Energy management in flour mills

by Urs Dübendorfer
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Global energy price increases are once again sensitizing the grain processing industry to the issue of power consumption in its value-adding processes. Since energy consumption accounts for up to 6% of the total cost of flour milling, flour and semolina producers are interested in finding new solutions to reducing power requirements.

In order to obtain energy management solutions, it is important to analyze 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.

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.

MONITORING POWER CONSUMPTION

In order to fine-tune power consumption in industrial processes, the first thing to do is get a clear idea of the cur-  rent 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 in 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. In flour mills this will only be possible for a small number of sub-processes. These include 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 analyze 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

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 cost value of new motors should never be below 0.9.

The quality of the motors as well as their starting characteristics must 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 amount of time.

The reliability of frequency converters has improved in recent years while their prices have dropped. Therefore, it also may be worth contemplating their use in applications where their cost was considered too high until recently. Besides optimizing the starting characteristics of motors, frequency converters also improve motor efficiency by almost 1.0 because they prevent phase shifts in the motor windings. Improved power efficiency of the overall plant will reduce costs for power factor compensation with regard to the capacitor bank. Consequently, it will cut the cost of the reactive (wattless) power itself.

PLANT DESIGN AND ENGINEERING

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 minimize 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 until now.

For example, some flour mills already have the first breaks located above the plansifters, eliminating the need for elevating the intermediate products. Although not as user friendly, this design leads to 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%.

Minor energy savings at many points in the process can 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, thus reducing 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 also is often underestimated, which is why 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, for example, the need for pressure vessels is eliminated.

As previously mentioned, minimizing 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, a more energy-efficient alternative may be to recover 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.

Urs Dübendorfer is product manager flour milling for Buhler AG. He can be reached at  Urs.duebendorfer@buhlergroup.com.

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