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Digital Re-print - May | June 2010 Feature title: Flour mill design

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The fact that this earthquake was stronger than anything felt in the region for some

time just goes to demonstrate that there will always be situations that we cannot have foreseen.

Having said this there are many other factors to take into account when designing mill buildings.

First, we must consider what materials we shall use to construct the building with. What are our criteria, how fast do we want to be in operation, what are the ground conditions, what are the local laws?

For example, imagine we want to build a mill as a new entrant into the industry in somewhere like West Africa.

As a new entrant we obviously need to have dock side facilities for offloading wheat and if we cannot acquire sufficient land with-in a port we may wish to consider a separate facility for wheat intake and storage and then take into account the need to transfer wheat to the mill at a later date when the vessel has left port.

We also need to take a good hard look at the available draft within the port and approaches to it to make sure there is suf-

ficient depth of water to accommodate our chosen size of vessel. It may be that dredging is an economical answer or possible using dolphins far enough from the shore line where the water is deeper.

Concrete or steel?When it comes to the mill building

itself then we really have just two options, concrete or steel. Concrete is invariably the favoured option since raw materials are almost always available locally and with reasonable supervision local labour can be used to build such a mill building.

However, there may well be circum-stances where steel has distinct advantage, particularly when the site is of poor struc-ture. Steel is lighter than concrete and can be fabricated and erected much quicker. Overall build time to full commissioning can be reduced significantly by using a steel building, and it is easy to modify to accom-modate changes as it is being fabricated and erected.

Frequently, piles are used to strengthen the foundation of a building where it is weak and piling often pays for itself in the long run because the building steelwork can also be reduced in profile.

As an aside it is often common practice in concrete buildings to erect the wheat and flour silos from Concrete as well although there is no reason why steel silos cannot be used for wheat.

Students of the old correspondence course will know, however, of the advan-tage and disadvantages of using steel silos for flour in comparison to concrete. Temperature changes for one thing are not transferred as quickly into the finished flour when concrete is used as compared to steel which allows such temperature change to be almost instantly reflected in product temperature.

Obviously, whatever the circumstances and whichever is used then adequate ventila-tion is a must. Similarly, mill buildings made of steel require adequate ventilation, not only to keep the employees and operatives cool – and hopefully productive - but also so the process is not adversely affected by swings in temperature and humidity.

Forward planningOnce the building is erected and the

installation of machinery is commenced it is to be hoped that forward planning has been satisfactory and there are sufficient

Flour mill design by Jonathan Bradshaw

There can hardly be a more graphic illustration of a mill not being built to match the likely circumstances of an earthquake than the recent events in Haiti where the mill building was reduced to a pile of rubble in literally seconds. I don’t blame anyone for this since the building was originally

built several years ago and those who designed and built it adopted best practice at the time.

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holes in the floors, in the right places to accommodate spouting and pneumatics and other pieces of plant that require access through the floors. It is easier to accommodate changes in steel buildings than in concrete but it is also easier to inadvertently weaken the building structure by making too many structural changes in a steel building after the building has been erected.

A simple layout of a milling facility would be to have the intake and wheat silos at one end, close to the water and the supply vessels followed by the main mill building housing the wheat cleaning plant and then the mill.

Flour and bran will normally be blown into finished product silos at the opposite end of the mill building to the wheat silos and these will, likely as not feed through packing machines into the main storage warehouses. All this arrangement will ideally be in a straight line to avoid using unneces-sary power transferring products from one point to another when gravity can do the job far cheaper.

Other items, such as offices, maintenance workshops, staff welfare facilities and labora-tories will need to be allowed for and apart from them being near to the mill there is nothing to be gained by having them in any particular places, suffice to say that the wheat laboratory may well be close to the intake point and the flour laboratory close to the packers.

I am obviously referring to a mill where bagged flour is the prime product but I real-ize that there are more and more mills being built with bulk facilities and several are also being built in certain developing countries with plant bakeries very close by, if they are not built as an integral part of the whole facility.

Certainly this is the case within pasta plants and semolina mills invariably being built side by side to obtain the greatest economies overall.

Economies on maintenance, quality con-trol, staff welfare and general administration can all be maximized by incorporating as many operations as is feasible and practical on the one site.

Traffic flow and layoutSeveral sites I have worked on over the

years have adopted traffic flow as the main criteria for their premises layout. With one incoming weighbridge and one outgoing weighbridge, a circular route for vehicles around the perimeter of the site often proves to be an economical layout with minimum supervision of vehicles. Cameras and traffic notice boards serve to control traffic very well and also provide a good record of vehicle turn round time and precise data as to when vehicles arrived and departed.

The mill building itself is usually designed by the milling engineers and is then checked over by local structural engineers who may or may not be retained until the building is complete and machinery is installed. Use of contract managers is invaluable if you have nobody on staff who has carried out major works before, either civil or mechanical.

Attention to detailElectrical controls are usually supplied by

milling engineers as well but transformers,

Grain&feed millinG technoloGy may - June 2010 | 13

Feature Flour mill design

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And finally may I say that there are two ways to design a mill, or a site, the first being to obtain a good layout from a practical point of view and the second one being to start with the customers specification for flour and then work backwards through the flow sheet requirements and then building criteria to house such a mill.

I generally favour the latter but the former can often work well particularly when local bakers and traders are the customers and the mill is producing general all purpose bakers’ flour as its main product.

In both cases however the plant must be laid out with plenty of access room to individual machines because if operatives cannot work on a piece of equipment easily without getting knuckles cracked and knees rapped they simply will leave it alone, with all the knock on effects that has on flour quality.

Getting involved from day one of con-ceiving of the mill design and layout will proves extremely beneficial and as I always advocate regular walk rounds of the mill that is in operation I can strongly advocate twice daily tours of the site whilst under construc-tion. If work is being done to the upper floors and for one reason or another your presence is not welcomed then take a pair of binoculars with you and take a good look from ground level.

Always hold regular site meetings to monitor progress and deal with other design issues that may crop up periodically. Always make sure contractors know where they stand on an issue, be clear with you instruc-tions and listen to the counsel of those wiser and more experienced than you are. You will find that the ‘Old Hands’ take great pride in their work and will help you along well so long as you look after them in return.

very few remote places in North America there are strict guideline on emissions and constant monitoring is now the norm with equipment being calibrated annually with the local authority being present at the time.

Noise levels are also closely monitored and it may prove beneficial to house certain items of plant which generate high noise levels separately. This is certainly becom-ing the norm and most blower rooms are positioned on the side of the mill furthest away from housing and almost always these blower rooms are well insulated.

It is easier and more economical to house such items separately when they are to be insulated since this greatly reduces the quantity of expensive acoustic insulation that is required.

Maintaining water volumes and quality

All mills require water for damping of wheat and whilst we do not have the problems nowadays of disposing of the contaminated water from washers and whizzers we do still have the problems of maintaining water quality and holding sufficient water in storage to pro-vide adequate pressure for operations.

Invariably we see the water tanks being placed on the top floor of the mill close to tempering devices and with separate feeds down through the mill to provide adequate pressure and supply to offices, laboratories, canteens, workshops, toilets and showers.

Sprinkler systems usually have their own supply of water which only need topping up whenever sprinkler tests are carried out.

In some instances it is however illegal to store water because it is such a scarce com-modity. In such circumstances it is usual to sink a bore hole to provide an adequate supply of potable water although I have seen some rather shallow bore holes which left a lot to be desired in terms of water quality, often only rectified by sinking the bore deeper.

SecuritySecurity is another consideration, par-

ticularly where there is a history of forced entry into premises in the area or where fraudulent practices are deemed acceptable.

Depending on the country arrangements for security may range from a van com-ing by the mill premises a few times each night through to armed patrols being on site at all times with locked gates, machine gun platforms and observation points being the norm. It very much depends upon the culture of the country as to what level of security is required. The choice is yours.

generators, field wiring within the mill and laying of high tension cables may be subcon-tracted with good efficiencies being achieved.

Invariably whoever carries out the electri-cal installation is the person who will subse-quently maintain the plant and to be in at “the ground floor” will help in their understanding of the electrical wiring and the protocols which have been adopted. Most new mills are highly sophisticated in their electrical controls and so attention to detail is important.

With auto roll gap adjustments, extrac-tion monitoring and power recording as the norm, along with possibly on line testing of flour and automatic adjustment of additive feeders and wheat blends it requires people with advanced knowledge and skill to design, install and maintain the electrical control system in the mills of today.

Fortunately most engineers have access to electrical contractors with the necessary capabilities and some even have a separate facility with suitably skilled staff within their own umbrella organization.

Having said this there are some countries where the client wants the simplest mill he can obtain with no degree of automation whatsoever. In my opinion there is little sense in taking this approach since even in places where all the operatives are illiterate it still pays to carry out tasks automatically rather then rely on individuals to remember when various tasks are to be carried out.

Noise pollution and dust emissions

Some other items which we need to con-sider are noise pollution and dust emissions to the atmosphere. In most places except the

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June 2010

• Flour mill design

In this issue:

• Grain storage - The Canadian way

• Simultaneous determination of

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• Hygienic Compact

Containment System

• The use of peripheral machines

in flour mills of today

• Horizontal twin-shaft paddle mixers

The ultimate in mixing technology

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