The design of a dispersion system begins by defining the batch size and product characteristics. From there we can begin to determine blade size, types, and number which will then allow us to define shaft rpm and horsepower requirements.
Dispersion Rules of Thumb
In its simplest form we can follow these rules of thumb in defining a dispersing systems utilizing a single dispersion blade. These recommendations are listed below:
- Blade Type – A Sawtooth disperser blade, F Blade, is the initial choice for dispersion configurations.
- Tip Speed – The blade tip speed is targeted at 5000-5200 fpm (feet per minute).
- Blade Diameter – The disperser blade diameter should be approximately 1/3rd the diameter of the vessel.
- Blade Location – The blade should be located in the center of the tank
- Blade Position – Position blade 1/2 blade diameter off the tank bottom
- Liquid Level – The liquid level should be 1-1/2 blade diameters above the blade
- Freeboard – Sufficient headroom should be allowed for product vortexing to prevent material from splashing over the top of the tank. 20% freeboard is recommended.
- Disperser Power – The typical power requirements are in the range of 10 HP per 100 gallons of product.
Changes and Exceptions
While these “Rules of Thumb” present a very good tool in designing your preliminary dispersion system, there are very good reasons to depart from this configuration.
Dispersion Blade Selection
The sawtooth or “F” Style Blade is the workhorse in the industry. These dispersion blades provide a high level of shear with reasonable flow for most dispersion operations. They are a cost-effective solution as the can be formed from a single piece of stainless. The sawtooth disperser blade can operate in a wide viscosity range.
For increased viscosity, a higher profile blade may be preferred. The larger teeth produce more pumping than the typical sawtooth blade but at a cost of reduced shear. Our "G" Blade is a good example of this.
For improved dispersions, a Ring Blade or CSI blade can move the dispersion further on the Hegman Scale. These blades carry a higher price as they are fabricated from multiple pieces and assembled into their final form. They also have a limited viscosity operating range.
We generally design a disperser’s tip speed for 5200 feet per minute. Depending on pigment characteristics, adjustments can be made in the field. Easier grinding pigments can be run at lower tip speeds. Tougher products may benefit from higher tip speeds. Certain blade styles operate better above or below the 5200 fpm mark. Temperature-sensitive products can be run outside the range; lower to run longer or faster to achieve grind quicker with less heat build-up.
For high viscosity products such as caulks and adhesives, it is typical to run at tip speeds around 4200 fpm.
Disperser Geometry Considerations
Viscosity and/or rheology may effect blade diameter choices. For products that are water-thin material may over vortexes, cavitates, or splashes from the tank. For extremely viscous products, insufficient flow may occur allowing the formation of dead zones and making a homogenous dispersion impossible. When these issues cannot be overcome with changes in dispersion blade selection blade diameter can compensate for the issues. Dropping to a smaller diameter dispersion blade can still deliver the required shear while reducing the vortex depth. For the high viscosity products, a larger bade may be more efficient at establishing flow at the outer edges of the tank eliminating stagnant areas and dead-spots.
So why not start with as large a blade as possible. First, power plays a big role. We will discuss that later. Also, as dispersion is a radial flow process, there are two distinct flow patterns that arise. One flow pattern occurs below the blade, the other above the blade. We count on the area outside the periphery of the blade to allow the exchange of material between these zones. If the disperser blade is too large this exchange is greatly hampered and it is difficult to get a homogenous mix.
The disperser blade should be centrally located in the tank. While this too is largely correct, there is a case for offset blade location. A centrally located disperser blade provides the best surface motion and deepest vortex for pigment wetting. An offset location will allow higher energy consumption and quicker dispersions provided adequate flow is present to ensure a homogenous mix.
But flat bottom tanks are not the only tanks used. Dish bottom tanks eliminate dead-spots at the interface between the tank wall and bottom. The curved bottom allows us to increase the distance from the bottom to 1 blade diameter. Viscosity also plays a role here with thinner products allowing larger distances. Cone bottom tanks present other challenges and deep cones are generally avoided.
For larger batches in fixed tanks, it can be advantageous to reduce the diameter and increase the tank height. This can afford the mixer a smaller footprint better utilizing the facility height. A second blade can be added to improve surface motion in the tank. Typically, horsepower demands are lower with this configuration than the short squat tank.
Like blade diameter and location, the liquid level can be adjusted to compensate for vortex depth and surface motion. Higher liquid level can cut down on an overly aggressive vortex. Lower levels can allow for additional surface motion for better overall mixing.
Lower viscosity, deeper vortexes require additional headroom. Reduced surface motion can operate with less. A freeboard of 20% typically allows enough leeway for most mixers.