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Selecting Dispersion Equipment
One requirement must be considered in every step of the equipment-selection process: Durability.
(Reprinted from Modern Paint and Coatings, May 1980)

By JEROME P. TIPPETT, Schold Machine Co., St. Petersburg, FL

A common-sense approach and a basic knowledge of mechanics are essential in properly selecting dispersion equipment. Selection of equipment depends on experience, the manufactured product, floor space, operating modes, and mechanics. These areas are of broad scope and are extremely important.

Personal or organizational experience with certain types or brands of equipment is priceless. But experience must be supplemented with a broadminded approach. For example, some previously acceptable equipment may have become outmoded and basing equipment selection on experience alone could result in the installation of out-of-date machinery. Thus, the supplier's experience becomes important. The manufacturer can aid in setting up the process and determining whether designs are up to date.

The product line (or products) is the key factor in determining the type of equipment needed. In selecting and justifying the cost of-new equipment, products that are being considered for future manufacture must be considered. Determine their qualities: viscosity, pigment, rheology, quantity, etc. Thus, considering conceptual or the latest process equipment is logical.

If future plans are not readily available, the next most logical approach is to opt for versatility. The more versatile the equipment, the more quickly its cost can be justified.

In all businesses, floor space is at a premium. Consider the total floor space required by all components. Some units will do an adequate job and take up far less space if all previously determined processes-present and future-have been satisfied.

Calculate the floor load by determining the total weight of the unit with all components. In some cases, multistory facilities cannot stand the strain and torsionals produced by some of the heavier-duty low-rpm, high-torque dispersers.

After the product(s), present and future, has been determined, space has been allocated to the project, and the experience of those involved has been reviewed, equipment choices must be carefully considered. All of the areas previously considered will be of no value if the wrong unit, or one that has all the outward signs of having price-conscious construction, is selected. Selection must be aimed at finding the best-suited and most durable equipment. The best-suited unit would be one that has -at least- a track record in processing materials similar to those in the application under consideration. The type of dispersion required (suspension, grind, emulsion, solution, high-shear, low-shear, extrusion, rolling, pulverizing, etc.) is important in selecting durable equipment.

Selection Example

The most universally used method is high-shear or high-speed dispersion. High-speed, high-shear dispersion usually takes place in the peripheral agitator speed range of 5000 to 7000 feet per minute. For optimum material turnover and best utilization of the prime mover (an electric motor), the diameter of the agitator of a saw-tooth (or similar) design should approach one-third that of the vessel. This easy sizing of the blade diameter will, of course, come after the volume or yield of the batch has been determined; in turn. volume will determine the vessel height and diameter.

Before a unit that will power the adequately sized agitator at the proper speed is chosen, the versatility of the unit (relative to other products and even other vessels) should be considered. In most cases, one high-speed disperser can be used in as many as four sizes of vessels, providing the largest vessel and highest viscosity material were used in the initial sizing of the unit and its prime mover.

Speed Range

Keeping in mind the optimum peripheral speeds of the agitator, check to see if the speed range of the disperser will cover the range of blade diameters to be used in different vessels. (Remember, one-third the diameter of the vessel for blade diameter.)

Changing the blade from batch to batch is not always desirable. Thus, the mean diameter of the blade should relate to the various vessel diameters. The unit must also be able to make up the difference in peripheral speed of the agitator by means of a broader speed range. Units having 4:1, 5:1, and 6:1 speed ranges are by far the most versatile. They can make up for the differences in blade diameter and double as pilot units for small batches, samples, lab work, and small orders. This added versatility can help justify the cost.

All manufacturers have techniques for determining the power required to disperse fluid materials adequately. In determining the horsepower necessary for an application, the manufacturer should be consulted. The manufacturer's process history records can be used to remove the guesswork.

Dispersers having wider speed ranges and higher speeds generally are of sturdier construction. They have large-diameter agitator shafts, larger bearings, greater bearing spread, and dynamically straighter totally machined shafts. The wide-speed-range units generally have larger base plates and are free standing. A free-standing unit should still be secured to the floor, but its fastening will remain tighter longer without imposing the worry of cracked cement or rusted fasteners causing damage or injury.

The bearing size of the unit is directly proportional to the shaft size and proportional to the bearing housing diameter and length.

With the same prime mover, the faster the agitator shaft is, the larger the diameters of the shaft, bearings, and related parts will be. The larger the components are, the larger the frame will be, providing greater durability at all speeds. This mass eliminates most of the vibration and the dynamic stresses produced during the dispersion process.

For larger batches when the agitator shaft is longer than normal, the bearing housing should be proportionally longer to provide greater bearing spread. Units with a narrower speed range can survive with a shorter bearing housing, smaller diameter shaft, and smaller bearings only because their agitator shaft speeds are much less-that is, less speed, less energy, less work.

For units with air-over-oil or true hydraulic raising capabilities, three basic considerations are important: piston diameter, cylinder bearing spread, and oil-seal construction. The piston function is basic and understood. Bearing spread is important by itself and directly related to durability and the life of the seals. The rigidity of the upper portion of the disperser is a function of the cylinder bearing and the torque rods. Torque rod diameters should be increased proportionally with the power of the unit.

The rise of the unit's piston should be calculated to encompass the tallest vessel used at present and any vessels contemplated for use in the future. Plan ahead for future products and their volumes. Do not undersize the unit in horsepower or physical size if products will be added in the future.

Belt Selection

Most variable high-speed dispersers have variable-speed belts. Generally, one is all that is needed. In larger horsepower units, two variable-speed belts are used. For the most part, using multiple variable-speed belts causes problems. Alignment is a problem, noise is considerable, and the replacement cost is high. And dual-belt systems must use an intermediate shaft or jack shaft to transmit power to V belts and then to the agitator shaft.

Higher-horsepower units (100 hp and up) perform inconsistently with this system. Until these variable-drive systems are perfected, hydraulic or two speed drive should be considered. In horsepower ranges from 75 and down, units in the 4:1, 5:1, and 6:1 speed ranges do a fine job with a single variable-speed belt. Choosing a unit with one belt will eliminate the intermediate shaft and the necessary V belts and additional bearings. The noise level will drop, and maintenance will be greatly reduced. A single variable-speed belt, when lined up properly during construction at the factory, will last as long as a short belt with a V-belt intermediate shaft.

Stay away from custom-designed belts. Replacement costs are high, and they are available from only one source. Timing belts and flat belts are unsuitable for most applications.

For units with a ram-type piston, valves and related hardware require close consideration. Usually, two separate valves should be used. As in an air-over-oil system, the most predominant, a separate brass valve should be used to charge the oil reservoir with air and another valve (preferably a foot type) of equal quality should be used to meter the pressurized oil evenly and slowly for accurate positioning. These valves should be of high-quality brass with tool-steel spools and the 0 ring sealed.

The same quality should be considered when an air-lock vessel holder is chosen. Vessel holders should be of total steel fabrication, not a series of pipes and levers. Because the vessel holder is locked and unlocked as many times as the unit is started, the quality will reflect durability. Vessel holders for higher-horsepower units should be air actuated and of heavy construction; they should also be adjustable to all the plant's vessels, present and future. For lower-horsepower units, a spring actuated chain type will be sufficient as long as there is at least 120 degrees of contact surface.

Generally, the size of the components that transmit power relate to strength. Strong transmission components are more durable.

Because the greater speed ranges in high-speed dispersers give more benefits to the user, these units are better suited to today's varied product lines.

A manufacturer that designs, fabricates, machines, and assembles its units normally provides higher quality. And such a manufacturer generally has the engineering expertise to furnish special designs for specific process demands.

The equipment manufacturer is a valuable tool in selection and sizing. But units require careful comparison. Not all dispersers provide a wide range of benefits, and some can perform well only in a given set of circumstances.

Versatility and durability are the keywords.