The types of the silos

The most common silos types - Storage and dosing of bulk materials in bakeries

Over the last few, or even more than ten years, the technologies of storage and bulk ingredients dispensing in the bakery and confectionery industry have changed significantly. There have been a number of new technologies for transportation and dispensing, new types of silos or modern systems to control the manufacturing process. However, the changes in technology have not been followed by professional literature, which would allow to get acquainted with these new technologies by new generations of bakers. This fact is used by some companies offering equipment for bakeries and bakeshops, which in addition to modern solutions that are consistent with applicable standards and regulations, routinely offer solutions that are at odds with the modernity and safety of installations. The lack of basic technological knowledge among the users of silo systems often leads to the popularity of technical solutions that attract customers by their low price but in fact can cause a drastic increase in the silo system’s hazards through the lack of compliance with applicable standards, often causing a real threat to the life or health of its users. The text below contains description and short information about usage and security of the most used silos types in Europe.

BASIC SILO TYPES:

Composite silos

These silos are made of glass-reinforced polyester composites of the highest durability approved for contact with food. Silos of this type are the only ones that withstand the impact of hurricanes or earthquakes and meet the standards required by explosion security regulations. The best producers of such silos calculate the strength of their walls such way, that in case of an explosion the whole energy is kept inside, released only via the venting (explosion) panels, without risk of the explosion or damage of the silo body. The finest versions of this silo type are made ​​in the form of a monolithic, single and seamless element devoid of thermal bridges and providing the best parameters for ingredients storage  As the only type of silos, thanks to the exceptional smoothness of their inner walls, they enable the most effective emptying of the silo and real retention of the principle ‘first in – first out’, which is essential in the food industry. They are available in capacities from 1 to 250 tons, mostly as external silos of circular cross-section. It is the most modern solution, which, due to its advantages, is gaining more and more popularity in the world. Composite silos are great for storing flour, sugar and salt, ensuring long-term reliability.

Figure 1: A battery of 8 modern composite silos with a capacity of 25T each in the vicinity of Prague (Czech Republic). Photo: Author.

Thanks to modern solutions, such as a cone with a greater slope, modern technologies that provide exceptional smoothness of the walls (up to seven times smoother than polished stainless steel), excellent insulation properties without the need for additional insulation and a very high level of security – these silos are effectively replacing the previously used concrete and metal technologies. Due to properties of the material from which they are made, these silos can be repeatedly moved from place to place without the risk of damage, which may be important for plants planning their long-term development. The most important added value of composite silos is the additional ATEX security, because of monolithic, seamless construction.

Metal silos

Metal silos are made of steel or aluminum. In their external version, they are silos of a circular cross-section, while the internal version of these silos often has a square or rectangular shape. Due to the thermal conductivity of metals and the problems of condensate formation resulting from it as well as the need for additional insulation of metal silos, particularly aluminum silos, the use of external silos made ​​of metal becomes less frequent in favor of composite silos. Because of their popularity in the late twentieth-century, they are still often found in industrial bakeries and processing plants.

The relatively popular silos made ​​of aluminum have an excellent thermal conductivity, which is often the reason for the formation of large quantities of condensate inside the silo and the ‘blackening’ of flour. Even further insulation of the silo made ​​of aluminum does not guarantee one hundred percent safety, since the elements supporting the insulation become thermal bridges, which give rise to condensation inside the silo or cause freezing up of flour to the inner walls of the silo. This is one of the reasons the reason, why the producers offering aluminium, steel or trevira silos prefer the indoor location of the silos.

Figure 2: An installation of indoor metal silos in a modern bakery plant. Photo: Author

Metal silos are usually available in two versions: with a cone designed to empty the silo or with so-called aerating (fluidization) bottom. Both versions have advantages and disadvantages. While in the case of cone accurate emptying of the silo is provided, then in the case of an aerating bottom usage, the silo flour is better aerated, but usually only in the immediate vicinity of the aerating bottom (about 3-10 cm). Unfortunately, in the case of silos with an aerating bottom, it is extremely difficult to obtain a real total emptying of the silo, which results in the need for periodic cleaning and disinfection. The fluidization bottom is also the element that requires the most and very expensive service in most silo installations using this type of extraction. It is important to confirm, that the aeration bottom works as expected only in very small silos, not bigger than 10T capacity. Flour stored in the bigger silos blocks the flow of the air: the air blown via the aeration bottom does not mix the flour anymore, just looks for the simplest way out creating “channels” in the flour or along the silos walls. In that case, aerated bottom is a way to lose money and energy without significant influence on the flour quality.

Metal silos are used both in their outdoor version (they are usually made in the form of cylinders), and in the version for setting them inside a building. Metal, internal silos are usually made in a modular technology, facilitating their transport, but at the same time, it extends the time needed to assemble them. The modular design of internal metal silos enables the use of them in almost each bakery plant.

Canvas silos

Canvas silos are usually made ​​of a material called Trevira with high strength, mostly stretched on a steel frame. This solution is particularly popular because of its cost – it is the cheapest option, especially while equipping the existing small bakeries with a silo installation. The solution developed many years ago as a temporary storage solution, simply to ensure the continuity of work in a production facility during its repair or reconstruction, has become a solution commonly used in some countries as the main place for flour or sugar storage.

Canvas silos are designed for most types of bulk materials. They are made in capacities from 2 to about 40 tons. The biggest advantage of canvas silos is their low price. They have a number of drawbacks, which include, among other disadvantages, a frequent lack of integrity, which results in the risk of dustiness and explosion in the silo room as well as a high porosity of their internal walls, which, in conjunction with the necessary linen tape reinforcements used inside silos, make a complete emptying of the silo virtually impossible. Therefore, it is not possible to retain the ‘first in – first out’ principle and hygiene in such solutions. The disadvantages also include the susceptibility of Trevira silos to deformation of their walls during their filling or emptying, often resulting in displacement or deformation of the silo unloading equipment, which in turn causes flour jamming and common remaining of its residues in the silo. An effective mechanical cleaning of a Trevira silo is virtually impossible, so if there is the need to clean this type of a silo, the simplest solution is to replace it. The often-used half measures, such as chemical disinfection of the silo interior do not ensure its purity, they cause only ‘gluing’ together the flour residues remaining inside the silo. The improper use of disinfectants may cause potential contamination of flour stored in the silo. In some countries, the use of canvas silos in food production is limited by local regulations regarding health and safety.

Figure 3: The vibrating bottom of a new Trevira silo with visible effects of the silo leaks and a visible deformity – a tilting of the vibrating bottom. Photo: Author

The lack of tightness of silos made ​​from permeable materials causes frequent penetration of flour particles into the silo room. In effect, the whole silo room should be treated as a potentially explosive zone (ATEX zone 20). Securing a silo by making its walls from a material preventing the formation of static electricity (a fabric containing metal threads discharging static charges) obliges the user to earth the silo installation. In the absence of earthing, metal threads will not help much. Moreover, the use of a conductive material does not release the user of a Trevira silo from the obligation to adapt the whole silo room to the standards of the ATEX zone 20. Most of the suppliers of those type of the silos declare, that the silos room fulfills the standards of the safe ATEX zone 22. This declaration must be obligatory confirmed by a specialized unit, most of the cases, because of very high dusting level in the silos room, the canvas silos causes obligations of the ATEX zone 20 (category 1 or zone 1). The dust in such room is continuously present during normal silos operation.

When buying a Trevira silo, it is also advisable to refer to the documentation presented by manufacturers of these silos to check whether the attestation or certificate concerning the silo protection was issued by an appropriately accredited organization (a list of organizations eligible for certification in compliance with the ATEX directive is available on the Internet, on the European Commission’s website). If the certificate presented by the supplier of such silos has been issued by an organization not present on the list, it is simply invalid, and these silos are treated as devices without a certification.

Figure 4: The dust around trevira silos in the bakery. Such conditions are dangerous – the whole room requires additional ATEX security and must be classified as zone 20 “permanent explosion risk”

The description above contains the most common silos types. Of course in many places also other silos types can be found: made of concrete, made of wood or other materials. Such solutions are not common, mostly those are the silos build many years ago and still in use. The principle of flour storage and dosing stays the same. In the next articles, we will talk more about silos security, flour transfer, and dosing systems and quality of dosing.