever an exchanger wasn't in service. Eliminating the unnecessary cooling water circulation enabled shutdown of at least one pump. At a pharmaceuticals and fine chemicals plant in Michigan, heat exchangers on the third floor of a process building sometimes suffered from inadequate cooling water pressure. Closing the cooling water supply lines of some heat exchangers on the lower floors not in cooling mode improved the supply pressure.
Tip 3: Consider staged cooling. At a pharmaceutical plant, we found that a newly added production building relied on chilled water to cool compressed air and for several other exchangers. The plant didn't install cooling water supply and return lines to avoid a cost over-run on the project. Management was surprised when we pointed out that operating the chiller for ambient level cooling is about three times more expensive than using cooling water. So, the plant made the necessary piping changes to send cooling tower water to the air compressor and other exchangers. This decreased the chiller load, which, in turn, led to a net reduction in cooling tower load. Overall energy use for process cooling also came down.
Tip 4: Don't reject heat to the cooling water system if a suitable process heating application is available nearby. Air compressors and chillers that are water-cooled and trim coolers handling substantially higher temperature process streams are good candidates.
Tip 5: Look for alternatives to rejecting heat to the circulating cooling tower water. The ground water in regions close to the Great Lakes is good quality and available throughout the year at temperatures of about 60°F. As many plants in states like Michigan have found, you can directly use this water as once-through cooling water and then send it for other process applications. It may suit modest process cooling loads like air compressors and smaller chillers.
Automatically controlling cooling water temperature can optimize fan power consumption in a cooling tower. When cooling demand decreases, less water flows through the partially loaded cooling tower. Such conditions also may require less airflow and, hence, lower fan speed. In one chemical plant that uses dual-speed cooling tower fans, when seasonal cycles permit and cooling water well temperature reaches the set point, the automatic control system changes to the lower fan speed. The same controller also may stop some fans in the multiple-cell cooling tower. These steps provide measurable energy savings. However, when cooling towers form part of a chiller train, lower fan speed may boost the load on the chiller compressor — sometimes resulting in a net increase in power consumption. So, carefully evaluate negative effects before stopping or reducing the speed of cooling tower fans.
A health-care-products manufacturer in Virginia was operating a large fountain in the middle of a pond in front of its main office building. It was a closed-loop system with a recirculation pump and a filtration system before the pump to remove any particulates brought in with rainwater collected in the pond. We recommended using that water as the cooling water for the condenser of the building's air conditioner whenever it is in service. Doing this allowed the site to reduce operation of a cooling tower installed exclusively to cater the air conditioner. Process engineers may find similar alternative cooling opportunities in their plants, too.
VEN V. VENKATESAN is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@putman.net