Energy management in flour mills

An Industry Insight by Urs Dübendorfer, Bühler

Global energy price increases are once again sensitising the grain processing industry to the issue of power consumption in its value-adding processes.

“As a result of utility price increases already made or still to be expected, monthly electricity bills have become a permanent issue in many companies”
-Urs Dübendorfer

Since energy consumption accounts for up to six percent of the total cost of flour milling, flour and semolina producers, too, are interested in finding new solutions to reducing power requirements. In order to obtain an integral view, the issue will be dealt with here from different perspectives. Before this, we will analyse the energy requirements within the process chain. The purpose is to show where investments for cutting power consumption will pay off and in which plant sections energy consumption is only of marginal significance. The article details analytical approaches and possible measures to conserve energy. Its primary goal is to contribute to the power consumption discussion. It will also show well-known and proven procedures to optimise energy usage.

Cost structure in industrial flour production
In a competitive business environment, much attention is paid to the operating costs of the entire production chain. It is safe to assume that most commercial flour producers have already streamlined their processes in terms of manpower requirements. This means that the potential for further cost reductions in this area is low. The focus must therefore be on energy prices, which are a substantial production cost factor. As a result of utility price increases already made or still to be expected, monthly electricity bills have become a permanent issue in many companies. Figure 1 shows an example of energy consumption in the various process operations of flour milling.

Monitoring/fine-tuning of power consumption through the process control system
In order to fine-tune power consumption in industrial processes, the first thing to do is to get a clear idea of the current situation. For this purpose, a plant will ideally be divided into plant sections or sub-processes that are detailed as accurately as possible. The energy requirements of these sub-processes can then be accurately determined by integral power measurements. This will provide an overview as a function of time over the energy used by the individual sub-processes at any given point of time. By dividing the contract with the energy utility into different time frames, it may be possible to move energy-intensive processes to periods with lower rates. It is needless to say that in flour mills this will only be possible for a small number of sub-processes. These include, for example, grinding of the byproducts from grain cleaning, bran pelleting, raw wheat transfer from the storage bins to the blending bins, etc. Another possibility is to analyse plant sections that operate simultaneously. As power rates often depend on peak power consumption, this may allow fine-tuning of power usage.

Power measurements are also a suitable instrument for pinpointing process operations with high power consumption. This means that any investments made should focus on sections where a fast return on the investment is ensured.

Power distribution / infrastructure
Before considering the processes in detail, we will briefly discuss the power supply system and the related electrical equipment used. Whenever possible, transformers should be installed as near as possible to the equipment that uses the power. The longer the cable routes, the higher the power losses. This is especially important today, since the current high copper prices will generate high costs if cable cross-sections are oversized.

Another important factor is the selection of the electric motors. In the recent past, most manufacturers of induction (asynchronous) motors have substantially improved the efficiency of their products. As a rule of thumb, the cosΦ value of new motors should never be below 0.9.

Starting characteristics of motors
Not only must the quality of the motors but also their starting characteristics be considered in the context of energy consumption reduction. Depending on the service hours, specific functions, and power consumption of drive motors, their circuitry may have an appreciable impact on power usage. It is therefore important in each application to determine whether it is worth the cost to buy a somewhat higher-priced system in order to conserve energy over a certain time. The reliability of frequency converters has improved over the past years while their prices have dropped. It may therefore be worth the trouble to contemplate their use also in applications where their cost was considered too high until recently. Besides optimising the starting characteristics of motors, frequency converters also improve motor efficiency up to almost 1.0 because they prevent phase shifts in the motor windings. On the one hand, improved power efficiency of the overall plant will reduce costs for power factor compensation with regard to the capacitor bank. In consequence, it will cut the cost of the reactive (wattless) power itself.

Plant design and plant engineering aspects
The design of a plant, and especially its flow of materials, has an impact on its energy consumption. Sophisticated plant engineering solutions allow energy to be saved. As a basic rule, the plant layout should minimise material conveying distances. Pneumatic lifts should only be applied if they offer true added value in the form of higher sanitation or flexibility. By adhering to this principle, solutions may be found which have been considered rather exotic up to now. For example, some flour mills already have the first breaks located above the plansifters, eliminating the need for elevating the intermediate products. This somewhat lower user friendliness is offset by the advantage of reduced energy consumption. Another excellent example is final flour sifting. Whenever possible, plant engineers should select gravity feeding for the flour screws. This will eliminate the need for elevating all the flour at the end of grinding for final sifting (rebolting) and weighing.

Additional potential exists in the design of the pneumatic intermediate mill stock lifts in the grinding system. Improved sizing of suction lines may reduce the air volume requirement by up to 25% and the pressure loss by up to 10%.

We all know that even minor energy savings at many points may add up to a respectable total reduction of power consumption. Thus, it may be possible to apply only a single motor to power the two superposed roll passes of eight-roller mills. On the one hand, this will cut the installation costs. In addition, a single drive will operate in a higher-efficiency range and therefore have a direct positive impact on the operating expenses.

The consumption peaks during starting and stopping of motors is also often underestimated. This means that more attention must be paid to continuous operation of equipment. Prime examples of this are the compressors that generate dust filter cleaning air or compressed air for the in-plant network. Pressure monitoring and speed variation by frequency converters may slash the electric power consumption of such auxiliary equipment by as much as 40%. This is on top of the lower installation costs, since – say – the need for pressure vessels is eliminated.

As mentioned above, minimising conveying distances is one of the most effective ways to reduce power consumption. This applies in particular to the handling of the finished products. An analysis of the finished products of a flour mill may reveal that more of them could be made on the grinding system itself and therefore would not have to pass through the flour blending section. This is a classic example of a reduction in conveying distance with an immediate impact on the power bill.

Heat recovery
Since we also use energy in our latitudes to heat buildings or recycled air, we must also briefly point to the possibility of energy recovery. In the grain processing industry, energy in the form of heat is generated in various sub-processes. Instead of exhausting this thermal energy into the atmosphere, it may be worthwhile to consider recovering it. Recovered thermal energy can be used to preheat the fresh air introduced into buildings during the cold season, or for preheating the process air used in thermal processes.

To contact Urs Dübendorfer, Bühler AG, Uzwil, Switzerland, please email mu.buz@buhlergroup.com or visit www.buhlergroup.com.