Re: Mixer Grinder Working Principle.pdf
Latrina Cobbett
This document discusses the physics behind mixer grinders. It begins with a brief history of the mixer grinder, originating from Herbert Johnson's invention in 1908. It then discusses key parts of the mixer grinder like the motor, blades, and jars. The working principle is that the motor converts electricity to mechanical energy using a coil and magnetic field to rotate the blades at high speed. Higher wattage motors between 500-1000W are recommended for home use. The document discusses construction, working, applications, maintenance and advantages and disadvantages of mixer grinders.Read less
A high-speed disperser is a type of mixer used to rapidly break apart lumps of powder material, uniformly distributing and wetting them in a liquid. It is also used to dissolve soluble solids in a liquid.
A high-speed disperser works on the principle of energy transfer. A disc-type blade is mounted at the bottom end of the mixing shaft and rotated at a relatively high tip speed. Tip speed is the speed at the outer tip or edge of the rotating disc. Tip speeds typical of high-speed dispersers are measured in feet per minute, calculated by multiplying the constant 3.14 times the diameter in feet of the disc times the revolutions per minute of the mixing shaft. The industry term for tip speed is peripheral velocity.
The solids and liquids are drawn into the rotating disc by the suction it creates. This suction usually results in a visible whirlpool from the top of the mixture down to the top of the disc. A similar whirlpool is created below the disc, extending from the bottom of the tank to the underside of the disc. The whirlpools are actually two individual vortices, although common industry practice refers only to the visible upper one as the vortex.
When the solids/liquid mixture enters the vortices and is sucked into the high-speed disc, the energy (horsepower used to drive the disc) is instantaneously transferred from the disc to the mixture. This intensely focused energy transfer creates tremendous, instantaneous velocity changes in the mixture as it progressively contacts the disc. Think of the mixture as a series of individual horizontal layers descending downward from the top and upward from the bottom onto the face of the rotating disc. As each layer contacts the disc, it is instantaneously accelerated from the slow-moving vortex into the high speed of the disc and projected outward away from the disc and toward the wall of the tank. The rapid tearing apart of layer upon layer of the mixture is shear force, commonly referred to as shear.
Moderate-shear high-speed dispersers operating at about half the normal blade speed of high-shear dispersers are sometimes used in place of agitators when some shear is required. Typically, the discs have larger teeth to promote better pumping and require about a third of the horsepower of a high-shear disperser but still three times more than an agitator.
A high-speed disperser will generate the shear force necessary to rapidly de-lump powders in a liquid. This de-lumping process is called dispersion. The agitator is an efficient mixer, but it typically cannot generate sufficient shear to disperse powders, regardless of how long they are mixed. This is because the forces holding the agglomerates (lumps) together are stronger than the force of the mixer trying to pull them apart. Mixers can do an excellent job of holding dispersed (sheared) mixtures in suspension, but they typically cannot disperse (shear) the mixture.
Adding supplementary agitation to help feed the high-speed disperser blade can extend the operating range of a disperser. This type of machine is typically called a dual or triple shaft mixer. It has a shaft with a slow-moving sweeper blade passing close to or scraping the tank wall to promote mass flow, and one or more additional mixing shafts with disperser blades to generate high-shear.
High-speed dispersers are available with single-speed, two-speed, and variable-speed mixing shafts. Some are directly mounted atop a tank and are fixed to operate with the blade in only the original mounting position. Other tank-mounted dispersers can raise and lower the blade by several feet (to better control the vortex) without exiting the tank.
Another design, perhaps the most popular, places the disperser on top of a hydraulic lift, similar to the ones used at gas stations to lift automobiles, that is mounted to the floor. The lift enables the operator to raise the blade completely out of the mixing vessel and change to another vessel. This technique uses small portable tanks (up to 500 gallons) that can be rolled away on wheels or picked up with a fork truck. Larger stationary tanks are often centered within the arc of rotation from the center of the hoist to the center of the mixing shaft.
The bridge containing the mixing shaft at one end and the motor at the other is then rotated from one tank to the next. Choosing the best configuration of available designs is a combination of functional need and economic justification. An experienced process engineer or consultant familiar with dispersers is a good investment.
The size of a high-speed disperser is generally thought of in terms of horsepower. However, there are dispersers that are dimensionally very large but use relatively small amounts of horsepower. These are exceptions to the rule.
The horsepower of the disperser is related to the blade diameter and the anticipated load the blade will create at a given speed and resistance. The resistance is a function of the rheology of the dispersion as well as the viscosity and density. However, as the blade diameter increases, the horsepower increases disproportionately. For example, if a 12-inch diameter blade were to draw 20 HP in a non-Newtonian system (viscosity changes with shear), doubling the blade diameter could increase the HP demand by a factor of 5. That means a 24-inch diameter blade of the same design, working in the same product, would require 100 HP. The larger blade would also pump considerably more, so it would lend itself to working in a much larger (perhaps 5 times the volume) tank and producing a much greater amount of finished product in the same time period.
Horsepower requirements are interrelated with blade diameter, tank diameter, batch size, rheology, viscosity, and density. Variations outside recommended operating parameters usually result in compromises in performance, such as poor particle separation, extended dispersion times, and a decrease in quality of the finished product.
The ideal manufacturing tank for most high-speed dispersers is slightly taller than wide. Dished or bowl-shaped bottoms aid in preventing solids from accumulating in sharp corners associated with flat bottoms. Equally as important, dished bottoms drain to the center, where a discharge valve can be installed. Flush bottom ball valves welded into the center of the dished bottoms further enhance the ease of discharge and cleaning. Optimum tank geometry is an integral part of several aspects that need be considered and are listed further on in this article.
The blade is sized based upon the flow characteristics of the product and the desired degree of dispersion. The thicker the product, the larger the blade diameter must be in comparison to the tank diameter. Conversely, the thinner the product, the smaller the blade diameter must be in comparison to the tank diameter. This comparison is called the blade-to-tank ratio. Thick products like heavy, flowable pastes may require a .5:1 ratio. Moderate products like paint require a .33:1 ratio, and thin products like stains can work with up to a .125:1 ratio.
Once the batch formula has been process optimized, the typical time required to reach maximum dispersion should range from 20 to 30 minutes after the last ingredients have been added. Longer times do not usually result in better dispersions and in some cases can be detrimental because of the higher batch temperatures generated by the high-shear disperser blade. As the blade begins to wear, longer and longer batch times are required to get to the optimized dispersion standard. Sawtooth-type high-speed disperser blades should be replaced once the blade tips are worn to half their original height.
high-shear disperser blades are available in a range of styles and sizes. They can be generally categorized into two groups: open sawtooth and ring type. Both categories work well when used under the proper operating conditions.
The open sawtooth blade is the most popular because of its low cost, ease of cleaning, and general utility. It is available in a wide range of tooth designs. As the teeth increase in size and become more aggressive in shape, the pumping ability of the blade increases. However, as pumping (turbulent flow) increases, shear decreases. A high-pumping saw blade still generates significant shear compared to a low-shear paddle blade agitator. This aspect is an important consideration when determining exactly what is to be achieved in the finished product.
The ring type blade is a powerful tool for optimizing disperser performance. It is more expensive to purchase and consumes more horsepower than the saw blade. It typically runs at higher tip speeds (5,700 + fpm) and performs more like a rotor stator.
Typically, high-speed dispersers perform best when the flow pattern is doughnut-shaped and the blade tips are traveling at about 5,000 feet per minute in a medium viscosity (1,500 to 5,000 centipoises). Lower tip speed may be acceptable at higher viscosities and higher tip speeds may be acceptable at lower viscosities to get to the same shear rate and stress. In other words, shear is a function of blade-tip speed and product rheology.
The high-speed disperser is a very fast and powerful machine. Serious and fatal accidents can occur in a split second of carelessness. Human reflex is no match for the instantaneous danger of operating a disperser unsafely. Never sacrifice safety for convenience.
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