The HCPS model can achieve high milling viscosities of 400,000cps. Two different sweep blades can be fitted in the HCPS mill, the sidewall scraper for thixotropic materials and the anchor helix scraper blade for heavier, tackier materials. The HCPS also has an auger extending from its upper draft tube. This allows the mill to pull material above the upper draft tube and feed it into the media field below for grinding.
Figure 1. HCPS scraper mill
Side Wall Scraper For Free Flowing & Thixotropic Products
The sidewall scraper includes one rod linked to a concentric hub rotating around the mill assembly. There is a stack of Teflon scrapers arranged atop each other continuously permitting coverage of the complete sidewall of the tank.
As the concentric hub rotates clockwise or counter-clockwise the scrapers are pressed against the tank wall by the milled material. As the scraper rotates clockwise it creates a flow away from the tank sidewall and into the auger resulting in heavier material being fed into the auger zone.
On reversing, the sidewall scraper behaves as a baffle and destroys the vortex of lower viscosity material, offering a more steady flow to the auger. The company’s new design uses the batch itself for pressing the scrapers against the sidewall eliminating the need for springs used in more traditional type scrapers. The purpose enables flow control for highly thixotropic, lower viscosity materials. The single rod scraper support helps them to be changed and cleaned easily.
The jacketed tank bottom is maintained film-free in spite of the turbulence created from the bottom propeller of the mill while milling this type of shear thinning material. Substantial heat transfer is promoted by the turbulent flow from the bottom propeller. There is a considerable improvement in heat transfer from the vessel wall to the batch as it behaves as a scraped surface agitator.
Figure 2. Sidewall scraper
Cooling with the Sidewall Scraper
The three heat transfer conditions are defined below:
- Convection is the heat transfer between comparatively hot portions and cool portions of the same liquid
- Radiation is heat transfer from or to a liquid surface to or from a gas above the surface
- Conduction is heat transfer via a fixed material like a tank wall into the cooling water jacket
The Anchor Helix Scraper: for Slow Flowing Products
Anchor Helix Scraper
The Anchor Helix Scraper features a helical design having one riser and a bottom leg spanning the complete tank bottom. The tank is scraped on both its sidewall and across the bottom. The Anchor Helix Scraper is developed for high viscosity and density products than for the Sidewall Scraper.
The agitation is provided by the Anchor Helix Scraper eliminating stratification and hot spots. The design of the Anchor Helix Scraper is such that it can scrape only in the counter clockwise direction. As the scraping of the tank walls are being performed, milled product from the tank bottom rides along the helical riser toward the tank top where it is pulled back into the mill by the auger. This enables easier cleaning, easier changing and highly effective milling.
Figure 3. Anchor Helix Scraper
Cooling with the Anchor Helix Scraper
The Anchor Helix Scraper is a very good cooling instrument for high-density slow flowing products. It is more effective in heat transfer when compared to the sidewall scraper under these conditions. A forced convection and forced conduction zone is created by the sidewall scraper while both the mill and scraper are in operation. These zones can be created by the anchor helix scraper without operating the mill. While considering the forced radiation type of heat transfer it becomes more effective due to the uniform top to bottom turnover of the batch provided by the helix.
Even though forced radiation is the least effective for cooling, it helps improve overall heat transfer. Combining three types of heat transfer and the further scraping of the cooling surface area at the bottom of the tank renders the Anchor Helix Scraper considerable benefit over the Sidewall Scraper on high density products that are not free flowing.
Choice of Scraper
For thixotropic, low density free flowing materials, the sidewall scraper is most effective. The Anchor Helix Scraper is most effective for higher density products that are slow flowing, not necessarily thixotropic and will not turn over without supplementary agitation.
The HCPS includes an auger in the mill’s upper draft tube permitting it to grab material from above the mill chamber and feed it into the media field. The mill efficiency is greatly reduced without the auger.
The effects of the auger on the immersion mill are:
- Feeds highly viscous material into the grind chamber
- Behaves as a governor to control feed rate into the grind chamber
- Behaves as a seal to prevent floating or “burping” of media as mill is immersed
- Entrapped air exits the bead field through the screens
- Prevents film formation inside upper draft tube, enhancing cooling
Figure 4. For illustrative purposes, auger is shown outside the intake upper draft tube of the mill. The auger is lowered half inside for operation
The upper draft tube has a wear resistant, hardened sleeve preventing abrasive material from wearing into the cooling chamber surrounding it. The auger comprises a much softer abrasion resistant synthetic material minimizing wear. The HCPS upper draft tube is cooled. Without the auger, film formation in the upper draft tube while the mill is running may take place, making the effect of cooling in the tube limited. Constant scraping of cool material is done and augered into the media field as it is replaced with incoming material creating both additional forced convection and conduction zones.
Engaging Problems of Scale-up Cooling
While scaling up from lab size machines to production size machines there is a need for special measures in order to maintain cooling of the batch. In comparison to a laboratory tank, the available cooling surface area of a production tank is much smaller per gallon of product.
The best way to control heat is by developing a forced conduction zone, forced convection zone and a forced radiation zone simultaneously in the tank. This is performed by scraping physically cool material from the tank walls, mixing the cool and the hot material and exposing the hot material to the atmosphere above the batch. Using internal tank cooling coils or baffles may interfere with flow at the time of high viscosity milling and be counterproductive.
Other Variables to be Considered in Heat Transfer
Other variables include the following:
- Conductivity of the material being milled – In case the batch has non-conductive or poor ingredients, temperature can build quickly and become tough to remove.
- Viscosity – The rise in viscosity, in case equipment and cooling variables remain constant, there is an increase in batch temperature.
- Shear - Tends to cause an increase in temperature with an increase in viscosity
- Solids Content - Increasing solids content tends to increase shear and temperature
- Cooling water temperature – Can be adjusted to help insure predictable temperature control
Adding the Anchor Helix Scraper or the Sidewall Scraper and the auger has significantly enhanced the immersion mill’s ability to effectively operate in viscosity ranges in which it could not function previously. The new HCPS High Viscosity Immersion Mills will enable companies to shift several products away from the roller mills. Not all roller mill formulations can be usedon the HCPS but many can be in case the formulas are adjustable.
About Hockmeyer Equipment Corporation
Hockmeyer Equipment Corporation has been providing process solutions for over 70 years. It is a leader in the wet grinding and dispersion industry and offers custom designed process equipment for mixing, blending, dispersing and particle size reduction as fine as 25 nano-meters. The Hockmeyer Team is dedicated to building and designing processing equipment that is the highest quality available. Our goal is to increase the profitability of our customers through continuous innovations in process technology.
We are committed to learning our customer’s process so that we can provide the best possible equipment for the application. We will build equipment that is as efficient as possible to reduce the energy requirements, and therefore, the environmental impact of the industries we serve. Hockmeyer offers a state of the art customer service laboratory for product testing and process development.
“We don’t sell machines, we sell solutions." Herman Hockmeyer.
This information has been sourced, reviewed and adapted from materials provided by Hockmeyer Equipment Corporation.
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