ENERGY EFFICIENCY (compressed air distribution, motors, motor controllers, machine capacity, boilers, grain drying)

An important area to consider in terms of achieving energy economies is the use of motive power. This is especially true in the energy intensive flour milling process. This is because of the large numbers of motors required in the process as well as the long running hours associated with the industry generally. Any potential savings here should be carefully reviewed.

There are a number of aspects to adopting an energy efficient stance to motive power. So called energy efficient motors are now readily available from a number of manufacturers. These offer considerably improved performance ( of the order of one or two per cent depending on motor size and manufacturer ) over standard motors. This is particularly true if motors are operating at considerably less than full load. The efficiency of motors at less than full load has specifically been targeted by motor manufacture rs. This is because most motors in service operate at less than full and typically at three quarters full load. This might be considered a terrible waste. However it must be taken into consideration the reasons for this typical over sizing of motors. It is a fact that while most processes are well understood and can be predicted relatively reliably, a number of unforeseen circumstances will always occur in transferring projects from the drawing board to the real world. An example of this might be the power requirement of a grinder used to grind wheatfeed and screenings in a mill. The contract may specify one per cent screenings in the dirty wheat and the engineer could quite happily size the motor required to handle this amount of product. However on commissioning of the plant, or some time after start up, wheat may arrive with two per cent screenings and the motor becomes undersized.

In order to avoid such problems engineers will over specify motor and even machine capacities to provide a ‘comfort’ or ‘safety ’ factor. In terms of motors this translates into a lot of motors operating at less than full load and in the case of standard motors, less than full efficiency. With the adoption of high efficiency motors most of these inefficiencies can be tackled successfully.

The better performance achieved by high efficiency motors compared to standard motors is achieved in a number of ways. There is a larger amount of material in the windings which provides less resistance to the flow of electricity. Thus less waste heat is generated by the motor. This is energy saved. Because less heat is generated by the motor, the cooling requirements are lower. This requires a smaller cooling fan than a standard motor. Energy is saved by using a smaller fan. The smaller fans mean that the fan cowls need to be redesigned to accommodate the fans. This redesign gives energy efficient motors their characteristic appearance. The lower air flows over the motor body also generate less noise which is another area where some of the input electric energy to a motor ends up. A number of other finer points also contribute to the superior performance of high efficiency motors.

Due to competitive pressure the premium s demanded for such motors are decreasing and the energy efficient option is becoming more attractive. The justification for adopting energy efficient motors in developed countries is highly dependant on the tariff structure applied to the electricity supply.

The billing system may include a capacity element, that is the electricity company charges the customer for reserving a specific amount of generating power for the user. The second element of the billing system may be a demand charge. This is a charge for the amount of the reserved capacity actually used during the billing period. The other elements seen in a billing structure are kilowatt hours used and power factor.

Energy efficient motors and other energy efficient technologies can minimise all aspects of the tariff structure.

In developing countries the argument may involve other factors also. In order to perform a certain amount of work less KVA is required to operate a plant employing energy efficient technologies than conventional ones. This could have implications for the size of generating plant required to power the plant. The savings on a new plant could be significant.

The energy efficient option for motors should be considered in all circumstances. The most attractive stage for investment in an energy efficient motor is when the new plant is being purchased. This is because the premium is the only part of the motor cost that needs to be justified. In fact the application of these motors will help to improve the pay back period of a plant because the pay back period for the energy efficient premium will be quicker than for most capital projects.

The second situation where energy efficient motors should be considered is where a motor has failed in service and requires rewinding or replacement. The cost that must be recouped here in a replacement scenario is the difference between the cost of a new energy efficient motor and the cost of repair of the old motor, minus the salvage value of the old motor. Though less attractive than the first scenario, the pay back period for such an investment can be worthwhile.

The third scenario to be examined is the replacement of an in service motor with its energy efficient counterpart. This scenario can actually be worth while in cases where the in service motor is quite old, running at low loads or for extended running hours. This last point is a critical one in any energy efficient philosophy. This is because while differences in efficiency can be small, coupled to long running hours the accumulated savings can be significant. A point to note about energy efficient motors is that it is now possible to get them in small kilowatt ratings. While the energy saving on one motor may not be significant, the installation of a number of them over a period of time can contribute significantly. Drifting from the central discussion for a moment, a side effect of installing energy efficient motors is the reduced noise emissions from them. In an area with a high concentration of motors this may be significant. An example would be the roller floor in a mill. This is a feature a health and safety officer may appreciate! Returning to the discussion on energy efficient technologies, the next area worth examining is motor speed controllers. Most people are aware of their existence and their capabilities in terms of machine and process control. Their use can be successfully extended to saving energy and ultimately money. Before the advent of inexpensive frequency inverters other types of speed controllers were available but were complex and expensive to run and maintain. These included DC drives and mechanical variable speed gearboxes. These types have largely been superseded by the frequency inverter because of simplicity, reliability and low cost.

The most obvious place to save energy with a speed controller is in fan speed control. This is so for two reasons. The first is that the common means for controlling airflow is through the use of a throttle or valve ( often a butterfly valve ) which is inefficient in energy terms. The second reason why speed control is successful on fans is because of the disproportionate relationship between airflow and power required, The so called fan laws ( see figure ). Thus an even modest reduction in the speed of a fan can achieve significant energy savings.

Many other examples exist where speed controllers can save energy for the consumer. One such possibility is where it is possible to operate a motor above synchronous speed to achieve higher throughputs through a plant. This leads to shorter running hours or greater throughput in a given tine. The benefits are obvious. The remainder of the plant is operating closer to full load and the base load will be approximately the same. This means that the specific energy consumption of the plant per unit processed (e.g. kWh / tonne) will be lower.

Compressed air distribution is also a minor area which is worth examining for energy savings. As mentioned in the last issue every bar of pressure drop achievable in a compressed air system is significant. Restrictions in air flow due to under sized pipe work, dirty filtration equipment, under sized air coolers and even half open valves are all pressure drop zones. In order to maintain satisfactory machine operation the static system pressure is forced upwards. To determine if any of the above is an issue, install a pressure gauge downstream of the suspected problem location. If the pressure drops significantly during machine cycles then a problem exists.

In mills where boilers may be required to generate hot water or steam, significant savings can be made through the specification of multiple pass boilers with good heat recovery features. On existing plants factors such as the temperature of the hot water supply should be examined to see if it can be reduced without impinging on the plant and operator requirements. Steam supplies and their operating pressures should also be examined .An empirical approach to this exercise follows. Reduce the pressure or other relevant parameter in gradual increments until a problem develops as a result of these adjustments. Back track the adjustments to such a point that the problem operation begins to operate smoothly again. Do not stop here however. The reason that a problem occurred is probably because the operation has an inherent fault ( for example a supply pipe that is too small for an application). Locate and tackle the problem and then resume the gradual adjustment procedure until another bottleneck is arrived at. Keep repeating this procedure until a point is reached where it is not possible to proceed any further. This procedure can be applied to many facets of flour milling to achieve the utmost from the plant.

Another area of interest though not to all mills is grain drying . Poorly maintained burners or inadequate control systems can be very expensive oversights. Indicators to watch out for are smoky exhaust stacks and ex- dryer moistures significantly below target moistures. . Accurate controllers are the key to efficient operation. Again the size of the operation will determine how worthwhile the actual operation is.

Finally, the one area where the greatest energy savings can be made is where machines are allowed to run unnecessarily. This can take two forms. One is where a machine runs idle for significant periods of time but all its auxiliary functions are still operating. Prime examples of this are compressors and exhaust systems. The other situation is where machines with excess capacity are run at less than full throughput possibly due to an upstream or downstream process limitation. Examples here include conveying systems. Vigilant operators and strategic plant control systems can successfully tackle these two areas.

Other specific items which will be examined at another point include air recycling, conveying, temperature and humidity control, insulation and heating among others.

Tip of the Month: To determine the suitability of a motor application for an energy efficient motor.

Situation 1 a new installation: The price difference between the standard and high efficiency motors is all that needs to be justified. Compare the efficiencies, convert this to a kilowatt difference and determine the annual kilowatt hour saving based on the predicted running hours for the motor installation. Using these two figures and the tariff structure of the power supplier, the annual economic saving can be determined. The simple pay back period is the price premium divided by the annual cost saving.
Situation 2 motor replacement:Calculate annual running cost saving as described above. The simple pay back period is the purchase price of the new motor minus the repair cost of the old motor and the old motor’s salvage value divided by the annual saving. The comfort afforded by having a new guaranteed motor has a value also.
Situation 3 replacing an in service motor: Pay back period is the cost of the new motor minus the resale value of the old motor divided by the annual savings.Warning: Make sure that comparable performance figures are used for estimating the potential energy savings of motors and other technologies. Conditions such as temperature can influence performances.

Definition:1 KVA = One thousand Volt Amps. This is the unit used for measuring power in Alternating Current ( AC) grids. It is different from kilowatts because of the influence of low power factors generated by motors, fluorescent lighting and equipment employing magnetic fields.

Reference:Energy Efficiency Report; B.B. Greenwood, 1987, Hawker Siddely Electric Motor Company, England. ( Brook Hanson motors ).