In this practical example, the development of an effective cleaning technology for a flour silo involved applying the different available cleaners and cleaning methods to ensure optimized hygiene.

About 7 billion people are currently living in this world, and by 2050, it is estimated that this number will rise to 9 billion. About 50% of the people who will populate Earth in 2050 are already living today. This has become possible because modern production technologies make it easier for all people to live better and healthier lives. In particular, advances in food manufacturing technology have contributed significantly to improvements in the quality, functionality and safety of food. 

While dramatic technological advances offer vast opportunities, they also present enormous challenges. The efficient and cost-effective production of high-quality food products, which new processing technologies foster, is necessary to meet the needs of a burgeoning global population. However, production efficiencies and cost-effectiveness cannot come at the expense of meeting the most stringent of cleanliness and safety requirements for food manufacturing and handling. Ensuring that all of these needs are met requires a very detailed yet broad process know-how. The comprehensive process expertise of engineers today must therefore be in sync with the food processing industry’s need to ensure that products are produced with the highest level of efficiency, hygiene and safety. 

Optimizing cleaning processes in a processing plant is one important way to accomplish these goals. As an example, GEA engineers worked closely with a well known manufacturer of bakery products to find effective solutions to the challenges of cleaning the operation’s flour silos.

Flour Silos: A Cleaning Challenge

Flour Silo
Component to be cleaned: an aluminium flour silo, 3.5 m in diameter, 33 m in height

The bakery manufacturer’s products are made to the highest quality standards and are in compliance with extensive ecological and economic requirements. The hygiene requirements placed on the process plant make it necessary to clean all systems and machine components used for this type of production to a level at which no residues remain. For these cyclically repeated production processes, the customer’s quality assurance staff was looking for an advanced cleaning system for the perfect interior cleaning of their flour storage silos. 

The cylindrical flour silos, approximately 3.5 m in diameter and 33 m in height, which were presented to engineers for examination and advice on cleaning do not have any internal structures. The silo walls are made of uninsulated aluminium and each has a conical outlet and a flat silo top with an eccentric manhole. Located outside near the production building, the storage towers are arranged in a silo farm.

The flour is discharged from the silo by gravity onto a conveyor worm, and compressed air is used for further conveyance downstream. Production runs 24 hours a day all year long, and as such, each silo is periodically completely filled with flour and is then emptied continuously or intermittently, depending on process requirements. As a result, the inside of the tank is irregularly contaminated with product residue deposits.

These contaminants build up at various points and various levels; in particular, lumps of flour form at all heights of the silo wall, which, after the level has risen to a certain point, tend to drop down uncontrollably and cause recurring blockages with subsequent standstill of the downstream flour conveying and production plants. This results in cost-intensive production downtimes for remedying the damage. The type and thickness as well as the adhesion behaviour of the contamination is largely determined by the quality of the flour; the flowing and emptying properties of the flour, depending on the discharge rate; the air humidity in the suppliers’ transport silos and in the storage silo itself; and the seasonal fluctuations in temperature and other parameters.

The previous cleaning process was such that hired cleaning workers/industrial climbers - equipped with manual lifting gear and watched by a safety supervisor, entered the silos in order to clean them. Flour residues, which vary from light dust to heavily encrusted or sticky residues, were then removed either using brushes or brooms for light contamination or with spatulas and scrapers, in miner's fashion, for stubborn residues. The main drawback of this solution not only was that the mental and physical strain for the workers, who had to be provided with breathable air, was extremely high, but cleaning also took several hours or even an entire day.

Additionally, the cleaning efficiency and results varied from cleaner to cleaner and the result was not repeatable. Due to the eccentric manhole, positioning of the personal safety and lifting gear for the cleaning workers was complicated and time-consuming. In order to minimize the time and effort described above as much as possible and to remedy existing problems, the company sought an improved cleaning process based on water with reliably repeatable results. An essential prerequisite was unreserved compliance with all customer requirements with regard to food hygiene regulations.

Cost-effectiveness, minimization of cleaning times, cleaning media, utilities and auxiliary materials, and system sustainability also were of foremost importance to the bakery product manufacturer. An inventory of requirements, technical details and on-site conditions were recorded during a personal visit to the site. These initial engineering considerations were subsequently translated into a cleaning concept, which was then put to a practical test (i.e., basic engineering).

A key initial step in the preparatory considerations was to clarify the basic approach; namely whether a low-, medium or high-pressure cleaning method should be used. 

The following methods were assessed:

  • Low-pressure cleaning is based on the effect of the chemical composition of the cleaning agent, the temperature and the volume flow rate of the cleaning medium, and the resulting cleaning velocity. This is an ideal application for spray balls and rotating jet cleaners.
  • Medium-pressure cleaning is based on the effect of the chemical composition of the cleaning agent, the temperature and the reduced volume flow rate of the cleaning medium at an increased cleaning pressure, and the resulting cleaning velocity. This is an ideal application for rotating nozzles and rotating jet cleaners.
  • High-pressure cleaning is based on a mechanical cleaning effect that is achieved by an intensive, direct cleaning jet. This is the typical field of application for orbital cleaners.

As a next step, a suitable nozzle and cleaning pattern for the selected pressure cleaning method was chosen from the following nozzle systems, in accordance with the type of contamination:

Static cleaners for the cleaning of vessels, tanks an  containers, such as storage tank and clean-in-place (CIP) tanks, are designed to work with low pressure. A fixed spray head sprays the cleaning medium onto the surface to be cleaned. Cleaning is achieved by rinsing or impingement of the tank walls. By adding appropriate cleaning agents, the cleaning effect can be enhanced while cleaning times are reduced. The flow rate ranges between 2.4-42 m3/h, at a pressure difference of 1 bar. The cleaning diameter is 0.8-8.0 m.

Rotating cleaners are used for the cleaning of tanks, vessels and containers with heavy product encrustations (e.g. larger storage tanks, fermentation tanks, tanks with internal agitators). These cleaners are designed to work with low pressure; a flow gear unit generates a fan-shaped jet, which slowly rotates in one plane, thereby wetting the entire surface. The flow rate ranges between 7.1 and 28 m3/h, at a supply pressure of 2.3-4.3 bar. The cleaning diameter is 2 to 10 m. Depending on the material, operating temperatures in the range between 80°C and 100°C are possible.

Orbital cleaners for the cleaning of tanks, vessels and containers that require special mechanical treatment of the inner surfaces by a concentrated jet (e.g. road tankers, product tanks and kegs), are designed to work with low, medium or high pressure. A flow gear unit generates a highly concentrated cleaning jet that rotates in two planes. The ideal jet geometry is produced by specially shaped round-jet nozzles and bevel gears that produce a dense orbital cleaning pattern which covers the entire surface to be cleaned. The flow rate ranges between 1.8 and 27 m3/h, at a supply pressure of 4.5-80 bar. The cleaning diameter is 2 to 14 m.

Orbital Tank Cleaner Tempest
Orbital Tank Cleaner Tempest

When the engineering considerations for the method to be selected were finally aligned with the customer’s requirements, the relatively low-priced spray balls were excluded right from the start due to the degree of contamination, which can be very high at times. The rotating jet cleaner would have worked in the upper section of the silo, but it would not have been possible to implement the optimum cleaning line near the bottom of a 33-m high silo. To avoid an additional investment on the customer’s part for the necessary pumps, considerations with regard to medium- and high-pressure cleaning were not pursued.

Due to on-site conditions, the type of contamination and the silo geometry, a low-pressure method was selected for optimized water-based cleaning, which typically works with a pump capacity of 8-9 bar, cold water. As no external utilities were available on the silo dome, a turbine-powered cleaner was selected for testing. For cost reasons, cleaning chemicals and thermal support for the cleaning process were not to be used.

Considering an installation height of more than 33 m, an orbital cleaner with four nozzles of 7 mm each was selected, which discharges approximately 12 m3/h cleaning water at a working pressure at the cleaner of approximately 5 bar. The engineers expected short cycle times for cleaning when the cleaning result was first assessed, so it was decided to discharge the cleaning water into the onsite waste water system.

To test the selected orbital cleaner under the given conditions, the cleaner was connected via a pressure hose to a centrifugal pump placed on the bottom of the silo and then introduced eccentrically into the silo and positioned at an immersion depth of 2500 mm and at a lateral distance from the wall of 500 mm.

After positioning the cleaner, the cleaning process was started and monitored. When the process was stopped after three minutes, a large part of the adhering, even critical, contamination had already been removed from those silo surfaces that were covered by the strong cleaning jets. This cleaning result, achieved just after a few minutes, confirmed that the selected path was correct. After an overall cleaning duration of just 15 minutes, all contamination, especially stubborn flour encrustations, were removed. Despite the cleaner’s eccentric position, it worked without any oscillating movement in the silo, while generating a jet pattern that covered the entire surface of the silo, even in the deeper zones.

After completion of a water-based cleaning of the silo another point to consider was drying, as this is an indispensable step from a process engineering point of view.

Due to the seasonally ideal conditions for the cleaning process described and as the silos are installed outdoors, it was decided to remove residual moisture by convection. Direct sunlight on the surface of the silo ensured sufficient drying from a technical and economical viewpoint. To allow any residual water to evaporate easily, the upper manhole and the connection in the bottom section of the silo outlet cone were opened to enable optimum venting and discharging of moisture.

In similar applications where it is not possible to dry the silos by solar radiation, using hot water as a cleaning medium lends itself as a supplementary solution. The hot water heats up the silo walls during cleaning and afterwards dries off the inside surface of the silo by convection.

If hot water is not available for cleaning, another feasible solution for the reliable drying of the silo contact surfaces is blowing filtrated hot air into the tank via the openings at the top and bottom. Attention must be paid here that sufficient air flow rates are selected.

Approximately 3,000 litres of cold water were consumed to achieve the cleaning result, which were discharged into the factory wastewater system together with the removed flour.