Green Series: Good to the Last BTU
November 7, 2012
For organizations able to commit capital to energy recovery systems, the realized efficiencies are the gift that keeps giving.
A been-there, done-that look crosses the faces of mechanical engineers when the topic of energy recovery and reuse comes up, and on one level, that is understandable.
If thermal energy is going to be recirculated, for example, it’s a good bet that a heat exchanger will be involved. Other than selecting the type of heat exchanger and whether the exchange should be air-to-air, fluid-to-fluid or a combination, system design is more plumbing work than engineering challenge.
A fresh perspective can change that, though, as was the case at Summer Garden Foods Manufacturing Co. The Boardman, OH facility, commissioned in 2008, was designed with an eye toward LEED certification. Empty bicycle racks and reserved parking for hybrid vehicles racked up LEED points. But the advancement that keeps on giving is a heat recovery system that defies conventional wisdom.
Minimizing pressure drops in the exchange and maximizing Delta T is the standard approach to design, but that sacrifices thermal change on the cooling side, notes Darrell Wallace, an assistant professor of industrial and systems engineering at nearby Youngstown State University who provided engineering consultancy on the Summer Garden project. By significantly slowing down the glycol flow rate through a plate and frame heat exchanger, 30 tons of waste heat a day is diverted and reused.
“We had big pressure drops, but the extra pumping horsepower was offset many times over by the energy savings,” says Wallace. “Most of the waste heat you get out of processes [with conventional recovery systems] is very low grade. We took a blank sheet approach to designing the system,” viewing heat recovery as an element of an integrated system, not a supplement. Suppliers of pasteurizers, cook kettles and other equipment had to alter their piping to accommodate the pressure drops and other changes, though some embraced the re-engineered designs and adopted them as standard elements.
Arrowhead Systems Inc., the Oshkosh, WI OEM that supplied Summer Garden’s pasteurizer/cooler, upgraded its pump motors to variable frequency drives and resized the pumps and impellers. The VFDs cost about $100 more than conventional motor starters, but the new pumps cost less, resulting in “a net zero change,” Arrowhead’s Sales Manager Jeff Kaplan says. “It wasn’t a 1,000-hour reengineering job,” he says, “just one day reviewing pump curves with Dr. Wallace and changing out pump selection and impeller selection.”
“We used conventional wisdom in sizing components,” Wallace adds. As a result, Summer Garden has a large mechanical chiller that loafs or idles much of the time. Chiller specifications were a hedge against theoretical expectations and actual performance. “At this point, I would be more amenable to using a multi-stage loading chiller that runs efficiently at low capacity,” he says.
Three on a therm
The food sector was the industrial leader in cogeneration in the 1980s, the last time the nation was serious about reducing dependence on foreign oil in the aftermath of the Arab oil embargo. In recent years, declining natural gas prices have dampened what little enthusiasm remained for cogen, but some food companies now are considering trigeneration, leveraging the energy potential of gas turbines to produce electricity, hot water and refrigeration.
Lexington, KY-based Gray Construction invested six months of design work on a trigen system for a beef processor in the Southeast. A decision was expected this month on whether to proceed with the trigen approach, with capital costs, not technical feasibility, the biggest obstacle. If trigen gets a green light, the plant will be off the grid when production begins in May, according to the client’s general manager.
Gas-fired turbines would generate the facility’s electricity at a cost less than half the rate charged by the local utility, says Michael Rach, Gray’s director of EPC, but that’s a cost efficiency. Energy efficiency resides in the two-thirds of the energy potential that is otherwise lost as waste heat. Some of the reclaimed energy will provide process and domestic water, plus space heating. The rest will power absorption chillers that replace evaporative condensers and cooling towers in standard mechanical refrigeration. Lithium bromide desiccant would be part of the absorption cycle, with an ammonia chiller completing the cascade refrigeration system.
The rule of thumb holds that only 30 percent of the energy input to the electrical grid is consumed by the end-user, says Rach. “We’ll make use in excess of 60 percent of the energy inputs.” Additional equipment costs would stretch ROI out over nine years. Normally, that kind of payback period would make trigen a nonstarter, but the client faces stiff community opposition for environmental reasons and is interested in burnishing its reputation as a forward-thinking organization.
Efficiency vs. quality
A three-year payback is enough to make most capital projects a nonstarter, though sustainability initiatives tied to corporate social responsibility programs are giving food engineers the latitude to consider energy recovery projects that otherwise never would be funded, points out Dan Poirier, director-process engineering at Buhler Aeroglide Corp., Cary, NC. “We have put in energy recovery systems with five-year ROIs, provided there were no risks on the product quality side,” he says.
Conveyor dryers account for most of the Buhler Group division’s output. “Dryers are huge consumers of energy,” Poirier readily allows, and ready-to-eat cereal is one of the food segments where Buhler dryers can be found. As big as they are, cereal dryers are too small to justify an energy recovery system, he says; besides, those manufacturers are more focused on product quality and sanitary considerations. Sectors such as pet food and fruit and vegetable drying are more fertile ground for energy reuse, in part because the dryers are magnitudes bigger. Air-to-air heat exchange to warm makeup air to 140°F is fairly common on those applications, though “there are issues with doing that,” he points out.
The main problem is fines and oils in the exhaust that can build up in the heat exchanger. Air-to-water heat exchange helps, but that doesn’t completely resolve the fouling issue. If there is a product cooling step after the dryer, “spent cooling air can be routed back to the dryer” to improve performance, Poirier says. “There’s no heat exchanger, just a little duct work to bring it back to the dryer.”
The portfolio of Mary Frances Stotler of Springfield, MA’s The Dennis Group includes recovery systems. “The paybacks are not what they used to be” when natural gas was involved, concedes Stotler, the engineering firm’s sustainability coordinator; on the other hand, the ability to validate the economic benefits has helped jump start a number of projects that languished when “best practices” was the main justification.
Upfront quantification of water recirculation’s benefits made the project a no-brainer. Water chilled to 33°F was used to wash lettuce. It was then filtered and pumped through a plate heat exchanger, where it lowered the temperature of incoming municipal water 17°F. Only another 5° of heat had to be removed. With 250-325 gallons of water per minute coursing through the loop, engineers calculated the system saved about 1.9 million Btus an hour, an annualized savings of approximately $79,000.
The Dennis Group has engineered a number of air-to-air heat recovery systems involving compressed air, but efficiency pales in comparison to Atlas Copco’s oil-free and water-cooled rotary screw compressors, Stotler concedes. Heat transfer in water “gives you much more flexibility in where you can use the heat,” points out Tom Poot, business line manager-oil free products, Atlas Copco Compressors Inc., Rock Hill, NC. Waste heat that is not captured in the water can be recovered by an optional desiccant dryer, which requires virtually no power and delivers extremely dry air.
The first installation of the firm’s CarbonZero system occurred in 2010 at Hormel Foods’ Dubuque, IA plant (see “Hormel’s Progressive Processing plant is built for the long haul,” Food Engineering, December 2011). Boiler feed, sanitary cleanup and space heating are among the uses of the hot water. Reduced maintenance of the compressor itself is a fringe benefit.
Poot estimates one-third to one-half of the oil-free compressors Atlas Copco is putting into service have the water-cooled feature, though the ratio is lower in North America than in Europe, where the company is based. “We were doing energy recovery back in the ‘50s,” he boasts, and improvements have been more evolutionary than revolutionary. For example, the temperature of the water delivered by the system is gradually getting hotter, reaching 190°F in some cases. Poot seconds Stotler’s point about quantified savings. “We can calculate the recovered energy, we can measure it, we can prove it,” he says. “A lot of the time, the business case is obvious, and the returns are staggering.”
Synergies can accompany energy reclamation, as demonstrated by the water-cooled, stainless-steel motor developed five years ago by Stainless Motors Inc. (see “Water cooled and stainless,” Food Engineering, February 2009). Chief Engineer John C. Oleson developed the concept at the request of Beef Products Inc., the Dakota Dunes, SD processor that produces lean ground beef from trim. Heated water pulled from BPI’s 400hp electric motors is about 120°F and provides an overabundance of boiler feed. A bigger benefit is reduced maintenance. Bearing failure was a frequent event when air-cooled motors drove ammonia compressors during long, hot summers. Oleson eventually installed 11 water-cooled motors, “and every one of those motors is still on line,” he asserts. “They will pay for themselves on the maintenance savings alone.”
“Several dozen” water-cooled motors manufactured by his Albuquerque, NM firm are providing power in food plants currently, but the piping, pumps and controls needed to support them discourage wider use. A listless economy hasn’t helped: An order for a 750hp unit was canceled when credit tightened. Nonetheless, Oleson sees a bright future. “One day, the large water-cooled motor will be common,” he predicts.
A waste-steam recovery system for saturated steam retorts from Allpax Products Inc., Covington, LA, is another example of apparently can’t-miss investments that defy easy calculation of financial savings. The system, which General Manager Greg Jacob wryly describes as “a pipe with a sprinkler system inside,” captures the steam vented from retorts during the come-up, vent-open cycle. The steam is condensed and stored in a tank for auxiliary use. Otherwise, it would be exhausted to atmosphere. If a manufacturer has a bank of half a dozen or more retorts, the payback seems intuitive. But intuition is no substitute for a solid ROI calculation, and the calculation is complicated.
Only one steam recovery system has been installed since Allpax introduced the concept last year. The manufacturer uses the hot water to heat gravy during the cooking process. “They had enough engineering staff to do some in-depth calculations,” says Jacob. “Everything is based on ROI, and they exceeded their numbers.” Most companies lack the engineering bench strength to measure variables such as pipe size, the degree of opening of valves and the point at which steam forms inside the retort. “It’s not a simple formula,” he concludes. “If it were more user-friendly, there would be a lot more systems going in.” Subsidies from government and utility companies can lower the ROI hurdle, but quantifying the financial savings from energy recovery is an ongoing challenge.
It’s a deal breaker for big ticket projects like biogas systems, which can introduce unanticipated levels of complexity (see related story on page 58). Intangibles such as goodwill and customer relations can’t be computed on a spreadsheet, but they can extend the payback period. “I hear, ‘It’s the right thing to do’ a lot,” says Robb Raney, project manager with Kansas City-based Burns & McDonnell engineering. In some cases, the right thing to do is defined by key customers that expect their suppliers to implement sustainable practices. Prodding in the name of sustainability doesn’t exempt energy recovery from realizing a return, but instead of two to three years, a manufacturer might stretch payback to four to 10 years, Raney says.
Many factors influence a decision to proceed with an energy reclamation initiative. Ideas that look great on paper may not survive closer scrutiny, and a convenient application for the recovered energy is a must. But interest in pursuing these projects is growing. As energy costs rise, and vendors and plant managers get a better grasp of the financials, recovery loops will become more common in food and beverage manufacturing.
The risk in energy recovery
Big capital projects usually stretch out the timeline for financial return, which is why third-party financing and operation of energy systems are gaining favor. Those arrangements carry risk, however, and the risk is borne by both the partner and the food company.
A biogas system at Coolbrands Dairy in North Lawrence, NY was the toast of the state’s Department of Environmental Conservation when it came on line in fall 2006. Built, owned and operated by Ecovation Inc., the anaerobic digester produced 150 million Btus of methane a day, replacing a third of the energy inputs needed to run the yogurt plant’s steam boilers. The dairy paid Ecovation for the gas at a cost lower than the 250,000 gallons of No. 6 fuel the methane replaced, Ecovation received an acceptable return on its $3.3 million investment, and the business model was applied to similar projects after Ecolab Inc. acquired Ecovation in 2008.
The win-win fell apart in April 2011 when the plant closed, and 132 workers were laid off. Upstate Niagara Cooperative Inc. subsequently purchased the plant and spent several million dollars on renovations before restarting production in October 2011, but not before the biodigester was decommissioned. Tanks, sensors, feeders and controls were removed by Ecovation, leaving a building shell and no prospects for restarting the system.
“The system became upset very easily if there was any change in the feed stream, but it produced a decent amount of gas for our boilers,” according to Matt Davis, plant manager of the renamed North Country Dairy. Despite the technical success and financial viability over four and a half years, the project underscores the vulnerability of long-term energy partnerships to unanticipated events.
For more information:
Greg Jacob, Allpax Products Inc., 985-893-9277, firstname.lastname@example.org
Jeff Kaplan, Arrowhead Systems Inc., 708-386-9770
Tom Poot, Atlas Copco Compressors Inc., 803-817-7000
Dan Poirier, Buhler Aeroglide, 919-851-2000
Robb Raney, Burns & McDonnell, 816-333-9400, email@example.com
Mary Frances Stotler, The Dennis Group, 413-858-3417, firstname.lastname@example.org
Michael Rach, Gray Construction, 859-252-5300, email@example.com
John Oleson, Stainless Motors Inc., 505-867-0224, firstname.lastname@example.org
Darrell Wallace, Youngstown State University, 330-941-3272, email@example.com