The high shear mixer falls into this category when operating at high speed in turbulent mode as it combines an intense hydraulic shear with its excellent blending capability. The uniformity of the high shear field in the mixing head means that some of the problems associated with stress uniformity in normal high shear mixers are avoided. The high shear mixer typically put 5 times more energy into fluid than equivalent high shear mixers and achieve up to 25% improvements in dispersion performance.
The gas dispersion required is a function of the mass transfer rate. A given time cycle can be attained by a high mass transfer rate with relatively small surface area contact (large bubbles) or by a slower mass transfer rate and a high surface area contact (small bubbles). The horsepower requirement is determined by the gas bubble size needed to give the desired performance. Horsepower increases with increasing surface area requirements.
The majority of products need to be recirculated through an inline high shear mixer – whether single or multistage – to get the desired result. Our inlines self-pump at a far higher rate than triple-stage units, ultimately giving the same result in the same or less time. In addition the simple, robust construction of our inline mixers makes them easier to clean, easier to operate and easier to maintain.
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Rotor-stator dispersers usually called
vacuum mixers, are the most common form of dispersing mixer. By placing a form of closely-fitting shroud around a high speed impeller, it is possible to create a shearing action between the blades and stator shroud. As material is centrifugally pumped through the mixing head, some of it will see this high shear zone and experience shear stressing that results in dispersive mixing. Where small or uniform dispersions are required, material must be cycled through the head many times to ensure statistically that all of the material has passed through the high shear zone at least once. The viscosity range of fluids to be sheared is restricted, especially in batch mode, as the mixer uses a centrifugal pumping action which is not effective at higher viscosities. Although their dispersion performance is limited,
vacuum emulsifying mixers are fairly flexible in terms of their duties and capabilities and are also available in both batch and inline forms.
Vacuum mixing systems are designed for relatively high fluid discharge velocities in order to disperse the gas into fine bubbles throughout the liquid. This will retain the gas in the liquid as long as possible and provide greater contact surface between the gas and liquid phases. The liquid in the vessel will expand by the volume of gas entrained and it is therefore necessary that adequate expansion space be provided above the normal ungassed liquid level. Under conditions of high gas input and/or low tank pressure the batch may expand considerably resulting in a reduced batch specific gravity.
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