As The Simpsons' Charles Montgomery Burns can attest, great power rests with those who generate power. Some of the utilities found in food and beverage plants qualify as localized power plants, with all the advantages and attendant challenges that Mr. Burns’ nuclear plant faces.
Steam power is a prime example. Steam is a very efficient heat-transfer medium, and balancing output with demand determines how cost effective a plant’s system is. That goes well beyond the stoichiometric efficiency of the boiler itself or the efficiency of piping, distribution and condensate return. Imbalance in any of those elements drives down overall system effectiveness. Steam flow is another variable that needs to be monitored and controlled, and that can be a challenge at low flow rates. Differential pressure devices suffer from a drop in accuracy when pressure falls below a certain point.
To overcome this limitation, Spirax Sarco Engineering Plc developed the target variable area (TVA) flow meter for saturated steam measurement. The unit has built-in density compensation, explains Mark Booker, manager-energy monitoring and management at the Blythewood, S.C.-based company. This eliminates the inherent inaccuracy of using the square root relationship between differential pressure and flow rate to calculate turndown flow.
Another advantage is TVA’s compact space requirements: While an orifice plate flow meter requires 20 pipe diameters of straight pipe upstream and three diameters downstream, TVA only needs six diameters upstream and three downstream.
“It’s a simple plug-and-play device, but the big advantage is reduced straight-run requirements,” says Booker. That allows installation at kettles and other steam-powered equipment, allowing system operators to get a clearer idea of where demand is coming from and how much is needed. Reduced operating cost is the ultimate benefit of more accurate flow measurement, even in 50:1 turndown scenarios.
Simplified use also is a plus, given the shrinking pool of steam-power experts available to manage systems. Ex-Navy engineers were a ready pool of steam operators, but a live steam-powered U.S. naval vessel hasn’t been commissioned since 1979. As the last of those seaman/operators approach retirement, industry is searching for replacements and looking for help to simplify operations.
Operational efficiency was a nonissue until natural gas prices shot up around the turn of the past century. Although fracking has ushered in a new era of cheap gas, system optimization remains a priority. Years ago, if 50,000 lbs. of steam per hour were needed, plants would generate 70,000 lbs. and install enormous boilers. Those boilers now are relics, replaced by batteries of micro-boilers that can be fired as needed, observes Scott Knight, director-engineering and development at Armstrong International Inc., Three Rivers, Mich.
Lower operating pressures are an industry trend that is forcing manufacturers to rethink efficiency strategies, such as placement of economizers on exhaust stacks, Knight observes. Lower pressure means fewer Btus going up the stack, resulting in less waste heat to recapture.
Many alternatives exist, with “good piping practices” at or near the top of Knight’s list. One example is re-use of condensate. “Condensate is like gold,” he says, yet many plants continue to dump it into drains.
Returning hot, pre treated condensate to the boiler typically reduces energy inputs 15-16 percent, but faster system start-up may be a more important benefit. One food processor estimated his plant gained more than 5 percent uptime as a consequence of a return-system installation, according to Knight.
New efficiency options
Too much condensate can result in water hammering, a potentially dangerous situation that increases maintenance costs. Hammering also can be an issue with compressed air, another plant utility that could be considered a local power system.
When Diageo Plc expanded its Plainfield, Ill., packaging hall six years ago, compressed-air buffer tanks were placed at strategic locations. One benefit was elimination of hammering when pressure surges occur, but the bigger payback was reduced system pressure. The plant was able to drop from 125 psi to 85 psi, slashing annual energy costs well into the six figures.
Buffer tanks still are used in industry, but “nowadays, people are moving to variable speed drive compressors,” reports Deepak Vetal, product marketing manager-oil-free screw compressors for Atlas Copco LLC, Rock Hill, S.C. While buffer tanks reduce banging during compressor cycling and lower energy consumption, there still is energy loss during machine unloading, points out Vetal. Higher energy prices and lower VFD costs are obsolescing that approach.
“Everyone knows compressed air is one of the top energy-consuming utilities in the plant, so any changes in how it is used are a good thing,” he adds. An example is the recirculation of waste heat from the compressors to drive desiccant towers that remove moisture from compressed air. Dryers traditionally drew their own power, but the inclusion of systems that capture latent heat from the compressors for that task now is a common feature on many compressors.
The vast majority of rotary screw compressors in operation are oil-flooded, with only a few industries, food and beverage included, applying air-cooled units, notes Michael Camber, marketing services manager at Kaeser Compressors Inc., Fredericksburg, Va. Some food companies have standardized on oil-free, and although Kaeser offers those units, he suggests the differences in air quality are minimal to nonexistent.
Regardless of the compressor used, compressed air must be dried and filtered before it is sent through the lines. Oil-flooded units meet ISO Class 1 air-quality standards, which caps oil levels at 0.01 ppm. Oil-free compressors may meet the user-defined Class 0 standard of 0.001 ppm.
“Here’s the problem: you can’t measure that level, and you can’t get all the oil out anyway,” Camber says. Using food-grade lubricants in an oil-flooded compressor is a more economical and realistic approach, he argues.
Oil-flooded units also run cooler. The inherent inefficiency of air-cooled compressors can be lessened by adding a water bath. Water coming out of the bath can top 140°F, Armstrong’s Knight claims, making it an excellent feed source for boilers. That becomes a plumbing project, and Armstrong, Spirax Sarco and Atlas Copco increasingly are getting involved in those types of projects in food and beverage plants.