Dan Messinger PE, PMP, Senior Project Manager, SSOE Group and Michael Eschenbrenner, Senior Mechanical Designer, SSOE Group (Aug/Sep 09)
In 2008, our firm published the results of a survey collecting data on sustainability goals across the industries we serve. The results indicate that overall, 45 percent of companies surveyed have formalized sustainability goals, and that those goals are reducing energy consumption; recycling; achieving sustainable facility design; and using energy-efficient products and equipment. Sustainable design, whether it relates to a facility or process, has become such a universal goal that instead of asking, "Should we.?" companies are asking, "How should we.?"
Food manufacturers and processors have discovered opportunities to execute a wide range of "green" initiatives, from "5,000 feet up" strategies such as energy and sustainability master planning and LEED certification to improving immediate results by managing utilities. Water and natural gas, in particular, are typically used in abundance in older plants. Yet there are effective strategies for sustainable management of these resources.
Cooling Water Usage
Thermal food processing plants can benefit from reuse of water and waste heat from their processes. Energy audits of frozen food plants, for example, often find a pattern of high water usage, both in processes and for cleanup operations.
Here is a case in point. In a ready-to-eat soup plant located in the Midwest, the once-through process they were using for immersion cooling of filled, sealed soup cans used approximately 2 million gallons of water per day. Engineers enabled the plant to reuse that water through design and implementation of a recirculation system. Specifically, the process cooling loop system recovers process cooling water after use and pipes it to plate-and-frame heat exchangers, where it is indirectly cooled by a cooling tower; if necessary, the water can be further cooled using chillers. Cooled water is then returned to the process. The capital cost of the system was approximately $4 million, and the project met the 2.5-year payback/40 percent return on investment (ROI) benchmark.
Another source of significant water usage and, even more important, change-over time is the use of low-pressure wash systems to clean food processing equipment. In many cases, engineers have replaced low-pressure wash systems with high-pressure wash systems, achieving a significant reduction - as much as 75 percent - in change-over time.
Returning Heat to the Process
As in the case of the soup plant, a Midwest condiment manufacturing plant cooled its batch product using a heat exchange system with once-through city water as the heat exchange medium; however, the resulting hot water went right down the drain. The utility cost of the water was $2.30 per thousand gallons; the sewer cost added another $2.35 per thousand gallons. In this case, use of a recirculation cooling loop saved $4.60 per thousand gallons of water. In this plant, the cooling-process improvement afforded an added benefit: the cooling water, which comes off the final step of the cooling process at 160 degrees F, is now piped to another heat exchanger to preheat the ingredients at the front end of the process, saving on the cost of natural gas. This $1.25 million project saves approximately $500,000 a year, achieving a 40 percent ROI.
In another food plant, in this case, a soup plant in the South, a heat exchange system was designed to take residual heat off the top of boiler stacks, using condensing stack economizers to recapture the latent heat in water vapor and use it in the manufacturing process. This system saved the owner $800,000 in the first year of operation.
More recently, engineers have identified an opportunity to redesign a vacuum cooling operation that utilizes steam educators and steam pumps as the vacuum generator, to vacuum pumps and low temperature chillers, the end result being a significant reduction in natural gas usage.
Part of the challenge of these types of projects in existing plants is to control the negative impact on plant production, particularly during installation. Smart project planning results in lower total cost and reduced downtime for the owner. In all of these cases, the engineering team successfully avoided operational interruptions during installation and production startup.