Electrical energy savings in flour milling
September 1, 2008
Modern gradual reduction systems for wheat milling and other grain processing are major expenditures for millers converting grains to basic ingredients. While it is a major component of mill conversion costs, it can be very difficult to manage and control these expenses.
That’s why it is important for mill managers to understand some introductory concepts of electrical energy management.
World total net energy consumption in billions of kilowatt hours is shown in Table I (page 47), as reported in the International Energy Annual 2005 by the U.S. Energy Information Administration. Energy consumption by country is quite varied depending on population and cultural practices with respect to consumption and conservation.
Energy use in milling extends from the beginning to end of the transformation process. Grain must be taken into the facility and delivered to storage bins. From the storage bins, it may be blended, cleaned and conditioned. The milling of grain is energy intensive no matter what type of grain is being processed. Finally, the grain processor must package and load out both the finished products and byproducts.
According to a report prepared by the United Nations Food and Agriculture Organization and the European Bank for Reconstruction and Development in October 1999, energy costs for the French milling industry were the third largest expense at 7.5%, behind labor-related charges and management/administration/office charges at 40.2% and 33.1%, respectively.
According to a report on the manufacturing industry prepared by the U.S. Census Bureau in 2005, energy use in the United States was approximately 4 to 7 kilowatt hours (KwH) per cwt of flour produced. Using the average cost of 6¢ per KwH, the total energy cost is 24¢ to 42¢ per cwt, or approximately $4 to $7 per tonne of wheat milled. Looking at these figures, it’s easy to see why energy consumption is a major part of mill conversion costs.
Energy costs can be reduced through demand reduction. The startup of a milling unit requires a considerable energy draw to overcome inertia. This energy must be must be provided and made available by the energy supplier. The energy suppliers must be committed to providing the electrical energy while insuring demand for neighboring facilities, business or homes. Managing operations and start-up sequence and time can be a start in reducing demand charges. Soft-start motors can also assist in reducing these expenses.
Obviously, a computer controlled and operated mill would assure proper startup to minimize demand. However, it is not impossible to manage for similar results in a manually operated flour mill.
Energy companies also offer reduced rates for those companies that are willing to reduce their energy consumption during times of peak use. There are peak times for energy consumption, such as in early morning when a city or country awakens and begins the business of the day.
In some countries where air-conditioning is prevalent in businesses and homes, hot weather can peak demand during the afternoon. Agreements can often be negotiated that permit lower energy rates if during these peak times a portion of the process can be shut down. Lead times, duration and cost reduction should be considered as you look at interruptible rates.
Electrical motors account for approximately three quarters of electricity use in the milling industry. Energy-efficient motors offer reduced energy use during operation. A high-efficiency motor is more expensive than a standard-efficiency motor. However, in high energy cost locations the increased installation expense can be offset with energy savings. Careful attention to efficiency ratings can result in hundreds of dollars of savings per year for a 20-horsepower motor operating continuously. In many applications, variable speed drives can result in more savings than high-efficiency motors, depending on the process.
The following are key points that should be considered during an energy audit:• Turn equipment off if it is not needed.
• Don’t forget motors and electrical equipment you can’t see such as computers, printers, FAX machines, compressors and fans.
• Adjust controls to lowest possible energy consumption
that gets the job done.
• Use equipment to full capacity, with full rather than partial loads.
• Clean and adjust equipment, keeping it properly
maintained.• Reduce demand charges.
• Use most energy-efficient replacement equipment sized
to do the job.
• Use energy cost as part of repair and replacement cost
justification.• Consider automated controls to aid energy conservation.• Consider using waste heat to advantage.• Manage your lighting needs. • Consider use of byproducts for cogeneration.
As part of the energy audit in the milling process, a review of air usage and control for pneumatic conveying and general suction systems should be considered in your process. Leaks in seals around sight glasses, rotary valves and connections can cost energy dollars. Tremendous reduction of vacuum loss was demonstrated by replacing sight glass seals above rotary valves in a commercial soft wheat mill. Filter sock maintenance must not be put off, since longer sock life may easily be offset through energy consumption associated with excessive back pressure.
Of course, compressed air and steam leaks should be repaired immediately. It is often difficult to hear these leaks during operation. When the mill is shut down, the loss of pressure in compressed air lines should be monitored. Faster rates of pressure loss suggest leakage.
When the mill is down and the compressed air system is at operating level, it should be possible to locate serious leaks. This brings up an important issue: the mill should never be allowed to start unless the compressed air is at designed level. Low air pressure could cause equipment to not run properly, such as compression of sifter press top holders or roll adjustments. All air is expensive to generate and move and must be put to efficient, effective use.
In 2002, Tokyo, Japan-based Nisshin Seifun Group Inc. planned to reduce electricity used for pneumatic wheat flour moving processes by 30% by fiscal year 2006. Undoubtedly, these savings were not achieved overnight but rather through a step-by-step process.
A first step to successfully control energy costs is to identify the components that make up charges on your electric bill and define a reasonable goal for cost reduction.
Determine through meeting directly with your energy supplier whether programs are available that allow you to save money through rescheduling mill operations or allowing portions of the
process to be shut down.
Conduct a plant inspection to identify energy waste reduction opportunities. Look at management policies and operational policies, and then seek out system design, engineering and equipment changes to achieve your energy saving goals.
Dr. Jeff Gwirtz is a tenured associate professor in the Department of Grain and Science at Kansas State University. Gwirtz is also chief executive officer of JAG Services Inc., a consulting company serving the grain and milling industries. He can be reached by e-mail at email@example.com .