All millers at some point wished they had a bit more milling capacity, and when changing from one grist to another, they wish there were ways to either compensate for the quality adjustments they have to make on the mill or utilize the areas where they find bare dressing.

Using booster mills enables millers to do precisely that. This technology is gaining popularity at a time when the number of operating mills are declining, particularly in Europe, and the remaining milling units are called on to mill a greater range of flours.

Flexibility seems to be the key word, and the booster mill has played a distinct role in both increasing the capacity of existing facilities with very little capital cost and enabling new mills to operate to maximum efficiency, producing a broad cross-section of flour types from both hard and soft wheat.

The conventional way of increasing mill capacity and throughput has been to purchase additional machinery and install it into the plant, modifying the existing diagram. The process can take many months in the planning stage and even longer when it comes to executing the project, as production facilities need to meet the continued demand for flour.

It is usually a traumatic time for all concerned, and especially since the original need for the increased capacity is due to the plant being stretched to its limit in the first place. Carrying out running remodels is fraught with problems, and those mills able to have any length of shutdown time are usually very limited.

The booster mill can be installed while the mill is still operational. Apart from a short shutdown to link the new unit to the existing flow, there is no disruption to the existing facilities. It generally requires very little modification to the existing mill, and most of the installation can be done on the run. Only the final connection requires any shutdown time, and this is limited to a series of days rather than any continued or protracted period of non-operation.

Essentially, the booster unit is independent of the main mills and incorporates head break and reduction purposes. Flour produced from the booster unit is usually fed down the same lines as flour from the main mill. Intermediate mill stocks are fed into the existing tail end break and reduction systems. The booster mill needs the existing mill to operate when it is running, but the existing mill can operate independently of the booster mill.

The capacity of most milling units is limited by the amount of stock that can be processed through the head break and reduction passages, namely first and second break and A, B and C passage. The tail end of the break system and the remainder of the reduction system are usually capable of handling more stock. With the booster mill in operation, this stock is provided to these lower passages without increasing the head end loadings. This load is taken up by the booster mill, which is basically a collection of head break and reduction passages that can be brought on line when required, feeding its intermediate stocks into the host mill’s tail end passages where there is spare capacity.

Most booster mills operate at around 120 tonnes of wheat per 24 hours and incorporate four roller mills and one sifter. A purifier can be used where very clean flours are required and disruptors would normally be fitted following the head reduction rolls, enabling these rolls to make flakes that are then broken down before the sifters. Separate pneumatic conveying facilities are required but are much less expensive than would be required for a full plant remodel. All of this equipment can be installed without the host mill having to cease operations.

In addition to the booster mill described above, it is also possible to incorporate debranning facilities into the system. This dramatically increases plant throughput but does need more modifications to the head breaks because debranned wheat will pass to the first break passage and subsequent stock characteristics will be different to subsequent break passage.

The most popular size of booster unit has been around 120 tonnes per day, approximately equal to five tonnes per hour extra to first break. This gives the best ratio of capacity against use of equipment and, hence, the most effective capital cost per unit of output.

It is possible to use smaller booster units, but generally speaking, the smaller the booster unit the higher up the host mill’s flow the stocks have to be fed. This reduces the effectiveness of the booster unit, particularly on mills that are already stretched in terms of machinery utilization. Generally, the host mill needs to have an installed capacity of greater than 220 tonnes per day, but on smaller milling units it is possible to utilize booster mill technology by adding an odd roller mill into the host mill’s existing flow in order to maximize the booster mill throughput. Interestingly, this often gives greater-than-anticipated performance to the host mill.


Building space is a consideration, but it is surprising how  easily a booster mill usually fits into existing buildings, even those with limited space. The pneumatic cyclones and airlocks can often be mounted on frames directly above the sifter, thus eliminating the need for separate pneumatic floors. Roller mills will often fit into basement areas, eliminating the need for additional steelwork support. Sifters can either be incorporated on the existing sifter floor or one floor below, especially when roller mills are placed in the basement.

Obviously, the flour and bran collecting systems need to be capable of handling the additional throughput, and this must be verified before additions are made. Similarly, the wheat supply and cleaning systems need to be evaluated, but it is surprising how an appraisal often exposes current shortfalls. Diligent use of gravity selectors and other stream-splitting machinery often allows existing equipment to be more selectively used on more concentrated split density streams. Invariably, far greater capacities can be achieved than are strictly required, often enabling running times in wheat cleaning plants to be reduced.

Booster mills lend themselves to automation and can be set to run with minimum human intervention. If the host mill has an existing automated control system, the booster mill’s control protocols can fit into the existing system.

Full automation of mills is becoming the norm these days, especially as labor costs escalate. Approximate power requirement for a 120-tonne-per-day booster unit is 350 kilowatts, although this can vary according to survey findings and local conditions. Operator interfaces can be whatever type is desired — remote, touch screen, PC in the main control room area, or any other kind of interface traditionally preferred by the existing plant owners and operators.

The technology of booster milling has a real application when grist changes are from one extreme to another; when both hard and soft wheat are to be milled on the same plant. Traditionally, soft wheat milling capacities are 10% to 15% lower than hard wheat, but the booster mill allows this shortfall to be eradicated. Similarly, when hard wheat are being milled and starch damage levels need to be met, the booster mill can be used to increase the available surface on the head reductions that enables the required grind to be achieved.

Jonathan Bradshaw is a consultant to the agribusiness and food processing industries, specializing in project management and bespoke training programs through his company, J B Bradshaw Ltd. He has extensive experience in flour and feed milling in Africa, the Americas, Europe and the Caribbean. He may be contacted at:  [email protected] .