Efficient grinding

With today's increasing emphasis on efficient use of energy and its effect on the environment, feed mill managers must take an objective look at their process systems in order to maximize output against power consumption.

The major sources of power consumption are clearly the grinding and pelleting processes.

Without any doubt, savings can be made in the pelleting system, but this article will only examine the grinding process.

When looking at efficiency, it is important to identify the criteria that have an effect on costs, such as:

High maintenance cost will generally be in the excessive use of beaters and screens together with the dust separation sleeves in the exhaust filter, as well as labour time in changing them. Frequent plate replacement and even bearing failures can also contribute to the cost.

High operating costs will generally be caused by excessive power usage in the grinder as well as idle time when the system is running but not producing. Poor throughput will contribute to a high operating cost but will stem from an inadequate system to support the grinder.

This will, in turn, produce inconsistent grist spectrums which will have a detrimental effect on the finished feed.

Manual labour costs are part of the maintenance of a machine, but also part of the running of the installation. This cost can be significantly reduced by an efficient central control system. The installation of the grinder and its associated equipment should also be examined to ensure that the mechanical components achieve their maximum life.

For instance, the design of the structure that the grinder is mounted on must be robust enough to withstand any movement due to both static and dynamic loads. The grinder itself should be supported on anti-vibration mountings with flexible connections to both the inlet feeder and the expansion hopper. Any excessive vibration exerted by the grinder will result in premature bearing failures and possibly major damage to the grinder itself, generating a cost in replacement parts and labour and production downtime too.

The feeder should be sized to extend across at least 90 percent of the grinder inlet to ensure that the beaters are fed an equal amount of material so that they wear evenly.

The remaining 10 percent allows for a five percent clearance at each end of the inlet to reduce the feed to the end set of beaters, where they are in proximity to the end plates of the body. This helps to reduce the wear caused by attrition at this point.

Product loss used to be a major problem when cyclones were used to remove dust from the exhaust air. The use of filters has virtually eliminated this, so any loss is now going to be through moisture reduction due to excessive frictional heat in the grinder. The operation of the exhaust system will have an effect on this.

In trying to establish the efficiency of a grinder, the first problem is by what performance criteria do you measure it? The wide range of raw materials used in a mill all react in different ways when subject to a size reduction process. While grinder manufacturer performance figures are usually based on a specific material, such as wheat with a specific moisture content through a three mm screen, this data has to be interpreted by the end users to give them the required mill production output. In practice, maize will achieve a higher throughput while barley and oats will be lower, all at the same grinder settings.

So, where do we start? Initially, when looking at the system, particularly in the case of existing equipment, the only certain improvement in power reduction is to use the latest energy efficient electric motors which will enable a saving in the cost of electrical usage. They will also benefit the environment by lowering absorbed power per tonne of output.

From this point on, there are many aspects of the process to consider. The grist or consistency of the ground product must be suitable for its further use or processing. For instance, a coarse meal will be required for layer hens while a fine meal is needed for pelleting and extrusion processes. If the mill is only used for specific finished products, then a grinding system can be selected from a manufacturer's range to give an optimum performance with a minimum cost. However, if a wide range of products is required, then various grinder screen sizes will be needed and the capacity of the machine will vary from low on the fine grist to high on the coarse grist, according to the material being handled and the screen size used.

It is essential, therefore, to use an effective control system to operate the feeder unit and to monitor the throughput of the machine, in order to ensure that neither the grinder itself nor the transport systems is overloaded. With regard to screen sizes, another criteria to consider is the open area of a particular perforation. A larger open area will give a higher throughput, but may result in a greater wear rate of the screen. The majority of current grinders have the facility of changing screens manually without stopping the machine. In multi-purpose mills, however, it can be beneficial to use a grinder with a integrated power operated screen change system to enable screen sizes to be changed remotely to suit particular raw materials. A further step is to use two speed or even inverter-controlled motors to reduce the grinder speed and thus the absorbed load. The same screen size will produce a coarser grist when running at a lower speed.

This brings us to the two main process systems employed in feed mills - pre-grind and post-grind. Pre-grind refers to the storage, after grinding, of individual raw materials which are held in bins before weighing and mixing in their specific formulation. With postgrind, the raw materials are weighed out in batches as required from their main storage bins before each batch is ground prior to mixing.

In a pre-grind system, the performance of a grinder can be optimized for the raw material being ground, potentially giving a power usage benefit. However, in order for this system to operate in a mill, it is necessary to have sufficient storage capacity for both the raw materials and the ground materials. Pregrind also limits the flexibility of the mill in handling pre-processed materials, which are supplied in either meal or pelleted form, or coarse materials which need to be ground in order to achieve an acceptable pellet quality.

Nevertheless, while the post-grind system can reduce the capital cost of a mill because only one set of storage bins is required, it does result in a grinding system which must be sized to take into account both the increased throughput inherent in the batching process and also the various characteristics of the different materials.

With a batching system, it can be seen that, given a nominal six minute cycle with one minute for filling, four minutes each for weighing, grinding & mixing & one minute for emptying, a 20 tonne/hour milling line will need a grinding system capable of handling 30 tonnes/hour. While fine tuning can reduce this overcapacity requirement, it cannot completely eliminate the downtime between batches.

However, various methods are employed to improve the efficiency of such a system. These include situating a live bin to provide some initial mixing prior to grinding and after weighing, in order to even out the consistency of the batch. Including a sieving machine before the grinder - to separate the fine materials which do not require grinding - and therefore reducing the batch size, is one method used. But using an additional machine at this point will increase the capital cost and also add to power usage and maintenance costs.

A further refinement can be incorporated in the control system - once the batch is clear, the power to the motor is switched off. The inherent inertia in the rotor will keep the grinder running until the next batch is ready, and it only takes a slight surge in power to bring the grinder back up to speed. This method is particularly useful when longer downtimes between batches are necessary.

Another option is cascade grinding where a high capacity grinder with a large screen size is initially used to break down the particles which are then screened to separate the oversize. The product is then passed through a smaller grinder to achieve the required grist. In some instances, a roller mill is used for the initial break. This is of benefit where coarser grists are required and will give a significant reduction in absorbed power.

However, the maintenance cost will need to be evaluated by comparing the cost of replacing rollers compared to beaters. A return flow duct may be incorporated into some machines. This enables oversize particles, which do not pass through the screen on first impact with the beaters, to be returned to the main inlet for a second or even third time. The benefit here is that it enables the material to be reduced by impaction rather than attrition, by rotating within the grinding chamber until it passes through the screen. This will reduce the absorbed power.

Finally, when considering the grinding process, the air exhaust requirement must be taken into account. Although some grinders operate without a significant air volume, most require a comprehensive fan and filter system to purge the screen area. While grinder manufacturers specify the amount of air and pressure required, it should not be forgotten that the volume of air being generated varies according to the resistance in the grinder. Resistance depends on the level to which the grinder is loaded and the type of material being processed.In order not to overload the filter, it is always preferential to include an automatic damper valve in the system, to control the air volume and thus to achieve optimum flow through the filter unit. The power absorbed by the fan motor will also be reduced. The size of the filter should be sufficient to give an air to cloth ratio of 5:1, or less if high fat diets are being processed. The receiving or expansion hopper must also be adequately sized to allow the ground material to drop out of the air flow before it passes through the filter, otherwise any excessive build-up on the sleeves will cause increased resistance to the air flow. This in turn will reduce air flow through the grinder and negatively influence its capacity and power usage. The expansion hoper must incorporate an adequate explosion panel with a duct to atmosphere and an emergency stop system in the event of an explosion in the grinder. If this happens, it is most likely due to metal entering the grinding chamber. The importance of an adequate cleaning system before the grinder cannot be emphasised enough.

There is no magic formula to guarantee a vast reduction in the power requirement for grinding. It is nonetheless possible to achieve significant savings by carrying out an objective assessment of each mill process to optimize the operating parameters.

After all, even a one percent reduction in annual power usage represents a significant saving on operating costs and also benefits the environment.