By Jim Kent and Wolfgang Haucke
It goes without saying that a modern food or beverage plant handles lots of powders, either as raw ingredients or finished products. We’re talking dairy or infant formula powder, sugar, salt, flour, a flavor or any of a myriad of dry-bulk ingredients available today. To handle all this stuff, plants are equipped with powder transport and conveying systems as well as powder silos, feeders, blenders, de-baggers and filling machines.
For those charged with the responsibility, a frequent question is whether this equipment should be wet-cleaned with a water-base cleaning solution. And once that decision is made, the next is whether effective cleaning-in-place (CIP) is possible.
Whether wet cleaning should be undertaken is purely the plant manager’s decision. But, as to whether effective CIP of the equipment is possible, the answer is a resounding yes.
For those unfamiliar with CIP, it is the cleaning of equipment by an automatic system with little or no disassembly or operator intervention involved. A system includes a CIP Kitchen, with water and cleaning solution tanks, pumps, heaters and valves; as well as pipes and valves for cleaning solution circulation to and from the equipment being cleaned.
When a powder-transport line is cleaned, it is usually flooded so that liquid velocity sufficient for cleaning is established. When a vessel is cleaned, appropriately designed nozzles inside it affect coverage within it.
Two wet-cleaning alternatives to CIP are manual cleaning, where equipment is opened up and cleaned by operators in its working location, and cleaning-out-of-place (COP), where equipment is disassembled and put in circulating baths to be cleaned semi-automatically. Both options are labor intensive and the results often inconsistent.
Equipment dry cleaning is also a valid alternative, and often used to extend operational cycles between wet cleanings. Dry cleaning, though, is labor intensive, raises operator safety concerns and introduces opportunity for contamination solely through the occasion of operator intervention.
Should food powder systems be wet cleaned?
The single, most-often-cited reason not to wet clean, and it is a valid one, is that if dry-powder equipment is never wet cleaned, bacteriological growth is almost impossible to initiate, unless caused by outside factors.
Never wet-cleaning dry-powder equipment is a valid approach to maintaining hygiene within a plant’s dry areas. Many plants operate this way safely for years. But if an episode of contamination occurs, cleaning must be done by other means.
Reasons that justify wet cleaning include:
• Product or batch segregation
• Allergen class segregation
• Bacteriological contamination
• Contamination due to out-of-spec product
• Product build-up causing a safety hazard
If reasons for wet cleaning, as outlined above, outweigh considerations for not doing so, and appropriate cleaning, operations and quality procedures are implemented to wet clean the plant, then the final decision is whether CIP cost outweighs the operational cost following from longer downtimes and additional labor for manual wet-cleaning or COP.
It has been proven time and time again that dry-powder equipment can be CIP cleaned effectively, given appropriate designs from the outset and clear goals for using CIP. Some plants do CIP once a year as part of a quality-assurance regime. For them, downtime and water usage may not be critical, but an effective, safe plant cleaning has value. A CIP regime like this may take 24-36 hours during an annual plant shutdown period.
At the other end of the spectrum are plants whose production schedules mandate changing batches and allergen classes several times per day. Equipment must be cleaned, dried and back in operation quickly. Typically, a CIP cycle — from off-product to on-product — needs to be completed in an hour or less.
Process of CIP cleaning
Using water and cleaning chemicals to clean-in-place stainless-steel equipment, five parameters must be understood and controlled:
• Duration of cleaning
• Cleaning solution temperature
• Solution pressure when spraying in an open vessel
• Solution velocity when cleaning a line
• Chemical strength
Sometimes high-pressure spray breaks down product build-up so quickly that only a low-temperature water solution is required. In others, product build-up accumulated over months is so severe that CIP solution is cycled to different areas, allowing chemicals to soak into a build-up layer for a period of minutes before solution is sprayed again, washing away one layer of build-up and exposing a new one.
CIP time: Surface spraying cleaning solution to remove thick or aged build-up takes time. When cleaning time must be reduced, the other four parameters come into play, or intervals between CIP cleanings must be shortened.
CIP solution temperature: Generally, the hotter the cleaning solution — whether plain water or with cleaning chemicals — the more effective the cleaning. However, cleaning chemicals have an upper temperature limit above which they begin to lose effectiveness. Products, especially proteinacious products, can be “cooked” at high cleaning temperatures, becoming insoluble or difficult to remove. The rule of thumb is to use the lowest temperature solution possible to affect the cleaning based on product involved and chemicals chosen.
Pressure: When cleaning an open vessel like a silo or feeder, i.e., something not flooded during cleaning, higher cleaning-solution spray pressures facilitate build-up removal. But it’s possible to get to the point where higher input pressures at the nozzle won’t deliver higher nozzle output pressures.
Velocity: When a line or small vessel is flooded for cleaning, adequate cleaning velocity is generally considered to be 1.5 meters per second. Simple calculations allow the system to achieve this. Stubborn or denatured build-up may more easily be removed with higher cleaning velocity, but generally the other parameters are adjusted to improve the cleaning.
Chemical Strength: Given their expense, reduce chemical use to the lowest level permitting effective cleaning. Chemical strength can be adjusted for particularly stubborn products.
CIP cleaning equipment
Regardless of installation particulars, design standards exist to ensure normal powder processing combined with CIP can be accomplished. As mentioned above, CIP equipment generally falls in two areas: 1) CIP Kitchen of tanks, pumps, heaters and valves for central preparation of cleaning solution and 2) field equipment of valves, return pumps and nozzles to affect required cleaning.
Given chemicals use, high pressures and elevated temperatures, personnel safety is paramount. Review plant procedures before implementing any technique discussed in this article, especially if CIP is new to you.
Equipment must be able to contain the liquid cleaning solution within its vessel. Don’t assume equipment that can contain powders will necessarily contain liquids properly. Powder-conveying and transport lines generally hold liquids at pressure, especially when equipped with fittings for vacuum or pressure conveying. Most other vessels are atmospheric and not meant to hold the pressure of a head of water, particularly if designed to pressure-vessel standards.
Before initiating CIP inspect:
• Gasketing of any powder blenders used
• Flexible connectors on vibrating bin bottoms, drop ducts or other construction elements
• Manways, including on lower sidewalls of hoppers and vessels
• Instruments and their connections
Focus on nozzles
Of particular interest are nozzles used in CIP of powder-handling equipment. They may be permanently mounted — thereby reducing set-up time — or removable and thus allow device maintenance during production.
Nozzles fall into three basic categories: static, rotating and orbital.
Static nozzles, often called spray balls, and looking like a ball on a stalk with holes drilled in it — clean vessels, tanks and containers under low pressure. Fixed heads spray cleaning medium onto the surface medium, with cleaning achieved by rinsing or impingement of the tank walls. Adding appropriate cleaning agents reduces cleaning time. 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.
Static nozzles are generally the lowest cost, easiest to retrofit into a vessel and come in a variety of sizes and spray patterns. Each aspect within a vessel or system can have a uniquely assigned nozzle best suited to its application.
It’s difficult to clean the largest vessels, like silos, with spray nozzles. They also protrude into the vessel and are therefore generally not left in place during production. Because this adds to set-up time, spray balls aren’t the best choice when time is a leading consideration.
Rotating cleaners are used on tanks, vessels and containers with heavy product encrustations (e.g. larger storage tanks, fermentation tanks, tanks with internal agitators). They work under low pressure; a flow gear unit generates a fan-shaped jet, which slowly rotates in one plane, thereby wetting the entire surface. 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 from 80 C to 100 C are possible.
Rotating cleaners can be removed with each cleaning or left in place. Today some are automatically retractable and flush mounted, staying in place without interfering with the product flow.
Orbital cleaners, for tanks, vessels and containers needing special mechanical treatment of inner surfaces by a concentrated jet (e.g. road tankers, product tanks and kegs), 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. 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.
The means to dry-out powder transport lines equipped with blowers are readily available. Turn on the blower for 10-15 minutes prior to introducing powder and the system will be fine.
However, most vessels and powder silos don’t have facility to create heat and airflow for effective drying in a reasonable amount of time. If an entire powder-handling facility need be dried at once and in a short time, then a dedicated arrangement of fans and heaters may be needed. If only one area of a system gets cleaned at a time, a portable fan and heater may dry out each area consecutively or as needed. Either way, how best to accomplish dry out of process equipment needs to be carefully considered when doing CIP.
Bakery goods maker case in point
Achieving effective cleaning for a flour silo involves applying different cleaners and cleaning methods to ensure optimized hygiene. GEA Tuchenhagen engineers worked closely with a bakery goods maker to find the best way to clean the operation’s flour silos.
Bakery goods are made to the highest quality standards, in compliance with extensive environmental and economic considerations. Given its cyclical production processes, the quality assurance staff wanted an advanced cleaning system to clean the insides of its flour storage silos. Hygienic requirements mandate no remaining residues for all systems and machine components used.
Located outside, near the production building, the cylindrical storage towers are arranged in a silo farm. Each is approximately 3.5 m in diameter and 33 m in height, and has no internal structures. The silo walls are made of un-insulated aluminum and each has a conical outlet and a flat silo top with an eccentric manhole.
Flour discharges 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 build up at various points and levels; in particular, flour lumps form at all heights of the silo wall, which, with the level 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.
Type, thickness and adhesion behavior of the contamination is largely determined by flour quality, its flowing and emptying properties, discharge rate, storage-silo and supplier transport-silo air humidity, seasonal temperature fluctuations and other parameters.
In the past, cleaners and industrial climbers entered the silos equipped with manual lifting gear and watched by a safety supervisor. Flour residues — from light dust to heavily encrusted or sticky residues — were removed using brushes or brooms for light contamination or with spatulas and scrapers, in miner’s fashion, for stubborn residues.
Unfortunately, workers needed to be provided with breathable air. The mental and physical strain was extremely high. Cleaning took several hours or even an entire day. Efficiency and results varied from cleaner to cleaner. Due to the eccentric manhole, positioning personal safety and lifting gear for the workers was complicated and time-consuming.
To minimize cleaning time and effort, remedy existing problems and ensure repeatability, the company sought an improved cleaning process. An essential prerequisite was unreserved compliance with all customer requirements regarding food-hygiene regulations.
Cost-effectiveness, minimizing cleaning times, cleaning media, utilities and auxiliary materials and system sustainability were important to the bakery-product manufacturer. An inventory of requirements, technical details and on-site conditions was recorded and initial engineering considerations were subsequently translated into a cleaning concept, which was then put to a practical test.
As a next step, an orbital cleaner was chosen, with a suitable nozzle and cleaning pattern for the selected pressure-cleaning method, in accordance with the type of contamination involved.
Once engineering considerations for the selected method were aligned with the customer’s requirements, decisions could be made. From the start, relatively low-priced spray balls were excluded due to the sometimes very high degree of contamination involved. A rotating jet cleaner would work in the silo’s upper section, but it wouldn’t have been possible to implement the optimum cleaning line near the bottom of a 33-meter high silo. To avoid adding the pumps that then would have been necessary, possibilities in regard to medium- and high-pressure cleaning weren’t pursued.
Due to on-site conditions, contamination type and silo geometry, a low-pressure method was selected for optimized cold water-based cleaning, which typically works with a pump capacity of 8-9 bar. 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 weren’t used. Given 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. Engineers expected short cycle times when the cleaning result was first assessed, so it was decided to discharge the cleaning water into the on-site wastewater system.
To test the selected orbital cleaner under actual 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 running the process for just three minutes, a large part of the adhering, even most critical, contamination was already removed from those silo surfaces covered by the strong cleaning jets. After an overall 15-minute cleaning, all contamination, including stubborn flour encrustations, was gone.
Despite the cleaner’s eccentric position, no oscillating movement was needed in the silo, and a jet pattern was generated that covered the entire surface of the silo, even in the deeper zones.
As for drying, due to the seasonally ideal conditions for the cleaning process and with the silos 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 economic viewpoint. To allow any residual water to evaporate easily, the upper manhole and connection in the bottom section of the silo outlet cone were opened to enable optimum venting and discharging of moisture.
Repeatable and effective
In similar applications where the benefits of the sun aren’t available, using hot water as a cleaning medium lends itself as a supplementary solution. The hot water heats up the silo walls during cleaning and the silo insides are dried by convection. If hot water isn’t available, another reliable drying solution for the silo contact surfaces would be 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 maintained.
Approximately 3,000 liters of cold water is used in the cleaning, and is discharged into the factory wastewater system together with the removed flour.
Final laboratory analysis of the silo surface samples confirmed that the expected results were achieved. The process is repeatable and effective. 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.
Jim Kent is business development manager with GEA Nu-Con Ltd, GEA Colby Ltd, Jim.firstname.lastname@example.org.Wolfgang Haucke is an applications engineer with GEA Tuchenhagen GmbH, Wolfgang.email@example.com.
GEA Process Engineering is a full-service engineering company offering systems and solutions for the food, dairy, beverage, chemical and pharmaceutical industries. The company is known for innovative processing systems for drying, granulation, evaporation, crystallization, filtration, heat and mass transfer, aseptic PET bottle filling, powder handling and packaging, tableting, mixing, containment, cleaning (CIP) and fermentation.