h-AJMixing-60-73:C-Masters_old 1/3/08 8:57 AM Page 1 Batch agitating mixer selection: Doing your homework A.B. Flower A&J Mixing International This article explains what you should know before selecting a new batch agitating mixer. Sections cover questions you need to answer before shopping for the mixer, how to interview mixer manufacturers, and common mistakes to avoid before you buy. I f your bulk solids processing plant is like most others, you’re probably only occasionally in the market for a new batch mixer. This makes it even more important to do your homework before you buy. Understanding your mixing application, talking with mixer manufacturers, and then investigating the most promising batch agitating mixers for your application are all part of the job. Before you meet with mixer manufacturers Knowing as much as you can about your mixing application before approaching mixer manufacturers will help you accurately define your project so you can streamline the mixer selection process. If you’re mixing a common product that most manufacturers have experience with, you may not need to provide more than a few basic details about the product. But if your product is unusual, you’ll need to provide several details, such as what ingredients your mixture includes and each ingredient’s percentage of the total mixture, particle size, and weight per cubic foot. You should also be prepared to answer the manufacturers’ more specific questions. Answering the following 15 questions before you meet with mixer manufacturers will lay the groundwork for selecting the right mixer. 1. Does your product mixture have any unusual characteristics, such as stickiness or lumpiness? For instance, if your mixture includes materials that tend to form lumps, such as pigment, zirconium, or titanium dioxide, you may need a mixer that applies shear or other techniques during mixing to eliminate lumps. 2. Does your product mixture include any liquids? If so, what is each liquid’s percent of the finished mixture? What is its temperature and viscosity? This information can determine what type of liquid sprayer your mixer needs and how much volume the mixer requires so it has the right amount of empty space above the material bed to promote efficient liquid addition. 3. Are you currently mixing this product? If so, what type of mixer are you using? What problems — such as with mixture quality, repeatability, reliability, and throughput volume — do you want to overcome? For instance, you may be looking for a new mixer because your current mixer doesn’t achieve the throughput volume you want and uses too much of an expensive ingredient, such as pigment in a grout mixture. 4. Does your plant receive the ingredients in bulk carriers, bulk bags, or small bags? This will determine whether the mixer will be automatically or manually fed and what type of upstream handling equipment or ancillary equipment must connect to the mixing system. 5. How will the ingredients be loaded into the mixer? How much time does your process allow for loading? For instance, feeding ingredients in bulk to the mixer requires a mixer with an automatic feeding system, while manually h-AJMixing-60-73:C-Masters_old 1/3/08 8:57 AM Page 2 feeding ingredients from small bags into the mixer requires an opening — such as an inlet door — in the mixer that’s integrated into the plant’s dust collection system with an appropriate dust capture hood or similar device. To avoid delaying your downstream process, the mixer and related equipment may also have to be designed to speed feeding. 6. What equipment or vessel will the mixer discharge to? This determines what kind of discharge the mixer will have and how much space it needs. For instance, on a horizontal mixer, a drop-bottom discharge is more compact than a discharge port at one end of the mixer, but the dropbottom discharge must empty into a hopper — which also consumes space — to control the mixture’s flow to downstream equipment. If the mixer will discharge to two destinations, such as into a bulk bag and to a small bag filler, the mixer must have discharge ports on either end, and they will consume even more room. 7. Will processes other than mixing, such as shearing, particle size reduction, heating, or cooling, be required in the mixer to make your finished product? For instance, a sticky mixture can require a mixer that can apply shear during mixing to eliminate lumps, and a heat-sensitive mixture can require a mixer fitted with a cooling jacket to avoid product degradation. 8. What product throughput volume do you require per shift based on the minutes worked in that shift? The mixer manufacturer will calculate the throughput volume per shift your mixer must provide based on the number of minutes your operator runs the mixer during one shift. So you need to explain whether your operator has an 8-hour shift with no breaks (480 minutes), an 8-hour shift with a 30-minute lunchtime (450 minutes), an 8-hour shift with a 30-minute lunchtime and two 15-minute breaks (420 minutes), or another schedule. 9. Does your company have a fixed budget for the new mixer? What return on investment does your company expect? This information can help you determine whether a more expensive mixer that provides a fast return on investment may be a better buy in the long run than a less expensive unit. 10. Does the area where the mixer will be installed have any space restrictions? Narrow aisles or low ceiling height in your plant can limit the mixer size, which is a good reason to check out smaller mixers that can provide high throughput volume. Be aware that space restrictions imposed by your application requirements — such as feeding ingredients from three supply hoppers to one mixer — can complicate the design and installation of even a small mixer. 11. If your plant has limited space, do you need the mixer manufacturer to provide computer modeling of the mixing process? Computer modeling (such as with Solid Works software) of the mixer and related equipment in your process layout can help ensure that the mixer you select will fit precisely into your tight space. As the mixer manufacturer adjusts various mixer dimensions during the design process to help fit the mixer into your plant, computer modeling can help the manufacturer calculate the throughput volume the mixer can achieve each time one dimension is altered. 12. What do you expect from the mixer manufacturer? If your plant is small, you may not have an in-house engineer who can choose and integrate the mixing system’s nonmixing equipment, such as supply hoppers, conveyors, feeders, and controls. In this case, you’ll need to work with a mixer manufacturer that can design and install the entire system. Depending on your available in-house expertise, you may also want the manufacturer to provide startup assistance at your plant after the mixer is installed. 13. Does your mixer application have any special requirements, such as for food-grade construction or washdown between batches? For instance, if your mixture is a food or drink product, the mixer you choose must have food-grade construction. If your mixture is a pharmaceutical or fine chemical product, you’ll need a mixer equipped with a clean-in-place system so it can be washed down between batches. 14. What construction materials does your mixer require? There are several options, including carbon steel, carbon and abrasion-resistant steel, stainless steel, or stainless abrasion-resistant steel, and which one is right for your mixer depends primarily on the product you’ll be mixing. For instance, if your product is a mineral mixture for animal feed, it will wear out stainless steel very quickly. A better option for this application is a mixer with interior parts constructed of carbon steel or an abrasion-resistant steel. 15. When will you be ready to place your mixer order, and when do you want the mixer to be up and running in your plant? This information will help the mixer manufacturer determine whether it can supply a mixer by the time you need it — an especially critical factor when you have a short project deadline. When you meet with mixer manufacturers When you interview mixer manufacturers, ask them about their hardware — that is, the mixer itself — and their mixing know-how. Hardware. Ask if the manufacturer will be able to supply a mixer with the features you need. Also ask how the h-AJMixing-60-73:C-Masters_old 1/3/08 8:57 AM Page 3 mixer will be constructed to minimize downtime and maintenance once it’s operating in your plant. Since the largest maintenance costs for batch agitating mixers are for servicing the mixer discharge and shaft seals, ask how these parts and the end plates, rotors, and shafts will be constructed and installed. Also ask about the mixer’s durability. Will it start under load without damage to the motor or other parts? Being able to safely start the mixer under load after a power outage or other interruption is critical to maintaining your process efficiency and maximizing the mixer’s service life. bly be around 1,000 inch-pounds of force per cubic foot. 2. Accepting wide tolerances and improper finishes on the mixer’s housing and rotor. Poorly fitting discharge doors, a wobbling rotor, and rough interior finishes on welds and other surfaces are signs the manufacturer is trying to build cheap. You’ll pay the price with product quality and mixer cleanout problems caused by material buildup over the life of the mixer, which is typically about 25 years. Ask questions to determine how the mixer will affect your operating costs. For instance, can the manufacturer customize the mixer to meet your needs, such as by designing the agitating elements to apply more shear or by installing a clean-in-place system that completely washes out the interior between batches? Such customization can greatly reduce your operating costs. Mixing know-how. Ask whether the manufacturer has had experience mixing your product, and based on this, what mixing performance level the manufacturer expects the mixer to achieve with your product. Ask the manufacturer whether it has test equipment that can run your product to demonstrate the mixing results. Before you decide to buy After interviewing various manufacturers, you’ll be able to narrow the list of mixers that are suitable for your application. The following are eight costly mistakes to avoid when making your final selection. This mixer’s high-quality construction is easy to see in the smoothly ground welds on its agitator elements and joints and in its polished and buffed interior walls. 3. Failing to notice a too-thin housing. A thin mixer housing will wear out prematurely and cause problems like preventing the mixer’s discharge from sealing properly. You’ll be living with the maintenance costs associated with these problems over the next 25 years, too. 1. Buying on price. If you’re buying the cheapest mixer, beware. There’s typically not a big price difference among comparably sized and equipped mixers. If one is markedly cheaper, chances are it’s because something was left out to reduce the mixer’s cost. For instance, the agitator elements may be designed with less steel. But this will also reduce the elements’ strength and durability. 4. Choosing a mixer that’s too small. If the mixer you buy is just large enough to handle your current product, how will you handle a recipe change? Chances are, the mixer will be used for more than one product over the years it works in your plant. Buying flexibility for future expansion in the mixer you choose today is a lot less expensive than replacing the mixer later. More often, the missing item is motor horsepower, producing an underpowered mixer that won’t be able to start under load. How will you start this mixer after a power outage or after you’ve topped off the mixer load with manually fed ingredients? The mixer’s horsepower rating alone can’t tell you what the mixer’s true power is. Instead, a good rule of thumb for calculating the true power is to look at the torque per cubic foot of capacity: torque = horsepower ⫻ 63,000/rpm/ft3. If a mixer’s torque is less than 700 inch-pounds of force per cubic foot, including that required by the mixer’s agitating elements, and will mix sticky materials or those with high internal friction (such as sand), be wary: The mixer’s torque should proba- 5. Depending on the mixer to operate at an overload to meet your production goal. If the manufacturer tells you that the mixer will work properly at 40 percent overload — that is, when filled 40 percent above its rated fill level — don’t believe it, and don’t depend on operating at this kind of overload to meet your production goals. It’s like buying a motorcycle to serve as a family vehicle. Buy a properly sized mixer that handles your throughput volume without having to operate at an overload. 6. Buying an overly large mixer with a long mixing cycle. You may think that you can achieve a consistent product batch by mixing a large quantity for a longer cycle in the h-AJMixing-60-73:C-Masters_old 1/3/08 8:57 AM Page 4 mixer. In this way of thinking, if you take a sample that reveals that more mixing is required, you assume you can adjust the ingredients, mix again for 20 minutes, and come up with a consistent batch. In reality, however, you may wind up with a batch that has segregated. The assumption that mixing a larger quantity for a longer time can produce a consistent batch is based on a misunderstanding of the natural forces that cause particles to segregate and how mixing dynamics affect these forces. One of the biggest contributors to segregation during mixing is a size or weight difference between particles in the mixture. One study1 has shown that in a mixture of particles of different sizes, all larger than 50 microns, particles just 20 percent smaller or larger than other particles in the mixture will roll away from the other particles. For instance, if you place equal amounts of salt and poppy seeds in a bottle and turn the bottle a few times in any direction, you will see that the finer salt always flows to the center and the larger poppy seeds flow to the outside, as shown in Figure 1. The larger particles roll faster to the outside and then occupy this outer area, preventing the finer salt from getting there. In mixtures of particles all smaller than 50 microns, differences in particle weight are the major cause of segregation. time will deliver the most consistent mixture before segregation can occur. But the longer it takes to mix a set of ingredients, typically the more the mixture tends to demix. Segregation can occur when ingredients in a mixture have different particle sizes and weights. Here, the larger, heavier poppy seeds and multicolored sugar beads in this segregation test device flow to the outside, while the finer, lighter brown salt flows toward the center. Figure 1 Salt and poppy seeds segregating by particle size and weight in a rolling bottle For this reason, you’re better off using a smaller mixer with an efficient mixing action that provides organized particle movement. This mixing action creates voids between particles that allow the particles to randomly roll together, as well as controls the mixing speed and the void sizes to move the particles in an organized way through the mixing chamber and toward the discharge. This action produces a homogeneous mixture by overcoming the particles’ tendency to segregate by rolling apart. The added benefit is that the smaller mixer is a better investment: Its smaller size not only reduces your initial investment but reduces the mixer’s operating costs. The smaller mixer also provides more efficient mixing, which means you may be able to reduce the amount of a costly minor ingredient, such as pigment, in your mixture. Be aware, too, that a well-designed mixer discharge helps to prevent segregation. A mixer’s action must overcome segregation to achieve consistent mixing. If operated long enough, any mixer can cause a mixture to mix and demix (that is, segregate by ingredient). In fact, every mixture has its own mixing curve that traces the mixture’s progression over time from a consistent mixture to a segregated mixture, then back to a consistent mixture, and so on. The trouble is, you can’t predict what this curve is for your mixture because the particle size difference that causes segregation is so small. The result? No logic can be applied to figuring out how much mixing 7. Buying a high-intensity mixer when you don’t need one. A high-intensity mixer is an agitated mixer that operates at more than 1,000 fpm at the rotor periphery. This mixer is relatively costly to purchase, but is also expensive to run because it uses a lot of energy. It creates friction heat, too. Choosing a high-intensity mixer makes sense if you need a lot of power to mix your product, such as with a milkbone product, or if mixing the product requires friction heat, such as for compounding polyvinyl chloride (PVC). But in most applications, you can get the job done better and faster with another mixer for a lower initial investment and with lower operating costs. h-AJMixing-60-73:C-Masters_old 1/3/08 8:57 AM Page 5 Using a high-intensity mixer when you don’t need one opens the door to other problems, too. The high-intensity mixer’s agitator elements (typically plows or heavy-duty paddles) apply high shear — that is, they impart energy directly to the particles, which can degrade them. These elements provide completely random particle transport in the high-intensity mixer, which makes mixing more difficult and lengthens the mixer’s discharge time. The plows or paddles also make the mixer harder to clean, and problems with material carryover from one batch to another are common with this mixer. A mixer that doesn’t apply high shear and instead creates organized particle movement and space for particles to move can effectively mix your product while handling your particles gently. 8. Testing mixers until you drop. In bulk solids processing and handling, a test or demonstration on the equipment you’re considering is priceless. Bear in mind, however, that tests are not only expensive and often time-consuming to run, but the results are often costly to analyze. Rather than having multiple manufacturers run mixing tests on your product, a more logical process is to first interview each mixer manufacturer you’re considering. The information you receive will help you narrow the field to just a few manufacturers whose mixers appear to be close to what you need. Then get price quotes from each of these manufacturers and further narrow the mixer choices to the unit most likely to satisfy your goals. Have the manufacturer test or demonstrate your mixture in this machine, and as soon as the results are successful, make your decision and buy the mixer. PBE Reference 1. J.C. Williams, “The mechanisms of segregation,” presented at a Postgraduate School of Powder Technology seminar at The University of Bradford, Bradford, West Yorkshire, England. For further reading Find more information on selecting mixers in articles listed under “Mixing and blending” in Powder and Bulk Engineering‘s comprehensive article index at www.powderbulk.com and in the December 2007 issue. A.B. “Bliss” Flower is president of A&J Mixing International, 8-2345 Wyecroft Road, Oakville, Ontario L6L 6L4; 905-827-7288, fax 905-827-5045 ([email protected], www.ajmixing.com). He has 27 years experience in designing mixers for dry bulk solids applications.
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