How do you clean a flour silo?
This overview of a practical application example demonstrates the development of an efficient cleaning solution for a flour silo. This first of all involved finding the right technical approach to ensure highly economical cleaning at the required level of hygienic safety. The subsequent selection and configuration of the right cleaner type was facilitated by the comprehensive GEA Cleaning Technology portfolio.
More than 8 billion people are currently living in this world, and by 2060 this number is estimated to rise to 10 billion. 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.
Even more efficient and cost-effective production of high-quality food products is necessary to meet the needs of a growing global population. However, efficiency and cost-effectiveness cannot come at the expense of meeting the stringent demands of cleanliness and safety in food manufacturing. Ensuring that all of these needs are met requires a very detailed yet broad process know-how. Comprehensive process expertise must be in sync with the food processing industry’s need to ensure production with the highest level of efficiency, hygiene and safety.
Optimizing cleaning performance 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, finding effective solutions to the challenges of cleaning the customer’s flour silos.
Flour Silos: A Cleaning Challenge
The bakery products made by our customer are produced to the highest quality standards and in compliance with today’s ecological and economic requirements. The hygienic standards in the process demand a high level of cleanliness of all systems and machine components connected to food processes. In order to optimize this cyclically repeated process, the customer’s quality assurance staff requested an advanced cleaning system for the interior cleaning of their flour storage silos.
The cylindrical flour silos, approximately 3.5 m in diameter and 33 m in height, were presented to our engineers for examination and advice on cleaning. The silos have no internal structures, with walls made of uninsulated aluminum. Each silo has a conical outlet and a flat silo top with an eccentric manhole. Located outside near the production building, these 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.
The production runs 24 hours a day all year long, and as such, each silo is periodically completely filled with flour and 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. They then 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 behavior 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 cleaning process over decades 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. This procedure not only meant extremely high mental and physical strain for the workers, who had to be provided with breathable air. Cleaning also took several hours or even an entire day. Additionally, cleaning efficiency varied from worker to worker and the results were not repeatable. Due to the eccentric manhole, positioning of the personal safety and lifting gear for the cleaning workers was complicated and time-consuming.
The company now sought a modern cleaning process based on water with reliably repeatable results. One essential prerequisite was uncompromising compliance with all customer requirements and regulations regarding food safety and hygienic production environment. But cost-effectiveness, meaning minimization of cleaning times, cleaning media, utilities and auxiliary materials, was also of great importance to the manufacturer, as was the sustainability of the system. An inventory of requirements, technical details and on-site conditions was 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).
Dynamic Sinner’s Circle
Assessing the factors for successful cleaning
When configuring effective cleaning solutions, GEA engineers refer to the “Dynamic Sinner’s Circle” to illustrate the interdependence of important technical factors. The areas in this circle representing the different factors vary in percentage from application to application, depending on how best to achieve the targeted cleaning result.
- The temperature of the cleaning medium is a factor that can reduce the need for mechanical action and chemical cleaning agents. However, the required heating of the water consumes energy and impacts the environmental footprint of the operation. In the case discussed here, the manufacturer of bakery products opted for a solution without heating, for cost and sustainability reasons.
- Chemical cleaning agents can support and accelerate the cleaning effect but will cause additional operating costs and possible safety issues – at the very least, they must be thoroughly rinsed out in a separate step. For this reason, the use of cleaning agents was also excluded from consideration in the presented case.
- Cleaning time can be extended to diminish the role of the other factors and improve cleaning results. However, this is only possible to a certain extent. In the case of a tank as tall as a flour silo, with residues that can at times be stubbornly adhesive (soiling class IV in GEA’s 4-level classification system), a high level of mechanical impact will be mandatory.
- Mechanical impact, optimized through sophisticated technical principles, can reduce or eliminate the need for high temperature and cleaning agents. GEA offers an extensive range of cleaner types and options to achieve this, no matter which soiling class is being addressed.
Aligning these engineering considerations with the requirements of the bakery products manufacturer led to the identification of the most suitable cleaner type. Low-priced spray balls were excluded as they are designed for easy-to-remove liquids (soling class I), not the degree of contamination typical of flour silos. Free rotating cleaners or slow rotating cleaners, designed for soiling classes II and III, would have been strong enough to work in a smaller silo, but not for the application at hand.
In an orbital cleaner, an internal gear arrangement generates a slow rotation of the cleaner body around the vertical axis and the nozzle carrier around the horizontal axis – thus, producing a dense cleaning pattern. By using specially shaped round-jet nozzles and stream straighteners, ideal and highly concentrated jets are produced which cover the entire surface to be cleaned.
GEA orbital cleaner Tempest
The orbital cleaners are driven by the cleaning medium that starts an internal rotor, which was suitable for this customer’s situation as there were no external utilities on the silo dome. The orbital cleaners are also designed to operate at low, medium, or high pressure as needed, which helped meet the customer’s requirement to avoid additional investment for high-pressure pumps. Considering an installation height of more than 33 m, the engineers selected an orbital cleaner with four nozzles of 7 mm each for each silo. This cleaner discharges approximately 12 m3/h cleaning water at a working pressure of approximately 5 bar.
To meet even more complex demands for cleaning performance and sustainable water consumption, GEA customers can turn to modular orbital cleaners such as the OC200 type that can be equipped with different nozzles and nozzle carriers tailored to the exact application – even changing applications and reusing the cleaner for different tasks.
GEA orbital cleaner OC200
Testing the selected cleaner configuration
To test the 4-nozzle orbital cleaner selected for the baking flour silo under the actual application conditions, the cleaner was connected via a pressure hose to a centrifugal pump placed on the bottom of the silo, then introduced eccentrically into the silo and positioned at an immersion depth of 2500 mm with 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, there was no oscillating movement in the silo, thanks to the balanced design of the cleaner. The jets generated a pattern that covered the entire surface of the silo, even in the deeper zones.
Repeatable cleaning performance and efficiency
The final laboratory analysis of the silo surface samples confirmed that the desired and expected results had been achieved. The water-based cleaning process selected and described in this article is repeatable under the conditions determined and achieves the desired result efficiently and effectively. In addition, the process defined allows for intermediate cleaning at any time in the event that contamination increases. Expensive external cleaning specialists are no longer required as water-based cleaning can easily be carried out by the customer's own staff and without any expensive production downtimes.
The right cleaner for the right task
Soiling classification systems determine the mechanical cleaning power required for effective cleaning. We have defined four soiling classes to describe the degree of mechanical force that should be used and the recommended type of cleaners.
Soiling Class I – Rinse cleaning
Easy cleaning conditions Water-soluble products with little or no tank adhesion Recommended: Static Cleaners
Soiling Class II – Low impact cleaning
Moderate cleaning conditions Water-soluble pro ducts with low adhesion Recommended: Free Rotating Cleaners
Soiling Class III – Medium impact cleaning
Difficult cleaning conditions Stubborn residues with medium tank adhesion Recommended: Slow Rotating Cleaners
Soiling Class IV – High impact cleaning
Highly difficult cleaning conditions Encrusted or dry products with high adhesion Recommended: Orbital Cleaners and Index Cleaners
